Moonlander: Let it Rain
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 what role liquid methane might be playing on Titan.
Astrobiology Magazine (AM): What are your initial reflections post-landing, only one week after Huygens reached the surface on Titan?
But successfully landing on a moon so far away, and one that seems more Earth-like and currently active than any other place in the solar system, with all that implies about what we might learn about Earth's own early history and the odds of life elsewhere in the universe - well, it's hard to describe how exciting that is.
If NASA is going to go in a new exploration direction in the coming years, how can Titan not be our number one long-term priority?
AM: Not to put too fine of a point on it, were you surprised by the first sights of 'alien mud'?
ADG: My thinking about Titan has been going up and down like a yo-yo in the past year.
Then after a few Cassini Orbiter flybys seeing few clouds outside the polar region and no sunglint at near-infrared wavelengths, I began thinking that the atmosphere and surface are drier than I had imagined, and that maybe the methane is locked up underground and only gets out occasionally.
But now Huygens has apparently seen evidence of a methane reservoir just beneath the surface.
It's still not clear to me whether it's really like mud, i.e. whether you've got solid water ice/organic particles suspended in a methane liquid or not. And of course we don't know if the landing site is typical of other places on Titan.
But the Huygens data make it pretty likely that it's rained sometime or other on that part of the moon, whether just yesterday, in which case it may really be mud, or 7 years ago when Titan was at its autumnal equinox and methane storms would have been more prevalent at the landing site, as is the case in Earth's tropics.
AM: The atmospheric composition seems dominated by nitrogen and methane. Does this chemical composition have implications for modelling global circulation, temperature profiles, etc.?
ADG: Yes, it has implications for both of those things, especially the methane.
From a purely observational standpoint, knowing the vertical profile of methane abundance will help the [Cassini] Orbiter scientists in several ways. First, when we look at near-infrared images taken at wavelengths where methane absorbs sunlight weakly, it will help us figure out how high the cloud tops are.
Second, Cassini will be indirectly measuring the temperature structure of Titan's atmosphere at many latitudes by observing how light is affected as it passes tangentially through the atmosphere. Knowing how much methane there is allows that [profile] to be converted into temperature information more accurately.
AM: What if it really does rain on Titan?
ADG: For modelling, we've been speculating for a long time about whether models of Titan need to include the methane equivalent of a "hydrologic" cycle. Now it's pretty clear that we do.
Then we can ask the question of whether without these processes, Titan's atmospheric circulation would be very different or not.
Rain, evaporation, etc. could just be a by-product of the atmospheric circulation, as it is for example on Earth when a weak low pressure system makes rain in New York in December.
Or it could be that the circulation itself is modified in an important way by methane rain, as is the case for example in the tropics on Earth where Amazon and Indonesian thunderstorms control what the large-scale circulation patterns look like.
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):