One of the most enticing-looking worlds in the Star Wars universe is the Ewok home of Endor; a forested landscape populated by hang-gliding teddybears. But Endor is not a planet. The green land is a moon orbiting the uninhabitable bulk of a gas giant. 1
While Ewok jerky (a popular snack across the Outer Rim) may not be on the menu, habitable moons are a serious prospect. Our own gas giant planets are mobbed by moons. If such satellites could support life, the locations where ecosystems could develop would boom. But orbiting a gas giant provides more than a boost in planetary real estate. Despite being far from the temperate zone, the moons around our giants are one of the most promising places for uncovering extraterrestrial life.
Our most massive gas giant, Jupiter, is circled by at least 67 natural satellites. Situated at more than five times the Earth’s distance from the Sun, every one of these mini worlds should be a frozen ice ball. Yet, all is not what it first seems. Our ecosystems on Earth suggest that life requires at least three ingredients:
1 Biogenic elements such as carbon, oxygen and hydrogen that build living systems.
2 Water for a liquid medium in which to construct complex molecules.
3 An energy source to power life’s metabolism.
Situated beyond the ice line, the domain of the gas giants is packed with frozen water. However, the Sun’s heat only delivers about 3 per cent of the power per unit area that we receive on Earth. To stand on the surface of Jupiter’s moon, Callisto, you would need to bundle up in clothing capable of coping with average temperatures of -139°C (-218°F)
This brings us to an anomaly. While Callisto is suitably cold and still for its distant location from the Sun, its sibling moon, Io, is a fiery mass of activity. The most volcanically active place in the Solar System, temperatures on Io range from more than 1,500°C (2,700°F) to -130°C (-200°F). Admittedly, neither moon looks fit for Ewoks, but it is clear that another source of energy must be powering Io. That source is the gravitational pull from Jupiter.
While Jupiter does radiate energy by reflecting sunlight and releasing heat during the gas giant’s very slow contraction, the amount is nowhere near enough to power Io. Instead, Io’s generator is tidal heating. As the moon circles Jupiter, it is squished by the planet’s gravitational pull just as a closely orbiting planet can be distorted by its star. Io’s orbit is slightly elliptical, so the bulges raised on the moon increase and weaken as the moon moves closer and further from Jupiter. The incessant flexing raises Io’s surface by more than 100m (328ft); the height of London’s Big Ben. It is this stress-ball treatment that heats the moon. If Io were a lone moon, its orbit would eventually circularise and this heat source would die away. This does not happen on Io because of its large siblings.
In 1610, Italian astronomer Galileo Galilei built a telescope with which he could observe Jupiter. While tiny by today’s standards, the 20-times magnification of Galileo’s instrument allowed him to spot the four biggest moons around the gas giant. These became known as the Galilean moons. Galileo himself referred to the moons by a number, which was scientific if a little dull. However, the moons were simultaneously discovered by German astronomer Simon Marius. Marius named these satellites after the lovers of Zeus, the Greek mythological counterpart to the Roman god Jupiter. Publishing in 1614, Marius referred to the four moons as Io, Europa, Ganymede and Callisto.
Moving outwards from Jupiter, the moons circle the planet in: Io, 1.8 days; Europa, 3.6 days (x 2 Io); Ganymede, 7.2 days (x 4 Io); Callisto, 16.7 days (comparatively dawdling).
Callisto is the only moon of this four whose orbit duration is not an integer multiple of the time for Io’s orbit. The synchronisation of the other three forms a 1:2:4 resonance. As we saw for migrating planets in Chapter 5, resonances are difficult to break. The three moons are therefore very resistant to changing their orbits, causing the trio to all maintain elliptical paths. This allows continual tidal heating from Jupiter as it kneads the moons.
The fierce tidal heating on Io prevents liquid water from forming, but the sibling moons of Europa and Ganymede are less brutally flexed. Orbiting further from Jupiter, these moons initially appear to have the opposite problem as their cold surfaces are covered with a thick layer of solid ice. However, there is evidence that the pair are not frozen to their cores.
While the outer Jovian moon of Callisto has one of the most heavily cratered surfaces in the Solar System, the surface of Europa is one of the smoothest. Both moons should have been pounded by meteorites (in this case, crashing comets) over their lifetimes, suggesting that Europa’s surface is far newer than that of its sibling. Europa’s pathetic crater count points to an average age of around 65 million years, just 2 per cent of the actual age of the moon. Somehow, the moon has been resurfaced like an impossibly large public skating rink. Images of Europa’s surface also reveal regions that appear to be expanding, without corresponding regions of contraction. Since the moon is unlikely to be undergoing a mad global inflation, another explanation is needed for this icy landscape.
As it happens, we see something very similar to this renewal and expansion on the Earth. Our planet’s surface regularly expands during sea-floor spreading, where new crust forms as magma emerges in ridges along the ocean floor. The Earth’s overall area remains constant, since similar amounts of material are subducted when one of the surface tectonic plates pushes underneath another. This comparison suggests that Europa might be the first world beyond Earth, and possibly Mercury, 2 to show evidence of current plate tectonics. Rather than silicate, Europa’s plates are made from ice sheets. Where the icy plates pull apart, a fresh new surface is created on the moon. The mobility of the Earth’s plates is evidence that the lower mantle is more plastic and moves within the Earth. Similarly, the shifting of Europa’s icy surface is evidence that the moon cannot be solid ice through to its core. Instead, the frozen surface sits on a deep ocean of liquid water.
Further evidence that Europa harbours a hidden ocean comes from the appropriately named NASA space probe Galileo, which orbited Jupiter from 1995 to 2003, sweeping past Europa in January 2000. During this flyby, Galileo measured a varying magnetic field.
Europa has no magnetic field of its own, but Jupiter has the most powerful field in the Solar System. Ten times stronger than the Earth’s magnetic shields, Jupiter’s field originates from the blisteringly high pressures in its outer core. Crushed under the atmosphere, a metallic liquid version of hydrogen forms that acts like the Earth’s molten iron centre. The flowing of this strange metal creates a current that generates a magnetic field. The inner part of Jupiter’s magnetic field extends outwards to roughly 10 times the huge planet’s radius, stretching to between Europa and Ganymede. The outer region is sculpted by the solar wind to form a teardrop shape, extending over 100 times the radius of the planet. As is the case with the Earth and pulsars, Jupiter’s field is not perfectly aligned with the planet’s rotation axis. The offset causes the enveloped moons to feel a varying magnetic field strength as Jupiter rotates.
Not only can moving charged particles generate a magnetic field, but the reverse can also occur. Changes in a magnetic field will cause charged particles to move and create an electric current. The induced current will then create its own magnetic field. This effect was discovered in 1831 by the British scientist Michael Faraday. It became known as electromagnetic induction. Electromagnetic induction only works if charged particles can move. That is, the charges must be in a medium that can conduct a current. Metal is an excellent example of a conducting substance, but so is salt water.
When the Galileo probe passed Europa, it measured a fluctuating magnetic field originating from the moon. An induced magnetic field due to electromagnetic induction always opposes changes in the imposed magnetic field. As Jupiter rotated, the magnetic field strength over Europa strengthened and weakened. The moon’s induced magnetic field flipped directions, attempting to weaken the strengthening field, then strengthen the weakening field. 3 The Galileo probe’s measurement of this flip-flopping confirmed that Europa’s magnetic field was being induced by Jupiter and not generated within the moon’s core. To achieve this, the moon had to be conducting a current. Beneath its icy surface therefore had to lie an ocean of salt water.
Ice and pure water are poor conductors of electricity since they contain few mobile charged particles. Dissolve salt in the water and the situation changes. The salt separates into positively and negatively charged atoms that feel magnetic fields. As Jupiter’s magnetic field waxed and waned over Europa, these salty charges flowed to create their counter field.
Due to the strength of the secondary magnetic field from Europa, this water was no briny puddle. Europa possessed a hidden global ocean where charged particles could flow freely. While it is difficult to be certain exactly how deep the ocean lies, a reasonable estimate would be 10km (6mi) of ice followed by a subsurface ocean 10–100km (6–60mi) deep.
The Galileo probe also investigated Europa’s gravitational field, which can reveal information on the internal structure of the moon. In his Principia, Isaac Newton showed that the gravitational force from a perfectly spherical distribution of matter was identical to a single concentration of mass at the sphere’s centre. By measuring the gravitational tug from different locations close to Europa, the probe was able to record the deviations from Newton’s law and recreate the true bumpy insides of the moon. These measurements suggested that Europa has an iron core surrounded by a rocky mantle beneath the deep ocean and icy lid. If the water is predominantly liquid, then Europa’s oceans amount to twice the water found on Earth.
With such a copious supply of water, could Europa be teeming with hidden life? It is a possibility that is being seriously considered, with both Europe and the US planning missions to explore the icy moon further in the next decade.
Europa’s hidden interior has access to water and tidal heat, but life would also require an organic starter kit. If the icy shell is thin enough to allow outside material to pass through cracks, then debris from meteorite impacts could add organic molecules to the system. A thin outer layer could even allow weak photosynthesis in the upper layers of water. High-energy particles trapped in Jupiter’s strong magnetic field also bombard Europa, breaking up surface water molecules into hydrogen and oxygen. The hydrogen is too light to be retained by the moon’s small gravity and escapes, leaving a supply of oxygen. This oxygen could be used in biological oxidising reactions that, similar to photosynthesis, can generate energy for organisms to survive.
If Europa’s icy top is thicker, the most probable place for life is on the ocean floor. Hydrothermal vents could nurture ecosystems similar to those that flourish in the Earth’s oceans. Whether these could exist depends on if Europa can summon up volcanic activity. The Jovian moon is slightly smaller than our moon, which is geologically dead. However, if the tidal heating from Jupiter is enough to melt part of Europa’s rocky mantle, then hydrothermal vents could exist.
If life does exist at Europa’s heart then detection will be a serious challenge. Our success would depend on the mixing of the more complex telltale organic molecules between the ocean bottom and icy shell. If hints of hidden life can be found in the ice, then we might be able to deduce what lies out of sight.
Should life be found on Europa, it is likely to be completely independent of any life on Earth. While the Earth and Mars could possibly have exchanged early microbial material via meteorites tossed between the planets, Europa’s distance makes such sharing far more improbable. Finding life on the Jovian moon would therefore tell us a great deal about how easy it was for life to begin.
Stepping out further from Europa takes you to the biggest moon in our Solar System, Ganymede. The third Galilean moon is a little larger than the planet Mercury, but only half as massive due to having nearly 50 per cent of its mass in ice.
Unlike Europa, Ganymede generates it own magnetic field without electromagnetic induction from Jupiter. The moon probably creates the field similarly to the Earth within a molten iron core. This also produces an aurora on Ganymede as the magnetic shielding channels charged particles towards the moon’s poles. With an icy surface similar to Europa’s, Ganymede has a tenuous atmosphere of oxygen from split-water molecules. Collisions with oxygen atoms would illuminate a splendid red aurora if you were to look up from a spot on the moon’s surface. It is the aurora that provides a clue to the moon’s internal structure.
Still under the umbrella of the outer extent of Jupiter’s magnetic field, Ganymede experiences both its own field and the fluctuating strength emanating from the rotating giant planet. As Jupiter’s magnetic field waxes and wanes over the moon, Ganymede’s aurora is tugged back and forth. The expected result was calculated to be a change of about 6 degrees, but observations by the Hubble Space Telescope show a smaller 2-degree shift. The difference can be explained by an additional Europa-like induced magnetic field within Ganymede, repelling the effect of Jupiter’s imposed variations. The existence of a second magnetic field on the moon is proof that Ganymede also has a subsurface ocean. Although this situation was suspected by the Galileo probe, the results were far less conclusive than for Europa. The aurora observations by the Hubble Space Telescope nailed down the speculation.
Could Ganymede’s hidden ocean be habitable as well? It turns out that the situation is less promising than for Europa. Three times more massive than its inner sibling icy moon, the pressures near Ganymede’s core are considerably higher, risking the freezing of the lower ocean into a thick icy layer. The result would make Ganymede a deep-ocean water world like Gliese 1214b, with the silicate sea floor sealed off from the water. Further from Jupiter than Io or Europa, Ganymede also has less tidal heating. Its surface ice is much older than that on Europa, dating back to several billion years, and with no evidence of recent geological activity such as plate tectonics. This suggests that the ocean may be deep under the surface ice, around 150–300km (90–185mi) down. The combination indicates that Ganymede’s waters may be both unable to benefit from surface organics or sunlight, and also be blocked from hydrothermal vents.
The third and final moon to harbour a possible ocean in the Jovian system is Callisto. The outermost Galilean moon, Callisto is not in resonance with its three large siblings. It therefore has to forego tidal heating and rely on the dwindling heat from its formation to warm its heart. Its icy surface is the oldest in the Solar System, with a heavily cratered visage that denies the presence of geological activity. It was therefore suspected that Callisto, at least, had to be an entirely frozen world. The moon’s internal heat should have been insufficient to keep its interior from freezing solid.
Despite this expectation, the Galileo probe revealed a surprise. Like Europa and Ganymede, Callisto has an induced magnetic field caused by Jupiter. This pointed to another subsurface salty ocean. It seems that the moon’s icy crust is better at retaining Callisto’s limited heat supply than had previously been suspected. That said, with a solid surface and small supply of energy, Callisto is the least likely of the three icy siblings to support life.
The tiny, shiny moon
While the hidden oceans of the Jovian moons had to be deduced from the subtle shifts of their interiors, Saturn’s moon Enceladus had no such modesty. The moon was observed rocketing 250kg (550lb) of water vapour into space every second through cracks in its icy shell. Such watery expulsions are known as cryovolcanoes, which spew ice and water rather than lava. The plumes stretch 500km (310mi) above the surface at the south pole, making the tiny moon the smallest volcanic body in the Solar System.
Like Jupiter, Saturn is not short of natural satellites. At least 62 encircle our second-largest gas giant, ranging in size from asteroid-sized moonlets only a kilometre across, to a moon approaching Ganymede in size. Smaller than the moonlets are the rings of Saturn, which comprise microscopic dust through to bodies measuring a few hundred metres. These extend over thousands of kilometres in a disc only about 10m (33ft) thick. Saturn’s major moons have sizes between 10 and 150 per cent of our own Moon, and all sit beyond the main rings.
In Greek mythology, the personifications of the Earth and Sky had two races of divine children known as the Titans and the Giants. The leader of the Titans was Kronus, who later bore the Roman name Saturn. He and his consort (and slightly disturbingly, also his sister) Rhea (Roman name: Ops), birthed the gods Zeus (Jupiter), Poseidon (Neptune) and Hades (Pluto). 4
The naming of Saturn’s major moons was proposed by the British polymath John Herschel in 1847, when he suggested that the large satellites take the names of the Titans and Giants who were the siblings of Saturn. Enceladus was discovered by John Herschel’s father, William Herschel, and was named after one of the Giants.
The sixth-largest moon of Saturn, Enceladus is tiny and very shiny. At only 500km (310mi) across, the moon is just over a seventh of the size of our Moon, and can almost fit within the borders of England or Arizona.
The moon orbits Saturn within its diffuse and outermost ring, which is fed particles from Enceladus’s geysers. The moon’s shiny surface is due to the continual resurfacing of highly reflective fresh ice from the spouting water, making the moon one of the most reflective bodies in the Solar System. Mirroring back even the small amount of radiation received near Saturn results in a particularly cold surface, with a noon average surface temperature of -198°C (-324°F).
Until the joint US and European Cassini-Huygens missions arrived at Saturn in 2004, very little was known about Enceladus. Being so small and further than even Jupiter’s moons from the Sun, it was assumed to be a dead and frozen world. The discovery of geysers revealed both the presence of water and the geological activity needed to drive the plumes. Enceladus clearly required a rethink.
It was initially thought that Enceladus’s subsurface water might be confined to the active geysers around the moon’s south pole. This region showed a tiger-stripe pattern of cracks where the water had burst through the ice. Then further observations picked up a slight wobble in Enceladus’s orbit that could most easily be explained by the sloshing of a global ocean about 26–31km (16–19mi) deep. Such a wobble is similar to that seen when spinning a raw egg. 5 The water depth is about 10 times deeper than the average ocean depth on Earth.
With water easily accessible as it spurted from Enceladus’s interior, the Cassini orbiter grabbed a sample as it flew through the plumes. It found a melange of water, carbon dioxide, methane, salt and ammonia crystals. Combined with the heat source that drives the moon’s geysers, this mix of organic compounds could create an environment for life.
Far from the Sun, Enceladus is heated with the same tidal flexing that fires up the Jovian moon trio. The tiny Saturn moon is in resonance with its sibling, Dione (named after a Titan in Greek mythology). Enceladus orbits Saturn twice in the time it takes Dione to circle once. Like the Galilean inner moons, tugs from Dione keep the orbit of Enceladus slightly elliptical so that Saturn’s grip tightens and weakens during its orbit. However, this does not entirely explain Enceladus’s power source. The expected heating from Saturn is lower than the energy that seems to drive the powerful geysers. Possibly the deficit is compensated by a residue of the internal heat generated during the moon’s formation, or left over from an era when Enceladus might have been on a more elliptical orbit.
The easy access to Enceladus’s water makes it a tempting prospect for exploring life on these icy moons. Unlike on Europa, a visiting probe would not need to land or drill through kilometres of ice to analyse the ocean’s contents. This is offset by the longer distance to the Saturn system, making the moon a seriously long hike from Earth. The Cassini-Huygens mission took seven years to reach Saturn, compared with the 2016 arrival of Juno at Jupiter, which took five years. Current planned missions are therefore focused on Europa, but Enceladus remains a tantalising target for future searches for non-Earth life.
The moon with liquid lakes
Enceladus may be small but Saturn’s moon, Titan, more than makes up for its dearth. While 62 moons have been recorded orbiting the ringed gas giant, Titan comprises more than 96 per cent of their combined mass. The second most massive Saturn satellite is Rhea, which has less than 2 per cent of Titan’s mass and a third of the size. This makes Titan the second-largest moon in the Solar System, being topped by a mere 2 per cent in size by Ganymede.
Like the watery ice moons, Titan has a slightly elliptical orbit that results in tidal heating from Saturn. Unlike the other moons, the origin of this eccentricity is not clear. Titan has no sibling moon large enough to keep its motion perturbed, and Saturn’s pull should have dragged the moon into a circular orbit. It is possible that Titan suffered a relatively recent collision and not enough time has passed to circularise its path around the gas giant.
Regardless of the origin, Titan’s eccentricity causes Saturn to flex the moon during its 16-day orbit. The distortions to the moon’s shape were measured by the Cassini probe and found to be far higher than expected for a solid rocky body. Titan’s surface bulged by 10m (33ft) rather than the anticipated 1m (3ft). For comparison, the Sun and our Moon cause the Earth’s crust to rise by about 50cm (20in) and our open oceans to rise about 60cm (24in). Titan’s squishiness suggests that this moon also harbours a subsurface ocean.
Due to the size of the moon, the pressure near Titan’s core makes it likely to be encased in a layer of frozen ice. As is the case with Ganymede and exoplanet water worlds, deep-sea life is therefore significantly less likely. However, the surface of Titan is completely different from the surfaces of the icy moons.
Rather than a top lid of ice covered by a thin envelope of gas, Titan has a thick atmosphere with a surface pressure 50 per cent higher than there is on Earth. This makes the moon one of four rocky worlds in our Solar System with significant atmospheres. Venus wins for the densest atmosphere, while the Earth and Mars both have thinner atmospheres than Titan. But as is the case with our planetary neighbours, Titan’s air is far from breathable.
Titan’s thick atmosphere was first perceived as early as 1908. The moon had been discovered on 25 March 1655 by the Dutch astronomer and physicist Christiaan Huygens. He and his older brother, Constantine Huygens, Jr, were interested in the manufacture of scientific instruments and had observed the moon using one of their own designs. Around 250 years later, Catalan astronomer José Comas Solà measured a change in the brightness from the centre of the moon’s surface to its perceived edge. He interpreted this gradient as the presence of an atmosphere.
In the 1940s, Kuiper (of Kuiper belt fame) examined the wavelengths of light being absorbed by Titan’s atmosphere. He concluded that the air contained methane but was not certain if this was the dominant gas. This was resolved during a flyby by the two NASA Voyager probes in 1980 and 1981. The pair confirmed that a thick atmosphere surrounded the moon, which obscured the view to the planet’s surface. The gases found were about 95 per cent nitrogen with up to 5 per cent methane. Interactions with the limited ultraviolet sunlight that reached the moon allowed the methane to form more complex molecules of hydrogen and carbon, such as ethane. 6 These heavier hydrocarbons then sank to the ground as solids or liquids, creating the orange haze that blocks the clear view of the moon’s surface. Collectively, these sinking hydrocarbons are known as tholins, after the Greek for sepia ink, due to their reddish-brown colour.
As on Venus and Earth, Titan’s atmosphere provides the moon with a greenhouse cloak to boost the surface temperature by about 10°C (50°F). This fails to protect the moon against freezing temperatures since due to its incredibly distant location it receives just 1 per cent of the sunlight that reaches Earth. Surface temperatures on Titan are therefore a staggeringly cold -180°C (-290°F). Such temperatures preclude the possibility of liquid water. Instead, liquid methane and ethane pool in lakes on Titan’s surface.
At a (rather warm compared to Titan) temperature of 0.01°C (32.02°F), water can exist as a solid, liquid and gas. This strange temperature is known as the triple point of water. That temperatures on the Earth are close to this triple point is what allows abundant quantities of ice, water and vapour to exist on our planet’s surface. Together, these phases give us the water cycle whereby water moves from clouds to rain to ice and snow. While temperatures on Titan are far from water’s triple point, they are close to that of methane. This can exist in all three phases at -182°C (296°F), and forms a methane cycle of clouds, rain and lakes on Titan.
Quite why Titan gathered a thick atmosphere when the similarly sized Ganymede and Callisto failed to is not entirely clear. One possibility is that comet impacts on the Jovian moons were faster and harder due to Jupiter’s stronger gravity. These major collisions could have stripped the gases from the larger Jovian moons. Alternatively, the colder environment around the more distant Saturn was more effective at trapping gases within ices during moon formation, allowing these to later vaporise into the atmosphere.
Given the more familiar environment of surface lakes and rivers, could life exist on Titan’s methane shores? The answer depends on whether methane could replace water as a medium for biological reactions. This is highly speculative and further hindered by the colder temperatures on Titan making it more difficult to dissolve the necessary organics into solution.
Evidence that Titan is indeed a barren world came from the Cassini-Huygens mission. While Titan’s smoggy gases scatter optical light and prevent a view of the surface, the longer infrared wavelengths can pass through to the ground. The Cassini probe’s instruments used this to map the moon from above. Meanwhile, the Huygens lander dropped down to Titan’s surface. The lander separated from Cassini on Christmas Day in 2004 to begin a three-week descent through the smoggy atmosphere.
Unlike on Mars, where the thin atmosphere is too thin to slow down an incoming spacecraft to a safe landing speed, the Huygens probe was able to touch down using parachutes. It landed in a cloud of hydrocarbon dust on 14 January 2005, to become the first probe to land on a world in the outer Solar System.
The Huygens lander was equipped with three hours of battery life, which was used during the final stages of the descent and 72 minutes on the moon’s surface. The lander sent back around 80 images from the surface, none of which showed any signs of movement or plant life. Assuming that life would spread to all regions of the moon as on Earth, Titan would seem to be devoid of macroscopic life.
While our Solar System’s outer moons may harbour life, their ecosystems are hidden below thick icy or opaque gases. Similar moons around extrasolar planets would appear lifeless rocks in observations from Earth. But what about giant planets that have migrated closer to their stars? Of the plethora of gas planets found in the temperate zone, could one of these host a more Earth-like moon?
Notes
2. The planet’s name is also Endor and it orbits a pair of binary stars, which are also called Endor. It is interesting physics, but spectacularly unimaginative nomenclature.
2. NASA’s MESSENGER space probe found evidence that Mercury may be contracting as the planet’s core cools. This would also indicate that the planet’s surface is tectonically active.
3. Electromagnetic induction is basically fuelled by a strong dislike of change.
4. Roman mythology adopted most of the Greek gods but switched to Latin names. The Titans and Giants appear predominantly in the Greek mythology.
5. Do not try this near the edge of a table.
6. Methane is CH4 (four hydrogen atoms and one carbon). Ethane is a longer chain of C2H8 (eight hydrogen atoms and two carbon).