was going on with that brightening. It's not hydrocarbon "snow," Jason said, because the haze can't fall out fast enough. So that bright material must represent the delayed deposition on the rain-wetted areas of some substance that can either sublimate or blow away. Jason said that it could be just about anything that has fine enough grains to appear white to the imaging spectrometer, which sees at a wavelength range of 1 to 5 microns. He advanced a plausible hypothesis: the rain wets the surface, darkening it, and ponding in some places. Rain falling on the uplands runs off to valleys, where it can saturate the soil. Wind blowing across these saturated surfaces will evaporate, and evaporation causes cooling. It might just cause enough cooling for methane frost to form on the surface, brightening the places that were darkened previously. But, he said, this is only a hypothesis, and asked the community to come up with quantitative versions of this model, or laboratory tests.
There was further discussion of rain on Titan today, in an interdisciplinary "planetary hydrology" session incorporating results from both Mars and Titan. Antoine Lucas showed evidence from Cassini RADAR images that there are "knickpoints" in Titan's polar rivers and terraces hidden beneath the waves of Titan's north polar seas that suggest that there have been recent changes in sea level beyond the cyclical, seasonal variation.
Ralph Lorenz talked about rainfall rates on Titan. Actually, though, the best part of his talk was right up front, when he addressed the criticism that some people don't like the adjective "Earth-like" being applied to Titan. Titan's surface geology bears strong superficial similarities to Earth's despite the huge differences in composition and temperature. So, Ralph presented a different way of looking at it, in the context of this interdisciplinary session on hydrology on Mars and Titan: "Mars is the second most Titan-like world in the solar system."
That particular session ended with a presentation by Jeff Moore, who challenged the room with a hypothesized Titan climate history that differs strongly from the situation that holds today. He called it the Triton-Titan-Mars hypothesis, and the central conceit was: what if Titan hasn't always had a thick atmosphere? What if it used to look like Triton (with all its volatile components locked in surface ices), and some relatively recent event -- the long-term brightening of the Sun, or a large impact -- suddenly triggered Titan's methane to sublime to gas? Geophysically, the results could be quite interesting. FIrst of all, you'd have massive global methane rainfall, carving valleys everywhere.
More importantly, though, the thick atmosphere is an insulator. An airless world like Triton has surface temperatures that are much warmer at the equator than the poles, a difference of about 25 Kelvin. But Titan, with its methane blanket, has nearly constant temperature everywhere. That sudden warming of the poles would actually thin the icy crust, which would then sink as a result of isostasy. (Why that is and what isostasy is, are subjects for another blog entry; just take my word for it that warming icy crust thins it and lowers its elevation.)
So you'd suddenly have a global topographic gradient from equator to poles; all that rain, eroding bedrock and washing away sediment, would run from equator to poles, carrying sediment with it, depositing it in the polar depressions. As time went on and more and more of Titan's methane rained out, it would stop raining at the equator first, with Titan dessicating slowly from equator to pole. So you'd get highstanding, valley-carved mountains near the equator; badlands-style water-carved topography at midlatitudes; and sedimentary deposits and lakes in lowstanding poles. Another thing that would happen as the poles sank is that you'd see compressional tectonics -- faulted blocks rising -- and near the equator you'd see extensional tectonics -- pull-apart basins and mountain ranges.
Like all of Jeff's stories, there are intriguing matches between 