Hot little ice moon
NOVA: How is it possible on a world that has a surface temperature of
-330°F that water could be spewing out of the south pole?
Carolyn Porco: Well, there's obviously something heating up Enceladus in the south polar region to the point where the heat flux coming out of the south pole per square meter is at least two and a half times greater than the average heat flux coming out of the Earth. So there's something going on there that's providing a surprising amount of energy.
By default it has to be something associated with flexing the body, because the only other present-day source of heat within Enceladus is radioactivity arising from its rocky components. Assuming it's got the normal contributions from radioactive elements, you can compute what the heating rate ought to be. We've done that, and it's not anywhere near capable of producing the heat flow that we observe. So the only thing left is flexure.
But when we use the standard models for a satellite interior, even with the amount of flexing we think Enceladus is undergoing due to its resonance with the moon Dione and possibly due to a resonance between its own spin and its orbit, throwing together everything we could think of with the old-fashioned models still doesn't give us the amount of heat that is being measured. So this is telling us that the old models are out.
What's taking their place?
Well, we're now looking at models that have at least part of Enceladus being molten. And if something is molten, then that changes the heating rates. You can get more flexure out of it and that can inject more heat into it by the same forcing.
Does Saturn have any influence on Enceladus in terms of heating it up through tidal forces?
Well, there is interaction between Saturn and Enceladus, but Enceladus's orbit is eccentric because of Dione. Dione perturbs the orbit, making it eccentric, and that brings Enceladus close to Saturn and then far away in the course of an orbit. That brings about a change in the differential gravity from Saturn across Enceladus, which then changes the moon's shape, flexing it like you would a paperclip. It's a common heating mechanism. Now it's a matter of trying to figure out how that mechanism is able to generate so much heat within Enceladus.
Enough heat to melt water-ice.
Yes. It's not really too ridiculous an extrapolation to say that if you go from this cold surface—now, mind you, the surface at the south pole is warmer than everywhere else because of this heat, but it's still cold, something like -150, -170°F—but even starting there, and assuming heat is getting up to the surface by conduction, you don't have to go too far down into the surface to get to a temperature that would melt water-ice ... only a few tens of meters.
So putting those simple geophysical arguments together with the analysis that the imaging team made of the plume coming off the south pole, we come to the conclusion that we may have liquid water close to the surface. From the images, we get a measure of the abundance of particles in the plume (because when we're looking at the plume in our pictures, we're not seeing vapor, we're seeing particles). And the abundance of particles is way too much to be explained by the other candidate models.
And all this led you to suspect that what is gushing out of the south pole are ice particles derived from liquid water near the surface?
Well, we were driven to this conclusion almost reluctantly, because we knew it was somewhat radical. But, yes, we were driven to the model whereby we actually had geysers that were liquid-water geysers. They're under pressure, but the liquid and vapor can leak out through cracks in the surface. And once the liquid explodes out into space, the water freezes immediately and you can get copious quantities of icy particles this way. That's what we think we're finding.
A place called home?
Could there be life in that water?
Well, this is the Holy Grail of modern-day planetary exploration. The cardinal goal is to find those places in the solar system that might have given rise to life. And that is life as we know it: carbon-based and developing and living in water. There may be other forms of life, but right now we are at a loss to even know how to look for them. So we're looking for life that is like us. That means looking for water.
"It could be snowing microbes at the south pole of Enceladus."
Water's not the only ingredient, though. You need organic materials, because that's what earthly life is all about. It lives off and is made of organic materials, the elements carbon, hydrogen, oxygen, nitrogen, phosphorus, and a few others. You also need heat.
Finally, you need a stable period of time over which life can develop. That is, you need to have an environment that had all of these ingredients going for a bit of time in order for life to develop. On Earth, it took at most about 300 million years. Geologically speaking, that's the blink of an eye. It's not at all far-fetched to think that what we're presently observing on Enceladus has been ongoing for at least that long.
So what might life on Enceladus look like if the moon had these ingredients?
You can draw an analogy with the hydrothermal vents of the Earth, where you have organisms that are living without sunlight, because they're at the bottom of the ocean, but they thrive on chemical energy. That chemical energy has to come from somewhere, and it comes from the heat that gushes out of those hydrothermal vents.
So if we want to make that analogy, we could have organisms elsewhere that live similarly: They developed, evolved, and live in water; they're organic; they're made out of the same elements we are; and instead of living off sunlight, they're deriving their food from heat. The waters coming out of the hydrothermal vents have been in contact with very hot rock, so minerals get dissolved in the water, and then the ocean organisms live off hydrogen sulfide and hydrogen and things like that.
So let's assume that we might have something like that in an extraterrestrial body of water. Then we'd need a source of substantial internal heat to provide the required chemical energy. In some sense, we have all three on Enceladus. (Don't ask us why—we don't really have that answer yet—but it is the way it is.) We have heat coming out of the south pole. We've got liquid, we think. And anywhere you look in the solar system basically there are organic materials. There certainly are the simplest organics in association with the fractures in the south polar region. That's where we see the organics on Enceladus; in fact, that seems to be the only place we see them, in association with those cracks.
It doesn't take a rocket scientist to put all this together and say, "My god, we could have an environment here that is potentially suitable for living organisms." I can't say that we definitely have life there. We won't know until we go there. And I can't even say right now that we definitely have liquid water. That will take more examination of what we've seen so far as well as further exploration with Cassini. But let's say right now it's looking very provocative.
So living things could be in that water just beneath Enceladus's south pole?
Yes, assuming all the conditions I've just mentioned really do exist and have existed on Enceladus for the required length of time for life to develop. Of course, this is all extreme speculation, but it's certainly fun to think about.
And here's a completely wild and crazy idea for you. If the fluids have living organisms in them, then the frozen particles coming out of these vents could have flash-frozen organisms in them. It stands to reason that if they're coming from the source where there's life, you could have particles containing organisms. It could be snowing microbes at the south pole of Enceladus, and that's quite a thought.
Absolutely. So how easy would it be to fly a spacecraft through that plume, collect some of that water, and test it for the presence of life?
Well, we'll be doing these things. We did it already with Cassini, because we have the instrumentation on Cassini to measure the composition of vapor and, to some degree, the composition of the particles, though the latter is difficult and much less precise.
Nonetheless, hopefully we'll have an extended Cassini mission, with Enceladus featuring prominently in that. I hope we have several repeated flybys of Enceladus, in which we take the spacecraft and go as low as possible in altitude to the south pole, and fly through the plumes and make as many measurements as we can.
"I think Enceladus may have just taken center stage as the target of astrobiological interest in our solar system."
Also, we'll need to map the surface in thermal radiation to get a much better feel for where the hottest spots are. Take more images of the plumes like we've gotten already, only better so we can refine our measurements there as well. We want to do as much as we can with Cassini.
So will space scientists now be itching for a new mission to Enceladus?
We should consider Cassini to be the precursor mission for a follow-on mission back to Enceladus hopefully to land and do there what scientists would do if they were trying to study, say, the Yellowstone geysers. It may be a ways in the future, but it's not as far into the future as it will be to try to do things like this on Europa. Even if we landed on Europa tomorrow, it would still be many decades in the future before we drill down to the liquid ocean, which is kilometers beneath the surface. Also, a spacecraft can't operate very long on Europa because of the intense radiation field around Jupiter.
We don't have these problems on Enceladus. For these reasons and the ones I've already mentioned, I think Enceladus may have just taken center stage as the target of astrobiological interest in our solar system.
Putting Europa in second place? Or will we still aim to go there first?
Well, this will all have to be discussed within the planetary science community, of course, and what I'm giving you now is my own personal take on this. Undoubtedly Europa has its fans, and people will still want to go there, even for other reasons than astrobiology. But if getting up close and personal with extraterrestrial bodies of liquid water is the goal, then Enceladus may be the place to go. It may even be better than Mars.
Understand that these are early days, and these results are brand new and will undergo examination and scrutiny by the scientific community. That is the way science goes; it's the way it has to go. But it will be very interesting to see what course our future explorations take once we've all had time to absorb the significance of what we've just found with Cassini.
Moments of discovery
Quite a striking development. How did you feel when you first learned of it?
Well, you know, this happened incrementally. We had seen the near-surface plume in images back in January 2005, but we weren't sure it was real. We saw it again in February, and we still weren't sure it was real. Some of my team members thought it was a camera artifact. So as a team we were very silent about the whole thing. But by late summer, I had done my own analysis of the images and finally convinced the rest of the team that the plume was indeed real.
"We've been able to take all the world with us on this fantastic voyage, and that makes me feel really good."
To get a better look, we planned a sequence of very high-phase images to be taken in November. By that time, Cassini had verified with the other on-board instruments that a plume of material was, in fact, coming off the south pole. Then came those spectacular images in late November showing in incredible detail many individual jets coming off the surface, going in all different directions, and feeding a much larger plume, as big as Enceladus itself.
That was positively smashing. That was just—it's hard to describe. We always expected something about Enceladus was producing Saturn's E ring. We'd even talked about discovering geysers on Enceladus before Cassini got to Saturn. But we didn't expect it to be so dramatic and to be such a large, spectacular phenomenon.
So that was a thrilling moment. As we started to analyze our data and put things together logically, we dismissed the more prosaic models, and finally we were staring the model in the face that said we had liquid water. So it happened incrementally, but when we finally said, "Yes, we're going to put in this paper that we've got liquid water," it was like, "Uh-oh. We know what this means." I even wrote in an e-mail to some of my team members, right after I submitted our final version of the paper to the journal, "Okay, now we sit back and wait for the world to tell us we're nuts."
Well, the late Carl Sagan used to say, "Extraordinary claims require extraordinary evidence." In some sense, this is an extraordinary claim, because it's an extraordinary and surprising inference we're making about this small little moon. So we could expect the scientific community to really take the whole idea through its paces and try to rip it apart and see if there's another, less exotic explanation.
Nevertheless, I'm finally settling into the idea that this is probably the most spectacular result that the Cassini mission has produced, and I'm thrilled about it.
As head of the Cassini imaging team, you must be extremely proud of all the mission has achieved so far.
Yes, we're very proud, believe me. I feel like a proud mama. It's enormously gratifying to know that after this mission is over, our legacy will be tremendous. We're leaving behind not only a treasure trove of scientific findings, but an enormous collection of the most beautiful and magnificent sights anywhere to be seen in our solar system. We've been able to take all the world with us on this fantastic voyage, and that makes me feel really good.