[X] My Assistant

[Guest_AlexBlackwell_*]

An interesting (and topical) paper in press with the journal Planetary and Space Science:

Titan's methane cycle
Planet. Space Sci., In Press, Corrected Proof, Available online 25 July 2006
Sushil K. Atreya et al.
Abstract
Preprint (451 Kb PDF)

[Mongo]

Yesterday I read a very interesting paper on the variations in cloud cover over Titan during the last eight or nine years. Unfortunately I rather stupidly forgot to bookmark it, and the webpage seems to not be in my browser history. Perhaps somebody else knows the paper I am talking about.

The gist of the article was that low cloud cover has almost always been one percent or less, with two known exceptions. The most recent was a massive cloud outbreak over the south polar region that lasted for a month or so, which was in its final few days and had already largely dissipated when Cassini took its first distant encounter images. You will recall that there still was significant cloud cover in the imagery from that first distant flyby, but the cloud cover would have been much more extensive a couple of weeks earlier. This would have been right around the time of maximum insolation over the south polar region.

The earlier outbreak happened one-quarter of a Saturnian year earlier, centered over the equatorial regions, and was HUGE. Evidence suggests that this may have been two separate outbreaks, located nearly opposite each other near the equator, with one outbreak peaking about a week after the other, in which case the three outbreaks would have each covered 8-10% of the disk of Titan. This earlier outbreak(s) happened right around maximum insolation over the equatorial regions.

Combined with other papers, we start to see the possible basics of Titan's atmospheric methane cycle.

It seems to me that the Titan atmosphere is normally 'unsaturated' with methane, and that evaporation from surface lakes, etc. gradually increases the partial pressure of CH4. Eventually, the partial pressure is sufficiently high that convective overturn can happen in areas with high insolation, and enormous 'cloudbursts' form. I have read them described as similar in intensity to desert cloudbursts, but instead of lasting for an hour or so, they last for a month or more. (This reminds me of the story of Noah and the '40 days and 40 nights' of rain -- should we call these mega-cloudbursts 'Floodbursts' or something of that sort?)

This amount of precipitation would be enough to fill many of the dark depressions we see -- the recent lakes in the north polar region would have presumably been filled during the north polar floodburst, and possibly by runoff from the more recent equatorial floodbursts. The reduced temperature since the equatorial floodbursts would presumably have slowed the rate of evaporation in the northern polar region enough for the lakes to still be partially filled. Ontario Lacus, on the other hand, has presumably just been refilled, and its surface would currently be evaporating at a a fairly rapid pace, compared to mid-winter, half a Saturnian year from now.

So Titan's surface may well experience infrequent (once every 30 years in the polar regions, once every 15 years in the equatorial regions) but lengthy, intense precipitation, with nothing much in between. What would the effects be on erosion rates? I know that in the Grand Canyon, almost all the erosion occurs during the highest flow rates of the Colorado River. It could be that extreme concentration of precipitation on Titan might result in a long-term erosion rate as high as, or possibly higher than, what we see on Earth. In spite of the much lower average energy available on Titan, if it is stored up in the atmosphere over several years and released all at once, it should have considerable erosive power.

Bill