All members of our inner rocky planetary family – Mercury, Venus, Earth and Mars – are unique and have very distinct differences that have ultimately determined whether or not they might bear witness to life. At the heart of this lie two key attributes: their distance from the Sun, or heliocentric position (which influences their temperature), and their mass. Although celebrated in every mythology throughout history and some even visible to the naked eye on a clear night, their physical characteristics only became known in the twentieth century with the birth of spectroscopy and photography, and subsequently through space exploration itself in the 1960s.
Beyond the orbits of Mars and the asteroid belt, we find the outer gas giants and icy bodies sitting well beyond the Solar System’s traditional Goldilocks zone. However, observing these worlds, and especially their moons, has caused us to re-think what actually constitutes a habitable zone for life, how far this zone reaches across the Solar System and, most importantly, whether there could actually be multiple Goldilocks zones located within different planetary families. The opening of our minds to the many extreme conditions life can endure, and all the extraordinary environments in which life on Earth has been found to be present, has meant that the cold, dark outer reaches of the Solar System have begun to intrigue us from an astrobiology perspective and may actually hold the most promising targets for finding alien life. Wherever there is liquid water on Earth there is life, whether it is in the oceans that envelop the planet, thousands of metres beneath the ground, inside a nuclear reactor, or hidden within glaciers. As long as there is water, there is life. Of course, it is mainly tiny, mostly microbial, life – but it is still life.
Messenger to the Gods
Baked in scorching sunlight by day, and deep-frozen by night, it is far too easy to dismiss Mercury in the search for life in the Solar System. Only slightly larger than Earth’s moon, Mercury is already the smallest of the Solar System planetary family and is still shrinking. It was long thought to be something of a relic, a stagnant world that had not changed in eons and eons. Like the Moon, it is covered with craters caused by billions of years of bombardment into its iron-rich basaltic crust. This is because Mercury has very little atmosphere to prevent impacts from meteorites and asteroids damaging the surface. The planet is actually home to one of the largest impact basins in the Solar System: the Caloris Basin. Mercury’s day-side is super-heated by the Sun to a sweltering 427°C (800°F), but at night temperatures drop hundreds of degrees below freezing. Its egg-shaped orbit takes it around the Sun every 88 days, although this orbit is quite odd – it takes longer for it to rotate on its axis and complete a day than it takes to orbit the Sun and complete a year. It also has only 38 per cent of the gravity of the Earth (a human would weigh 62 per cent less on Mercury than on the Earth) and coincidentally has almost the same gravity as Mars.
We do not know who first discovered Mercury but it has been known to exist since ancient times. Mariner 10 was the first spacecraft to visit in 1974 and was also the first spacecraft to slingshot past one planet on its way to visit another, as well as the first probe to visit two planets in one mission. Despite being a spacecraft that seemed to be behaving neurotically, posing problem after problem that confounded its designers and controllers, it still managed to reveal a small, bleak planet with a thin helium atmosphere, a weak magnetic field and a cratered surface reminiscent of that of our Moon. The spacecraft ran out of fuel in 1975 and today Mariner 10 is presumed to be silently continuing its orbit of the Sun. After the picture painted by this probe, it was another 30 years before humanity returned to Mercury – little did we know that the least explored of the inner planets in the Solar System was hiding a very lively personality …
When the spacecraft MESSENGER arrived at Mercury in 2011, it went down in history as the first spacecraft ever to orbit the closest planet to the Sun. To arrive at this point, MESSENGER soared through the inner Solar System, performing one fly-by of Earth, two fly-bys of Venus, and finally three fly-bys of Mercury. Before its self-destruction on 30th April 2015 with a planned impact into the surface, it was tasked to map Mercury’s surface geology, study its magnetic field and expose its internal workings. Apart from creating the most detailed and accurate 3D map of the planet to date, MESSENGER also revealed Mercury to be a unique, geologically diverse world. It has active geology: hollows formed when volatile materials – probably sulphur-containing compounds buried beneath Mercury’s surface – sublimated (turned straight from solids into gas), causing the terrain to sink by several tens of meters, and volcanic vents measuring up to 25km (15.5 miles) across, which may once have been sources for large volumes of very hot lava.
Owing to its small dimensions, many scientists believed that Mercury’s once hot liquid core would have long since cooled and in essence the planet had become a dead, roasted chunk of rock. We now know that it has a massive core for its size, and as such is still partially liquid. We know too that its dynamo is still functioning, creating a weak magnetic field. MESSENGER also solved another mystery about Mercury. It bounced laser beams off its surface to remotely collect information about the chemical elements that make up the planet’s surface. Incredibly, water ice was discovered to be hiding in regions in permanent shadow near the north pole. Even more excitingly, carbon-containing organic compounds were found, forming a thin layer of very dark organic material covering part of the frozen water. This material, which is somewhat like tar, coal and soot, is believed to be similar to what has been observed on icy bodies in the outer Solar System and in the hearts of comets. Scientists suspect that impacts as well as solar and cosmic radiation are triggering chemical reactions in the organic material, turning it about twice as dark as Mercury’s surface.
Although not previously the most logical choice for life, Mercury has shown us that it has a few nooks and crannies with the makings of a habitable environment. To be clear, no one in truth thinks that Mercury has microbes. What it does have, however, is evidence of volcanic activity in its past, a hot liquid core, a functioning magnetic field, a thin atmosphere and gloopy tar-like organic materials. Water ice has been found frozen at its poles that, combined with a hot core, may hint at the possibility of liquid water forming at depths below the surface. Mercury is not in the Goldilocks zone of our star but is a key witness to the delivery of ingredients for habitability from the outer Solar System to the inner. Even if Mercury is not itself a good candidate on which to look for ancient or current life, the planet may hold clues as to how life got started on Earth. Finding a place in the inner Solar System, where some of the same ingredients that may have led to life on Earth are preserved, is really very exciting. As a result, humanity is going back. Europe’s first mission to Mercury, BepiColombo, will launch in 2017, arriving at the smallest terrestrial planet in 2024 to carry out the most extensive exploration of Mercury and its potentially habitable environment to date.
Goddess of Love and Beauty – if You Say So …
If not Mercury, who is next on the list? With its similar size, mass and composition, Venus’s dimensions are very close to those of Earth, hence it is commonly called its twin and is likely to have a still-functioning internal heat source, perhaps from radioactive decay, similar to the Earth’s interior. There is, however, one major yet significant difference between these near-identical siblings. Venus’s thick atmosphere makes temperatures on the planet hot enough to melt lead, and therefore it is most certainly too hot to sustain life. Blanketed in clouds, the veiled planet once had oceans much like those on Earth, but these evaporated as Venus heated up. Today, Venus is about 100,000 times drier than Earth and is 460ºC (860ºF) at its surface. Its atmosphere is 96 per cent carbon dioxide with a thick atmospheric pressure of 89atm (9MPa) – a greenhouse planet where no life currently found on the Earth could ever hope to survive.
As one of the first planets to be visited by spacecraft, due to its relatively close proximity to the Earth and it being a necessary waypoint for missions to Mercury, Venus has witnessed and probably rolled her eyes at the many failed attempts to try and visit her. Despite these, however, more than 20 unmanned explorations have been successful, with Venus the target of a cornucopia of Soviet, American and European probes. Spacecraft have performed various fly-bys, orbits and landings, and even balloon probes have been sent to float through its atmosphere. The American Mariner 5 spacecraft can boast the first successful encounter, coming within 35,000km (21,750 miles) of Venus in 1962. As it flew past, it established that Venus has scarcely any magnetic field and with reasonable accuracy measured the planet’s temperature as being up to 316°C (600°F). The Soviet Venera fleet achieved notoriety in 1966 when the Venera 3 space probe crash-landed on Venus, becoming the first spacecraft to reach the surface of another planet, despite not being intact. In 1975, the descent vehicle from the Venera 9 orbiter was the first probe to take photos (and even black and white video) of the Venusian surface from the surface itself, revealing shadows, no apparent dust in the air, and a variety of rocks up to 40cm (16in) in size that did not appear eroded by wind or water, as would be found on Earth. Venera 9 also detected clouds that were up to 40km (25 miles) thick, with bases from 30km (18 miles) in altitude; acidic chemicals in the atmosphere, including hydrochloric and hydrofluoric acid, bromine and iodine; surface pressures of about 90atm (9MPa); a surface temperature of 485°C (905°F); and light levels bathing the world that were comparable to those at Earth’s mid-latitudes on a cloudy summer’s day.
Two notable missions for the purpose of understanding the possibilities of life on Venus are those of the USA’s Magellan and ESA’s Venus Express. Magellan arrived in 1990 and with its radar mapped 98 per cent of the surface with a resolution of approximately 100m (330ft). The resulting maps are comparable to visible-light photographs of other planets, and are still the most detailed in existence. Magellan greatly improved scientific understanding of the geology of Venus: a world covered in volcanic rocks and vast lava plains, lava channels more than 6,000km (3,730 miles) long, fields of small lava domes, and large shield volcanoes. The probe found no signs of Earth-like plate tectonics, although the scarcity of impact craters suggested the surface was relatively young (less than 800 million years old).
Venus Express arrived 16 years later in 2006 and took up a polar orbit, focusing on long-term observation of the Venusian atmosphere from the surface right up to the ionosphere. Venus Express confirmed that eons ago, Earth’s twin must have had substantial oceans. It observed lightning on Venus happening more frequently than on Earth, and witnessed a colossal double vortex swirling over the planet’s south pole. The probe photographed a night glow, an eerie radiance in the night-time atmosphere of Venus, seen as the Sun’s ultraviolet light hit the atmosphere, and infrared light energy was released by high winds from the swirling of oxygen (O2), hydroxyl (OH) and nitric oxide (NO) molecules. This was also the first detection of hydroxyl in the atmosphere of any planet other than Earth – important because it is created by a reaction between oxygen and water. A major question in Venus science is whether it is still geologically active today. In 2015, nearly 10 years after the arrival of Venus Express at our sister world, tantalising evidence was discovered in a rift zone for hot spots that change in temperature from day to day, and are the best evidence yet for active volcanism on present-day Venus. On Earth, rift zones are the result of fracturing and cracking of the crust and are often associated with the upwelling of magma from below the surface. This process allows hot molten rock to be released through fractures as a lava flow. Although the surface of Venus is thus proving to be still geologically alive and so could provide an energy source for biological reactions, the hellish environment means that life would be hard-pressed to survive.
Despite all this, Venus does sit in a very privileged position at the inner edge of the Solar System’s Goldilocks zone. Having undergone a runaway greenhouse effect, the surface has become far too hot for liquid water or organic molecules to be stable, and therefore is not a habitable environment for life as we know it, at least not today. High above the surface, however, is a potential habitable zone where temperatures lie in the range between freezing and 120°C (248°F) and clouds offer long-lasting droplets of water, although they are highly acidic. The lower and middle cloud deck of Venus may therefore have the ability to support an aerial biosphere; whether it actually does remains a mystery to be solved.
The God of War
The most Earth-like planet known is our reddish dusty sibling, Mars. Half the size of Earth and lacking a magnetic field or thick protective atmosphere, this world is, believe it or not, currently our best hope of finding life elsewhere in the Solar System due to a number of similarities. Its day is only 29 minutes longer than that of the Earth and it takes 1.88 Earth years to orbit the Sun. It has seasons just like on Earth, 38 per cent of our gravity, and has more than five million cubic kilometres (almost 1.2 million cubic miles) of water ice, mostly hidden just below the surface. A toxic atmosphere of 95 per cent carbon dioxide, minimal oxygen and an average temperature of –63°C (–81.4°F), however, makes the surface of Mars appear rather inhospitable to life, especially human life. Astrobiologists are most interested, however, not in the possibility of life existing on Mars today, but in the past. Could life once have existed here and if so, where did it go? Where might it be hiding if it were still present? Mars is a planet that we are becoming as familiar with as our own, and so no astrobiological discussion can be started without first acknowledging the many missions that have provided us with this window into such a familiar yet alien world.
Mars Mission History
As we saw in Chapter 1, popular culture had it since the nineteenth century that Mars is or was an inhabited planet, crisscrossed with canals of liquid water built by some advanced civilisation that might or might not be on the verge of colonising the Earth. Once technology caught up with the desire for exploration, however, satellites were tasked to orbit the planet and take the first ever close-ups of the Martian surface. After several catastrophic failures, many of which occurred even before the spacecraft left Earth’s atmosphere, in 1965 NASA’s Mariner 4 finally flew by Mars after a 7.5-month journey through 54.6 million km (33,926,870 miles) of open space, snapping the first pictures of the Red Planet. This eagerly anticipated arrival shattered any imaginings of a lush, Earth-like world with flowing rivers and cities full of humanoid Martians. Instead, it was clear that Mars is a rocky, barren world, scarred with impact craters and cavernous valleys – a world that in many ways is more reminiscent of the airless, lifeless Moon than the Earth. It also discovered that Mars has no global magnetic field, which would be necessary to protect any life forms on its surface against dangerous solar winds of charged particles. We now know that Mars’s magnetic field disappeared around 4 billion years ago, but we do not know why. With the loss of its magnetic field, the planet’s atmosphere was no longer protected and was stripped away, exposing the surface to solar and cosmic radiation, gradually making it even more inhospitable.
Viking
Undeterred by the disappointment of Mariner 4, humanity returned to Mars. In 1976, NASA’s Viking Project became the first US mission not only to land a spacecraft on the surface of Mars in one piece, but also to return images of the surface. Twin spacecraft, both consisting of a paired lander and orbiter, entered Mars’s orbit before detaching the landers to begin their fiery descent to the planet’s surface. The Viking 1 lander touched down on the western slope of Chryse Planitia (the Plains of Gold) just north of the equator, while the Viking 2 lander settled down at Utopia Planitia, the largest recognised impact basin on Mars, and indeed the Solar System. Besides taking photographs of a vast number of rocky vistas, the probes became renowned for finding evidence of water action on Mars, including sweeping valleys and deep fluvial erosion patterns. Most famously, however, the landers conducted three biology experiments designed to look for possible signs of life. These discovered unexpected and mysterious chemical activity in the Martian soil, but were unable to provide clear, undeniable evidence for the presence of living microorganisms near the landing sites. The conclusion made was that Mars is self-sterilising: that the solar ultraviolet radiation saturating the surface, the extreme dryness of the soil and the oxidising nature of the soil chemistry combine to prevent the formation of living organisms in the Martian dust. As depressing a result as this seemed to be, scientists now claim that the method by which the samples were collected could actually have destroyed the evidence of life they were looking for … bad news for Viking, good news for astrobiology.
The lack of life found on Mars by Viking was an enormous blow to the global community, and it took another 20 years before we successfully went back to Mars. From 1996, Mars Global Surveyor (MGS), accompanied by Mars Pathfinder and its little rover Sojourner, orbited and mapped the entire planet. MGS achieved so much in its seven-year life, including the characterisation of surface features and geological processes; the determination of the composition, distribution and physical properties of surface minerals, rocks and ice; and the mapping of the global topography, planet shape, and gravitational field. The mission also monitored global weather and, importantly, imaged possible landing sites for the 2007 Phoenix Lander and 2011 Curiosity rovers. Since then, the space around Mars has become rather full, as NASA’s Mars Odyssey (2001), ESA’s Mars Express (2003), NASA’s Mars Reconnaissance Orbiter (MRO, 2005), ISRO’s Mars Orbiter Mission (MOM, 2013) and NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN, 2013) have now joined MGS, with ESA’s ExoMars Trace Gas Orbiter (TGO), scheduled to arrive at the party in 2016. A total of 13 orbiters to date has been sent to circle the planet and map and explore the surface, while several rovers have scoured its landscape searching for clues that might indicate life is, or once was, possible.
MAVEN
The Mars Atmosphere and Volatile Evolution mission, or MAVEN mission, launched in 2013, is currently orbiting Mars to explore how the Sun may have stripped the planet of most of its atmosphere, turning a world once wet and habitable for microbial life into a cold and barren desert. Scientists want to know what happened to the water that once flowed across the surface and also where the planet’s thick atmosphere disappeared to. Each time MAVEN orbits Mars, it plunges temporarily into the ionosphere – the ion- and electron-laden atmospheric layer lying uppermost, at 120–480km (75–300 miles) above the planet’s surface. This layer serves as a form of shield around the planet, deflecting the intensely hot, high-energy particles of the solar wind. Today, MAVEN has shown a plume of atmospheric particles breaking free of the planet’s gravity, escaping from the polar region, extending behind Mars like a tail. MAVEN has also detected a long-lived layer in the electrically charged ionosphere of Mars, made up of metal ions (iron and magnesium) that are the remains of incoming comet dust and meteorites. The spacecraft has also seen the Red Planet glow under the impact of violent Coronal Mass Ejections (CME) sent from the Sun. These blast billions of tons of solar material into space at millions of kilometres per hour but because Mars is not protected by a global magnetic field as is the Earth, CME particles directly impact the Martian upper atmosphere, driving the escape of atmospheric gas into space. This generates some stunning displays of aurora.
Roving on Mars – Sojourner
The great-grandfather of Mars’ rovers, Sojourner (meaning ‘traveller’) was the first moving robot on Mars, and indeed the first wheeled vehicle driven on any other planet in the Solar System. It travelled just over 100m (330ft) within the ancient floodplain of Ares Vallis, snapping over 550 photographs of the rocks it encountered. The first one it chemically analysed was dubbed Barnacle Bill and this changed our view of Mars forever. The rock was found to have more silica in it than the surrounding environment, a clear sign of past thermal activity. Suddenly Mars’s geological history became a great deal more interesting. Further rocks, nicknamed Yogi and Scooby Doo, were recognised as being not just volcanic rock (basalt), but also sedimentary. On Earth, sedimentary rocks are made by deposition of material on the surface and, importantly for Mars, within bodies of water. Images beamed back supported this, exposing rounded pebbles and conglomerates that told a story of rock movement by water in the past. A more water-rich planet was starting to reveal itself, and where once there was water there may have been life.
Spirit and Opportunity
In 2004, siblings Spirit and Opportunity bounced on to the surface and finally delivered conclusive proof that liquid water had once been present on Mars. Spirit landed in a possible former lake within a giant impact crater given the name of Gusev, while Opportunity (fondly known as Oppy) headed to the flat plain of Meridiani Planum, where satellite data had found a high level of the mineral haematite, an ore that requires liquid water to form. Their goal was to search for signs of past water activity on the Red Planet and this did not take long at all. As soon as Oppy opened its panoramic camera eyes, scientists knew they had struck gold – actually, haematite. Landing in a shallow impact crater, Oppy was facing a layered sedimentary rock wall, surrounded by marble-sized iron-rich mineral balls dubbed blueberries. This amazing rover, on its first day of operation, had found an area that had formed in an ancient acidic and oxidising shallow lake. Its mission was already a complete success. The rover had discovered the evidence needed to prove that ancient Mars may have been habitable for life for potentially millions of years. Spirit was not letting Oppy take all the glory, however, as it also completed its initial mission in record time. At a location in Gusev Crater dubbed Home Plate, Spirit discovered opaline silica, which would have formed in volcanic fumaroles or hydrothermal vents, showing that water had interacted with magma in the past at that site. It also finally discovered the elusive carbonate rocks, which, given its atmosphere of carbon dioxide and evidence of water, scientists had been expecting to come across far sooner. Equally interesting, Spirit also observed complex coatings on olivine basalts, created by modern-day water on Mars, or possibly frost.
Phoenix
Although not a trundling rover, a hugely important astrobiological mission to Mars came in 2008 with the arrival of the Phoenix lander. Phoenix was designed to study the history of water on Mars and the habitability potential of the Martian Arctic’s ice-rich soils. It landed in a flat landscape shaped into 2–3m- (6.5–10ft-) wide polygons, and had been sent there because such geometric features are created on Earth by ice expanding and contracting inside soils when the temperature changes. Phoenix found water ice on Mars just a few centimetres below the surface in the middle of the polygons, and amazingly the ice was photographed slowly sublimating (turning straight from solid ice into gas) when exposed to the Martian atmosphere. Phoenix also observed snowfall on Mars, and found calcium carbonate in the soil, indicating a wetter past environment at the landing site. It also located something pretty dangerous for life – perchlorate salts. The big question regarding the presence of organic compounds in the soils surrounding Phoenix was left open, since heating of samples containing perchlorate would have caused any organic materials present to break down and be destroyed. Under certain conditions perchlorate can inhibit life, but all is not lost since microorganisms do exist on Earth that can obtain energy from it by anaerobic reduction. Also, the chemical, when mixed with water, can greatly lower the liquid’s freezing point, just as salt can when it is applied to roads to melt ice. Thus, although a potential problem for life itself, perchlorate may allow small quantities of liquid water to form on or beneath the surface of Mars today and therefore, ironically, provide microhabitats for life.
Curiosity
Finally, we come to the best-known, and it must be said, most Twitter-savvy rover to date: Curiosity. The centrepiece of NASA’s Mars Science Laboratory (MSL), this BMW Mini-Cooper-sized robot was detailed with finding out for certain whether Mars is, or was, suitable for life. Its immense size has allowed it to carry a suite of instruments designed to crush, bake and photograph any rock within 2m (6ft) of its robotic arm. Weighing in at 900kg (1,984lb), it can rove up to 200m (650ft) per day, faster than any rover before it (although still literally at a snail’s pace), and is powered by a radioisotope thermoelectric generator (or nuclear-powered generator, using plutonium-238). The $2.5-billion MSL spacecraft launched from Cape Canaveral, Florida, on 26th November 2011, and arrived on Mars on 6th August 2012, after a daring landing sequence that NASA dubbed the seven minutes of terror. This intricate sequence used a supersonic parachute, rocket thrusters and the now famous skycrane, which allowed the landing assembly to dangle the rover beneath the rockets on a 6m (19.5ft) tether, gently positioning Curiosity on to the ground while simultaneously severing the link to crash-land elsewhere on the surface. Fun fact: as Curiosity trundles across Mars, it leaves in its tracks a message from home in Morse code. The wheels contain embedded cut-outs of dots and dashes that the rover can use as reference points to estimate how far it has travelled. In dot-dash notation, however, each wheel carries three characters (• – – – / • – – • / • – • •), which just so happens to spell out JPL, the acronym of the Jet Propulsion Laboratory in Pasadena, California, which manages the rover mission for NASA.
Curiosity is tasked with searching for habitable environments at the landing site of Gale Crater, a 154km- (95.5-mile-) wide impact crater and central 5.5km (3.5 miles) geologically layered mountain called Mount Sharp, formed by a meteor strike some 3.5–3.8 billion years ago. Scientists chose Gale as the landing site for Curiosity because it displays many signs that water was present over its history, and as we are all aware by now, water is a key ingredient for life as we know it. Some of these indicators are minerals manifested as clays and sulphates, formed only in the presence of water. They are also exciting to study, as on Earth many preserve signs or biosignatures, of past life.
To achieve its goal, Curiosity has many instruments and experiments set up on board, including one that bombards the surface with neutrons whose speed would slow if they encountered hydrogen atoms: one of the elements of water. It has a robotic arm that can collect samples from the surface, an oven to bake them inside its main body and the ability to test the gases that are given off, analysing them for clues about how the rocks and soil formed. It has high-resolution cameras surrounding the rover that, besides taking great selfies, can take pictures as it moves, providing visual information about the landscape that can be compared to analogue environments on Earth. Panoramic images especially are taken to help scientists select promising future geological targets and to help the rover drivers steer Curiosity to those locations to perform on-site scientific investigations.
Within six months of arriving at Mars, Curiosity had its answers and the mission was deemed a complete success. Gale Crater had the right ingredients and environments to support ancient microbial life, should it ever have arisen. The landing site itself contains at least one lake that would have provided a deliciously habitable environment. Surprisingly, the clays drilled out from inside some rather informative mudstones at a site known as Yellowknife Bay were found to be much younger than expected. This discovery does not just prove that Mars was habitable but also extends the window of time when it may have been suitable for life. If that were not exciting enough, powder from the very first drill samples Curiosity obtained from the surface of Mars included the elements sulphur, nitrogen, hydrogen, oxygen, phosphorus and – you guessed it – carbon! Finally, we have unequivocal evidence that Mars has the chemistry for a habitable environment and the basic elemental building blocks for life.
In December 2014, Curiosity went one step further and made the first definitive identification of organics on Mars. It found chlorinated hydrocarbons such as chlorobenzene, dichloroalkane and chloromethane. Also in December 2014 came the news that Curiosity had detected wafts of methane in the Martian air. From time to time, Mars belches out a gas that on Earth comes largely from life forms (from one end or the other). This may well hint at communities of microbes living under the Martian surface and churning out the gas. Nonetheless, any number of other non-biological, probably more likely, processes can and do make methane. Rocks on Mars contain the mineral olivine, which can react with water to release methane. Also, clathrates or molecular cages harbouring methane in the icy sub-surface could be a source, releasing the gas in bursts over time. Unfortunately, detecting methane alone is not enough to claim life.
In addition to its main mission, Curiosity has been carrying out radiation observations to determine how suitable an area like Gale Crater would be for an eventual human mission. Curiosity operates its Radiation Assessment Detector for 15 minutes every hour to measure radiation on the ground and in the atmosphere. In December 2013, NASA decreed that the radiation levels were viable for future human crews heading to Mars. A mission comprising a 180-day journey each way to and from Mars and 500 days on the planet would administer a dose of 1.01 sieverts. This is only a touch over the total lifetime limit of 1 sievert currently set for ESA astronauts, and is associated with only a 5 per cent increase in fatal cancer risk over a lifetime, so is tolerable …
Despite the fact that organics have finally been found on Mars, the question of whether it has or had life still remains. That revelation, and I believe it will come, still lies ahead of us, and will hopefully be provided by the next rover to head to Mars in 2018 – ExoMars.
ExoMars
The Exobiology on Mars mission (ExoMars), set to launch in 2016 and again in 2018, is the first space operation designed chiefly to search for biosignatures of past and present life on Mars. This hotly anticipated astrobiology-led mission is currently under development by the European Space Agency (ESA) in collaboration with the Russian Federal Space Agency (Roscosmos). Due, at time of writing, to launch in early 2016, the ExoMars Trace Gas Orbiter (TGO) and an Entry, Descent and Landing Demonstrator Module (EDM) stationary lander called Schiaparelli will head to Mars first. After delivering the lander to the surface, the TGO will stay in orbit to map the sources of methane and other gases on Mars, and will confirm the ultimate landing site for the ExoMars rover, scheduled to head to Mars in 2018. It will also act as a crucial communications relay between the ExoMars rover and Earth. The unique selling point of the ExoMars rover is its drill. This little rover will be able to drill 2m (6ft) into the Martian sub-surface, cutting through and exposing millions of years of Martian history, and possibly buried life forms.
Where would you send ExoMars if you had to pick just one spot to visit on Mars? At the time of writing, this is the decision being made – where could ExoMars land that would offer the best chances of finding life? As with every space mission, it comes down to a delicate yet familiar balance between engineering constraints and scientific goals. Unfortunately, owing to the way we have to land the rover on Mars the engineers win, and so most of the planet is already ruled out as being too dangerous. The landers need to touch down at as low an elevation as possible to give them more time to come through the atmosphere and therefore more time to slow down (so as not to crash into the surface). To help narrow this already small list of landing choices, a further engineering requirement is for the rover to access as much sunlight as possible since it runs on solar power. ExoMars therefore needs to land in a latitude band straddling the equator of Mars that is a meagre 30 degrees wide from top to bottom. Finally, landing on another planet is never possible with pinpoint accuracy; it is much more likely that ExoMars will touch down somewhere near its intended target rather than directly on it. This area is called the landing ellipse, and for ExoMars is the equivalent of landing anywhere within a 104km by 19km (65 mile by 12 mile) area of where you aimed.
With all these requirements met, the scientific goals start to come into play. Four possible sites were identified for ExoMars to make its stand and hopefully achieve its mission to find signs of past or present life – Mawrth Vallis, Oxia Planum, Hypanis Vallis and Aram Dorsum. Over the coming years, one of these will become the most studied and talked about spot on Mars. Found clustered within the same equatorial region of Mars, but covering an area the size of Western Europe, these sites feature ancient rocks containing a record of the environment on Mars over 3.5 billion years and, of course, display evidence that liquid water once flowed there.
Mawrth Vallis is a veteran landing site, having also made it to the final four for the Curiosity rover. Named after the Welsh name for Mars and the Latin for valley, it is an ancient channel carved by catastrophic floods. It has layered cliffs resembling Neapolitan ice cream that are rich in clay minerals. Such minerals, called phyllosilicates, form in the presence of neutral pH water and tell us that habitable conditions for life once existed and, as said before, are also good at preserving signatures of long-dead life. Oxia Planum, 400km (250 miles) away from Mawrth, is also made up of layers of clays and has an ancient channel emptying into a now dry shallow lake. Hypanis Vallis is thought to represent ancient river delta deposits. Here, sediments were built up slowly and may have concentrated the evidence for life, making it easier to find. Finally, Aram Dorsum is an inverted river system, with a hill-like relief instead of a stereotypical depressed river channel. This is quite common on Mars. It happens when water carves a channel and deposits sediments that, once cemented and hardened, survive intact while everything outside is worn away by billions of years of erosion, leaving evidence of an ancient river system, now rising above the landscape instead of below it. Water was flowing throughout this region about 3.8 to 4 billion years ago, a period when life was probably just getting started on Earth, and possibly also on Mars. It may be that evidence of past life on Mars is hiding just beneath the surface at one or all of these sites. At least for now, it looks as though ExoMars will be heading to Oxia Planum, Mawrth Vallis lost out again, so let us all hope we have made the right decision.
Mars 2020
NASA’s Mars 2020 rover is a mission under development and forms part of a long-term campaign to bring Martian rocks home to Earth. Based on the successful design and operation of Curiosity, Mars 2020 will be sent to investigate an as yet undecided, astrobiologically relevant ancient environment on Mars to uncover its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures. It has been proposed that the rover collect and package as many as 31 samples of rock cores and soil for a later mission to bring back to Earth. The Mars 2020 rover will also help pave the way for future human explorers by investigating the ability to use natural resources available on the surface of the Red Planet. Designers of future human expeditions can use this mission to understand the hazards posed by Martian dust and demonstrate technologies to process carbon dioxide from the atmosphere to produce oxygen for human respiration, and potentially as an oxidiser for rocket fuel (see Chapter 10 for more information on humans travelling to Mars).
When on Mars …
Although scientists have not (yet) found life populating the Martian surface, we have not lost hope and are looking harder than ever. This is because in so many respects Mars is the most Earth-like planet in the Solar System, and with its much warmer and wetter habitable environment in the past, the conditions for the emergence and persistence of life may have been present on Mars just as they were on the Earth. Mars has famously undergone global climate change over its 4.5-billion-year life, so we are not just looking to environments on Mars today that might support life, but also at different points in its history. Mars has transitioned through three climatic stages: from relatively wet to semi-arid to hyper-arid conditions, and consequently the surface habitability has deteriorated greatly over its planetary lifetime. The history of Mars can thus be divided into Early, Middle and Present Mars phases. Early Mars covers roughly the first billion years of its lifespan, when liquid water was still presumed to be present on the surface. This was an environment similar to the Earth, supporting the hypothesis that life may have had the chance to flourish on Mars as it did on Earth during this time. However, after the first few hundred million years the environmental histories of these two planets diverged drastically. Mars underwent a global climate shift that resulted in a drop in surface temperatures and loss of liquid water. This is the onset of Middle Mars, defined as the second billion years when a global cryosphere (global ‘ice age’ during which the entire surface froze) took hold of the planet and paved the way for the Mars we know today. Present Mars started some 2.5 billion years ago and is characterised by a hyper-arid climate. The existence of life on the Martian surface today seems unlikely, given the extremely cold and desiccating conditions, high UV radiation levels received on the surface or flux, and the lack of magnetospheric shielding against ionising radiation. Beneath the surface, however, partially protected from radiation by rocks and dust, and where it is slightly warmer so that isolated pockets of liquid water may remain, who knows what (or who) might be hiding …
King of the Planets
The terrestrial planets do not present many especially habitable conditions on their surfaces today. Mars may contain preserved biosignatures within surface materials, or active communities deep in the sub-surface, and Venus may offer habitable conditions high in its atmosphere. That said, they are a much better bet than the planets beyond the asteroid belt. Once we cross this rotating barrier of rock, we enter the realm of the gas and ice giants, such as Jupiter. While there have been no samples taken that could test for microscopic life on Jupiter, there is considerable and compelling evidence against life as we know it existing or ever having existed there. Composed mainly of hydrogen and helium, there is virtually no water present that could support a life form. The planet does not have a solid surface anywhere for life to develop and the only real (tiny) possibility of finding it would be in floating microscopic form high up in the atmosphere. However, this too has its problems. The atmosphere of Jupiter is in constant chaos, so even if life somehow held on near the lower pressure regions in the upper reaches, and could resist the harsh solar radiation found there, it would eventually be sucked down into realms where there is 1,000 times Earth’s atmospheric pressure and temperatures are over 10,000°C (18,000°F); it would be almost instantaneously destroyed. No life on Earth could survive in anything close to these environments.
Known, therefore, as completely inhospitable, the gas giants are not truly included in the search for life, although it is fascinating to think about ways in which it might work. The most intriguing astrobiological targets in the Solar System are actually found on the moons orbiting these gaseous behemoths. The first is Europa, which on the face of it does not look or sound particularly appealing to life as it is constantly bombarded with ionising radiation owing to its location within Jupiter’s magnetosphere. Temperatures at its surface range from –187°C to –141°C (–304.6°F to –221.8°F), far below the lowest limits for microbial growth – not surprising since it is an average 805 million km (500 million miles) from the Sun. In 1979, the Voyager 2 probe whizzed past and spotted a network of cracks on Europa’s surface, confirming earlier theories that the moon was coated in a thick shell of ice. When the Galileo probe showed up in the 1990s, it became clear that the cracks were occurring because the ice was moving, floating on top of a hidden layer of liquid encircling the moon. Beneath Europa’s estimated 100km- (62-mile-) thick icy crust we believe there resides a liquid ocean with more water than covers the surface of the Earth. Within the cracks and fractures of the ice is a salty dark material, quite possibly the same as regular table salt, sodium chloride, which has risen up from the ocean beneath. Suddenly, Europa is a tantalising world for astrobiologists – if there is liquid water, where might life, if it exists, be hiding?
The surface ice itself is not an environment that any currently known terrestrial life could withstand, so we will not be seeing microbial igloos popping up there any time soon. However, the ice could provide just enough protection from the intense bombardment of radiation and encourage more favourable temperatures beneath it, to allow for the preservation of organics and even life forms. Just as a layer of ice over a pond allows the water beneath it to stay liquid and aquatic life to go on living through a freezing winter, Europa’s rind of ice shields its enormous ocean and helps to keep it warm enough to remain fluid in spite of the moon’s great distance from the Sun. Yet as Europa orbits Jupiter, the moon is contorted by the giant planet’s gravitational field, generating an interior heat, by far the dominant heat source, which also keeps its water from freezing altogether. Potentially, active volcanoes and vents may exist at the base of the ocean, further heating the water, and providing sites where bacterial life may congregate, as it does on Earth. Plumes have also been observed erupting from the southern hemisphere but interestingly, and perhaps worryingly, these now seem to have vanished – are they simply sporadic events or were they ever really there at all? Europa has many elements thought to be key for the development and even persistence of life, such as water and heat energy; astrobiologists are now keen to detect the presence of organic chemicals.
The much-awaited Jupiter Icy Moon Explorer (JUICE) mission is an ESA spacecraft planned as part of the Cosmic Vision science programme, scheduled to pay a visit to the Jovian system in 2030, to study Ganymede, Callisto and Europa (Io will be left out this time). Hopefully launching in 2022 and taking eight years to reach the system, its aim will be to analyse the character of these three worlds and evaluate their potential to support life, as all are thought to have significant bodies of liquid water beneath their surfaces. In particular, the focus on Europa will be on the chemistry essential for life, including organic molecules, and on the non-ice material criss-crossing its surface.
Lord of the Rings
If life is near impossible on Jupiter, you can guarantee the same can be said for Saturn. Comprised almost entirely of hydrogen and helium, with only trace amounts of water ice in its lower cloud deck, it has no surface upon which life could live. At the top of the clouds, the temperatures are around –150°C (–238°F), and although it gets warmer as you descend through the atmosphere, the pressures increase too. Sadly, once temperatures are warm enough to have liquid water, the pressures are simply too high for life. It is also extremely windy up there, with speeds of up to 500m/s (1,640ft/s). As with the Jovian system, the quest to find life near Saturn is turning its focus away from the planet and towards the moons.
Titan, Saturn’s largest, haziest moon, is getting scientists really rather excited. Deceptively Earth-like, Titan has a dense nitrogen-rich atmosphere (the only moon known to do so), complete with clouds and seasonal rainstorms that soak the surface. Sunlight and electrons stream across Titan from Saturn’s magnetosphere and break apart the nitrogen and methane in its atmosphere, setting off a cascade of reactions that produce organic compounds, and creates a solid organic haze that fills the atmosphere and shrouds the surface from view. It has a very familiar landscape beneath this seemingly impenetrable veil, with mountains, dunes, riverbeds, shorelines and seas. Indeed, Titan is the only place in the Solar System, besides the Earth, that has liquids pooling and flowing across its surface. It is likely that Titan would be a promising place to look for extraterrestrial life in the Solar System, if not for its coldness; Titan is far too chilly for life as we know it. All water on Titan is found as rock-hard ice. In fact, the many rocks that litter the moon’s surface are not made of rock at all but actually water. At Titanian surface temperatures (–179°C/–290°F), phospholipids – the chemical compounds that provide structure to cell membranes – cellular water bodies would be frozen solid. Any life that evolved on Titan’s surface would need to be made of a very different set of chemicals and not be reliant on water, as it is locked in a state inaccessible to it. But potentially lucky for life, Titan’s puddles are not filled with water; the surface is instead soaked with hydrocarbons. Methane and ethane, which on Earth are gasses, are able to flow as liquids across the surface owing to Titan’s frigid environment. The volume of liquid hydrocarbons resting in Titan’s second largest sea, Ligeia Mare, is actually 100 times greater than all the oil and gas reserves on Earth combined. Could a bizarre non-Earth-like life form exist on Titan that uses these slick, liquid hydrocarbons in a similar way to how life on Earth uses water?
During the 1990s, the Hubble Space Telescope (HST) offered hints that Titan was a wet world, but this was not confirmed until the NASA-managed Cassini mission allowed scientists to get a good look at the moon. This was a collaborative mission that included 16 European countries together with the US. On 14th January 2005, after a seven-year voyage, the Cassini spacecraft sent the Huygens probe parachuting through the haze to a spot on Titan’s equator, to become the first terrestrial robot ever to land in the outer Solar System. It then sent transmissions from the surface for another 70 minutes before Cassini moved out of range. Cassini is the fourth space probe to visit Saturn but the first to enter orbit and is still sending data at the time of writing. It has revealed a world on Titan that looks very much like ours – but with a completely different chemistry. As well as a hydrocarbon-drenched realm, Titan also may have a deep sub-surface ocean similar to that within Europa and another of Saturn’s moons, Enceladus. It may prove to be a water-ammonia mixture, which could be an environment habitable for organisms with biochemistry similar to that of terrestrial life – although requiring them to power a metabolism at the temperatures present on Titan would be a real challenge, even if the chemistry were usable. At this point, we do not really know what kind of life might be able to survive on Titan, as we have no examples of similar life on Earth. But this does not mean it isn’t possible.
Saturn’s sixth-largest moon, Enceladus, discovered by William Hershel in 1789, is a tenth the size of Titan, and is covered by fresh, clean ice with a surface temperature at noon of –198°C (–324.4°F). When Cassini flew by in 2005, it drew renewed interest from astrobiologists with the sighting of present-day geological activity occurring at its surface. Jets of fine icy particles and water vapour were observed erupting from cryovolcanoes at the south pole. Over 100 of these jets have been seen so far, feeding into a large plume that soars several thousand kilometres into space and containing not only water vapour but also simple organic compounds and volatiles – such as nitrogen (N2), carbon dioxide (CO2) and methane (CH4) – similar to the chemical make-up of comets. Some of the water vapour actually falls back on to Enceladus as snow, while the rest escapes and supplies most of the material making up Saturn’s E-ring. The southern polar terrain surrounding the source regions of the plume is surprisingly warm considering it is made of ice, and analysis of icy particles within Enceladus’ plume strongly suggests the presence of a salty sub-surface alkaline ocean. Most models regarding the origin of this plume include a sub-surface liquid water aquifer, and it is this aquifer with its potential to support the origin and evolution of life that is of particular interest for habitability. A plausible sub-surface ecosystem on Enceladus would be unlike many terrestrial biomes, as life forms would have to be independent of oxygen and not rely on organic materials produced by photosynthesis.
Gods of the Sky and Sea
To sustain life on Uranus or Neptune, these distant planets would need a source of energy that even the simplest life could exploit, as well as some type of standing liquid water. A sister ice giant to Uranus, the surface of Neptune dips to a glacial –218°C (–360°F), while the cloud tops are –224°C (–371°F) in temperature. They are both far too cold to host bodies of liquid water and have no solid surface on which they could form in any case. Uranus is composed mostly of methane, water and ammonia ices enshrouded by an atmosphere of hydrogen and helium; it is methane that gives it its blue-green colour. Tremendous pressures inside Uranus created by the overbearing atmosphere raise the planet’s temperature to more than 4,700°C (8,492°F), and would instantly crush and burn life. Add to this the lack of sunlight and internal heat and there is an absence of essential energy for life. Even though it seems impossible, technically there remains a chance some bizarre incarnation of life might be able to survive on Uranus but we are unlikely ever to be able to send a spacecraft down into the planet to check. Voyager 2 is the only spacecraft to have flown by Uranus, back in 1986. The planet revealed few secrets but there were hints that there exists an ocean of boiling water some 800km (500 miles) below the cloud tops. The five natural satellites in orbit close to Uranus, such as Titania, would be more likely to support life, but currently are not deemed to do so and humanity is in no desperate hurry to explore further in the Uranus Planetary System.
Neptune similarly offers little hope of life. It is a cold and dark world, whipped into a frenzy by supersonic winds. About 4.5 billion km (2.8 billion miles) from the Sun, it is mostly composed of a very dense atmosphere of hydrogen and helium, with ices of water, ammonia (NH3) and methane (CH4) over a possibly heavier, approximately Earth-sized, solid core. As is the case for the appearance of Uranus, Neptune’s blue colour is also the result of methane in the atmosphere. There is very little water in the cloud tops, but the percentage increases as you descend towards the core. Perhaps there is a band on Neptune where there is enough pressure and temperature for liquid water to form into an ocean layer. The only spacecraft ever to have visited Neptune is the same one that flew by Uranus – Voyager 2 – passing by Neptune three years later in 1989.
A slight glimmer of hope may exist within Triton, Neptune’s largest and backward-orbiting moon. It is tremendously cold with temperatures on its surface of water ice of about -235°C (-391°F). Its unusual orbit, essentially heading the wrong way round, implies that it did not form around Neptune but was captured after being ejected from the Kuiper Belt. Triton is quite dense, suggesting that, unlike its parent planet, it may have a solid core of silicate rock. In spite of its frigid state, Voyager 2 found geysers belching icy matter into space for over 8km (5 miles). This, as with other moons in this distant neighbourhood of the Solar System, could imply that a liquid ocean is hiding beneath an icy crust, kept fluid by tidal friction and the decay of radioactive isotopes, as happens on the Earth. Similarly recorded by Voyager 2, Triton’s sparse atmosphere has also now been detected from Earth and is growing warmer – we do not yet understand why.
The increasing number of sub-surface oceans on icy Solar System bodies could provide potential habitats for primitive extraterrestrial life forms, yet astrobiologists do not expect to find these inhabiting either Neptune or Triton, as neither fit the standard definition of a habitable world. If the ammonia that may well be present in Triton’s subsurface ocean were able to lower the freezing point of water, however, it might be a more suitable host for life. There is nothing to say that life (but not as we know it, Jim) could not be thriving on either body, just waiting to be discovered.
The Underdog
Now that the New Horizons spacecraft has completed the first Pluto fly-by (in 2015) after a nine-year journey, we have finally visited every member of the Solar System and the secrets of this dwarf planet too are starting to be revealed. But could this once-upon-a-time planet have the theoretical potential to support life when it is more than 4.8 billion km (3 billion miles) away from the warming embrace of the Sun?
It is amazing what, until 2015, we didn’t know about Pluto. For starters it is larger than we first thought. About two-thirds the size of the Moon, it is 2,370km (1,470 miles) across and could comfortably fit inside the area of Russia. The fact that we were wrong about something as simple as its dimensions demonstrates the importance of visiting a world to get accurate information, but is also significant in showing that Pluto is less dense than we thought – it turns out to be made of more ice than rock. Ancient surfaces, like those on the Moon, record the history of impacts – and therefore the history of the Solar System – in the form of craters. Pluto, it was assumed, would also be covered in a very old cratered crust but, amazingly, it has areas that are smooth and unscarred by impact craters and composed of much newer icy deposits as seen in the heart-shaped region of Tombaugh Regio. This tells scientists that the surface can only be about 100 million years old – fairly young in geological terms. How is this possible? To smooth away the craters created during Pluto’s history, as they were on most of the planets and moons in the Solar System, Pluto would need to have some kind of internal heat to soften or melt the icy surface. We have no idea what this source of warmth might be. It is probably too small to generate much radioactive heat inside its body, and there is no larger parent world to squeeze it and generate tidal energy; yet it is obviously geologically active. Figuring out this conundrum will be a huge revelation for planetary science and astrobiology alike.
Pluto is also losing its atmosphere (yes, surprisingly it has one) and as such has merely 1/100,000th of the atmospheric pressure at sea level on Earth. It has 3.5km- (2-mile-) high mountains made of rock-solid water ice, a frozen copy of the Earth’s Rocky Mountains, and a surface that in appearance resembles boiling milk. The smooth plains of Tombaugh Regio have officially been titled Sputnik Planum after the first Russian satellite, launched in 1957. An ice sheet within these plains appears to have flowed in a similar way to glaciers on Earth and may actually still be flowing. There are surface patterns resembling the convection cells seen in steadily boiling milk (yes, another milk analogy). One interesting feature is that Pluto has a tail rather like that of a comet, as it is losing an estimated 500 tonnes (550 US tons) of nitrogen into space every hour. New Horizons flew through this nitrogen tail, which extends for 109,000km (67,730 miles) away from Pluto, and is sculpted by electrically charged particles that have travelled all the way from the Sun and are continuing past.
Unsurprisingly, there are no signs of life on Pluto, yet it is showing hints of the ingredients we normally use to describe a habitable environment. There is an as yet unknown heat source, there is water – albeit frozen as ice – and the most exciting find, organic carbon-based molecules. Pluto’s tenuous atmosphere has haze layers where methane molecules (CH4) are broken apart by the Sun’s UV radiation. These recombine in various ways to form larger, more impressive molecules, but eventually group into solid specks called tholins. These have at best been described by Sarah Hörst, an Assistant Professor in the Department of Earth and Planetary Sciences at Johns Hopkins University, as ‘abiotic complex brown organic gunk’. These fall as tar-like rain on to the surface, giving Pluto its surprisingly Mars-like reddish-brown colour. With flowing ice, exotic organic surface chemistry, mountain ranges and a vast carbon-rich haze, Pluto is showing a diversity of planetary geology and even astrobiology that is truly thrilling and highly unexpected this far out in the Solar System – Pluto’s payback for being demoted to dwarf status!
Comets
In the quest to find evidence of life elsewhere in the Solar System, comets have been implicated in a number of stories, from the extreme notion of transporting living cells or even fully formed microbes from planet to planet, seeding each new world with life, to the more plausible idea of icy rocks carrying the basic organic building blocks to Earth, which led to the origination of life. It is theorised that 4 billion years ago the Earth was bombarded with rocks and balls of ice carrying organic molecules which, through their violent impacts with the newly formed Earth, were split up into the elements needed to form sugars, and ultimately DNA and life.
It is widely known that there are organic carbon-based molecules in interstellar space, with large quantities trapped in interstellar clouds and comets. When a European spacecraft analysed dust particles from Halley’s Comet in 1986, it turned out to be some of the most organic-rich material ever measured in the Solar System, even though meteorites that have hit Earth already contain a whole suite of molecules, including amino acids. We now know for sure that comets could have provided the raw ingredients that the Earth would have needed for life. A regular visitor to the inner Solar System, 67P/Churyumov-Gerasimenko was the chosen target for the ESA catch-a-comet Rosetta mission. It has a short orbital period of 6.45 years, controlled by Jupiter’s gravity, and is believed to have originated from the Kuiper Belt. When these Jupiter-family comets cross the orbit of Jupiter, they gravitationally interact with the massive planet. Their orbits gradually change as a result of these interactions until they are eventually thrown out of the Solar System or collide with a planet or the Sun.
In November 2014, everyone’s favourite comet lander, Philae, hopped, skipped and jumped its way into history. Instead of the planned single landing, Philae had an initial bouncing touchdown followed by a collision with a crater rim and two further touchdowns. Important for astrobiologists, analysis of data from the UK-led instrument Ptolemy discovered molecules that can form sugars and amino acids. Ptolemy sampled ambient gas and detected the main components of the coma gases (those in the halo around the nucleus of the comet), including water vapour, carbon monoxide and carbon dioxide. Smaller quantities of carbon-bearing organic compounds were also identified, such as formaldehyde and acetone. Formaldehyde is implicated in the formation of ribose, which ultimately features in molecules such as DNA, and acetone is best known as the chief ingredient in nail polish remover (both equally important creations). While this is a long, long way from finding life itself, Philae has shown that the organic compounds that eventually translated into organisms here on Earth were present in the early Solar System and within moving bodies that could have transported them to the newly formed planets. So far there are no signs in 67P of amino acids, the building blocks of proteins. However, they are probably there somewhere, since they appeared in samples from NASA’s Stardust mission, which returned material to Earth from the tail of comet Wild 2 (81P/Wild) in 2004, and have also been traced in meteorites that crash-landed on Earth.
There are no life forms as yet identified within comets but there are the ingredients for life. Comets therefore act as messengers, delivering water and organic-rich dust throughout the Solar System – sowing the ingredients for life far and wide. The challenge now is to discover where else they may have ended up.
Every day, our understanding of the envelope of life expands and with it the possible places in which life might exist in the Solar System. Not just that, but after a century of certainty the Solar System itself can still surprise us. At the time of writing, two astronomers from the California Institute of Technology have found evidence of a possible ninth planet lurking in the farthest reaches of our planetary neighbourhood. Planet Nine, as it has been dubbed, has not actually been seen yet but its existence has been inferred. Should this mysterious world existing as far away as 1200 AU be proven to be in fact real, who knows what it might be like or if it might have habitable environments. One day we could find the envelope of life stretched to even greater extremes. Nearly every planet and moon observed so far has the potential to support a habitable environment (not that this proves they ever have had or currently host life), so future studies of these worlds will be extremely exciting. But why stop at the edges of the Solar System when there is an entire galaxy of potentially habitable worlds beyond …