Chapter 9
Habitable Worlds in the Solar System and Beyond
On the summer night of July 16, 1969, Neil Armstrong (1930–2012), Buzz Aldrin, and Michael Collins blasted off from Kennedy Space Center in Florida for humanity’s first visit to the surface of another celestial body. Three (Earth) days later, the spacecraft dropped into Lunar orbit, from whence Armstrong and Aldrin split off from Collins and took the Lunar module Eagle down to the surface. On July 20, uttering his historic—if slightly misspoken—words, “That’s one small step for [a] man, one giant leap for mankind,” Armstrong stepped off the Eagle’s ladder and onto the face of our Moon. Aldrin joined him shortly thereafter, and after a scant two and a half hours of exploring the dusty Lunar surface on foot, the duo returned to the Lunar module. Some 18 hours later, after an awkward, noisy “night’s” sleep, the two launched the Eagle back into space to rendezvous with Collins. Firing up the command module’s single rocket, the reunited astronauts spent three more days on the return home, ending the historic Apollo 11 mission by splashing down in the Pacific on July 24.
On their return, the three astronauts were immediately ushered into a fully self-contained and hermetically sealed isolation chamber (a converted recreational vehicle!), physically cut off from the rest of the world. Heroes they may have been, but they would remain behind tightly sealed doors and windows as if they, themselves, were some exotic Lunar samples. The reason? A fear that “Lunar bugs” that might have infected them could escape to wreak havoc on our planet.
After three days, the trailer arrived in Houston, Texas, where the three astronauts joined three others (a doctor, a NASA public affairs officer, and a film technician who was accidentally exposed to some Lunar dust while handling a film canister) in more spacious quarantine facilities at the Lunar Receiving Laboratory. While the doctor kept close watch on the health of the astronauts, scientists in other, equally well-isolated laboratories incubated Lunar dust in nutrient broths and injected it into mice to see whether any microorganisms could be cultured. After two and a half weeks, neither the astronauts nor any of the mice seemed the worse for their exposure, and the quarantine was lifted. The astronauts were released to a world tour, months on the banquet circuit, and changed lives.
In December 1969, some six months after Apollo 11 achieved President Kennedy’s goal “of landing a man on the moon and returning him safely to the Earth,” the Apollo 12 mission set off for the Moon’s prosaically—if inaccurately—named Oceanus Procellarum (Ocean of Storms). One of the many goals of this mission was to demonstrate that the Apollo technology could achieve a precision landing (Apollo 11 had missed its planned landing site by several kilometers) so that future missions could explore more interesting, but more difficult, terrain. The all-Navy crew pulled it off and landed within 200 m of their target, the no-longer-functioning, robotic Surveyor 3 spacecraft that had landed on the Moon just two and a half years earlier. The astronauts, Pete Conrad (1930–1999) and Alan Bean (1932–2018), spent almost eight hours exploring the Lunar surface, during which they collected 34 kg of rocks and soil.* They also snipped off several pieces of Surveyor 3 for return to Earth so that engineers back in Houston could see how the various materials had fared after so many months under the harsh Lunar conditions. And what happened when the Apollo 12 astronauts returned? They too were ushered into quarantine. Once again, though, no Lunar life was found, save, perhaps, for some Terrestrial bacterial spores that may (or may not; there is some debate as to whether they were picked up after the sample was returned to Earth) have survived in hibernation, buried under some insulation deep within Surveyor’s camera.
The lack of Lunar life was, in reality, to be expected. The argument against life on the Moon was and remains, basically, that if you wanted to build a really good sterilizer, you’d make something like the Moon’s surface: no atmosphere, no water, extremes of heat well past the boiling point, intense radiation, and intense UV light. Still, even with the insights provided by the intervening decades, the quarantine precautions seem prudent. After all, what do we really know about the range of conditions that life in its broadest scope might find suitable? How does the broad range of environmental conditions to which life has adapted on Earth correspond to the conditions that exist elsewhere in the Solar System and beyond (again, given the caveat that the set of environmental niches that life can evolve to fill may be much larger than the set of conditions under which life can arise in the first place)? Our question is, in a nutshell: are there habitats—that is, habitable environments—beyond Earth? Here we explore this issue in detail for the Solar System and, in necessarily less detail, for the rest of the cosmos.
Potential Abodes of Life Elsewhere in the Solar System
Let’s start the search in the immediate neighborhood. Although the first astronauts to return after a Moon landing were placed under quarantine for fear of an infection with Lunar life forms, it is now clear that our satellite never had the rich mix of volatile elements that are almost certainly required for life. If there are any microbes on the Moon, it is safe to assume that they traveled there with the astronauts and that they’ve been dead for several decades now.
Moving inward from our home planet we have Venus, the planet most like Earth in terms of mass and composition, then tiny, rocky Mercury. Being so small and so close to the Sun, Mercury is hard to observe and thus was never much in mind during early speculations on life beyond the Earth. Just as well, since we now know that Mercury is a hot, dead world devoid of any appreciable atmosphere (see fig. 3.9). Venus, in contrast, is our closest neighbor in space and thus relatively easy to scrutinize through a telescope. Yet our sister planet presents a featureless, cloud-covered visage when viewed in visible light, an observation that encouraged endless speculation: the obvious interpretation being, of course, that clouds mean abundant water! Basing his hypothesis solely on Venus’s thick cloud cover and relative nearness to the Sun, Svante Arrhenius (you’ll remember him as the Nobel laureate and panspermia promoter) speculated in 1918 that “everything on Venus is dripping wet” and that “the vegetative processes are greatly accelerated by the high temperature; therefore, the lifetime of organisms is probably short.”* Sadly, though, spectroscopic observations conducted in 1922, just a few years after Arrhenius’s optimistic statement, failed to detect any water vapor in the Venusian atmosphere. As we now know, the clouds of Venus are composed not of water but of sulfuric acid droplets. The problem, as we explored in chapter 3, is that due to photolysis and the escape of hydrogen to space, Venus lost its water long ago, and the runaway greenhouse effect that ensued raised its surface temperature well above the melting point of lead. Precisely how long this took is still a matter of speculation. But the consequences for the possibility of life on our nearest planetary neighbor are clear: we don’t know of any chemistry that is of sufficient complexity to support life that would be able to withstand Venus’s hotter-than-an-oven conditions (see fig. 3.10). And while there have been suggestions of cooler, more favorable conditions in the Venusian clouds, we should probably conclude that the overall probability of life on Venus is very low. Indeed, if there are any other denizens of the Solar System, it seems likely that they live out beyond the Earth’s orbit.
Mars: From Canals to Rovers
From early on it was clear that, after Earth, Mars is the most promising potential habit in our Solar System, a designation that it continues to hold today. It is, for example, the second closest planet to the Earth. Even the modest telescopes available in the late eighteenth century were sufficient to let astronomers define the length of its day (just 37 minutes longer than ours), identify clouds and polar ice caps, and discover that Mars’s axial tilt is similar to Earth’s, giving it Earth-like seasons. More tantalizing still, by the late nineteenth century astronomers had noted that with these seasons came a “wave of darkening” occurring on a global scale. Similar variations are seen on Earth in, for example, New England or Scandinavia in the autumn. Could the Martian color changes also represent seasonal variations in plant coverage?
Attempts to actually detect life on the Red Planet, rather than simply speculate about it on philosophical and theological grounds, date back to at least 1877. In that year, the Italian astronomer Giovanni Schiaparelli (1835–1910) turned his telescope on Mars and thought he spied extended networks of trenches, which he termed canali, the rather innocuous Italian word for “channels.” And although there is no indication that Schiaparelli meant to imply that these structures were the handiwork of intelligent Martians, others—in particular, the independently wealthy and exceptionally self-promoting American astronomer Percival Lowell (1855–1916)—eagerly translated the Italian canali into the English canals. That is, artificial structures. In 1894, Lowell built himself one of the finest observatories of the era in the mountains of northern Arizona and spent decades visually mapping out the features on Mars and concocting a detailed hypothesis about a dying, intelligent race that had heroically built the canals to carry water from the poles as their planet fell into drought. This theory, he felt, explained much about the appearance of Mars. Indeed, in 1916, Lowell summed up his more than two decades of work on the subject by boasting, “Since the theory of intelligent life on the planet was first enunciated 21 years ago, every new fact discovered has been found to be accordant with it. Not a single thing has been detected which it does not explain. This is really a remarkable record for a theory. It has, of course, met the fate of any new idea, which has both the fortune and the misfortune to be ahead of the times and has risen above it. New facts have but buttressed the old, while every year adds to the number of those who have seen the evidence for themselves.”*
Given what the scientists of the day were saying, it’s not surprising that Mars became a popular location of extraterrestrial life-forms in the imagination of science fiction writers in the first half of the twentieth century. The invaders trying to colonize Earth in H. G. Wells’s classic War of the Worlds (1898) were only a vanguard of the many different civilizations placed on Mars by writers. In 1908, nearly a decade before the Russian revolution would bring forth another kind of red star, the Russian author Alexander Bogdanov (1873–1928) published the novel Red Star, in which the Red Planet is used as a canvas to describe a socialist utopia. In 1917, Edgar Rice Burroughs (who later created Tarzan) published his very first novel, A Princess of Mars, in which a Civil War–era prospector is mysteriously transported to Mars, where many a daring deed ensues. Even as late as 1938, an American radio broadcast of Wells’s War of the Worlds was so believable that it drove much of the country to panic. The possibility—nay, the certainty—of life on Mars was part and parcel of nineteenth- and early twentieth-century thinking about our next neighbor from the Sun.
Sadly, with the advent of improved telescopes and, perhaps not coincidentally, photographic methods that replaced visual observations, the case for intelligent life on Mars dropped away. Lowell’s canals have never been captured on film (fig. 9.1): in this enlightened—if less romantic—age, we know that they were the creation of an overactive imagination, poor “seeing,” and no doubt a sincere belief that we are not alone. Still, the same telescopic observations that killed the canals supported the observation of widespread seasonal color changes mentioned above, suggesting that, even if it lacked civilizations, Mars at least harbored simple plant life. But cracks were apparent in this hypothesis, too, as far back as 1909. In that year, William Campbell (1862–1938), who’d had a number of impressive “fights” with Lowell in the literature, led an expedition to the top of California’s 4,421 m Mount Whitney to collect spectra of the Martian atmosphere.* From this vantage point, situated above half of the Earth’s atmospheric mass (Whitney was the highest point in the then 46 US states, and remains the highest point in the United States outside Alaska), he first collected a spectrum of sunlight reflected by the Moon, which he knew to lack an atmosphere, as a means of correcting for any absorbance caused by the water in our atmosphere. Then turning his instruments toward Mars, he found that its water “absorbance bands” were the same despite the fact that the light had passed through the Martian atmosphere twice: once on the way in from the Sun and once on the way back out to the observers on the mountain. From this it was clear that the water content of the Martian atmosphere was far less than even that in the thin, dry atmosphere above his perch, casting serious doubt on Lowell’s vision of a planet covered not only with life, but with a globe-spanning civilization.
Mars is our second-closest neighbor in space, though, and has weather and seasons like the Earth’s, so surely it might be an abode for life? Although spectroscopic studies following on Campbell’s pioneering work put the carbon dioxide content of the Martian atmosphere at about 1% of the total atmospheric pressure on the surface of Earth, most of the Earth’s atmosphere is nitrogen, which is invisible in the infrared and thus not easily detected. Might Mars have a thick—if largely invisible from Earth—nitrogen-rich atmosphere that contains only a trace of carbon dioxide? Even early users of telescopes had noted clouds, suggesting weather, and the annual waxing and waning of the polar ice caps, hinting at Earth-like conditions. And, of course, there is that seasonal wave of darkening. Even as late as 1958, the American astronomer William Sinton (1925–2004) reported the spectroscopic detection of organic material on Mars, quite possibly as chlorophyll. Sadly, however, all this proved untrue. Sinton’s spectroscopic “bands,” for example, were soon to be argued away as experimental artifacts caused by similar features in the spectrum of deuterated water (water with one hydrogen atom replaced by a deuterium) in the Earth’s atmosphere. And that seasonal wave of darkening? Improved telescopic observations throughout the 1950s proved that much, if not all, of the proposed color change was due to seasonal changes in the distribution of dust associated with massive, sometimes globe-spanning dust storms. There are neither green fields nor golden autumnal forests on the Red Planet.
Figure 9.1 Percival Lowell used his state-of-the-art observatory to map out the canals of Mars in exquisite, hand-drawn detail. As shown in the inset, though, Lowell’s linear canals sadly did not survive the advent of astrophotography. (Courtesy of NASA/STScI)
Table 9.1
Past successful robotic missions to Mars
Mission |
Flyby dates or dates active in orbit or on surface |
Notes |
Mariner 4 (flyby) |
July 15, 1965 |
First Mars flyby; 22 images of southern hemisphere reveal cold, thin atmosphere |
Mariner 6 (flyby) |
July 31, 1969 |
Confirm thin atmosphere, low temperatures, and crater-saturated southern landscape |
Mariner 7 (flyby) |
August 5, 1969 |
|
Mariner 9 (orbiter) |
November 13, 1971 to October 27, 1972 |
Medium-resolution map of entire planet indicates northern hemisphere more geologically active, shows signs of fluvial (fluid-carved) features |
Viking 1 (orbiter) |
June 19, 1976 to August 17, 1980 |
Higher-resolution images confirm abundance of fluvial features |
Viking 2 (orbiter) |
August 7, 1976 to July 25, 1978 |
|
Viking 1 (lander) |
July 20, 1976 to November 13, 1982 |
First successful Mars landers; carried cameras, meteorological instruments, and suite of life-detection technologies |
Viking 2 (lander) |
September 3, 1976 to April 11, 1980 |
|
Mars Pathfinder(lander and rover) |
July 4, 1997 to September 27, 1997 |
Technology demonstrator |
Mars Global Surveyor(orbiter) |
September 11, 1997 to November 5, 2006 |
Technologically advanced orbiter carrying an altimeter (to measure topography), an infrared spectrometer, and cameras with 1.5 m resolution |
Mars Odyssey (orbiter) |
October 24, 2001 to present |
Carrying an imaging infrared spectrometer and a γ-ray and neutron spectrometer |
Mars Express (orbiter) |
December 25, 2003 to present |
Carrying a 2 m resolution camera, a high-resolution infrared spectrometer, and a subsurface-sounding radar to search for groundwater |
Mission |
Flyby dates or dates active in orbit or on surface |
Notes |
Spirit (rover) |
January 4, 2004 to March 22, 2010 |
Explored Gusev Crater for almost five Earth-years before becoming stuck in soft sand and continuing operations at that fixed location |
Opportunity (rover) |
January 25, 2004 to June 18, 2018 |
Explored hematite deposits of Meridiani Planum |
Mars Reconnaissance Orbiter |
March 10, 2006 to present |
Carrying a 0.3 m (!) resolution camera, a high-resolution imaging infrared spectrometer, and a shallow-subsurface radar |
Phoenix (lander) |
May 25, 2008 to November 2, 2008 |
Stationary lander; explored the soil chemistry at far northern latitudes |
Curiosity (rover) |
August 6, 2012 to present |
A nuclear-powered, 900 kg rover with spectroscopic and chromatographic equipment for chemical analyses and sniffing out potential habitats; findings suggest conditions at Gale Crater were once wet; discovered most significant organic chemistry yet found on Mars |
Mars Orbiter Mission |
November 5, 2013 to present |
Indian Space Agency’s first interplanetary spacecraft; technology demonstrator |
MAVEN (orbiter) |
September 24, 2014 to present |
Mars Atmosphere and Volatile Evolution mission’s goal is to determine how the Martian atmosphere and hydrosphere, presumed to have once been substantial, were lost over time |
ExoMars Trace Gas Orbiter |
March 14, 2016 to present |
This European Space Agency mission includes high-resolution spectrometers aimed at assaying the trace gas components of the Martian atmosphere |
InSight (lander) |
November 26, 2018 to present |
Landed in the Elysium Planitia region; investigating Mars geophysics |
The final nail in the coffin of Mars’s romantic, life-filled image came with the first close-up investigations of the planet. In 1962, after one American and several Soviet attempts had failed, the US spacecraft Mariner 4 flew 9,800 km above the planet’s southern hemisphere (table 9.1 provides a summary of humanity’s successful robotic missions to Mars). Along the way, it snapped 22 black-and-white images, 200 × 200 pixels, each of which required nearly 10 hours to send home via the spacecraft’s eight bits per second radio link. With this, centuries of speculation were supplanted by the first, close-up glimpses of the surface of Mars.
Sadly, far from the dynamic, life-filled world of lore, the pictures revealed an ancient, heavily cratered landscape reminiscent of the Moon (fig. 9.2, left). The Mars of Mariner 4 appeared geologically dead, without even the dynamism to erode away the many craters dotting its surface. Equally troubling, as the spacecraft passed behind Mars (from the viewpoint of Earth), refraction of its radio signal through the Martian atmosphere provided measures of the temperature and total pressure of the Martian atmosphere. The temperature, it found, was a chilly −100°C, far colder than previous, Earth-based estimates. Worse, the total atmospheric pressure, less than 1% of Earth’s, corresponded closely to the carbon dioxide pressure determined spectroscopically via telescopic observations—no thick nitrogen atmosphere for the Red Planet. Disappointingly, this pressure is so low that water cannot exist on the surface of Mars in the liquid state: under present Martian conditions, water sublimes directly from ice to vapor without passing through a stable, liquid phase. Two more flybys, by Mariner 6 and 7 in 1969, returned another 70 close-up pictures of ancient, heavily cratered terrain across the southern hemisphere and further strengthened the case for a thin, cold, predominantly carbon dioxide atmosphere. So much for Lowell’s heroic civilization.
But perhaps not all is lost. The generally pessimistic funk that Mars research fell into after the flyby missions lifted a little bit in 1971, when, after a launch failure left Mariner 8 resting on the bottom of the Atlantic Ocean, Mariner 9 became the first spacecraft to orbit another planet. Upon the approach, the Martian disk appeared strangely featureless; the planet was engulfed in a dust storm of global proportions. When the dust cleared a few weeks later, however, the outlook was brighter than expected. Mariner 9 found evidence of a far more active planet than dreamed of after the earlier flybys; only the southern hemisphere of Mars is old and cratered (even now, a half century later, the origins of this massive, global asymmetry remain an open question). Much of the remainder of the planet shows abundant signs of past geological activity, including extinct, or at least dormant, volcanoes up to twice as high as Mount Everest and a canyon system that would dwarf the Earth’s Grand Canyon.
Of particular interest to us, some of the exciting geology that Mariner 9 turned up seems to have been created by a fluid, perhaps water (fig. 9.2, right). This evidence included what looked like vast sedimentary deposits in the polar regions and extensive networks of valleys that looked very much as if they were formed by rivers. But the orbiter also made detailed studies of the Martian weather and confirmed that, although the Martian tropics can top out at a downright pleasant 24°C during the very warmest summer days, the temperature on Mars is typically quite far below freezing, and the planet’s atmospheric pressure is too low to allow water to exist as a liquid. The post–Mariner 9 view was of a now frozen and exceptionally dry planet with only ancient relics of a presumably warmer, wetter past.
Figure 9.2 Mariner 4, the first successful Mars flyby spacecraft, returned pictures of a bleak, geologically dead landscape reminiscent of the Moon (left). The more global picture provided from orbit by Mariner 9 is quite different (right). Mars’s northern hemisphere is covered with the remnants of once active geology, including many seemingly water-carved features such as the putative river channel shown here. For scale, the crater in the image on the left is approximately 150 km across, and the image on the right covers approximately 400 km of the 600 km long Nirgal Vallis. (Photos courtesy of NASA/JPL)
Given the Mariner 9 views, optimism about Mars ran fairly high during the 1970s when follow-on studies from the Viking 1 and 2 orbiters (we discuss their associated landers in the next chapter) carried much better cameras into Martian orbit. With the higher resolution provided by these cameras, scientists back on Earth identified a good many apparently fluvial (fluid-formed) features, including presumably water-carved, teardrop-shaped “islands” within massive outflow channels that themselves appeared to have been carved by intensive, if brief, floods, perhaps exceeding by a factor of 10,000 the flow rate of the mighty Mississippi River. As further evidence of these massive floods, the orbiters spied vast areas of chaotic terrains reminiscent of the scablands of the Pacific Northwest, which were generated at the end of the last ice age when an ice dam broke and a volume of water the size of Lake Superior flooded large parts of what are now Idaho and Washington states. There were even claims that the Viking orbiters had spied fossil remnants of the shorelines of some ancient Martian ocean. Based largely on these images, the warm and wet early period of Martian history seemed assured.
Unfortunately for those of us who would like to find Mars inhabited, the tide has once again largely turned against the warm, wet early Mars hypothesis. One of the most fundamental problems with this idea is that, as we discussed in chapter 3, the early Sun is thought to have been some 20% dimmer than it is today. And if sunlight cannot now heat Mars above the freezing point of water, how could it have done so billions of years ago? A thicker carbon dioxide atmosphere could have bumped the temperature up in the past, only to be lost later. Mars lacks a magnetic field, and thus the solar wind collides with the Martian atmosphere, providing a mechanism for such loss. The MAVEN spacecraft, in orbit since 2014, has measured the resulting erosion of the atmosphere and found it amounts to a ton a day, rising to 10 tons a day during solar storms. Even still, though, going through the numbers in detail, it has proven hard to come up with a physically plausible scenario by which Mars remained largely above freezing during the earliest days of the Solar System only to fall later into its present-day cold funk.
Further evidence against the warm, wet early Mars scenario was provided by the high-resolution infrared spectrometer on board another orbiter, Mars Odyssey, launched in 2001 and thus named after the film 2001, A Space Odyssey. Mapping the mineralogy of the entire planet, the spectrometer failed to find any carbonates. Given that carbon dioxide forms these (e.g., limestone) whenever liquid water is present on Earth, their absence suggests that Mars has never hosted significant bodies of surface water. Following on Odyssey, the Mars Reconnaissance Orbiter arrived in orbit around the Red Planet in 2006 and continues to map the planet with the highest-resolution cameras ever sent into deep space (its surface resolution is 30 cm) and a high-resolution imaging spectrometer that should be wonderful for spotting the signatures of—to speculate a bit—hydrated minerals surrounding some ancient hydrothermal vent. Despite many years of searching, though, it has not turned up any clear signs of water-derived minerals other than some clays and a few small areas containing hydrated silica and hydrated sulfates. None of these, however, is thought to require long-term liquid water to form, much less oceans of it. It now seems generally accepted among planetary scientists that Mars never had a long, warm, wet spell.
If Mars never harbored long-term liquid water, then where did its fluvial erosion features come from? There’s been no end of speculation about this. The suggested possibilities range from wind erosion, to temporary running water—either underneath thick, insulating ice caps or after a meteor strike vaporized some ice and cranked up the atmospheric density—to sapping (erosion from within) caused by the eruption of geothermally heated groundwater. Mars does, after all, have enormous and relatively fresh-looking volcanoes, so a liquid, subsurface aquifer is definitely within the realm of possibility, even if Mars never had a consistently warm and wet period in its history.
This vision of intermittent and brief bouts of liquid water on Mars has gotten some boosts in recent decades. In early 2004, abiding by its Mars creed “follow the water,” NASA landed solar-powered rovers at two seemingly water-modified landscapes in Mars’s northern hemisphere. The first, called Spirit, touched down (bounced to a stop, actually, on airbags) on January 4 in the Gusev Crater. Located at the end of the 10 × 900 km Ma’adim Vallis (valley), Gusev appeared from orbit to have once been a lake. After extensive exploration, however, Spirit found little but the most indirect evidence that the crater was ever filled with water. Some three weeks later, Spirit’s sister craft, Opportunity, bounced to a stop on Meridiani Planum,* a smooth, flat plain the size of the state of Colorado that had been selected because spectroscopic investigations from orbit indicated it was decorated with the mineral hematite. Hematite, a type of iron oxide, can be formed by several mechanisms, but on Earth it is most commonly deposited in aqueous environments. Cushioned by its airbags, Opportunity bounced a dozen times on the Martian surface before finally coming to rest in a small crater. When the rover shook off its landing and turned on its cameras, it found itself staring at thick, layered beds (fig. 9.3). The Opportunity rover, the fifth successful Mars lander, was the first to find bedrock.
Figure 9.3 In 2003, the Opportunity rover bounced down onto the plains of Meridiani and struck a cosmic hole-in-one by landing in a small crater dubbed Eagle. Detailed imaging and spectroscopic investigations of the bedrock suggest that it is sedimentary and was laid down from liquid water—briny, highly acidic water, but liquid water nonetheless. (Courtesy of NASA/JPL)
On Earth, the majority of layered rock is sedimentary, laid down over successive seasons or successive floods by liquid water. But successive volcanic eruptions (either as lava flows or as successive layers of ash) can also form layered rock, as can shifting patterns of windblown dust. So, which was it at Meridiani? The bedrock in Opportunity’s crater showed clear evidence of cross-bedding, or layers formed at angles, which often formed in stream beds on Earth due to turbulent flow. Some of the rock surfaces also showed clear signs of polygonal cracks, reminiscent of the hexagons that sometimes form in drying mud. The layers were also filled with the type of small, round rocks that littered the ground at Meridiani. Termed “blueberries” after the fruit they seemed to resemble in shape and size, if not color (somehow “grayberries” just sounds wrong), the small spheres littering the crater are composed of hematite—again, most likely formed in situ in the rocks by the action of water.
Perhaps even more revealing than the gross structures of the strata are their chemical compositions. The rover’s α-particle and x-ray spectrometer (APXS), which can identify the elemental composition of rocks (see sidebar 9.1), indicated that the rocks at Meridiani contain large amounts of magnesium, calcium, and iron sulfates along with traces of chlorine and bromine. On Earth, such salts form preferentially by aqueous deposition, and on Mars, are a likely sign that liquid water was once present. Similarly, Opportunity’s Mossbauer spectrometer found evidence that some of the iron at Meridiani is tied up in the mineral jarosite, a hydrated form of potassium iron sulfate that, on Earth, is invariably formed by the aqueous leaching of iron minerals under acidic conditions. In short, it looks as if we have very firm evidence that at least parts of Mars were wet sometime in the distant past (albeit with rather acidic brine) but that Mars then took a very different environmental turn.
Following up on Spirit and Opportunity, in 2011 NASA launched a far more sophisticated rover toward Mars, the 900 kg Curiosity. Landing in Mars’s Gale Crater in August 2012, after a journey of more than half a billion kilometers, it touched down within 2.4 km of the center of its intended target zone. Equipped with the most sophisticated chemical analysis equipment yet sent to another planet, Curiosity, whose mission continues as of late 2020, is pursuing two primary goals: determining the nature and inventory of any remaining carbon on Mars, and mapping out Mars’s climatic history over the last few billion years. To achieve this, the rover was targeted to the base of Mount Sharp, an isolated peak reaching 5 km above the floor of the 150 km wide crater. Images from orbit indicated that Mount Sharp is composed of layers of exposed sediment containing, geologists hoped, a multibillion-year record of sediments washed into the crater by what appeared to be one of Mars’s many ancient river channels. On its route up Mount Sharp, the rover explored clays and imaged putative, pebble-filled former streambeds (fig. 9.4). But any water in the Gale Crater was long in the past; Curiosity’s Russian-built dynamic albedo of neutrons (DAN) instrument (see sidebar 9.1), which uses neutrons to probe for hydrogen within the first two-thirds of a meter of soil or rock below the rover, has identified only a single, small area along Curiosity’s multiyear trek to date in which the soil water content even approached 4%.
Figure 9.4 In 2011, the Curiosity rover landed in Gale Crater, at the foot of the 5.5 km high Mount Sharp (top). Composed of sedimentary layers, Mount Sharp is believed by geologists to hold a multibillion-year record of the climate of Mars (bottom left). Shown is a close-up image of seemingly water-polished stones, providing putative evidence for water flowing down the mountain’s slopes (bottom right). (Images courtesy of NASA/JPL)
Where, then, is the water on Mars now? Some of it was lost: Curiosity’s mass spectrometer measured Mars’s deuterium to hydrogen ratio and found it to be five times higher than the Earth’s, indicating significant Jeans escape of hydrogen from the planet. Some of what remains is tied up in the polar caps, which consist of a seasonal mix of water ice and frozen carbon dioxide (Mars is so cold that a significant fraction of its atmosphere condenses out at its poles each winter). But the polar ice caps do not account for much water: the larger north polar cap is estimated to contain 1,200,000 km3 of water ice, which is less than half the size of the Greenland ice cap and a mere 4% of the volume of the Antarctic ice cap. To search for possible reservoirs of water outside the polar caps, the Mars Odyssey orbiter carried a neutron spectrometer, an instrument that maps out the distribution of hydrogen within the first few tens of centimeters of the Martian surface. And what did it find? It found high concentrations of hydrogen in the soil at high latitudes and progressively less toward the equator. In fact, the soil in wide swaths of the northern and southern latitudes contains so much hydrogen that, if (as seemed likely) it is water ice, the soil must be at least 50% water by weight (fig. 9.5).
Confirmation that the hydrogen in the Martian soil is, indeed, in the form of water ice was provided by NASA’s Phoenix lander. This relatively inexpensive craft assembled from ideas and spare components from previous failed missions (thus its name) touched down on May 25, 2008, and operated for 157 Martian days before the onset of the harsh Martian winter inevitably killed it. Phoenix landed in the Vastitas Borealis formation, a large, low-lying plain near the Martian arctic circle at 68° north; the Terrestrial equivalent would be landing in Barrow, Alaska, or Tromsø, Norway. Orbital images indicated a smooth landscape dotted with subtle troughs tracing out polygon shapes as is seen in the Earth’s Arctic, where the alternating freezing and thawing of the soil causes it to break into such shapes. And, of course, thermal neutron data from on orbit indicated that the first half meter of soil was dense with hydrogen. The first sign that Phoenix had found its prey was entirely unplanned: upon touchdown, the exhaust from Phoenix’s landing rockets had blown away some of the soil beneath the craft, exposing a large, white patch that resembled ice (fig. 9.6). Subsequently, Phoenix used its robotic arm to dig down a few tens of centimeters into the soil, where it found the long-sought ice, confirmed it to be water via its onboard mass spectrometer, and then watched it sublimate back into the atmosphere after having been exposed.
Figure 9.5 A map of the abundance of hydrogen in the first few tens of centimeters of the Martian surface as determined by neutron spectroscopy from the orbiting Mars Odyssey spacecraft. The soil across wide swaths of Mars’s northern and southern hemispheres seems to consist of at least 50% water by weight. (Courtesy of NASA/JPL)
Exciting as these discoveries are, permafrost is not the stuff on which life is founded. But if there is water under the surface of Mars as ice, is there some as liquid as well? When the European Space Agency’s Mars Express, in orbit since late 2003 and still going strong, mapped the upper reaches of several of the planet’s larger volcanoes, it found them so free of craters that the terrain must be less than a few million years old. That’s a long time for us, but it’s less than 0.1% of the age of the planet, suggesting that Mars is probably still geologically active today (after all, what are the chances that Mars was active for more than 99.9% of its existence and then its geological fires, coincidentally, died immediately before we humans started to poke around?). Consistent with this, the seismometer on NASA’s InSight (Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport) mission, which landed on Mars’s Elysium Planitia in November 2018 and continues to operate as of this writing, has confirmed that Mars is seismically active, though far less so than the Earth. And if Mars is active, could there be liquid water, and perhaps even life, under its crust?
Figure 9.6 The landing rockets of the Phoenix lander blew away the thin Martian soil, leaving behind a white mass that that appeared to be ice. The craft’s later chemical investigation of the nearby soil confirmed the presence of permafrost. (Courtesy of NASA/JPL/UA)
Putative (albeit still disputed) evidence for the existence of subsurface water on Mars today came in 2000, when NASA’s orbiting Mars Global Surveyor, which remained active until November 2006, spotted features that point to ongoing fluvial activity. Surveying selected swaths of Mars at about 1 m resolution, the spacecraft found what appear to be rather recent (some much less than a million years old) erosional gullies emerging from cliffs throughout the higher latitudes (fig. 9.7). And while none of these gullies, now called “recurrent slope linea,” have been observed precisely in the act of forming, follow-on observations of several hundred of them have seen a half dozen that changed from year to year; therefore, whatever formed them is still active on the surface of Mars today. On Earth, such gullies are quite common and result from the spring runoff of snow that has accumulated on a cliff face. Because liquid water cannot exist on the surface of Mars for very long (seconds to minutes, depending on the volume and surface area), a different mechanism is probably occurring there. Perhaps the most popular of several current theories on the origins of these gullies is that they are formed when ice plugs on springs rupture, briefly liberating a torrent of groundwater (kept liquid by geothermal heat) before the plug freezes over again and the flow is stopped. This potential juxtaposition of groundwater and geothermal energy suggests that, although the surface of Mars is cold and dry today, the planet may still contain viable habitats deep within its crust. Others have pointed out, however, that “Mars is not Earth.” Who knows whether, on its cold, exceptionally dry surface, some processes such as flowing dust or rapidly evaporating dry ice are creating these gullies? Consistent with the dust-flow hypothesis, recurrent slope linea are only seen on slopes greater than 27°, which is more or less the angle of repose for dust under the cold, dry, low-gravity conditions found on Mars.
Figure 9.7 In 2000, the Mars Global Surveyor orbiter photographed features such as these approximately 100 m long streaks that have widely been interpreted as water-carved gullies. Whatever they are, the features are modern; a handful have been seen to change from year to year. Whether are carved by emerging liquid groundwater, however, remains contested. (Courtesy of NASA/JPL/MSSS)
Some of the evidence against the gullies being formed by water was provided by instruments on the Mars Express and Mars Reconnaissance orbiters, both of which carry ground-penetrating radar aimed at taking stock of underground ice layers or water tables within the first kilometer of the planet’s crust. The dry Martian soil should be relatively transparent to the radio wavelengths employed by these two instruments, but discontinuities, such as hitting a water table, should cause some of the energy to reflect back to the spacecraft. Early results from these instruments confirmed that the permanent (i.e., summer) polar caps are pure water ice up to 3.7 km deep and that if they were to melt and spread evenly over the entire planet, the resulting “ocean” would be 11 m deep (for comparison, the average depth of the Earth’s oceans is more than 3,600 m). The orbiters’ instruments have also found evidence for remnant glaciers nearer the equator, where they are protected from sublimation by thick overlayers of dust and debris. The case for Mars as the most habitable place in the Solar System outside Earth received another boost in July 2018, when researchers using the Mars Express orbiter’s radar instrument reported observations which they interpreted as signs of a 20 km wide lake trapped 1.5 km below the surface of the ice and dust of the polar layered terrains at 81° south. The temperature at the base of the polar deposits is estimated to be −90°C, and thus, if the interpreted lake really exists, it must be filled with brine.
So what’s next on the Mars exploration docket? As we write this in mid-2020, three new rovers and two new orbiters are in the works (table 9.2). First up, with a landing date of February 18, 2021, is the Perseverance rover. This near twin of the currently operating Curiosity will explore the 45 km wide Jezero Crater, which, based on the presence of what appear to be river valleys leading into the crater, a large fan delta, and significant deposits of clays, is thought to have been a lake some 3.5 to 3.9 billion years ago (fig. 9.8). In addition to the instrument suite of its sister craft, Perseverance will be carrying a ground-penetrating radar to image subsurface structures and, if present, underground ice or brine down to depths of 10 m. It will also include an instrument named SHERLOC (scanning habitable environments with Raman and luminescence for organics and chemicals), which will employ spectroscopy to assess the presence of life-relevant elements (carbon, hydrogen, nitrogen, etc.), to identify potential organic biosignatures, and to provide detailed mineral analysis. In addition to exploring Jezero, Perseverance will also cache interesting samples onboard in the hopes that, someday, a future mission can return them to Earth for more detailed study, though the date of such a sample return mission is ill-defined and has had a habit of moving inexorably into the more distant future. And while Perseverance is the most sophisticated rover slated for the next batch of Mars missions, it will not be alone. In April 2021, the Chinese Space Agency is planning on landing their Huoxing-1 rover at Utopia Planitia (near where Viking 2 touched down almost 45 years earlier), where it will “perform chemical analyses on the soil, and look for biomolecules and biosignatures.” In parallel, the United Arab Emirates has launched the Hope mission, a demonstration spacecraft expected to reach Mars orbit in February 2021, when it will begin to study the Martian weather. Finally, in 2022, the European Space Agency intends to launch Rosalind Franklin, a solar-powered rover that will also employ spectroscopy and chemical analysis to better understand organics on Mars. No landing site has yet been reported.
Table 9.2
Currently scheduled robotic missions to Mars
Mission |
Arrival |
Notes |
Hope (orbiter) |
February 9, 2021 |
A climate-focused mission lead by the United Arab Emirates |
Mars Global Remote Sensing (orbiter and rover) |
February 10, 2021 |
The stated goals of this Chinese Space Agency mission are to search for evidence of current and past life and to assess the Martian environment |
Perseverance (rover) |
February 18, 2021 |
Due to explore Jezero Crater, a landscape shaped by water, using a Curiosity-like instrument suite; also expected to collect and cache samples for a possible future sample-return mission |
Rosalind Franklin (rover) |
Mid-2023 |
A joint European Space Agency / Russian Space Agency lander and rover, with the latter carrying a Curiosity-like instrument suite to explore Oxia Planum, a valley system site with extensive deposits of clay |
Figure 9.8 On February 18, 2021, NASA’s Perseverance rover is scheduled to set down in Mars’s Jezero Crater. A putative dry river channel leading into this 45 km wide crater deposited what appears to be an alluvial fan some 10 km across.
The Moons of Jupiter
Out beyond Mars we find the gas giants Jupiter (see fig. 3.1), Saturn, Neptune, and Uranus. The physics of these planets effectively rules out any processes that we would describe as life. Jupiter and Saturn lack solid surfaces (because of the massive bulk of these planets, even their metal-and-rock cores are thought to be liquid; they haven’t cooled enough since accretion to solidify), and the solid surfaces of Uranus and Neptune, if there are such surfaces, are deep below hot, highly convective seas of supercritical water and ammonia. Life in the atmospheres of the gas giants is probably not possible (for reasons outlined in chapter 1), and life beneath the hot, turbulent seas of Uranus and Neptune is probably equally precluded by the instability of the environment. There are other environments out among the gas giants, though, that might be more conducive to life. Each of the four planets has a retinue of dozens of moons, and some of the larger ones may be among the more promising prospects for extraterrestrial life in our Solar System.
The four largest companions of Jupiter were discovered in 1610 by the Italian astronomer Galileo Galilei (1564–1642), who made himself unpopular with the church authorities by pointing out—correctly—that the existence of moons orbiting Jupiter ran counter to the Ptolemaic worldview that everything revolves around the Earth. These four “Galilean satellites,” Io, Europa, Ganymede, and Callisto, remained little more than points of light for the next four centuries. Indeed, even our first robotic emissaries to the outer Solar System, Pioneer 10 and 11, which flew by Jupiter in 1973 and 1974, failed to return much data on the satellites. (See table 9.3 for a time line of missions to the outer Solar System.) It was not until Voyager 1 and 2 passed Jupiter in 1979 that we began to see these moons as worlds unto themselves, and much of what we know is based on measurements made by the Galileo mission, which orbited Jupiter from 1996 to 2003 before being sent to a fiery death in Jupiter’s atmosphere to avoid accidentally contaminating any of the Galilean satellites—potential habitats that they may be—with Earthling microbes that might have hitched a ride.
One of the more stunning discoveries of the Voyager missions to Jupiter was confirmation of a theory regarding the innermost Galilean satellite, Io. Stan Peale (1937–2015) from the University of California, Santa Barbara, and Pat Cassen and Ray Reynolds of NASA’s Ames Research Center had been pondering the ramifications of an observation made several centuries earlier: that the orbits of the inner three Galilean satellites are resonant. For every orbit of Ganymede, Europa makes two orbits and Io makes four (table 9.4 lists their orbital parameters and other physical properties). This means that the gravitational tug of Europa on Io builds up; it happens every other time at exactly the same place in Io’s orbit, forcing Io into an eccentric (out-of-round) orbital path. An eccentric orbit this near Jupiter would not usually be a stable state of affairs; orbital eccentricity should lead to enormous tides in the solid material of the moon, and the friction caused as the tides flexed the moon’s solid rock should dissipate orbital energy until the eccentricity was damped and Io returned to a circular orbit. But the resonance with Europa prevents this; while the tides of Io try to damp the eccentricity, counteracting tides raised in Jupiter tend to push the orbit of Io outward, causing the resonant interaction with Europa to kick up the eccentricity once again. In turn, a similar resonance between Europa and Ganymede forces Europa’s orbit out of round, again causing tides and subsequent damping although, because tidal amplitude drops off with the third power of the distance from the source, Europa’s tides are much smaller than Io’s.
Table 9.3
Past successful and future planned missions to the outer Solar System
Mission |
Nature of mission |
Flyby date, dates active in orbit, or launch/arrival date |
Notes |
Pioneer 10 |
Jupiter flyby |
December 3, 1973 |
First spacecraft to venture beyond the asteroid belt; first flyby of Jupiter |
Pioneer 11 |
Jupiter flyby |
December 2, 1974 |
Flyby of Jupiter followed by first flyby of Saturn |
Saturn flyby |
September 1, 1979 |
||
Voyager 1 |
Jupiter flyby |
March 5, 1979 |
Discovery of Io volcanoes, Titan’s thick atmosphere, and Enceladus’s high albedo |
Saturn flyby |
November 12, 1980 |
||
Voyager 2 |
Jupiter flyby |
July 9, 1979 |
First (and only) in situ exploration of Uranus and Neptune; discovery of Europa’s smooth surface and Triton’s geysers |
Saturn flyby |
August 26, 1981 |
||
Uranus flyby |
January 24, 1986 |
||
Neptune flyby |
August 25, 1989 |
||
Galileo |
Jupiter orbiter |
December 7, 1995 to September 21, 2003 |
First outer planet orbiter; discovered oceans under the ice of Europa, Ganymede, and Callisto |
Galileo probe |
Jupiter atmospheric probe |
December 7, 1995 |
Characterized Jupiter’s atmosphere for 58 minutes until the probe descended to crush depth |
Cassini |
Saturn orbiter |
July 1, 2004 to September 15, 2017 |
Extensive studies of Saturn and its moons; discovery of Enceladus’s geysers and Titan’s lakes; intentional final descent into Saturn to avoid risk of contaminating its moons |
Huygens |
Titan lander |
January 14, 2005 |
Studied Titan’s atmosphere and surface during its 2.5-hour descent and for 1 hour on the surface |
Dawn |
Vesta and Ceres orbiter |
July 16, 2011 to September 5, 2012 (Vesta) March 6, 2015 to October 31, 2018 (Ceres) |
Detailed studies of the geology and minerology of the two largest asteroids; identified potential, apparently water-driven “volcanism” on Ceres |
Rosetta |
Comet 67P/Churyumov-Gerasimenko orbiter |
August 6, 2014 to September 30, 2016 |
First comet orbiter; performed detailed studies of the composition, morphology, and activity of the comet as it passed the closest point in its orbit to the Sun |
Philae |
Comet 67P/Churyumov-Gerasimenko lander |
November 12 to 14, 2014 |
First soft landing on a comet, albeit its anchoring harpoons failed to deploy, limiting the ability of its instruments to contact the surface; performed limited chemical analysis of the surface |
New Horizons |
Pluto flyby |
July 14, 2015 |
Investigated Kuiper belt objects Pluto and Arrokoth |
Arrokoth flyby |
January 1, 2019 |
||
Juno |
Jupiter orbiter |
July 4, 2016 to present |
Aiming to understand origin and evolution of Jupiter, map magnetic field, measure water and ammonia in deep atmosphere, observe auroras |
Lucy |
Multiple asteroid flyby |
Expected launch October 2021 |
Will flyby one main belt asteroid and five trojans (asteroids sharing Jupiter’s orbit but either leading or lagging the planet by 60°) |
Psyche |
Psyche (asteroid) orbiter |
Expected launch/arrival 2022/2026 |
Orbiting the metallic asteroid Psyche, the goal of this mission is to explore the origin of planetary cores |
JUICE |
Ganymede orbiter |
Expected launch/arrival 2022/2029 |
European Space Agency’s Jupiter Icy Moons Explorer will conduct repeat flybys of Callisto, Europa, and Ganymede before finally orbiting the latter |
Europa Clipper |
Jupiter orbiter with multiple Europa flybys |
Expected launch/arrival 2024/2030 |
Will conduct 44 flybys of Europa at altitudes from 25 to 2,700 km during 3.5 years in orbit around Jupiter |
Dragonfly |
Titan autonomous aerial vehicle |
Expected launch/arrival 2026/2034 |
This 450 kg autonomous drone will explore Titan’s surface for 2.7 years, traversing up to 8 km each 192-hour-long Titan “day” |
Table 9.4
Jupiter’s Galilean satellites
Name |
Orbital period (Earth-days) |
Radius (km) |
Mass (relative to Earth’s Moon) |
Density (g/cm3) |
Composition |
Io |
1.77 |
1,818 |
1.22 |
3.53 |
Silicates |
Europa |
3.55 |
1,561 |
0.65 |
3.01 |
Silicates, small amount of water ice |
Ganymede |
7.15 |
2,634 |
2.02 |
1.94 |
½ silicates, ½ water ice |
Callisto |
6.69 |
2,408 |
1.46 |
1.83 |
½ silicates, ½ water ice |
The net effect of all this gravitational tugging is to convert Ganymede’s orbital energy into massive tidal flexing of the crust of Io and lesser tidal flexing of Europa. When the scientists worked out the precise numbers, they realized that this tidal effect dumps enormous amounts of energy into Io’s crust—orders of magnitude more heat per square meter than the geothermal flux of the Earth, then thought to be the most geologically active body in the Solar System. Their conclusion was clear: “although the morphology of such a surface cannot be predicted in any detail, one might speculate that widespread and recurrent surface volcanism would occur.”*
On March 8, 1979, exactly one week after the volcano prediction appeared in print, Voyager navigation team member Linda Morabito was looking at some images of the limb (edge) of Io. These images were overexposed to bring out faint stars in the background for navigation purposes. Oddly, an image showed a crescent beyond the edge of the moon (fig. 9.9). At first Morabito thought it was another of Jupiter’s satellites peaking over the edge, but a quick check of the locations of the other Galilean satellites nixed this idea. Instead, the crescent was a stunning confirmation of the tidal heating theory; it was the plume of an extraordinarily active volcano rising 260 km into space. In short order, seven violently active volcanoes were identified on Io (including a second obvious one in the picture Morabito was using), making it far and away the most volcanically active body in the Solar System.
Figure 9.9 Io, the innermost of Jupiter’s main satellites, is intensely volcanic. The crescent rising in the upper left-hand corner of this Voyager image is a volcanic plume jetting some 260 km into space, and the bright spot nearer the center of the disk is a second plume rising from the dark of night into bright sunlight. (Courtesy of NASA/JPL)
While it is a fascinating place in terms of geology, our interest in Io from the astrobiological perspective is limited; the incessant volcanism—and subsequent loss of volatiles to space—has baked Io completely dry. The same, however, cannot be said for the next moon out: Europa. Measurements of the slight deflection induced in the Voyager spacecraft as they flew by Europa allowed scientists to estimate the mass of this moon, which, when combined with knowledge of its size, indicated that the satellite has a density of 3.02 g/cm3. This is a little low compared with the density expected for the Solar System’s typical mix of silicates and metals (see table 3.2). More perplexing still, Voyager 1 and 2 found that, at the resolution their cameras, Europa’s is probably the smoothest surface in the Solar System; the moon is almost free of craters, and its topography doesn’t vary by more than a few hundred meters across its entire globe (fig. 9.10). Taken with the spectroscopic evidence on the composition of Europa’s surface, these observations led astronomers to the conclusion that the surface of Europa is a thick, relatively recently reworked layer of ice.
Recently reworked, of course, once again implies “geologically active.” Consistent with this, a few months after their prescient paper on Ionian volcanism, the same theory team published a second paper, with the provocative title, “Is There Liquid Water on Europa?”* Unlike their earlier prediction of volcanoes on Io, which took less than week to confirm, the accuracy of this second prediction, which also implicated tidal heating in causing subsurface melting, wouldn’t be nailed down for another 21 years. In the year 2000, the chief scientist on the Galileo magnetometer, Margaret Kivelson of the University of California, Los Angeles, finally announced conclusive proof of an under-ice ocean on Europa. During Galileo’s December 1996 flyby, the magnetometer found that Europa has a magnetic field, and it is pointed perfectly opposite to the strong magnetic field of Jupiter. While the opposing field could have been coincidental (Galileo could have just happened by at the very moment when Europa’s rotation brought its field into alignment with Jupiter’s), follow-on flybys showed that the Europan field is always opposed to Jupiter’s no matter where Europa is in its orbit. An opposing magnetic field can be generated by eddy currents, which are the currents induced in a conductor when it is moved through an external magnetic field. But what could the Europan conductor be? Based on the density of Europa’s crust and the conductivity necessary to generate the observed magnetic field, the only compelling answer is that it’s a global ocean of saltwater many tens of kilometers thick, a sub-ice ocean kept liquid by tidal heating. Even though Europa receives about one-tenth as much tidal energy as Io, that seems to be enough to prevent freezing of its ocean, insulated as it is from the cold of space by a thick layer of ice.
Figure 9.10 A crescent Europa, showing the crisscrossing lines that decorate its surface. On every other body in our Solar System, mountain tops jut into daylight and valley (or crater) floors fall into darkness, breaking up the line between night and day. On Europa, in contrast, the topography varies by only a few hundred meters, explaining the razor sharpness of the line separating daylight from the dark of night. (Courtesy of NASA/JPL)
Although its existence is considered confirmed, details of the “Europan ocean” remain sketchy. Galileo’s gravity measurements suggest that the combined ice and water shell can be no more than 70 to 170 km deep but cannot distinguish the liquid from the solid phase (their densities are too similar) and thus cannot estimate how much of this thickness is liquid ocean and how much is the icy shell that lays on top. Simulations of impact-crater formation in ice suggest, however, that the ice shield must be at least 3 or 4 km thick, and perhaps as much as 25 km. Still, some indirect studies suggest that the ice may, on geological time scales, sometimes become rather thin. For example, Galileo has photographed what appear to be “ice rafts” frozen into place, as if the surface ice had once temporarily melted and broken into icebergs (fig. 9.11). Likewise, telescopic observations from Earth have seen water vapor in the neighborhood of Europa, corresponding to release rates of a couple of tons per second, although whatever is causing this has proven sporadic; the vapor was detected on only a single night in 17 observations scattered over the years 2016 to 2017.
Figure 9.11 This image of raftlike elements, captured by the Galileo spacecraft in 1998, suggests that Europa’s crust, which floats on a sub-ice ocean, is sometimes thin enough to break up like pack ice in an Arctic summer. For scale, the base of this image is 34 km across. (Courtesy of NASA/JPL)
The likely presence of a liquid ocean beneath the thick, icy crust of Europa suggests that the moon may be a potential habitat, but is it truly habitable? As we have discussed in earlier chapters, water alone is not a sufficient criterion for habitability; we also need a source of energy to drive metabolism. Several possible sources have been suggested for Europan life. For example, even though the intensity of sunlight that strikes Europa is about one-thirtieth of what we receive on the Earth’s surface, meltwater near Europa’s surface could support photosynthetic organisms. Alternatively, and perhaps more appealingly, the geothermal (tidal) energy that keeps Europa’s ocean liquid could provide a source of energy for organisms living on the ocean floor. Whether this is a sufficient source of energy, however, has been rather hotly debated in the astrobiology community. Finally, Chris Chyba of Princeton University suggests yet another potential energy source, this one rather specific to Europa. Europa, Chyba notes, orbits deep within Jupiter’s intense radiation fields, and this radiation (which would kill an unprotected human in minutes) breaks down the ice chemically on the moon’s surface. That is, when the high-energy protons and electrons that make up the radiation impact the ice, they tear its molecules apart and create highly reactive species. This “radiolysis” has, in fact, been observed: spectroscopic studies of Europa from the Hubble Space Telescope indicate that radiolysis provides Europa with a tenuous oxygen atmosphere (albeit only 100 billionth as dense as Earth’s).* Other important products of this reaction probably include the simple organic compound formaldehyde and the oxidizing compound hydrogen peroxide. From estimates of the rate with which these species are formed on Europa, Chyba has calculated that this energy source could support up to 500 tons of microorganisms. Not a huge biosphere by Terrestrial standards—the Earth’s is 10 billion times larger than this—but enough to push Europa into the very short list of potentially habitable places in the Solar System.
And it appears that Europa might not be alone: there is some evidence that Ganymede, the third Galilean moon, might also hide an ocean beneath its icy crust. Slightly larger in diameter than the planet Mercury, Ganymede is the largest moon in the Solar System. Its composition, however, is a mixture of rock, metals, and ice, rendering its mass only about half that of Mercury. Magnetometer readings from the Galileo spacecraft have proved that, like Europa, Ganymede produces a small magnetic field counter to that of Jupiter, a clear fingerprint of a sub-ice ocean. This was unexpected as Ganymede is the outermost of the three participants in the orbital dance that pumps energy into Io and Europa and thus should be receiving far less energy than its inner two partners (remember: tides drop off with the cube of the distance). Much of the energy required to maintain Ganymede’s liquid ocean instead seems to arise from within Ganymede itself. Direct evidence for this was also provided by Galileo’s magnetometer, which registered that, in addition to the induced magnetic field produced by the ocean, Ganymede also generates a permanent magnetic field about 2% as strong as that of the Earth. The Earth’s magnetic field is thought to arise from the churning of the planet’s liquid iron core, suggesting in turn that Ganymede also contains a liquid iron core. Heat escaping from this core—which is presumably created by the decay of radioactive elements working in concert with what little tidal energy the moon receives—would be sufficient to maintain a liquid water ocean, albeit only if there were significant “insulation.” Consistent with this, the heavily grooved—and presumably once geologically active—surface of Ganymede (fig. 9.12) exhibits far greater topography than the smooth surface of Europa as it sports mountains up to 3 km high. Together these observations suggest a rigid, heavily insulating ice crust 100 to 200 km thick. This, in turn, suggests that the energy flux through Ganymede is small, likely far too small to support an ecosystem.
Figure 9.12 Ganymede’s icy crust is crossed and hatch-marked with icy grooves that speak of a once (if long ago) active geology. This image is 390 km across. (Courtesy of NASA/JPL)
And it doesn’t stop with Ganymede. Callisto, the outermost Galilean satellite and the third largest moon in the Solar System, also shows signs—in the form of an induced magnetic field—of an under-ice ocean. This is particularly odd, for a number of reasons. First, Callisto does not take part in the resonant dance of the inner Galilean satellites and thus is not subject to any appreciable tidal heating. Of course, as we just noted, Ganymede’s tidal heating is also too small to account for its ocean, which instead is thought to result, at least in part, from the heat of radioactive decay. Indeed, gravitational measurements by the Galileo spacecraft indicate that, unlike the other Galilean satellites, Callisto never differentiated into a metallic core, rocky mantle, and icy crust, suggesting that the moon’s interior has always been quite cold. This observation is likewise supported by the battered appearance of Callisto’s ancient surface: unlike those of the inner Galilean moons, the surface of Callisto is saturated with craters and shows no signs of having undergone any significant geological activity (fig. 9.13). The origin of Callisto’s ocean, which like that of Ganymede is thought to be hundreds of kilometers below the surface, thus remains a mystery. Most probably, however, given the poor thermal conductivity of ice and the presence of salts and incorporated volatiles such as ammonia that suppress the freezing point of water, even the feeble heat released by radioactive elements in Callisto’s rocky components is sufficient to maintain a liquid ocean.
Figure 9.13 The battered face of Callisto argues that it is the least geologically active of Jupiter’s four Galilean satellites. Its icy crust is nevertheless thought to cover an ocean of liquid brine. (Courtesy of NASA/JPL)
The Moons of Saturn and Beyond
For all his success with the Jovian system, Galileo (the Renaissance astronomer, not the robotic spacecraft) failed to discover any of Saturn’s numerous moons (the larger of which are listed in table 9.5) or, indeed, to recognize its rings for what they are. With regard to the former, his telescope lacked sufficient resolving power. With regard to the latter, Galileo had the bad luck of making his observations right before and then during the time that the Earth passed through Saturn’s ring plane, rendering the rings edge-on and thus invisible. At first it appeared that Saturn had two large companions on either side—the unresolved rings—and then months later they disappeared! Clearing up this mystery, the Dutch scientist Christiaan Huygens (1629–1695) described the rings in detail in 1659. He also discovered Saturn’s largest satellite, Titan, which is slightly bigger than Mercury and second only to Ganymede in size among the Solar System’s moons. Titan, though, was long considered the more interesting of the two from the astrobiological perspective. The reason for this interest dates back to 1925, when James Jeans (of the “Jeans mass” limit to star formation described in chapter 2) used his “dynamical theory of gases” to predict that, even given Titan’s modest gravity, the low temperatures found at Saturn’s distance from the Sun would allow the moon to retain gaseous molecules as heavy as or heavier than methane. Measuring the infrared spectrum of Titan, the Dutch American astronomer Gerard Kuiper (1905–1973) confirmed Jeans’s prediction in 1944 by identifying methane at a pressure of a few percent that of total atmospheric pressure on Earth. Among the many dozens of moons in our Solar System, Titan alone retains a thick atmosphere.
Table 9.5
Saturn’s major satellites
Name |
Orbital period (Earth-days) |
Radius (km) |
Mass (relative to Earth’s Moon) |
Density (g/cm3) |
Composition |
Mimas |
0.9 |
200 |
0.0005 |
1.17 |
Predominantly water ice |
Enceladus |
1.4 |
250 |
0.001 |
1.24 |
⅛ silicates, ⅞ water ice |
Tethys |
1.9 |
530 |
0.009 |
1.21 |
Predominantly water ice |
Dione |
2.7 |
560 |
0.015 |
1.43 |
⅓ silicates, ⅔ water ice |
Rhea |
4.5 |
764 |
0.031 |
1.33 |
¼ silicates, ¾ water ice |
Titan |
15.9 |
2,575 |
1.83 |
1.21 |
Predominantly water ice |
Hyperion |
21.3 |
~140 |
0.0002 |
~1.2 |
Water ice with voids |
Iapetus |
79.3 |
718 |
0.022 |
1.21 |
Predominantly water ice |
Our detailed understanding of Titan and its atmosphere began during the Voyager flybys in 1980 and 1981. In images taken by the Voyager craft, the moon presented only a featureless yellow-orange orb—Titan’s atmosphere is filled with a thick haze that prevents observation of its surface at visible wavelengths (fig. 9.14). Refraction of Voyager 1’s radio signal as it passed behind Titan (from the perspective of Earth) provided a means of characterizing the moon’s atmosphere. Surprisingly, Titan’s surface pressure is 1.5 times that of Earth’s; given Titan’s weaker gravity, this corresponds to an atmospheric density some three times ours.* Kuiper’s results, however, indicated that the pressure of methane in Titan’s atmosphere is far lower than this, suggesting that the bulk of the atmosphere is made up of a species invisible to his infrared observations. We now know this to be nitrogen, which makes up 97% of Titan’s atmosphere, with the remainder largely being the methane that Kuiper had observed. The discovery of methane and nitrogen in Titan’s atmosphere solved the mystery of its atmospheric haze: Carl Sagan, then at Cornell University in upstate New York, studied the effects of UV light and ionizing radiation on such a mixture and found that it reacts to form a high molecular weight mix of what he named “tholins,” from the Greek thol, for “mud.” Tholins, which are largely composed of long-chain hydrocarbon molecules terminated by nitrile groups (−C≡N), are generally reddish-yellow and probably constitute the orange fog that renders the surface of Titan impossible to image from space using visible light.
Figure 9.14 The haze-filled atmosphere of Titan, Saturn’s largest moon. (Courtesy of NASA/JPL)
The radio data from Voyager 1 also provided a means of estimating the temperature of Titan’s atmosphere, which was found to be a chilly −179°C at the surface. This is curious because it is near the temperature at which the atmosphere’s methane should condense out as a liquid. Indeed, at a concentration of a few percent, the methane “relative humidity” in Titan’s atmosphere is approximately 50%, suggesting that the gas could condense and fall as rain as water does on Earth. Together with the observation that the haze-producing, photolytic creation of tholins is so rapid that the resulting loss of hydrogen to space should deplete Titan’s atmospheric methane in a few tens of millions of years, this suggests that the methane is being replenished from some reservoir on the surface. And that reservoir? By analogy to water on Earth, the Voyager data were taken to imply that, beneath its thick covering of haze, Titan’s surface is dotted with lakes or even oceans of liquid methane. Indeed, the Voyager data suggested that, since Titan has just the right range of temperatures for methane to exist as a liquid, solid, or gas, methane might drive weather on Titan closely analogous to Earth’s water-driven weather.
Methane clouds billowing up above a frozen landscape; methane storms raining down to form methane rivers that in turn cascade into methane seas—it’s a very pretty image, but one that would have to wait more than two decades for confirmation. Not until the arrival of Cassini, which fell into orbit around Saturn in late 2004, and its companion, the Huygens lander, which parachuted to the surface of Titan in January 2005, would our knowledge of Titan take its next significant leap forward.
The first hints about the nature of Titan’s surface came when, during its 2.5-hour descent, Huygens snapped pictures of what look very much like dry river channels and dry lakebeds (fig. 9.15) before finally gently settling down on a flat, cobble-covered landscape reminiscent of a flood plain (fig. 9.16). Indeed, during the hour or so that it continued to relay data from Titan’s surface, Huygens’s warmth drove methane out of the surface in such large volumes that the ground must be saturated with the stuff, just as sand on a beach is saturated with water. Further hints about the nature of methane on Titan came from Cassini as it periodically flew by on its orbit around Saturn. Using Cassini’s radar to peer through the thick haze, the first thing scientists noted was that the moon bears very few impact scars, perhaps because craters erode rapidly under the onslaught of the hypothesized methane rains and rivers. Likewise, radar images of the moon’s north pole, which was cloaked in winter darkness at the time, showed that wide swaths of its surface were dark to radar and perfectly smooth (to within the measurement precision of just 3 mm) over hundreds of kilometers as would be expected for a windless lake (fig. 9.17). Finally, in 2009, during the Saturnian equinox, Titan’s north pole moved from night to day and, as it did so, Cassini captured a spectacular photograph of low-angle, early morning sunlight glinting off a mirror-like surface (fig. 9.18). It is now clear that, at least at its poles, Titan’s surface is spattered with lakes, and the physical properties of its atmosphere and surface are dominated by a “methane cycle” analogous to the hydrological (water) cycle in the Earth’s atmosphere.
Figure 9.15 Titan seems to have a complex, liquid methane–based fluid cycle reminiscent of the Earth’s hydrological cycle. During its 2.5-hour descent to Titan’s surface, for example, the Huygens probe spied what are probably river channels carved by methane rains. (Courtesy of NASA/ESA)
Figure 9.16 After a seven-year, 5-billion-kilometer trip, Huygens landed on Titan’s frozen surface to become, for an hour and a half, humanity’s most distant outpost on solid ground. In the distance, beyond scattered cobbles of water frozen to rock-hardness by the deep cold, lies the Titanian horizon. (Courtesy of NASA/ESA)
As fascinating as the river-carved and lake-dotted surface of Titan may be, it is Titan’s atmospheric chemistry that is of interest to the astrobiology community. The reason is that Titan’s atmosphere, although extremely cold, is in some ways a rough analog of the early Earth’s as postulated by Harold Urey. That is, the atmosphere is reducing and nitrogen rich. Under the influence of UV light from the Sun (and perhaps lightning in those methane clouds—although, despite significant effort, none has yet been detected), these dominant atmospheric components, so it is thought, must be reacting to produce life’s precursors, which form the thick haze and no doubt rain down to the surface. Over billions of years, perhaps hundreds of meters of complex organics have accumulated on the surface of this frozen world. The Huygens probe, alas, was not equipped to search for such molecules on the surface. Still, some of the Cassini flybys have been close enough for its mass spectrometer to detect, as the craft passed through the moon’s outermost atmosphere, the high molecular weight hydrocarbons and nitriles predicted by Sagan.
Figure 9.17 Ligeia Mare, one of the largest of the many methane lakes on Titan, covers about twice the surface area of Earth’s Lake Superior. (Radar image courtesy of NASA/JPL/USGS)
Figure 9.18 Early morning sunlight glinting off a methane lake near Titan’s north pole. (Courtesy of NASA/JPL/UA/DLR)
But is Titan a potential habitat? In that light, the moon’s excessive cold is a real problem. Temperatures on the surface of Titan are so low that all oxygen-containing compounds, which of course play an absolutely critical role in life on Earth, are locked into their solid form and would not be available to participate in Miller-Urey-type chemistries, much less in life. But perhaps not all is lost. Sagan argued, for example, that meteorite impacts must have imparted enough energy to the moon to ensure that every part of it has seen liquid water during at least some part of its history.
And what of the rest of Saturn’s retinue of 82 (and counting) moons? Saturn’s second largest moon, Rhea, is quite small, coming in at only one-third the diameter and one-sixtieth the mass of Titan. Moreover, judging by its heavily cratered surface, it appears to be geologically dead. Moving down in size from Rhea, Saturn has a bevy of other, still smaller icy moons, all of which, with two exceptions, lack any signs of activity. The first of these exceptions is fairly minor: the surface of Saturn’s fourth largest moon, Dione, sports a few “fresh”-looking cracks hinting at some sort of tectonic activity. The other exception, in contrast, appears far more active. That exception? Enceladus.
Enceladus is small; at only 500 km in diameter, it is about one-tenth the diameter and 1/2,000 the mass of Titan, suggesting that it, too, should be a dead world. Oddly, though, when Voyager 1 finally observed the moon up close in 1980, Enceladus was found to be as white as freshly fallen snow, reflecting almost 100% of the light that strikes it. In fact, Enceladus has the most highly reflective surface in the Solar System. Given that meteorite impacts and “space weathering” (the cumulative effects of radiation damage and meteorite impacts) tend to darken anything exposed to space, the snow-white appearance of Enceladus suggests that its surface is constantly being replenished. A much closer flyby by Voyager 2 the next year only added to the mystery: while parts of Enceladus’s northern hemisphere are at least modestly cratered, much of the rest of the moon is nearly crater free, and its south pole is crisscrossed with tectonic cracks. The two Voyager missions, however, failed to provide any insights as to how the surface of Enceladus’s southern hemisphere is replenished. The answer would have to wait for the next mission to Saturn.
Unlike the Voyager spacecraft, the next mission to Saturn, Cassini, was an orbiter, so we had the pleasure of viewing Saturn’s moons from a variety of perspectives as it slowly orbited the giant planet. In 2005, while behind Enceladus (from the perspective of the Sun), Cassini spied light scattered from geyser-like plumes of ice particles streaming from cracks at the moon’s south pole (fig. 9.19). Later flybys took the orbiter directly through these plumes, where Cassini’s mass spectrometer confirmed that, apart from water as their dominant component, they also contain a bit of ammonia and traces of several simple organic compounds, including methane, propane (C3H8), acetylene (HC≡CH), and formaldehyde (H2C=O). Cassini’s mass spectrometer also detected salts (NaCl, KCl, MgSO4) and silica (SiO2) in the plumes, implying that their source must be pools of liquid water interacting with rock, and not just solid ice sublimating directly into water vapor.
Figure 9.19 The geysers of Enceladus as they slip into a long winter’s night. (Courtesy of NASA/JPL/MSSI)
Infrared measurements from Cassini have shown that that temperatures at the south pole of Enceladus are far milder than the −210°C temperature found elsewhere on the moon’s icy surface. Indeed, some areas reach −90°C, but as these measurements were averaged over the smallest length scales resolvable by the instrument, it is likely that there are still smaller spots at much higher temperatures. All told, the geysers on Enceladus are pumping out 5 GW of heat (a gigawatt is equal to 1 billion watts), which is about 1,000 times the output of Old Faithful, the best-known geyser in America’s Yellowstone National Park. The source of this energy remains subject to debate. Enceladus is in a weak orbital resonance with Dione, but the resonance should be able to produce only about 1 GW of tidal heating over the long term, and Enceladus is so small that radiogenic heating isn’t thought to produce more than a fraction of a gigawatt. A possibility is that Enceladus’s orbit was, until recently, more eccentric than it is currently and that the geysers are driven by leftover tidal heating from that era.
Later flybys of Enceladus during Cassini’s second and final mission extension, which lasted from 2010 to its intentional destruction in September 2017, showed that the plumes are extended curtains of relatively constant emission, presumably from a crack in the surface, rather than geyser-like jets. The closer investigation of the plumes also brought the discovery of larger organic molecules, such as benzene (C6H6), which added to the appeal of Enceladus as a target for future astrobiology-focused missions.
Any more habitats out there beyond Saturn? Perhaps. Though demoted to “dwarf planet” status, Pluto is still fairly large, as is Neptune’s moon Triton, which is only slightly smaller than our Moon and ranks as the seventh largest satellite in the Solar System. Moreover, at the approximate 38 K (−235°C) temperatures found at these distances from the Sun, even Triton’s and Pluto’s weak gravities are sufficient to maintain nitrogen-rich atmospheres, albeit at densities of only 10 and 15 millionths those of the Earth’s, respectively.
When Voyager 2 flew past Triton in 1989, it observed more than a dozen geyser-like plumes spraying 8 km above the moon’s icy surface before they drifted 150 km downwind (fig. 9.20). Given that all the plumes were observed in a narrow latitude belt around 50° south, which during the Voyager flyby was the region of Triton pointing directly toward the Sun, it is thought that the likely energy source behind these geysers is solar heating; that is, visible light penetrating the ice, which absorbs infrared and thus serves as a greenhouse “lid,” warming the subsurface.
Pluto too, seems to be more active than one might have imagined; when New Horizons flew past in 2015, it found a surprisingly fresh surface marked by mountains of rock-solid ice (fig. 9.21). And while Pluto lacks any tidal heating (as it is tidally locked to its moon Chiron), theoretical studies indicate that, like Jupiter’s Callisto, which also lacks a tidal heat source, Pluto may harbor an ocean of briny or ammonia-containing water under its icy crust. For example, simulations run by Shunichi Kamata of Hokkaido University in Japan suggest that, despite Pluto’s being just one-eighth of Callisto’s mass (increasing the rate with which it cools and reducing its sources of radioactive heat), a thick layer of gas hydrates—crystalline solids formed by gas molecules trapped within molecular water cages—could insulate Pluto’s interior enough such that some liquid remains there since accretion. But could distant Pluto harbor life? The paucity of available energy sources suggests that this is unlikely. And given the good many decades that will pass before a spacecraft lands on Pluto, we can make this claim without fear of being proven wrong anytime soon.
Figure 9.20 When Voyager 2 flew past Neptune’s largest moon, Triton, in 1989, it observed more than a dozen geyser-like plumes (arrows) erupting 8 km above the icy surface and drifting up to 150 km downwind, leaving dark streaks of dust in their wakes. (Courtesy of NASA/JPL)
Figure 9.21 On July 14, 2015, exactly 50 years to the day after Mariner 4 sent back the first close-up images of another planet, the New Horizons spacecraft flew past Pluto, completing the initial, robotic exploration of the Solar System. This near-sunset image taken a few minutes after closest approach is about 380 km across at the base. Across the top are faint bands of haze in Pluto’s thin nitrogen atmosphere, which is approximately 100,000 times less dense than the Earth’s. The smooth plains on the right are a glacier of frozen nitrogen; Pluto’s temperature range is such that its nitrogen cycles between the atmosphere and the crust. The mountains on the left and on the skyline, named Tenzeng Montes and Hillary Montes, respectively, after the first team to successfully summit Mount Everest, are comprised of water ice and rise up 6 km above their bases. They are thought to be less than 100 million years old; otherwise their weight should have caused them to “relax” back into Pluto’s icy crust. The energy source driving their relatively recent formation remains a mystery. (Courtesy of NASA/JHUAPL/SwRI)
Planets beyond Our Solar System
Although long popular on television and movie screens, even the very existence of planets around stars other than our own was mere conjecture until the 1990s. The conjecture usually revolved around arguments we’ve seen before: given the many billions of stars that are very much like our own, it seems highly unlikely that the Sun is the only one that has planets, blah, blah, blah. The indirect arguments were based on the observation that young stars are often observed to be surrounded by gas disks resembling the one from which our planetary system is thought to have formed (as discussed in chapter 3). Thus, based on the assumption that the gas disks around young stars would tend to consolidate into planetary systems, most scientists believed that planets are common around other stars, even in the absence of any direct evidence.
The dearth of direct evidence came to an end in 1995, when Michel Mayor and Didier Queloz at the Geneva Observatory announced that they had discovered a planet half as big as Jupiter whizzing around the yellow dwarf 51 Pegasi, orbiting some eight times closer to its star than Mercury does to the Sun.* They made this discovery by analyzing the spectra of 51 Pegasi for Doppler shifts induced when the massive planet traveled around it, alternately pitching the star a little bit toward and a little bit away from the Earth as the two bodies orbited around their common center of mass once every 4.2 days (fig. 9.22).
Mayor and Queloz’s discovery of an extra-solar planet, or “exoplanet,” unleashed a rush. Within weeks, other astronomers had used their “radial velocity method” to not only confirm the existence of a planet at 51 Pegasi, but to also identify two further candidate solar systems. Within a few years, the list of exoplanets grew to more than a hundred, and by 2019, when Mayor and Queloz received the Nobel Prize in Physics for their discovery,** it had surpassed 4,000.
The radial velocity method is not the only way exoplanets can be detected. A second is to observe their transits. This is when the planet, its star, and the Earth line up such that the planet is in front of its star from our perspective on Earth, producing a slight, but measurable, dimming of the star’s light (fig. 9.23). The transit approach has proven able to detect smaller planets than the radial velocity method. Observing a transit, however, necessitates a bit of luck. Specifically, it requires that the Earth lie in the planet’s orbital plane, which, using a bit of trigonometry, we can calculate would happen only about one in 210 times for a planet in an Earth-like orbit around a Sun-like star. Likewise, transits are brief and occur only once during a planet’s orbit, and thus the approach requires large amounts of observation time and automated data analysis. And while this can be done from the Earth (David Charbonneau of Harvard reported the first example in 1999), it is better done from space, where the lack of a day/night cycle renders uninterrupted observations straightforward and the lack of atmosphere makes it easier to detect small fluctuations in a star’s light (an Earth-size planet passing in front of a Sun-like star dims its light by just 84 parts per million). To this end, NASA’s Kepler mission, a space telescope launched into solar orbit in March 2009 and named after the discoverer of the laws of orbital mechanics, was commanded to stare continuously at patch of sky about 50 times larger than the full Moon. During its nine-year mission, Kepler monitored more than a half million stars and identified at least 2,662 planets in orbit around them.
Figure 9.22 A planet and its star both orbit around their common center of mass. Thus, if a planet is moving away from us along its orbital path, the star will move toward us. This causes the velocity of the star along our line of site, its radial velocity, to oscillate sinusoidally over the course of the planet’s orbit. Monitoring this via the Doppler effect provides one of the means by which we can detect exoplanets. Shown are radial velocity data for the star Gliese 581, a red dwarf about 20 light-years from Earth. (Lower panel courtesy of European Southern Observatory)
Figure 9.23 Transits, in which a planet passes in front of its star relative to our line of site, dim the star’s light, which provides a second means of detecting exoplanets. The magnitude of the dimming tells us about the radius of the planet relative to that of its star, and the frequency of the transit tells us the orbital period. Shown are data for the transit of Kepler 6b, a gas giant two-thirds the mass of Jupiter in a 3.2-day orbit around the star Kepler 6. (Lower panel courtesy of NASA/Kepler)
Radial velocity and transit measurements do more than just detect planets; taken together, they can be used to tell us quite a bit about the planet’s nature. When coupled with the knowledge of the host star’s physics (we know enough about stars to infer their diameter from their color and gravity), the length of the transit (how long a planet takes to pass) tells us the planet’s orbital velocity, and the extent to which the star’s light dims tells us the planet’s radius. Given knowledge of the orbital velocity, the mass of the star (again derived from our well-established understanding of stellar mass and evolution), and Kepler’s laws of orbital motion, we can derive the planet’s orbit. Using the mass of the star and its Doppler-derived radial velocity, we can determine the planet’s mass. Finally, from mass and diameter we can calculate density, which we can use to constrain models of their bulk composition.
Clearly, we humans have gotten the hang of planet hunting and have even garnered some clues regarding the bulk compositions of some of the exoplanets in our neighborhood. Nevertheless, as valuable as they’ve proved to be, radial velocity and transits are indirect detection methods. That is, while they provide proof of a planet’s existence, they don’t allow us to actually see the planet. The problem is that planets normally shine by reflected light and thus are easily lost in the glare of their parent star. Note, though, we said “normally.” Newly formed planets emit copious infrared radiation as they cool down from accretion (even after 4.5 billion years, Jupiter still emits 1.6 times more energy as heat than it receives from the Sun). At infrared wavelengths, a hot, freshly accreted, Jupiter-sized planet is “only” a million times dimmer than a Sun-like star. And while blocking out a million photons from a star for each photon ejected from its planet is not easy, it can be done—provided the planet orbits far enough away from its star.
To date, a dozen or so gas giant planets in a half dozen solar systems have been imaged directly, and, in a few cases, had their spectra recorded. Among the best characterized of these are the four known planets orbiting HR 8799, a young (approximately 30 million years) star in the constellation Pegasus that, under ideal conditions, is just barely visible to the naked eye. Using a coronagraph coupled with adaptive optics, which corrects for atmospheric flicker, Christian Marois of Canada’s Herzberg Institute of Astrophysics and his team have collected multiple images of four planets circling HR 8799 at orbital radii ranging from 16 to 68 AU (by comparison, Pluto orbits at 39 AU) and with masses ranging from five to 10 times that of Jupiter (fig. 9.24).
Tallying up the results of radial velocity measurements, transit observations, direct imaging, and a few other, more esoteric methods, as of March 2020, there were 3,105 confirmed solar systems beyond our own, containing a total of 4,187 well-established exoplanets.* Together, these planets form a veritable zoo, ranging from small, rocky worlds through “super-Earths” and “mini-Neptunes” to the ice and gas giants (fig. 9.25). Terrestrial planets, ice giants, and gas giants we’ve seen since these are well represented in our Solar System. Super-Earths and mini-Neptunes are not. The former, which range from about 1.1 to about 1.7 times the diameter of Earth and about 1.4 to five times the mass of Earth, exhibit densities—and thus a bulk composition—similar to that of our planet. Planets weighing in at more than about five times the Earth, however, are generally of much lower density, suggesting that this is the approximate cutoff above which a protoplanet can start to collect hydrogen and helium from the protoplanetary disk, ultimately to become an ice or gas giant.
Figure 9.24 Shown are merged infrared images taken in November 2009 and May 2016 of four young (and thus still glowing) planets orbiting the star HR 8977, illustrating their orbital motion. To prevent the light from the star overwhelming that of the planets, it was suppressed using a coronagraph; the position of the star is indicated. (Courtesy of Christian Marois, NRC Canada)
Figure 9.25 Plotted are the diameters and masses of the first few hundred planets (both exoplanets and planets in our Solar system) for which both values are known, illustrating the wide range of forms planets can take. Planetary scientists have roughly ordered this planetary zoo into terrestrial planets, super-Earths and mini-Neptunes, ice giants, and gas giants. Super-Earths and mini-Neptunes are without representation in our Solar System. The largest of the gas giants weigh in at 1,000 times the size of the Earth. Objects still greater in mass support nuclear fusion and thus are stars (the smallest being “brown dwarfs”) and not planets. (Data courtesy of http://www.exoplanet.eu/)
A noteworthy example of the many solar systems that have been characterized is that of the star TRAPPIST-1. An amazing seven rocky planets have been identified in orbit around this extremely small, dim red dwarf, which weighs in at less than a tenth of the mass of the Sun. But although the luminosity of TRAPPIST-1 is 2,000 times less than that of the Sun, this planetary system is so compact (even the outermost known planet orbits six times closer to its star than Mercury does to our Sun) that two or three of the TRAPPIST-1 planets are thought to be in the habitable zone (fig. 9.26). Moreover, all seven are perhaps surprisingly Earth-like, with diameters ranging from 20% less to 15% greater than that of the Earth and masses ranging from 33% less to just 10% greater than the Earth. Consistent with this, the bulk densities of the TRAPPIST-1 planets range from slightly higher than that of Earth to slightly lower than that of Mars, implying typical, terrestrial bulk compositions. Finally, maintaining stability in a solar system containing seven planets all crammed closer to their star than Mercury is to the Sun requires orbital resonances akin to those that keep Jupiter’s Galilean satellites in line. Thus, as is the case for the Galilean satellites, the tidal energy flux on these planets is greater than the geothermal energy flux of the Earth, providing another potentially life-supporting source of energy. This said, given their extremely close orbits, it is almost certain that all seven planets are tidally locked, greatly reducing the habitability of their surfaces.
Figure 9.26 The TRAPPIST-1 solar system is comprised of seven, fairly Earth-like planets; their densities, for example, suggest that they are of similar bulk composition to the terrestrial planets in our Solar System (shown for comparison). And despite the dimness of their star, their close orbits mean that at least three of the planets are thought to reside within the habitable zone, where liquid water, if it is present, would be stable on their surfaces. This said, their close orbits also render it likely that all seven of these planets are tidally locked, greatly reducing their potential habitability. (Data courtesy of http://www.exoplanet.eu/)
Unfortunately, few if any of the 4,000 plus currently confirmed exoplanets are likely inhabitable. The tidal locking of the TRAPPIST-1 system illustrates one reason: a significant majority of the exoplanets discovered to date orbit within tidal locking distance of their parent stars (fig. 9.27), including four of the five exoplanets known to date to be reasonably Earth-like in size and temperature (counting two in the TRAPPIST-1 system). Likewise, the large majority of known exoplanets are ice or gas giants. This said, the seeming preponderance of planets that are (too) close to their stars and/or (too) massive to support habitable surfaces is due to observational biases and does not necessarily reflect the true diversity of plants and orbits. Specifically, both the radial velocity and transit methods of detecting exoplanets are more likely to identify large planets that are in close orbits. Larger, closer planets cause their companion stars to move more rapidly, improving the chances of observing a statically significant Doppler shift. Similarly, the probability of a planet being positioned such that it is seen to transit its star from the perspective of Earth is inversely proportional to its orbital distance (planets in closer orbits are more likely to pass in front of their stars), and larger planets block more of their star’s light. For these reasons, the transit method is, like the radial velocity method, biased toward large planets in close orbits.
Figure 9.27 A plot of the masses and orbital periods of the first few hundred planets for which both values are known illustrates the strong “observer bias” in the data. That is, radial velocity and transit methods are most sensitive to large planets orbiting close to their stars, and thus “hot Jupiters” and “hot Neptunes” are overrepresented in the data. The extent to which the paucity of Earth-like exoplanets in Earth-like orbits reflects the rarity of such planets versus the difficulty of detecting them remains unclear. (Data courtesy of http://www.exoplanet.eu/)
Things may be looking up, though. Specifically, the database of confirmed exoplanets is large enough that we can now start to use statistics to correct for the effects of the known planet-hunting biases in order to derive a truer picture of planetary populations. Doing so, we find that, for example, although so-called “hot Jupiters” (a gas giant planet with an orbital period of less than a week) are quite common among the known exoplanets (see fig. 9.27), they are actually rather rare, with only about 1% of Sun-like stars sporting them. In contrast, mini-Neptunes and super-Earths in intermediate orbits (10 to 100 days) are much more common; although our Sun does not host planets in either class, about half of all stars are thought to.
But what about potentially habitable planets? That is, on average, how many Earth-sized planets orbit in the habitable zone of a Sun-like star? Estimates of this vary widely. In part, this is because the “big-and-close” biases in exoplanet observation methods render the statistics poor: look again at fig. 9.27 and you’ll see how few planets of Earth-like mass and orbit we’ve identified to date. But it also varies depending on how you define Earth-sized and habitable zone. For example, one survey of data collected by Kepler suggested that between 15% and 30% of Sun-like stars harbor in their habitable zone at least one Earth-sized planet, where habitable zone was defined as the region receiving between one-quarter and four times as much light flux as the Earth receives from the Sun, and Earth-sized was defined as having a planetary diameter falling between one and two times that of the Earth’s. This definition of habitable zone, however, seems far too broad as it would place the inner edge of our Solar System’s habitable zone inward of Venus’s orbit and its outer edge beyond Mars. Likewise, this definition of Earth-sized is too broad since a planet larger than 1.7 Earth diameters is more likely to be a mini-Neptune than a super-Earth. Given these caveats, it’s not surprising that other surveys use other, often narrower definitions of habitable zone and Earth-sized and, in doing so, come up with other probabilities. Indeed, even a quick review of the literature turns up values ranging from 124% (i.e., on average slightly more than one Earth-sized planet per habitable zone) to just 1.3%. Still, the argument that even 1% of all Sun-like stars harbor least one Earth-sized planet in their habitable zone is one of the more optimistic things we’ve written in this book relative to the possibility of life elsewhere, no?
And, of course, scientists are not done looking. NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, is monitoring 200,000 bright stars, covering 85% of the entire sky, for indications of planetary transits with the aim to discover a range of exoplanets, including Earth-sized ones as well as gas giants. And while TESS is less sensitive than Kepler was, rendering it even more biased toward large, close-in planets, the greater brightness of the stars it is investigating renders ground-based follow-up studies far more accurate. By early 2020, TESS observations had already yielded more than 1,700 “objects of interest,” including at least one Earth-sized planet, TOI700d, orbiting within the habitable zone of its star. A year after the start of the TESS mission, the European Space Agency launched its Characterizing Exoplanets Satellite (CHEOPS), whose goal is not to discover new exoplanets but instead to determine the size of known exoplanets, which will improve our knowledge of their density and composition and, with that, our understanding of planet formation and habitability. Looking farther ahead, in 2026, the European Space Agency anticipates the launch of the PLATO (Planetary Transits and Oscillations of Stars) mission, which will use 34 separate small telescopes to scan a million stars, searching for new exoplanets with sensitivity approaching that of Kepler.
Conclusions
What, then, is the bottom line on habitable places in our Universe? It is clear that, in the Solar System, neither Venus nor Mars lies in the continuously habitable zone. And while Mars (and perhaps even Venus) may have hosted liquid water once upon a time, the surfaces of both planets are far too dry (and cold and hot, respectively) to support life now. Still, big questions remain. Did life arise on Mars when it was more clement and then, perhaps, flee to still-populated habitats beneath the surface? And when nonsolar energy sources (e.g., tidal heating) are available to keep things warm and moist, can life arise in places, like Europa, that fall far outside the classical habitable zone? And what of life around other stars? In the next chapter, we detail humanity’s efforts to answer these exciting questions by searching for evidence of life in these far-flung places.
And what about the Apollo astronauts? Obviously, they—and the rest of the biosphere—survived. Six months after the Apollo 11 mission, the astronauts of Apollo 12 were similarly quarantined. When they emerged from their three-week isolation unscathed, the decision was made that the Moon posed no threat, and none of the four remaining Apollo missions subjected their astronauts to the same isolation. Not that these “planetary quarantine” issues are behind us. While the highly tentative launch date of NASA’s Mars sample-return mission is “sometime before 2030,” its proposed date keeps moving forward at more than one year per year (i.e., it is receding rapidly into the future), but someday, we presume and hope, we’ll face a decision about the quarantine of samples from our neighboring planet. As far back as 1997, a National Research Council report argued that, although the probability that such samples will contain pathological or environmentally dangerous organisms is low, we should not simply assume the risk is zero. The consensus then and now is that such samples should be delivered to a combined quarantine and research facility unlike any other in existence—one that is capable of protecting the scientific integrity of the extraterrestrial samples (preventing their contamination with Terrestrial substances) while protecting Earth’s environment from exposure to potentially dangerous organisms from Mars.
Further Reading
Venus
Grinspoon, David H. Venus Revealed. New York: Helix Books, 1996.
A Lake on Mars
Orosei, R., S. E. Lauro, E. Pettinelli, A. Cicchetti, M. Coradini, B. Cosciotti, F. Di Paolo, et al. “Radar Evidence of Subglacial Liquid Water on Mars.” Science 361, no. 6401 (2018): 490–93.
The Galilean Satellites
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Europa as a Potential Abode for Life
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Titan
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Exoplanets
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- * Conrad, ever the joker, snuck an automatic timer for his camera with him to the Moon, hidden in a rock sample bag, so that he could perch the camera on Surveyor 3 and snap a picture of himself and Bean together. His hope was that, after the film was developed, people would eventually, with shock, ask the question: “If both astronauts are in the picture, who took the picture?” Alas, though, when the time came, Conrad could not find the timer in the sample bag where he’d hidden it.
- * From his book: Svante Arrhenius, The Destinies of the Stars (New York: G. P. Putnam and Sons, 1918).
- * From his article: Percival Lowell, “Our Solar System,” Popular Astronomy 24 (1916): 427.
- * Two notes for our Californian readers: (1) If you’ve ever climbed Mount Whitney, the stone structure still there at the top was erected for Campbell’s expedition and paid for by the Smithsonian. (2) Campbell went on to become the tenth President of the University of California, which is why there are buildings named “Campbell Hall” on some UC campuses.
- * Both rovers exceeded their planned missions spectacularly. Spirit ran into deep sand in May 2009 and failed to get out again. In January 2010, mission control redefined Spirit as a stationary instrument before losing contact later that year. Opportunity remained active until June 2018, exceeding its projected three-month lifetime by more than 14 years.
- * Peale, S. J., P. Cassen, and R. T. Reynolds. “Melting of Io by Tidal Dissipation.” Science 203, no. 4383 (1979): 892–94.
- * Cassen P., R. T. Reynolds, and S. J. Peale. “Is There Liquid Water on Europa?” Geophysical Research Letters 6, no. 9 (1979): 731–34.
- * Speaking of Europan spectroscopy, Corey Jamieson of the University of Hawaii claims (perhaps not too seriously) that we’ve already seen evidence for life on this moon: the closest laboratory match he’s found to the infrared spectra of Europa is provided by Terrestrial extremophilic bacteria frozen to liquid-nitrogen temperatures! Most members of the astrobiology community are betting that a more prosaic source for the Europan spectral features will be found.
- * The atmosphere on Titan is so dense, and the gravity so low, that were you visiting it, you could strap wings to your arms and fly like a bird. You would need to dress warmly, though.
- * More precisely, this was the first confirmed exoplanet around a “normal” star. In 1992, after a similar, false report the year before by another group, Aleksander Wolszczan and Dale Frail announced the discovery of two planets in orbit around a neutron star, the pulsar PSR B1257+12. These and the half dozen “pulsar planets” discovered since likely formed from the debris blasted out in the supernova explosion that created the pulsar.
- ** They shared the prize with Jim Peebles, the cosmologist whose work we discussed in chapter 2.
- * For the latest numbers, go to: http://www.exoplanet.eu/.