A distant sea

Enceladus (pronounced en-SELL-a-dus) is one of the moons of Saturn.  It's quite small, just three hundred miles in diameter -- one-seventh of the diameter of Earth's moon.  Despite its modest size, it's one of the more interesting objects in our solar system, and likely to be the target of intensive study in the future.

The surface of Enceladus consists of ice -- ordinary water ice just like the ice cubes in your freezer.  But beneath the ice is an ocean of liquid water covering the whole moon, an ocean we estimate to be between fifteen and forty miles deep, far deeper than any ocean on Earth.

The water ocean remains liquid, even though Saturn and its moons are ten times further from the Sun than Earth is, because Enceladus is geologically active.  Tidal and other gravitational effects from Saturn and its other moons constantly stress tiny Enceladus, which generates heat -- quite a lot of heat.  We believe that on the ocean floor there are thermal vents regularly releasing molten magma into the water, just as we see in some places on the ocean floors of Earth.  This provides the warmth that prevents the ocean from freezing.  The outer layer of surface ice, which is what we see from outside, is just the upper few miles of the ocean which are frozen because they are directly exposed to the near-absolute-zero temperature of space so far from the Sun.

The ice layer is thinnest at Enceladus's south pole, probably only one to three miles thick.  From the south polar region, plumes of water from the ocean below are frequently vented into space through cracks in the ice.  In 2005 the Cassini spacecraft was flown through these plumes several times and was able to analyze their composition.  They -- and by implication the ocean beneath the ice -- are rich in trace materials necessary for life as we know it, such as nitrogen, phosphorus, carbon dioxide, and sodium chloride.  The analysis also detected complex organic molecules which implied that amino acids may exist in the ocean.  This is not, in itself, evidence of life there, but it tells us that Enceladus's ocean is an unusually favorable environment for the emergence of life, perhaps the most favorable in the solar system after Earth itself.

Our failure to find any sign of life on Mars has been a disappointment, but Mars is, after all, an arid desert.  Life on Earth originated in the oceans, and didn't emerge onto the land until relatively late in our planet's history.  The warm and chemically-rich ocean of Enceladus is, realistically, a more promising place to look.

Unfortunately, doing so presents frustrating difficulties.  A future probe could perhaps fly through the water plumes again and do more sophisticated analysis, but this is unlikely to provide real proof of the existence of life, even if there is any.  To really settle the question would require a probe which could land on Enceladus and release some kind of combined digging machine and submersible, which would burrow down through the ice (one to three miles thick even at its thinnest, remember), explore the ocean below, and then report back to the lander on the surface, which would transmit the information back to Earth.  But this would really be pushing our technology to the utmost limits of its capabilities.

Nevertheless, someday we will be able to do it.  The results would tell us much about life in the universe more broadly.  If there are two life-bearing bodies (Earth and Enceladus) in a single solar system, this implies that life in the universe generally is abundant.  If we find large, multicellular forms there -- animals and plants or similar -- this would imply that such complex life is also abundant elsewhere.  If Enceladus turns out to be lifeless, that would tell us that even if all the necessary chemicals and an energy source are present, life is not inevitable, and may be rare to nonexistent in most of the universe.  There is perhaps no more vital goal for our space science than the future exploration of this distant sea.