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Plunge To Methane Lake?

Earlier fly-by image of Titan, taken by Cassini, pinpointing Huygens' landing site (yellow x). Credit: NASA.
by Astrobiology Magazine
Pasadena CA (JPL) Jan 13, 2005
Imagine descending through hurricane-like conditions where wind speeds can reach 400 miles per hour and the ground temperatures drop as low as -300 degrees Fahrenheit. A choking haze envelopes everything. If all goes well, on January 14, a tiny capsule will take this plunge in hopes of sending back data and pictures near the surface of the Earth-like moon, Titan.

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, Saturn 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 to gain a more fundamental understanding of how their meteorology differs from that of Earth.

As a gas giant, Saturn and its extraordinary weather are not altogether surprising, particularly given the planet's enormous diameter, rapid rotation and supercold atmosphere.

But data on another planet's changing climate present unique research and modelling opportunities. For instance, the orbiting Cassini probe has already made major discoveries, such as lightning bolts roughly one million times stronger than lightning on Earth.

A piggyback probe called Huygens was bolted to Cassini and has been riding along during the nearly seven-year journey to Saturn largely in a "sleep" mode.

The probe descent will begin at 4:07 AM on January 14, EST (USA), and its Descent Imager experiment hopes to have a couple of images taken during the descent ready that evening. Huygens will be the first human-made object to explore on-site the unique environment of Titan, whose chemistry is assumed to be very similar to that of early Earth before life formed.

Del Genio contributed his thoughts to Astrobiology Magazine as he explained why Saturn and Titan might be of interest to an earth scientist wanting to observe atmospheric high-drama.

Astrobiology Magazine (AM): What would be considered a warm day on Titan, on a relative scale?

Anthony Del Genio (ADG): 95 Kelvin (about -290 degrees Fahrenheit) is a good beach day in the tropics on Titan. As far as we know it never gets much warmer than that at the surface.

AM: Given the thick smog, is there uncertainty about the Huygens descent timeline after atmospheric entry, since that will depend on wind, temperature and daily pressure?

ADG: Yes, the atmospheric density profile especially is an uncertainty that will affect the probe descent and even its stability. I don't think that's as much an issue about the haze as it is simply a matter of the atmospheric composition and temperature not being known precisely going in.

AM: Cross-winds are expected to cause parachute drift. What kind of wind speeds are possible?

ADG: In the early part of the Huygens descent it's expected that winds of 100-200 meters per second (220-440 miles per hour) could be encountered, gradually decreasing to tens of meters per second (22 miles per hour) once it reaches the troposphere.

Our early Cassini image analysis suggests winds of 30-40 meters per second (66-88 mph) at altitudes of 20-30 kilometers (16-24 miles).

AM: Will Doppler radar give atmospheric information during descent or is that primarily intended for navigation and tracking?

ADG: The Doppler Wind Experiment on Huygens uses the Doppler shift of the radio signal to infer wind speeds as a function of altitude. This has been done on several previous planetary probes, e.g., Pioneer Venus and Galileo.

We're very interested in those results because they will provide the most accurate information on the structure of Titan's "superrotation", albeit at only one place and time.

Combined with the mapping of winds at different latitudes and times we get from the Cassini images we hope to put together a reasonable picture of Titan's global circulation.

AM: When one sets out to model a supercold methane atmosphere that is thick and dense, what are the biggest unknowns for understanding its changes?

ADG: Right now I think the two big questions are: What is the abundance of gaseous methane in Titan's atmosphere, and what is the nature of Titan's surface? And these are related questions.

The favored story going in was that the presence of methane gas, which is destroyed by ultraviolet light to make the stratospheric haze, implies that there must be a source of methane at Titan's surface to replenish it.

That in turn gave rise to the idea that there might be liquid methane on Titan's surface in the form of lakes or seas.

But so far we haven't seen any indications of surface liquid (though there's much more observing to be done before we could rule that out). And so far methane clouds have been widely scattered, very much unlike water clouds on Earth.

That might mean that the relative humidity of methane is at the low end (about 30% or so) of pre-Cassini estimates, i.e., the atmosphere is too "dry" to make clouds most of the time. (Other explanations are possible, e.g., there's lots of methane but no aerosol particles suitable for seeding cloud formation.)

But taken together the scattered cloudiness and the failure to find liquid on the surface may be pointing us to a scenario in which the methane is mostly trapped beneath the surface and is only released sporadically to the atmosphere in geysers, cracks, vents.

The Huygens probe will give us a definitive estimate of tropospheric methane abundance in one part of the atmosphere, which should really help constrain our ideas.

AM: Do these models have to include the boundary condition of the surface itself, and does it matter globally whether that is liquid or solid in patches?

ADG: Yes, the models do have to include surface conditions, for two reasons: The surface is the source of methane, so we have to be able to estimate the evaporation (or outgassing) rate of methane into the atmosphere if we are going to get the right amount of cloudiness and precipitation.

Also, liquid surfaces respond more slowly to seasonal changes in sunlight than do land surfaces, so predicting seaonal changes in the circulation, explaining the presence of primarily polar clouds, etc., requires that we know the type of surface we're dealing with.

AM: From the flyby missions, what has surprised you most seeing Titan up close?

ADG: Well, we now have 3 flybys completed. What has surprised me most about the atmosphere is how infrequently clouds occur.

We knew that in ground-based images clouds were only seen in the polar regions until recently. But I fully expected that once we got close to Titan with our cameras we'd see lots of little clouds that were too small to be resolved from Earth. And every now and then we do see that kind of thing.

But very rarely - it's like looking for a needle in a haystack. And we also started seeing long, zonally-oriented cloud streaks a few months ago unlike anything we'd seen previously from earth or on approach to Saturn.

Initially we thought those might be very high altitude clouds that had been stretched out by Titan's fast winds. But in the most recent flyby we found that in at least one case the cloud streaks are at a lower altitude (where the winds should be weaker) than the more common polar cloud. So right now it's a mystery as to why Titan makes clouds with that particular shape.

AM: What finding from the Huygens descent would most intrigue you as a modeller?

ADG: If the probe lands smack in the middle of a methane lake that we've already taken images of!

Barring that, and that scenario seems less likely as time goes on, I'd be very intrigued (and perplexed!) if Huygens measures no strong wind speeds.

But that's the beauty of all this - initially we're disappointed when the data don't match our expectations, then we get excited because we have new information to play with. It's satisfying when the data prove you right, but it's much more interesting when the data send you back to the drawing board. That moment of discovery is what keeps us going.

AM: What future work will you be doing on understanding what is happening on Titan after the mission?

ADG: I think the science of modeling the 3-dimensional climate and circulation of Titan is only in its infancy. The general circulation models that we use at GISS to predict future global warming on Earth are great tools to adapt to other planetary atmospheres.

Titan has a lot in common with Earth - a similarly thick nitrogen atmosphere, a hydrologic cycle, a surface with possibly several different surface types, organic chemistry, a photochemical haze.

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Go Huygens!
Pasadena CA (JPL) Jan 12, 2005
This map illustrates the planned imaging coverage for the Descent Imager/Spectral Radiometer, onboard the European Space Agency's Huygens probe during the probe's descent toward Titan's surface on Jan. 14, 2005. The Descent Imager/Spectral Radiometer is one of two NASA instruments on the probe.

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