Kudos for Cassini
The U.S./European Saturn mission takes home a trophy.
- By Heather Goss
- Air & Space magazine, March 2012
Bob Mitchell of NASA’s Jet Propulsion Laboratory is the Program Manager for the Cassini-Huygens Saturn mission, whose flight team took home this year's National Air and Space Museum Trophy for Current Achievement. Mitchell spoke to Air & Space Associate Editor Heather Goss in February.
Air & Space: During the eight years or so that the Cassini mission has been studying the Saturn system, what has been the most thrilling discovery or biggest surprise?
Mitchell: I’d say the most thrilling discovery and the biggest surprise are probably two different things. I think the most thrilling thing was seeing the surface of Titan. This wasn’t such a surprise because we knew that we didn’t know what was there; we knew we were going to see something different, but we didn’t really have much of an idea what we were going to see. So to actually see it, I think that was the biggest thrill.
The biggest surprise had to be the plumes on [Saturn’s moon] Enceladus. We fully expected Enceladus to be another icy satellite like several others that orbit in that vicinity. So to get there and find the plumes, that was a very pleasant surprise. And we’re still working on understanding exactly what’s happening there: what it is that produces them and the effects that the material has on the rest of the Saturn system.
What else has Cassini discovered about Enceladus? NASA scientists have talked about Enceladus having “astrobiological potential.” Do you have hopes that Cassini will uncover evidence of life there?
There’s still a lot we don’t know, but we hope to learn a fair amount more before we’re done. We’ve developed some models and ideas about it. Some of the questions are: What is the energy source that drives the plumes? The scientists think that Enceladus should be too small to produce such energy. But obviously it does somehow. Another question is whether there’s a reservoir of liquid water under the surface. And, do the plumes vary with time? You might think the mechanism is something like what drives Old Faithful in Yellowstone, but every time we’ve looked for the plumes they’ve been there, always more or less the same density and height; whereas at Yellowstone, Old Faithful only goes up about once every hour. So it’s quite different from what’s happening at Enceladus.
The question about the possibility of life is always a very exciting one. We have found, at Enceladus, organic compounds. There’s obviously an energy source. And the scientists think very likely there’s liquid water beneath the surface. So with those three things, that’s everything you need for life as we know it on Earth. We haven’t found life on Enceladus, and we don’t really expect to [with Cassini] since we aren’t really instrumented or in a location to be able to do that. But Enceladus would certainly be an exciting place to go look for life.
You mentioned that getting a look at Titan’s surface was a thrilling moment. Would you describe the first glimpse from the Huygens probe?
It was so Earth-like that it was eerie. There were mountains and dry river beds filled with rocks that were polished smooth, just like in river beds here on Earth; clear evidence of erosion, weather, flowing liquids, and even smog – and by smog I mean hydrocarbon haze, but it’s made up of many of the same things as smog here on Earth. Huygens didn’t see flowing liquid, but it was very clear that there had been something flowing before. It wouldn’t have been water, because they found the temperature to be around 90 Kelvin, so water would have been very solid ice. Rather, it was liquid methane. So there’s a weather cycle that’s very much like Earth’s happening on Titan, but with liquid methane instead of water.
There had been a lot of speculation before we arrived that Titan might be covered by an ocean of liquid hydrocarbons. Huygens certainly disproved that, but the Cassini orbiter subsequently found that there are very large lakes – larger than our own Great Lakes – full of liquid hydrocarbons on Titan, mostly at the poles.
How does Cassini study Saturn’s rings, and what have you learned?
We use cameras and spectrometers, and with those we can take pictures and measure composition, measure temperatures, and study the dynamics. When the spacecraft passes behind the rings, as seen from Earth, that means the radio signal from Cassini goes through the rings, and it’s really quite impressive what the scientists can deduce about the size and density of the particles and even the composition just from how the rings affected the radio signal.
We’ve learned a lot about the dynamics: how the ring particles tend to clump under the influence of their own gravity, and then disperse and break up due to Saturn’s gravity. We’ve learned a lot about how ring particles interact with nearby moons that orbit in ring gaps, and how they shape and affect the edge of the gap from their mutual gravity. You can also see waves that propagate clear across the rings – this is an interesting phenomenon – and we can use them to study general properties of the particles, particle size distribution, and have uncovered a wealth of information that we didn’t have pre-Cassini.
What other contributions to space science has Cassini provided?
A couple of things that we’ve done relate to the theory of relativity. One is that we looked for gravitational waves during the cruise phase from Earth to Saturn. We had radio equipment very sensitive to perturbations of the kind a gravitational wave would make. The scientists are still processing the data that we got from that, but so far they have not found anything. I’m not sure what their expectations were, but gravitational waves are, presumably, not real common occurrences, and so to be listening at exactly the right time is perhaps a bit of a long shot.