In The Field

AGU: Three from Titan

Lots of interesting stuff from Titan on Tuesday (forgive late blogging — my computer was knackered). The Cassini spacecraft has finished its primary mission, and its science team is understandably proud of its discoveries. What caught my eye, though, were three not-yet-quite-discoveries: things to follow up on with further data analysis and more observations in the extended mission.

Volcanoes! Robert Nelson of JPL talked about data from Cassini’s infrared instrument, VIMS, which show what seem to be brightness changes in a couple of regions. Rosaly Lopes described radar images of the regions that seem to show lobate flows of some sort of stuff. Together, they make a case for cryovolcanism — eruptions of water/ammonia lava. Here’s the JPL press release. Not so fast, said Jeff Moore, a planetary geologist from NASA Ames happy to play devils advocate at the associated press conference. Things can look volcanic without being volcanic, he warned, and things can change their brightenss, too — look at the wind streaks on Mars, for example. Listening to him it was hard to see the volcano case as beyond reasonable doubt. A nice bonus after the press conference was listening to Moore and Lopes discussing which of them was using Ockham’s Razor better: Moore, who was suggesting that everything on Titan can be explained with just the three surface and atmospheric factors known to be at play (wind, methane rain, and impact cratering) without invoking internal factors? Or Lopes, who was seeing ways to explain with one cause, volcanism, things that Moore would need to invoke a variety of causes for?

Titan as Mars Mars’s orbit and inclination mean that the southern hemisphere’s summer is shorter and hotter than summer in the north. As a result there’s a net transfer of volatiles — water — from one end of the planet to the other. Saturn’s orbit means that something similar applies to Titan, and Oded Aharonson of CalTech has a nice story to tell about how that, too, could lead to a net transfer of volatiles — in this case, methane.


Titan’s lakes are apparently restricted to high latitudes, and there are many more of them in the north than the south. At one point people wondered if the methane moved from one pole to another on a seasonal basis, but it turns out that the seasons on Titan are not long enough for that to work. They’re long season’s by earthly standards, to be sure — it takes Saturn almost 30 years to get round the sun, and that’s what sets Titan’s seasons — but transporting methane through Titan’s atmosphere takes a very long time; the time it takes a methane molecule evaporated from a Titanian lake to fall back as a raindrop is centuries. Instead, Aharonson suggests, the volatiles move from pole to pole according to a 30,000 year cycle that swaps the short hot summers from one hemisphere to the other. Nice supporting detail: this would explain why the only big lake in the south, “Lake Ontario”, is rich in ethane — which doesn’t evaporate, and thus would make up a sort of lag deposit as the methane evaporated away over the millennia. Neat implication for further research: could this explain why Titan’s craters seem to be mostly in the tropics, more or less — does the methane flip flop mean that erosion/resurfacing is orders of magnitude faster at the poles than at the equator?

Titan is Buffalo. In the north there are clouds of ethane and also, it appears, of methane. Is it possible that the methane clouds are “lake effect” clouds like those you get when cold wind passes over the warm great lakes in winter and hits high ground on the other side? This idea of Mike Brown’s got some press when it was first suggested, but I missed that, so I came across it fresh. Whether this particular idea holds up or not, there’s something about seeing people like him and Aharonson and Moore and many others take ideas from one planet with hydrology and weather and apply them to another that is really inspiring.

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