It is thought that one or more of the icy moons of the outer solar system could support life. Image Credit: NASA Planetary Photojournal
Until relatively recently, it was thought that the best, or perhaps only, place to look for life elsewhere in the solar system was Mars. The other inner planets were much too hot while the outer gas and ice giants were far too cold – the chances of any kind of life being found, even microbes, was considered extremely unlikely at best.
That view, however, is now starting to change. The more that various space probes have studied some of the far-off worlds in the outer solar system, the more it has become apparent that the conditions for life (at least as we know it) could indeed exist on some of them.
But it's not the giant planets that have renewed the interest in this possibility, it's their moons. With very little to no warmth from the Sun, they were assumed to be just frozen balls of ice and rock.
The Cassini Spacecraft. Image Credit: CNES/Illus. David Ducros
As the saying goes, however, truth is often stranger than fiction.
One of those seemingly "boring ice balls," Jupiter's little moon Europa turned out to be a lot more interesting, when evidence was found for the existence of liquid water beneath its bleak, frozen surface. Not just some water either, but an entireocean of it! Europa is actually now thought to contain a lot more water than all of Earth's oceans combined. As it turns out, tidal flexing caused by Jupiter's gravitational pull keeps Europa's interior warm enough for liquid water to exist.
This deep underground ocean is also thought to be quite salty like Earth's, sandwiched between a rocky core and the outer icy shell; the rocky bottom could provide minerals and organics, just like on Earth. There is also now evidence for lakes just under the surface which may be connected to the ocean farther down below.
Europa is the first true water-world found beyond Earth, and as such, has captured the imagination – what might be swimming in this ocean or the lakes? Even with no sunlight, life is still possible, as has been found in the deepest ocean depths here on Earth.
More recently, other moons have also become contenders for being potentially habitable – perhaps, in some ways, even more so than Europa. Enceladus, a moon of Saturn, is even smaller than Europa, but is more geologically active. Huge water-ice geysers were discovered by the Cassini spacecraft erupting from huge cracks in the icy surface at the south pole.
These plumes contain ice particles, water vapor and a variety of organic molecules. Most current planetary models favor a smaller sea beneath the south polar ice feeding the geysers, although a larger ocean is also possible.
Heat emanating from the cracks indicates that it is relatively much warmer deeper down. Cassini has already sampled some of the plumes directly, confirming the existence of organic material in the geysers. Although this doesn't prove there is life down there, not yet anyway, it certainly strengthens the possibility. On Earth at least, wherever there is water, heat and organic material, there is life.
Cassini arrived in orbit around Saturn in 2004. About a year later it deposited the Huygens lander on Saturn's largest moon, Titan, it has since been determined that the moons orbiting both Saturn and Jupiter are far more dynamic than previously believed. Photo Credit: NASA/JPL/Space Science Institute
Both Europa and Enceladus are now considered prime targets for further exploration, but Enceladus provides an easier means of doing so; future missions now being considered could sample the plumes again, but this time would specifically look for possible traces of actual life in them, which Cassini can't do. Accessing Europa's ocean or lakes however would likely require drilling very deep through the ice.
Some of the other moons which may also have hidden oceans include Jupiter's moon Ganymede and Saturn's moon Titan, which begs the question – if these water-worlds are common in our own solar system, as they seem to be, how many more might be out there, among the thousands of exoplanets now being found orbiting other stars…?
Paul Scott Anderson is a science writer whose work has appeared on a variety of web-based media outlets. He also operates his own blog, The Meridiani Journal.
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