January 25, 2005
Locating Methane Rain on Titan
As part of the Cassini Imaging team studying the atmosphere on Saturn, NASA's Anthony Del Genio explained in this part of his interview, how to make sense of a moon potentially making methane rain.
Astrobiology Magazine -- Since the remarkable landing of the Huygens probe on the surface of Saturn's largest moon, Titan, the community of planetary scientists has wondered anew about the discovery prospects in our own solar system. As part of the Cassini Imaging team studying the atmosphere on Saturn, Anthony Del Genio explained to Astrobiology Magazine his interests in the giant ringed world and its strange moons.
Del Genio is a research scientist at NASA Goddard Institute for Space Studies , (GISS) New York, and an Adjunct Professor in the Columbia University Department of Earth and Environmental Science. His interests in the terrestrial atmosphere have led him to study storms on other planets such as Jupiter, Saturn and Titan, fundamentally to gain greater understanding of how their meteorology differs from that of Earth.
Del Genio contributed his thoughts to Astrobiology Magazine as he explained in this part of the interview how to make sense of a moon potentially making methane rain.
Astrobiology Magazine (AM): Is there a sense of surprise about what you have seen so far from the surface pictures particularly?
Anthony Del Genio (ADG): Ecstasy is more like it. I think we had all been hoping that there would be liquid methane on the surface, that we'd see evidence of a hydrologic cycle, and that there would be some hint of what differentiates the dark and bright areas we can see from space.
There had been suggestions from ground-based data that water ice was exposed on the surface, and that this was a likely candidate for the bright areas. Then the early Cassini results made me, at least, start to wonder about what we'd really get - we saw hints of all these things from the morphology of the surface features and the presence of clouds, but some confusing things as well.
We were really fortunate in that Huygens landed right smack in a region where there is a dark-bright boundary in the Cassini images. That gives us the variety of surface types in the same scene that allows the viewer some perspective about what we're looking at.
AM: So to paraphrase, on the wishlist for Titan there would be listed the three items: liquid methane on the surface, rainmaking and whether the dark-light boundary were really surface features?
ADG: As for the things I'd been hoping for, it looks we got two out of three at least, and maybe all three.
We're already pretty sure that at least in this part of the moon the bright stuff is ice bedrock and at a somewhat higher elevation. And we're pretty sure that the dark stuff is lower and is the place where stuff collects after flowing down the drainage channels. And the channels themselves, at least some of them, say that it rains there.
The jury's still out on whether there's currently liquid on the surface, or whether it's mostly organic residue from previous flow events.
But the story in these images says that if surface liquid is not present there and then, then somewhere, sometime on Titan.
AM: Just from what we know about Saturn's distance from the Sun, can one give a relative or approximate sense of the illumination (solar power) beyond the two-billion-mile marker, relative to the earth's 93 million miles?
ADG: It's pretty dark there. The illumination by the Sun scales as the inverse square of distance. Titan is almost 10 times farther from the Sun than Earth is, so the sunlight it receives is about 1 divided by 10 squared times, or about 1 percent, what the Earth receives. And that's at the top of the atmosphere.
Much of the incoming sunlight is reflected back into space by the hydrocarbon haze that covers Titan, so only a fraction of that 1% actually reaches the surface.
The Huygens DISR instrument will eventually tell us exactly how much. But Marty Tomasko, the DISR PI, described imaging the dark areas on Titan's surface as being something like taking a picture of a blacktop driveway at dusk.
AM: Others have mentioned that Huygens had a fairly bumpy ride through the atmosphere (20 degree or more tilt windward) but landed softer than predicted (although still a 15-G collision or so). Can you suggest anything about the relation, if any, between high winds and what the Cassini Imaging team's models might try to pinpoint for superrotation or global circulation?
ADG: Just speculating, Huygens might have encountered some "clear air turbulence" as we get sometimes on plane rides here on Earth. Turbulence like that often results from what we call gravity waves, waves that oscillate in the air as the buoyancy of up-down moving air fluctuates, analogous to the ripples that form when you throw a rock onto a pond.
When gravity waves propagate upward, the decreasing density of the air with height makes the amplitudes of the waves larger, i.e., for the same kinetic energy of the wave motion the air motion is stronger where the air is thinner. It's like the effect of cracking a whip - you move the thick end of the whip a little, and you get a large movement at the thin end.
So high up the winds associated with these waves can get strong enough that sometimes they induce turbulence.
Now where the superrotation may come into the picture is that as the superrotation gets stronger with height, the wavelengths of any gravity waves may get smaller, i.e., the wave crests and troughs may get closer together. We think we may see that in some of our Cassini images. And the closer they get, the more likely that they become turbulent.
AM: Some have suggested that volcanism of some kind may be shaping Titan's surface. Any of this fit into a planetary model of its history with a hot core for Titan?
ADG: There's been speculation about the possibility of "cryovolcanism" on Titan, in other words, volcanic emissions of ice and gases rather than lava. And the images we've gotten to date suggest the possibility of cracks, striations on the surface that might indicate tectonic activity on Titan. And that in turn would suggest an active interior.
Now we hear from Huygens data that argon-40, which results from the decay of potassium in the interior, is present in the atmosphere, and that, along with some apparent ice extrusions seen in the Huygens images, seems to be pointing the geologists toward cryovolcanism.
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):
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