Titan: brown and blue
Posted by Emily Lakdawalla
28-08-2007 11:24 CDT
One of the things that's been clogging my email inbox (which is down to 250 items, so I've been making some progress) are a pile of articles about Titan from a special issue of the journal Planetary and Space Science that I requested from various scientists about three months ago. ESA issued a press release about these when they were published in June, and still I didn't get to them. With our website down yesterday, I was able to sit down and start doing some reading. Many of these represent the end product (or at least an end product) of work that I've written about before, when the researchers presented progress reports on their work at various conferences.
Larry Soderblom's name was first on two articles. One was on the topography and geomorphology at the Huygens landing site, which they are deriving by doing stereo analysis of images taken by Huygens' DISR imaging system. Reading this article, I was reminded of the extremes of luck that smiled upon -- and plagued -- Huygens. In the former category, Huygens was extremely fortunate in that as it drifted down to its landing site, it just happened to emerge from the haze right over a major boundary between Titan's bright and dark terrain, affording incredibly lucky views of drainage channels dissecting the bright highlands, emptying into a dry, dark basin. In the latter category, the loss of one of Huygens' two data channels, and the nearly vertical descent (rather than the horizontal drift predicted by Huygens' designers), resulted in there being very few pairs of images useful for stereo analysis. The paper presented some digital terrain models that were improved slightly from ones presented at past conferences, and also made the interesting point that the flat region in which Huygens landed has experienced runoff from those drainage channels but has also experienced what appears to be "major west-to-east floods across the plains with flow parallel to the highland-lowland boundary," an event that occurred in a wetter period in Titan's recent past.A related paper, on which Larry is also first author, discusses some possible explanations for why -- other than one set of dark dunes that can be matched -- "there is remarkably little correlation between features seen in the [Cassini RADAR] SAR images and in DISR." Larry's favored model goes like this. Optically, we see bright regions and dark regions on Titan. In infrared wavelengths, you can see evidence that the dark stuff has more water ice than the bright stuff. (Read that sentence carefully. Most people assume that the bright stuff is water ice, but it's the dark areas on Titan that are more ice-rich.) But if you look at Titan with a spectrometer like Cassini's VIMS, you find that the dark regions come in two varieties; one is richer in water ice than the other. The water-ice-richer dark stuff is referred to as "blue" by the VIMS team because that's what color it appears when you make an RGB image from VIMS data using the 2.0, 1.6, and 1.3-micron channels as red, green, and blue respectively. The water-ice-poorer stuff appears brown, not blue, in the same color composite. Larry and his coauthors show that the brown areas (relatively poor in water ice) "correlate strongly, if not uniquely, with the vast dunes seen in Cassini RADAR images." So, they say, the dunes are probably made of hydrocarbon and nitrile stuff with less water ice mixed in than the non-duney dark plains (the blue areas). The bright regions are probably dark underneath, but are mantled by "bright aerosol dust." This mantle looks bright to VIMS (and DISR) but might be transparent at much longer radar wavelengths, which is why the correlation between VIMS and RADAR images is often not very good. In the drainage channels the bright mantle has been eroded away, leaving behind dark channel floors. If true, this too was a fortunate circumstance for Huygens. The dark floors of the channels are what make them leap into view from the DISR images; on hazy Titan, there is virtually no topographic shading to tell us what's up and what's down.
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