Emily LakdawallaMar 15, 2006

LPSC, Wednesday morning: Titan

This morning at the Lunar and Planetary Science Conference began with Titan, and then later in the morning I had to choose between skipping Titan and going over to rover sessions, or staying with Titan. I elected to stay with Titan because there was a suite of interesting-sounding VIMS talks up against the rover talks, but I'm not at all sure I made the right choice. In fact, there was not a whole lot in the Titan session that struck me as being very new or a deeper understanding of ideas I'd previously heard before.

Ralph Lorenz gave a talk about the dunes on Titan (here's the abstract). Since much of the material from his talk is in a paper that is apparently in press in Science, it was a little better developed than I'd heard before. For example, in the past I have heard Ralph cite examples of dune-forms developed in snow in Antarctica where the dunes have almost no topographic expression but are visible to imaging techniques that are sensitive to ice grain size. Now, however, Ralph was reporting measurements of the heights of the Titan dunes -- they average 150 meters high, with a 2-kilometer spacing from crest to crest. He's now citing examples from the Namib desert. He showed some really beautiful Space Shuttle photography of those features.

Chuck Wood gave a talk about craters on Titan, but he had little new to say because after nearly two years in Saturn orbit Cassini has seen only two unambiguous craters on Titan (here's the abstract). He pointed out some very small features in the radar data that he said looked much like craters, but they were so small (5 to 10 kilometers diameter) as to be under the lower size limit that is imposed by the present density of the atmosphere (which would shield Titan from smaller impacts). He said that this could be evidence that either we aren't modeling the impactors coming in to the atmosphere correctly or maybe the current pressure of the atmosphere is transient, but I don't think anybody thought that the "craters" were compelling enough to force such a reevaluation.

Jani Radebaugh talked about the mountains on Titan observed by Cassini radar (here's the abstract). She performed radarclinometry to try to estimate the heights of the mountains and found them to have mean slopes around 8 degrees and heights around 300 meters, none (of 50 she measured) over 600 meters, over a mean basal diameter of 5 to 25 kilometers. So far this is just sort of basic fact information, not Earth-shattering, but of course it's significant to get any topographic information at all from Titan because the highly scattering atmosphere prevents us from using shape-from-shadow to figure out topography as we can do on all other solid surfaces in the solar system except Venus. I expect that the story of topographic information that you can get from radar data will get much more interesting as they get more overlapping data between radar and the imaging and spectrometers -- and as that data enters the public domain so that the different instrument teams can start looking at each other's data. (I thought it was very interesting that Radebaugh's was the only one of the radar talks that employed Cassini imaging data as context for radar data -- everyone else was using the crude Hubble albedo map as their base map. Cassini folks don't share their data as much as rover folks do!)

Guiseppe Mitri presented an interesting modeling study where he asked the question: Are the observations of atmospheric methane relative humidity and thunderstorms/cloud frequency consistent with a desert planet containing tiny fractional lake coverage? (here's the abstract.) According to his calculations, he said, a 50% relative humidity of methane in Titan's atmosphere could result from lakes covering only a small fraction, 0.2 to 4 percent, of the surface. (This was assuming "tropospheric overturning scales of 10 to 100 years" but I don't know what that means.) I also noted that his calculations implied that if such lakes exist, they evaporate at a rate of 3 to 10 meters of elevation per year.

One talk later in the Titan session that I thought was pretty interesting was the one given by S. Rodriguez (I did not catch his first name), who was attempting to use Huygens DISR spectral information to correct for the atmospheric scattering that is giving the VIMS team such difficulties in trying to pull compositional information out of their Titan data (here's the abstract). According to his talk, he claimed to have some success with this approach. Here's the deal: typically, when you want to find out information of the spectral properties of a surface -- how its reflectivity changes as you go from one wavelength to another -- you calculate a ratio image, where you divide an image taken in one wavelength from an image in another wavelength. You can do this because digital images actually are really just grids of numbers, each pixel represented by a number. What's cool about calculating a ratio is that the ratio usually removes the effects of any process that changes the brightness but not the color of a surface. For example, when the Sun illuminates a surface, it makes some parts look brighter and some parts look darker because of shadowing, across all wavelengths. When you calculate a ratio, you divide out this effect of light and shadow, and you're left with color differences.

However, when you calculate ratios for Titan using VIMS images, the ratio images look awfully similar -- with the same bright and dark patterns -- to the original images. That indicates that there is something going on that can't be canceled out by the ratio. Rodriguez argued that what's going on is atmospheric scattering that is adding brightness at all wavelengths, and you can't calculate out this additive component with a ratio. He was able to use the Huygens data to get an estimate of what this additive component might be, and when he subtracted that component out of the VIMS image of the Huygens landing site and then calculated VIMS image ratios, suddenly those bright and dark patterns disappeared, and instead he started seeing different spectral units pop out (that is, regions that have different relative brightnesses in different ratio images). It seemed pretty impressive. In one of the units that he mapped, he argued that he identified the spectral signature of water ice. I honestly don't know if other people in the room agreed with or disagreed with his methods and conclusions, but it was an interesting presentation anyway.

So, so much for Titan. All in all, there wasn't a lot that was new, either new data or new insight. I wish now that I'd skipped the latter part of the Titan session and gone to the rovers, but it was too late for that. The radar story will get more interesting, because after a long hiatus in the acquisition of radar data they are going to be getting a lot more radar passes beginning with the T13 flyby on April 30, so there is much to look forward to there.

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