Before March 2005, few outside of the planetary community had likely ever heard the name
Enceladus, much less could pronounce it ("en-CELL-a-duss"). But now, following the announcement
by the Cassini Imaging Team that this tiny moon of Saturn appears to have reservoirs of liquid
water beneath its south pole, the name Enceladus is known round the world. Why? Because liquid
water is the sine qua non of life as we know it. Indeed, Enceladus may have become, essentially
overnight, the go-to body in the solar system in the search for extraterrestrial life, says
the Imaging Team's leader, Carolyn Porco. In this interview with Dr. Porco, hear the latest on
Enceladus and its startling potential.
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.
NOVA: What's taking their place?
Porco: 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.
NOVA: Does Saturn have any influence on Enceladus in terms of heating it up
through tidal forces?
Porco: 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.
NOVA: Enough heat to melt water-ice.
Porco: 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.
NOVA: 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?
Porco: 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?
NOVA: Could there be life in that water?
Porco: 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.
NOVA: So what might life on Enceladus look like if the moon had these ingredients?
Porco: 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.
NOVA: So living things could be in that water just beneath Enceladus's south
pole?
Porco: 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.
Going there
NOVA: 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?
Porco: 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.
NOVA: So will space scientists now be itching for a new mission to
Enceladus?
Porco: 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.
NOVA: Putting Europa in second place? Or will we still aim to go there
first?
Porco: 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
NOVA: Quite a striking development. How did you feel when you first learned of
it?
Porco: 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.
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.
“We’ve been able to take all the world with us on this fantastic
voyage, and that makes me feel really good.”
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."
NOVA: Why?
Porco: 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.
NOVA: As head of the Cassini imaging team, you must be extremely proud of all
the mission has achieved so far.
Porco: 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.
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