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.
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.
Instead, Enceladus has turned out to be spectacularly alive. To begin with, the clues were subtle. One intriguing thing about Enceladus is that it orbits Saturn right smack in the middle of Saturn's largest ring. This could be coincidence, but on closer inspection, it becomes clear that the ring shouldn't be there at all. Saturn's rings are an enormous, complex structure. From edge to edge, the ring system would not even fit in the distance between Earth and the moon. The seven main rings are labeled in the order in which they were discovered. Starting from the planet outward, they are designated D, C, B, A, F, G and E. Saturn's remarkable E ring is the largest planetary ring in our solar system, spanning 621,370 miles. The ring is quite diffuse, and seems to be composed mainly of tiny, icy particles. The problem was that, according to all of our best calculations, the ring particles shouldn't be in stable orbits around Saturn; they should either fall into the planet or be thrown out of the system all together. If the E ring was continuously being depleted, something must be replenishing it, or the ring would have disappeared entirely billions of years ago. Could Enceladus have something to do with this mystery?
The next big discovery came when NASA's Cassini spacecraft got a good look at Enceladus earlier this year. The surface was much more dramatic than we guessed it would be. Parts of it were pitted with old impact craters, but other parts looked very strange. The south polar region was scarred with huge, blue cracks that looked like raw wounds in the icy surface. Not only did the cracks, called "tiger stripes" by the Cassini team, look very new, they were also warm. Cassini's infrared camera recorded temperatures in the cracks that were hundreds of degrees higher than those of the frigid surface of Eceladus. What was going on here?
Interest aroused, Cassini turned its battery of science instruments toward the little moon, ready for an all-out campaign. Enceladus was certainly up to something. Right off the bat, Cassini saw a huge cloud of water vapor over the mysterious south pole area. Something was venting water vapor right in front of our eyes. When Cassini swung around Endeladus and took a picture of it back-lit by the sun, the features jumped out immediately: active volcanoes spewing not lava, but water.
To be fair, a better comparison would probably be something like a geyser here on Earth. In fact, the amount of water pouring out of Enceladus at any given time is comparable to the Old Faithful geyser in Yellowstone during an eruption. But there, easily seen in the image, were several towering plumes of rapidly vaporizing water. Some of the water gets caught by Enceladus's gravity and is pulled back down, resulting in the beautifully pure snow we see on the surface. But some of the water gets squirted out with such force that it escapes from the moon entirely and falls into orbit around Saturn. The mystery of the E ring is solved. Amazingly, the Old Faithful-scale geysers of Enceladus have probably been erupting continuously and replenishing it for millions of years. That also makes Enceladus only the fourth place we've detected active volcanism. The others are Earth, Jupiter's moon Io, and Neptune's moon Triton.
Have we actually seen liquid water on Enceladus? Not yet, but this time we know it's there for sure. There must be pockets of water feeding the geysers, and they're probably not far under the surface either. And yes, Endeladus is now high on the list of places to search for life. After all, Enceladus's hidden oceans must be fairly substantial to have been able to feed these geysers for so long. When you add in the fact that another of Saturn's moons, Titan, has not only a thick atmosphere full of organic molecules but also river systems made of liquid methane (it's too cold for water), the Saturn system starts to look pretty beckoning. Come on in, the water's fine.