Moonlander: Let it Rain

The haze of an atmospheric layer on Saturn’s moon, Titan. Credit: Voyager Project, JPL, NASA

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 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?

Anthony Del Genio (ADG): The first thing to say is that last Friday [January 14] was arguably the most exciting day in the history of planetary exploration.

True color and surface infrared images of Titan Image Credit: NASA/JPL

One could make the case for the first ever planetary flyby, Mariner 2 Venus, in the 1960s, or the Viking landings on Mars and the Voyager Jupiter flybys in the 1970s.

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.

Surface orange pebbles from Titan. Click image for larger view. Credit: ESA

Before we got there, we knew about the dark and bright regions, the apparent need to have a surface source of methane to explain its continued presence in the atmosphere, and the ground-based radar detection of specular reflection (the equivalent of sunglint, but for radar waves), all pointing to at least a partially wet surface.

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.

Shoreline horizon during descent to Titan. Click image for larger view. Credit: ESA

The nitrogen was inferred from Voyager data and from our knowledge of the solar abundance of nitrogen and associated expectations of what a cold planet’s atmosphere should mostly be made of. We haven’t heard yet from the Huygens people what the actual abundance of methane in the troposphere is, and what the relative humidity of methane is.

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.

Titan continues to puzzle scientists who speculate about whether the darker regions are actual shadows, illumination changes, or humidity.
Credit: NASA/JPL

Once we know exactly how much methane there is, and have a better idea of what the surface is really like, we’ll have to include the possibility of rainmaking clouds and an interactive surface in our models, one that gets wet when it rains, lets methane infiltrate into the "soil" (or whatever one wants to call the stuff that Huygens landed on) and then evaporates at a rate that depends on how often the methane gets exposed to the atmosphere, how turbulent the air is near the ground, what its relative humidity is, etc.

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):