[X] My Assistant

[Guest_BruceMoomaw_*]

http://www.planetary.org/blog/article/00000495/ :

(1) "Ralph Lorenz gave a talk about the dunes on Titan. 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."

Lorenz also says -- in both his LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1249.pdf ) and his similar EGU abstract ( http://www.cosis.net/abstracts/EGU06/09468/EGU06-J-09468.pdf ) -- that longitudinal dunes only occur in places where the wind speed fluctuates a lot, which says very interesting things about Titan's weather.

(2) "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? 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."

Mitri added ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1962.pdf ) that "The rapid shoreline changes predicted here suggest that repeat Cassini observations of putative lake-like features should be performed at intervals of a year or more: any shoreline changes would provide evidence that candidate lakelike features are actually lakes." The prime target, presumably, should be Ontario Lacus. (I imagine that "tropospheric overturning scales" refer to the average time between a methane molecule evaporating from surface liquid into the air, and its raining back again to the surface as liquid. The longer that time is, the more you can combine high atmospheric humidity with a small amount of liquid actually on the surface. Titan looks more and more like an eerie cryogenic parody of the American Southwest, where its "sands" and its rare but violent rainstorms are concerned -- but its reasons for that storm pattern are somewhat different.)

[Guest_BruceMoomaw_*]

Actually, in neither abstract did Lorenz say just what he meant by "fluctuating".

As for Titan's rainstorms, there has been a suspicion for a long time that the combination of high tropospheric methane humidity with infrequent rain clouds can be explained by the startling but true fact that Titan's atmosphere, smog or no smog, is on the whole much CLEANER of solid particles than Earth's is. The only reason Titan's atmosphere is opaque in the visible to outside observers is that, while its smog content is very low, it towers up to such an amazing height above the surface. Earth's air has far more dust in it -- not only picked up from dry land, but salt particles spattered off ocean-wave foam and then drying out in the air.

Thus, with fewer solid nuclei around which a saturated vapor can condense (water in Earth's case, methane in Titan's), Titan's methane tends to remain gaseous until it rises to a REALLY high humidity level in some local place -- at which point droplets finally start to condense out anyway, and themselves serve as the nuclei for very rapidly mushrooming big drops of methane that then thunder out of the sky in a brief but violent cloudburst.

[Guest_AlexBlackwell_*]

Actually, in neither abstract did Lorenz say just what he meant by "fluctuating".

I didn't read the two abstracts you referenced. I thought maybe you had seen his T11-T14 TOST RADAR presentation which, as I quoted, did use that specific term. I guess great minds think alike, huh?

In any event, his TOST presentation had the Shuttle-based imagery of the Namibia dunes. They are strikingly similar to the ones on Titan.

This post has been edited by AlexBlackwell: Mar 17 2006, 01:02 AM

[Guest_BruceMoomaw_*]

In any event, his TOST presentation had the Shuttle-based imagery of the Namibia dunes. They are strikingly similar to the ones on Titan.

They sure are. Take a look at the pictures in the LPSC abstract -- you literally cannot tell them apart.

[The Messenger]

They sure are. Take a look at the pictures in the LPSC abstract -- you literally cannot tell them apart.

If it looks like a duck...

[Guest_AlexBlackwell_*]

If it looks like a duck...

Nick Hoffman, who has pushed (variations of) his White Mars model for several years now, has an amusing quip: "For those who persist in saying that if it walks like a duck and has a beak like a duck, then it must be a duck, I suggest you read up about the platypus. These marsupials even lay eggs but they don't quack!"