Here’s something to think about the next time you’re on the water. It doesn’t matter whether you’re on a lake or the ocean; or whether you’re floating on a yacht, an ocean liner, or even just on your back. Take a look at that animated, sparkling surface that surrounds you. It’s the exquisite boundary between our planet’s liquid shroud and its gaseous envelope, and it stays there because the water and the atmosphere are in a state of perfect equilibrium.
In the grand scheme of things, that’s a very special circumstance – so rare that, at present, we know of only two places in the universe where it is found. One is here on Earth and the other is…well, we’ll get to that.
It’s not so long ago that our forebears took seas and oceans completely for granted, assuming they would be commonplace on other worlds. The darker regions of our own Moon – easily visible to the unaided eye – have long been known as ‘maria’, Latin for seas. Some later sky-gazers were so fixated on the immutable perfection of the cosmos that they rationalised the Moon into having a mirror-like surface that reflected an image of our oceans back to us. But the invention of the telescope in the early 1600s quickly demonstrated that the lunar maria were quite unlike anything on Earth. We now know they are frozen lava flows – seas of a very different kind.
With the dawn of the space age, our horizons broadened to include not just the planets of the Solar System, but also their moons. Fly-bys of Jupiter and Saturn by the Pioneer and Voyager spacecraft in the 1970s allowed accurate estimates of the densities of their larger moons, suggesting a high proportion of ice in their make-up.
Speculation quickly arose about the possibility of oceans existing beneath their icy surfaces – something that would have seemed like pure science fiction a decade earlier. Today’s investigations extend even further into the realm of science fiction, with some querying whether living organisms might have developed within these subsurface oceans – a question we are yet to answer.
A comparison portrait of Jupiter’s four Galilean moons. Image credit: NASA Planetary Photojournal
The planet Jupiter boasts more than 60 moons, and the four largest are significant worlds in their own right. Three – Europa, Ganymede and Callisto – may have a thick surface layer of ice overlaying a liquid-water ocean, all overlaying a rocky core.
How do we know they have oceans under their surfaces? The major evidence comes from their magnetic properties, since their water generates a weak magnetic field in response to Jupiter’s extreme magnetism. That, together with the detection of hydrogen atoms by NASA’s Galileo spacecraft (in orbit at Jupiter from 1995 to 2003), suggests large bodies of liquid water. It is the gravitational pummelling of the ice-moons by massive Jupiter’s tidal forces, pulling and pushing on their rocky cores, that creates frictional heat and keeps the oceans liquid.
The demonstrated existence of large bodies of liquid water within the Solar System’s ice-moons excites astrobiologists looking for proof of living organisms, because water is essential for life on Earth. It is indeed possible that life may, for example, have evolved in Europa’s mineral-rich ocean nurtured by the tidal heating that keeps it liquid.
A first step in finding out whether any of Jupiter’s moons offer a habitat suitable for life is already on the drawing board. The European Space Agency’s (ESA) JUICE (JUpiter ICy moons Explorer) mission is slated for launch in 2022 to visit Ganymede, Europa and Callisto. The spacecraft won’t land, but the exploration of their subsurface oceans is high on the list of mission objectives. This will be achieved by imagery, laser altimetry and ice-penetrating radar, together with spectrum analysis of the surface to determine composition.
Beyond that, a NASA concept study has considered a robotic lander to investigate the rusty brown cracks that criss-cross Europa’s surface. This is to see what has been dredged up from the depths below, and how it has survived Jupiter’s harsh radiation environment.
It has even been suggested that a small nuclear heat source, like those used for power-generation on deep space missions – such as New Horizons to Pluto – could power an ice-penetrating robot. By melting the ice below it, the robot would slowly descend through Europa’s thick ice crust, reporting back on what it finds – perhaps emerging into the ocean beneath. Such heat-drilling is already a proven technology on Earth – although without the need for a nuclear heat source. That’s how dozens of 2.4km-deep holes were drilled into Antarctic ice for a neutrino particle observatory at the South Pole.
View of Saturn’s moon Enceladus taken by NASA’s Cassini spacecraft. Image credit: NASA
More evidence for a subsurface ocean comes from Saturn’s moon Enceladus. This has an icy surface like Europa, but with bluish-green linear features dubbed ‘tiger stripes’ in the region around its south pole. Erupting from these are geysers of ice that were discovered in recent years by the international Cassini probe, which has since flown through the ice-plumes and detected mineral salts. This hints that a moon-wide liquid reservoir from which the geysers emerge is in contact with a rocky surface and not totally encased in ice.
The warmth that keeps Enceladus’s ocean liquid has yet to be fully explained. Cassini observations show surface ‘hot spots’ in its south polar region, with temperatures up to –116oC, some 65oC above the ambient temperature. Saturn’s tidal influence is part of the story, but Enceladus seems warmer than can be explained by this alone.
Of all the ice-worlds of our Solar System, none is more bizarre than Saturn’s biggest moon, Titan. It is the only known moon with a thick, hazy atmosphere, which stabilises its surface temperature at about –180oC and gives it some extraordinary attributes. Most notable is that Titan is the only place in the universe known to have seas both above and below the surface.
A long-held suspicion that Titan might have pools of liquid on its icy surface was confirmed by Cassini’s smog-penetrating radar. By 2007 this had provided definitive evidence of methane-filled lakes. Further verification came from clever observations of sunlight glints. Located mostly near Titan’s poles, the lakes pool in basins in the ice ‘bedrock’. They are the only stable bodies of surface liquid known anywhere in the universe beyond Earth.
Three of the lakes are particularly large – comparable in area with North America’s Great Lakes – and echo ancient terminology by being described as maria. Some 30 smaller lakes, ranging in length from a few kilometres up to 200km, have also been identified. Most are thought to be fed by methane rain draining into river-like features. But a few in Titan’s equatorial region – in places where the ice bedrock is porous – could be fed by springs from a hydrocarbon ‘water table’. Depths vary from 2–3m for the smallest lakes, to 10s of metres for the polar seas, with a maximum depth of more than 200m for Ligeia Mare, Titan’s second-biggest sea.
The hydrocarbon seas hold many mysteries. What are the temporary surface features that have been observed in the three largest? Some scientists believe they are the surface ripples reflecting Cassini’s radar signals. But methane icebergs, which form on or near the surface and then sink as the conditions change, might also be responsible. It is thought cyclones may occur during Titan’s frigid summer. And strong currents in the so-called Throat of Kraken (a narrow neck of liquid in Kraken Mare, Titan’s largest sea) may even generate spectacular whirlpools.
Because observations have suggested a rich chemistry of ‘organic’ (carbon-containing) compounds on Titan’s surface, some scientists believe this distant moon is an analogue for the early Earth – with an atmosphere similar to that here before life evolved. Others go further, proposing there could already be life forms thriving in the hydrocarbon lakes. These would be quite different from the water-based life we see on Earth and use liquid methane as their working fluid, breathing hydrogen and feeding on acetylene. Tantalisingly, both chemicals are depleted to low levels in Titan’s atmosphere.
How could we explore the potentially rich submarine environment of a world that is 1.4 billion kilometres from Earth? The answer could be close. We already have much experience using unmanned underwater vehicles (UUVs) in our own planet’s oceans – for scientific exploration, military purposes, resource surveys and, most recently, searching for the wreckage of downed flight MH370.UUVs provide a sound basis for further development.
A UUV for exploring Kraken Mare, for example, would be feasible, although it would need an onboard nuclear generator for power. Its sleek form would hide an array of specially developed sensors (see opposite), including chemical and biological samplers, imagers and sonar. To relay data to and from such a submersible would require it to work, as the Mars rovers have done, in conjunction with an orbiting spacecraft.
A Kraken Mare submersible has already been the subject of a detailed NASA study, with thought given to such subtleties as dumping excess heat from the craft into its liquid methane-ethane surroundings. But there are still open questions. How, for example, would you deliver it from Titan’s orbit to sea level without damaging delicate equipment? The moon’s murky atmosphere might help. A winged atmospheric entry vehicle, not unlike the US Air Force’s ‘secret’ X-37B mini-shuttle (AG 128), could bring the sub close to the surface before deploying the craft on a parachute. The prospect of a torpedo-shaped submarine floating down onto the waves of an alien sea is fantastic beyond belief. But there is a good chance this will become reality before the middle of the century.
In late 2015, new findings from NASA’s Mars Reconnaissance Orbiter provided the strongest evidence yet that liquid water flows intermittently on present-day Mars. In this false-colour image, dark, narrow, 100m-long streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. Image credit: NASA/JPL-Caltech/Univ. of Arizona
If there is one thing we have learnt from the robotic exploration of the Solar System, it is that water and ice have played a fundamental role in its history. The Earth’s oceans, for example, are thought to have arrived, at least partly, during impacts by icy comets from the Solar System’s fringes. They, in turn, are composed of the debris of the giant gas cloud that gave birth to our star and its planets, assembled by gravity. Even the bone-dry landscape of Mars shows clear evidence of having been shaped by water. Most planetary scientists agree Mars’s northern hemisphere has every sign of having once harboured an ocean; its low elevation and paucity of impact craters suggesting a regenerated surface contrasting strongly with the rugged terrain to its south.
That is not to say Mars is now devoid of water. Much of it is still there, locked up as ice in the polar caps, or beneath the surface soil as permafrost at lower latitudes. Ground-penetrating radar aboard orbiting spacecraft has revealed glaciers overlain by a thin layer of soil, even at temperate latitudes. And the overall quantity of ice on Mars is far from limited. Data from the ESA’s Mars Express orbiter have revealed that if just the southern polar cap melted, it would produce enough water to flood the entire planet to an average depth of 11m.
It is believed that wet conditions on Mars lasted well into the planet’s so-called Hesperian era, which occurred between 3.7 and 2.9 billion years ago. This is the period during which we know life was beginning on Earth. The oldest undisputed fossilised terrestrial bacteria date from 3 billion years ago, with speculative evidence of micro-organisms existing another half a billion years earlier.
Perhaps further robotic exploration of Mars will find evidence of past, or even present, biological activity. That is exactly what ESA’s two-stage ExoMars mission, scheduled for launch in 2016 and 2018, will be looking for. It will include a rover capable of drilling 2m into the soil of Mars, where microbes could be producing the mysterious methane emissions that have been detected in the planet’s atmosphere.
Artist’s illustration of New Horizons as it passes Pluto. Image credit: NASA
There is still much to learn about the Solar System’s oceans. The dramatic New Horizons fly-by of Pluto in July raised questions about the dwarf planet’s internal heat source – questions that might also have a bearing on our understanding of the warmth of Saturn’s moon Enceladus. Being an isolated world, Pluto is devoid of any tidal heating. Yet data from New Horizons astonished scientists by revealing a surface that has been geologically renewed relatively recently.
That suggests an unknown heat source – such as a radioactive core, or perhaps even the heat given up by a subsurface ocean as it slowly freezes. An ocean under the ice of Pluto would be a discovery indeed, and the still-incoming New Horizons data will be intensely scrutinised for any evidence of this.
Beyond the Solar System is our wider Milky Way galaxy, in which we now know planetary systems are commonplace. Of the 2000 or so ‘exoplanets’ currently catalogued, only a handful are Earth-like, and none are proved to have oceans – although several, including a planet orbiting a red dwarf star, Gliese 581, are within the ‘habitable zone’ where the temperature is right for liquid water. Our capability to discover such worlds is currently limited by technology that is still in its infancy. But within the next decade or so, it is likely there will be evidence of liquid surfaces on some exoplanets, and a better understanding of the occurrence of ice-moons throughout the galaxy. If our Solar System is anything to go by, they could number in the hundreds of billions.
Proof of oceans on exoplanets will not come from their exploration in the near future because the distances are simply too great. A recently discovered Earth-like planet in the habitable zone of its parent star – known as Kepler 452b – is relatively close at 1400 light-years away, but even the fastest spacecraft ever launched would take 30 million years to reach it. So for now – until we’ve found a way to travel faster than light – we’ll need to rely on the next generation of large telescopes to look for spectral signatures of water in light reflected by these worlds.
With such a wealth of proposals aimed at investigating the Solar System’s seas and oceans, the future for this kind of research is bright. Meanwhile, Cassini remains operational and will return more data about the strange ice-moons of Saturn before its mission ends in 2017.
There’s much to be excited about in our exploration of waterworlds. And that is certainly quite something to reflect on, when you next find yourself floating over the limpid, life-filled waters of our own blue planet.
