In the winter of 2005, icy plumes were seen geysering from the surface of Enceladus, one of Saturn's moons. Other moons and planets in the solar system—Mars, Titan, Europa—are candidates for being places where life could have arisen. But Carolyn Porco, the leader of the Cassini Mission Science team to Saturn, believes that the moon Enceladus is the best candidate for finding simple organisms in the solar system in our lifetime.
GEYSERS ON ENCELADUS
NOVA: When were the plumes on Enceladus dicovered, and before the flybys began did you expect to find anything like that?
Carolyn Porco: 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.
"We now strongly suspect that the jets are coming from liquid water."
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.
Does Europa also have geysering?
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.
What is the evidence for that?
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.
HEAT ON ENCELADUS
What is the energy source on Enceladus that causes the ice to melt?
We've investigated various possibilities. Outside of Titan, Enceladus has more rock within it relative to its size than any other moon in the Saturn system. But despite the fact that it's, whatever it is—60 percent rocky by mass—there's not enough rock there to make radioactivity a plausible explanation for all the heat that we see coming out of Enceladus. It's way off. You can't do it with radioactivity alone. So the only other plausible mechanism left is tidal flexure. Because Enceladus is in an eccentric orbit as it goes around Saturn—sometimes it's close to Saturn, then it's farther away, then it's close again—this periodic change in its distance from Saturn means that the tidal bulge raised in the body of Enceladus by Saturn also changes in magnitude. If you think about it, that means that the body of Enceladus is being flexed, and if it's being flexed that means there's internal friction, and internal friction leads to heat.
Now, in truth, the amount of heat that we see coming out of Enceladus at this very moment is even more than we can explain with the present-day flexing. So it really becomes interesting because then you have to say, okay, Enceladus must have had an even larger eccentricity in the past and its flexure was even greater, and so the heat production at that time was greater than what we see now. But all that energy must have been stored in the interior of Enceladus and only now is it making its way to the surface where we can measure it. So we think today we're seeing the energy stored in the interior of Enceladus over the course of its past. How far in the past we're not really sure.
Does that mean it was warmer in the past and you would have expected more liquid water in the past?
It could be. There are two big questions. Are we seeing a monotonic decrease in heat output, with Enceladus getting colder and colder with time so that eventually all the liquid freezes and it becomes entirely solid and it never can get warm enough again to become liquid? Or is it in a cycle? It's orbital eccentricity gets very high, a lot of heat is produced, and then it starts to cool off. There's still a lot of heat coming out of it, and that stored heat takes a while before it all comes out. But it never loses so much heat that it becomes completely solid. Then it goes through a phase where its orbital eccentricity increases again. So its orbital eccentricity increases, decreases, increases, decreases, and all that cycling makes the heat coming out of Enceladus vary over time. But it never gets completely frozen. Those are some of the models that are being proposed now.
"The question is, has the liquid persisted long enough for life to even get started?"
Everyone studying Enceladus right now is getting better at what they're doing. The people who have the data in their hands are getting better at analyzing it, getting more familiar with what it means. The same applies to the modelers, the people who've been thinking about the interior of Enceladus. Their models are getting more and more sophisticated, and they're pushing them farther and farther. They are now coming to the conclusion that it is very possible a regional sea beneath the south pole of Enceladus never freezes entirely, and that it can be in this cyclical behavior. So that means there's always some liquid water under the south pole.
Does that mean if there is life there it never died out? Am I getting too far ahead?
We haven't gotten that far. 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.
"Science fiction writers can get as crazy as they want, but we can't get too crazy or we'll wreck our reputations."
There isn't very much data.
There isn't very much data. So the arguments that the liquid water on Enceladus might not have been there long enough for life to get started is important to consider. But there's no evidence to say what the shortest time period for life to develop on the Earth is, so we can put that argument on hold. The models indicate that it's very likely that liquid water does persist under the south pole of Enceladus. So life could have firmly taken hold on Enceladus. There could have easily been enough time. Even if the ocean or the sea on Enceladus lasts only a few hundred million years, if life only took 10 million years to get started, it could have gotten started there.
Does this mechanism of the flexing source of energy suggest that there are more habitable zones out there than we had previously thought?
Sure it does. We've been so slow to realize these things. It's kind of been a failure of imagination. Scientists don't go off and think completely wild and crazy things unless they have some evidence that leads them to that. It's almost like our contract says that we're not supposed to do that. Science fiction writers can get as crazy as they want, but we can't get too crazy or we'll wreck our reputations.
But once we know how much heat tidal flexing can produce, that means that anywhere there's tidal flexing and you have a body that's made largely of water ice—or a good fraction of it is water ice—you can have this situation of internal liquid. The other ingredients for life—the presence of organic materials and a continuous supply of the right kind of chemicals that organic-based organisms would want to consume—is pretty common. We have that on Enceladus. It's pretty widespread throughout the solar system, and it's probably pretty widespread throughout the cosmos. So the answer to your question is, yes, this really has broadened our view of what kinds of environments we might find biotic chemistry in.
How did you find out that there are organic materials on Enceladus?
The discovery of organic materials came from an instrument called the Ion and Neutral Mass Spectrometer, which was originally put on the spacecraft to sample the upper atmosphere of Titan. This instrument literally scoops up the atmosphere, takes it into its internal chambers, and can measure the mass of the molecule. And by having a mass spectrum of the constituents it has scooped up, it can identify what kind of molecules are there.
The instrument was pressed into service as soon as we realized that there was a plume of material coming off Enceladus, and it has found water vapor and simple organic compounds and traces of ammonia and other astrobiologically interesting compounds, very likely formaldehyde and propane and hydrogen cyanide and so on.
THE CASE FOR ENCELADUS
What is the thinking about the source of the organic molecules in these plumes? How were they were produced?
Everywhere you look in the interstellar medium astronomers find organic compounds. They find, for example, amino acids. We even find amino acids in meteorites right here in our solar system. The basic building blocks of life are present. It's sort of ordinary. It should not be surprising that we find organic materials in the outer solar system. The question is, do they go on to form more complex chains, like complex proteins or the long molecules that can encode instructions for building living organisms? When and where do those things happen? We don't know if any of those complex molecules exist on Enceladus because Cassini is not equipped to search for those kinds of ingredients. Some of us are hoping that we can soon go back with a spacecraft to Enceladus to properly sample the plumes, perhaps land on the surface, take seismic measurements to listen for the rumblings of a subterranean geysers and so on. There's a lot we could do if we had the opportunity to go back.
So Enceladus has all the ingredients that we knew were basic to this question of a habitable zone. It's got liquid water as far as we know, and it definitely has organic compounds. It also has excess heat up the wazoo, more heat than we know what to do with. And the fact that we see salt in the solid particles tells us that the subterranean liquid is in contact with rock. That means that it's kind of like these environments miles beneath the surface of the Earth where you have hot water flowing across hot volcanic rocks. So there's enough heat, you have water leeching important compounds out of the rock, and there are organisms that eat things like sulfur and hydrogen. They don't require sunlight. They don't even require anything that's been made by sunlight. So those are the analogous ecologies we think of when we imagine what kind of organisms could possibly exist under the surface of Enceladus.
To me, everything we have found with Cassini really makes the case to go to Enceladus first, above any other place in the solar system, if you're in a hurry to investigate a habitable zone. Enceladus seems to have all the things that people have known about, or thought to look for, in asking the question, did life get started elsewhere in our solar system? And it doesn't take a whole lot of speculation to say we could literally have organisms living under the surface of Enceladus, maybe even shot out in these ice particles. Because we know of ecologies right now, living under the surface of the Earth, that could live, could persist, could survive in this sea under the south pole of Enceladus.
People will say, well, we might have life on the surface of Titan. But any life living in the pools of liquid hydrocarbons on the surface of Titan is complete speculation because we don't know of any organisms living now on the surface of the Earth, or within it, that could survive in the hydrocarbon lakes on Titan, because the temperatures there are way, way, way too cold. To say that the lakes on Titan are places where we could have living organisms is much farther along the axis of speculation than our statements about the possible presence of life on Enceladus.
"With Enceladus, accessibility is the operative word. All you have to do is sample the plumes, and you've got what you came for."
How would the sample collection need to be different from what it was in the past in order to see if there are any remains of living organisms?
On Cassini we can only measure the masses of compounds up to a certain limit. If any compound were much more massive, the Cassini instrument would not be able to detect it. So we would want to take a more capable Ion and Neutral Mass Spectrometer, one that could measure far heavier molecules so that we could see if there are any, and what kind of molecules they are. That's just one kind of experiment.
You can also measure what's called chirality. Chirality is a fancy word for handedness. It's the difference between the way your left hand and your right hand are configured. Organisms on the surface of Earth preferentially use molecules of only one handedness. So if you came across a soup of biologically interesting molecules, and you sampled and found the chirality was 50/50—half was one handedness, half was the other—you would be pressed to conclude that there's no living organisms of the type we know of, because a soup of Earthly-type organisms wouldn't look like this. But if you found preferentially one handedness and not the other, then you know that there's some process driving that handedness out of equilibrium, and one very plausible explanation would be living organisms. You could test for that. By the way, those are simple measurements to make. You don't have to invent any new wild-and-crazy and currently unavailable instrument to make those measurements. We could do that right now.
Well, this is pretty far out on the axis of speculation, but could you imagine that Enceladus might be able to support multi-cellular life, more complex life?
I wax on excitedly about this all the time. When you talk about this you have to always issue the caveat that what we have on Enceladus right now is extraordinarily exciting and encouraging, but it does not mean that there are living organisms there. It just means that it is right now the go-to place in our solar system for examining a habitable zone. And with Enceladus, accessibility is the operative word. All you have to do is sample the plumes and you've got what you came for. You don't have to drill 10 kilometers under the surface, like you would on Europa, which incidentally will never happen in our lifetime. Unless they find some kind of jetting activity on Europa, we'll never in our lifetime get up close and personal with the ocean there.
On Mars they're talking about possibly finding fossilized life. I've heard it said that the most likely place for living organisms would be under the polar icecaps. But then you'd have to dig under the icecaps to get to them. Titan surface life, as I've said, is just pure speculation. So Enceladus is the best place to go in our solar system right now to ask the question, did life get started anywhere else?
If you found preferential chirality, would there be any way to know if those organisms were fossils or were living?
If you had living organisms that were forcing a system out of its chirality equilibrium and all those organisms died, eventually over time you would expect it to revert back to being 50/50. I'm not an astrobiologist, but I would venture to say that if you found a system that was out of equilibrium, then you could and would conclude that there were living organisms there now. I think that's how the logic would go.
What about the question of complex life?
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.
Is there a timeline for going back to Saturn to collect samples?
If you're paying attention to what's going on regarding NASA right now, there's a lot of consternation. The JWST [James Webb Space Telescope] looks like it's going to eat everybody's lunch unless Congress gives it extra money. And I don't realistically see that happening. And, of course, the general state of the country's finances also puts us and everybody else in a tremendous state of uncertainty. I can't answer anything about timelines.
Can you talk about how you think our solar system fits into all the other possible planetary systems out there in terms of habitability? Is our solar system in a calmer part of the galaxy that makes it more conducive to life?
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.
The way I feel about it, we should go to Enceladus. We should go directly to Enceladus, we should not pass Go, we should not collect $200. It's that simple.