The icy Old Faithfuls of Enceladus
Liquid water is special stuff. Here on Earth, we take it for granted that liquid oceans cover most of our planet, and rain falls out of the sky. In fact, liquid water should boggle our minds. We know that liquid water was almost certainly responsible for the beginning of life - so much so that when astronomers began the search for life outside our world, they started by looking for other places with liquid water. We ended up finding out that liquid water is very rare indeed in the rest of the universe.Skip to next paragraph
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This is not to say that water in any form is a rarity; we've got plenty of ice stored up in millions of comets and the new "planetoids" we keep finding out past the orbit of Pluto, and there's plenty of water vapor in the atmospheres of the outer planets. But getting water to behave as a liquid takes a very special combination of temperature and pressure. It's much easier to create a solid or a gas. It has to be just warm enough not to freeze, but just cool enough not to boil. Air pressure has to be just right too. Even at nice moderate temperatures, water will quickly boil away if there's no air.
Amazingly, we have never directly observed liquid water anywhere except Earth. That's not to say that we don't have indirect evidence of water, however. Mars seems to have had open oceans long ago, but now all the water seems to be frozen either in the polar caps or underneath the soil as permafrost. It's not impossible that seasonal melts produce a little liquid water, but with Mars' thin atmosphere, it's gone almost immediately.
The other places with the best evidence for water, interestingly, are not planets but moons. Two moons of Jupiter, Europa and Ganymede, have icy, smoothed-out surfaces which may indicate the presence of liquid water beneath thick layers of ice. Something seems to have erased impact craters and other old features from their surfaces, so these moons must have been geologically active in the recent past. In the case of Europa, the surface ice is broken up into chunks that seem to be able to move independently of one another, almost like vast icebergs floating on the surface of a hidden sea. Large circular spots on the surface may be due to water welling up from a warm interior. Most planetary scientists think that Europa does indeed have a subsurface liquid ocean, but they aren't sure about just how far under the ice it is. Europa's hard exterior may be only a mile thick, or perhaps a hundred miles. We also don't know how active the ice is today. Do the features we see form slowly over thousands of years, or is the ice still shifting and cracking even today? To be sure, we'll be keeping a close watch for years to come.
But Europa may have to take a back seat for a little while. Enter Enceladus. Enceladus (pronounced "en SELL uh dus") is a small, icy moon of Saturn that's only about as wide as Arizona. We've known it was interesting since the Voyager spacecraft flew by the Saturn system years ago, and recorded the fact that it was the most reflective object in the solar system. The moon's surface is literally as white as newly fallen snow, which means that it reflects almost all the sunlight it receives. With no sunlight being absorbed by any dark rocks or soil, the moon must be extremely cold, and scientists assumed it would be frozen solid - a frigid, dead world.