Great Throughts Treasury

"We had observations planned so we could look to see if Enceladus had any geysers. And, in fact, some of the very, very early observations in which the geysers—or the jets, as I call them—were first seen had been given a title with the word plume in it, because these particular observations were planned to search for any material directly coming off the surface of the moon. Also, one of the papers we wrote summarizing our goals for the imaging experiment at Saturn—which should have been published long before we got into orbit, but we were busy so it didn’t get published until about 2004—contained a statement that I deliberately put in that paper that said, “Enceladus could be the Europa of Saturn.” That was my way of saying that Enceladus could provide that kind of excitement and be the astrobiologically interesting moon of Saturn."

"It has not been found on Europa—we even looked when Cassini flew by Jupiter in December 2000 but didn’t find any. However, realize that geysering will be more unlikely on Europa because it’s a larger moon with stronger gravity, and it would be harder for liquid droplets to reach the surface. But Europa turns out to be the moon in the Jovian system that was first realized to have a body of water in its interior. We suspected that there might be something of this ilk on Enceladus, and the first discovery that there were jets or plumes coming off Enceladus were our images in January and February 2005. We now strongly suspect that the jets are coming from liquid water."

"There’s been a lot of confusion about this. People seem to think that we were completely surprised to discover that Enceladus had any geysering activity at all, and that’s not true. Going back all the way to the Voyager mission, and even prior, people were trying to figure out how Saturn's E Ring, in which Enceladus was clearly embedded, formed. There were various ideas for that. One was that the moon was being hit by high-energy particles that ended up releasing tiny ice grains into orbit around Saturn. But there was some speculation that it could perhaps be geysering coming from the moon—literally water droplets that were frozen and came off the moon with such energy that they escaped the gravity field of Enceladus and went into orbit around Saturn to form the E Ring."

"In our images we see icy particles. Other instruments, like the ultraviolet imaging spectrometer, can measure the abundance of water vapor coming off the south polar cap. And if you take the mass in icy particles that we see and compare it to the mass in the vapor that that instrument sees, the abundance of icy particles is more than you would expect if this whole phenomenon were arising from sublimating ice. You would expect fewer particles, fewer solids if it were coming from sublimating ice. So by default the next possibility is that it’s coming from liquid water. Another really fantastic piece of evidence that’s very hard to refute is the presence of salt in these particles. There’s so much salt there that it could only be that the solids were previously droplets of salty water and then got frozen. If, instead, it were coming from ice that had salt in it, the salt would generally be left behind when the molecules leave the surface of the ice. You wouldn’t expect to see the kind of saltiness that we see if it was sublimating ice. There is other evidence more along the lines of the models that people have proposed for how you get heat coming out of Enceladus. These models suggest pockets, or a whole regional sea, of liquid water under the south pole. So it’s becoming harder and harder to escape the conclusion that there’s a body of liquid water within Enceladus."

"The question is, has the liquid persisted long enough for life to even get started? And that brings up another issue, the collision of two ideas really. One is, how long does the liquid in Enceladus persist, and the other is, how long does it take life to develop? Just playing devil’s advocate someone might say, well, you might have liquid water there, but you don’t know that it’s been there long enough for life to develop because life might have taken hundreds of millions of years. Maybe the liquid water only persisted tens of millions of years. Now, we suspect, as I’ve already said, that the liquid water probably persists there indefinitely. But this brings up the issue, how long does it take for life to develop? Based on the record on the Earth for life to develop we think the maximum is on the order of a few hundred million years, maybe 300 million years. That’s the time separation between the appearance of the first fossilized organisms and the end of what we call the Late Heavy Bombardment, when the conditions on the surface of the Earth would have been hellish because big chunks of solid material were still raining down on the Earth. After this Late Heavy Bombardment was over, the conditions on the surface of the Earth became quiescent and stable enough for life to get started and not get wiped out by some large impact. It’s at that point, we believe, that life took a firm foothold on our planet. But all this doesn’t mean that life did, in fact, take as long as a few hundred million years to develop. That’s the upper limit. The lower limit could have been much lower: It could have been 10 million years. We just don’t know."

"I would imagine that if you got single-celled organisms going you could get evolution going, if it works like life has worked on Earth. But that’s why we’re so incredibly interested in this, because we don’t know! We don’t even know if life originating in water would end up looking like earthly life. That’s why we want to go. That’s what’s so intriguing about this question. We only have a statistic of one right now, and it would help enormously to have another example."

"I don’t think that’s an issue. In our local neighborhood astronomers are discovering planetary systems all the time. I can’t keep up with the numbers of them. I think there’s several hundred by now discovered by ground-based observers. And Kepler [a NASA mission to look for planets beyond our solar system] is discovering many more, and they vary in how they look—that is, the distribution of the planets and the sizes of the planets. But what we’re finding is more or less what we’ve always suspected, that there’s nothing at all unusual about our solar system: that there are very likely planets that look like Jupiter, and like Saturn with its rings, like Uranus tilted on its side, and like Earth covered in water, and probably lots of examples of types of planets that aren’t represented in our solar system. Of course, if we ever discover that genesis has occurred independently twice in our solar system, no matter where we find it, then that means that the spell has been broken, the existence theorem has been proven, and we could infer from that that life is not a bug but a feature of the universe in which we live, and has occurred a staggering number of times throughout the 13.7 billion-year history of the universe. And that would be a huge scientific result. I don’t think there’d be any question about it. It probably wouldn’t be the socially cataclysmic event that the discovery of intelligent life would be. But scientifically it would be a radically phenomenal event."