Titan's methane cycle Options

[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)

[Guest_AlexBlackwell_*]

There are a few Titan-related papers in the July 27, 2006, issue of Nature. For links, see the Editor's Summary: It's raining methane.

[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

[Guest_AlexBlackwell_*]

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. Perhapse somebody else knows the paper I am talking about.

Are you referring to one of the recent papers that Mike Brown wrote or co-wrote?

Interesting post, Mongo. I'll respond to the rest a little later.

[Mongo]

Are you referring to one of the recent papers that Mike Brown wrote or co-wrote?

Yes! In particular, to this one.

Thanks for remembering who wrote that paper, from a rather general description.

Bill

[Thorsten]

There are a couple of nice publications on Titan’s exotic weather, including the possibility of a persistent methane drizzle that might reach the surface and the possibility of severe methane connective storms accompanied by intense precipitation, comparable to flash flood events on Earth.

http://www.nature.com/nature/journal/v442/...ature04933.html

http://www.nature.com/nature/journal/v442/...ature04948.html

See also the News and Views

http://www.nature.com/nature/journal/v442/...ll/442362a.html

After having seen all those superficially Earthlike features, like weather and storms, rivers and lakes (?), hills, mountains, possible volcanoes and great sand seas, I almost forgot how “enticingly alien” and “intriguingly foreign” this world really is. Great that there is a place around like Titan.

[Guest_AlexBlackwell_*]

There are a few Titan-related papers in the July 27, 2006, issue of Nature. For links, see the Editor's Summary: It's raining methane.

These results were published online yesterday in JGR-Planets as part of a special section entitled "Coordinated Earth-Based Observations of Titan During the Huygens Mission."

For those without access to Nature and JGR-Planets, Emily has a nice summary.

[Guest_AlexBlackwell_*]

For those without access to Nature and JGR-Planets, Emily has a nice summary.

Here's a EurekAlert press release on one of the papers in Nature.

[Mariner9]

If this theory of periodic precipitation proves correct, is there any indication just how long we might wait until the next cloudburst? 15 years and 30 years were mentioned for different regions, but was the south pole cloud patch noticed early in the mission part of that predicted cycle? Or do smaller, and shorter lived cloud formations, occur more frequently?

What I'm getting at, is if this is true, what are the chances that Cassini will live long enough to see the next strom?

[elakdawalla]

Cassini has seen clouds that were gone later; I think that's the only data you can use to infer storm activity unless the VIMS team's recent detections of surface changes have something to do with storm activity (see the thread on the EuroPlaNet abstracts). I suppose you could also look for changes in drainage patterns in two overlapping SAR swaths but the amount of overlap they'll get over the course of the mission is pretty small and I worry also that the difference in viewing geometry between the two overlapping swaths will always make you wonder whether you're seeing actual surface change or just features that look different when the radar look angle is different.

It's actually easier to monitor long-term cloud formation and dissipation from Earth than it is to do from Cassini, at least for relatively big cloud systems; the JGR issue has a couple of papers from Imke de Pater's team using the Keck II telescope to do that. But you can't actually see rain falling with either Cassini or Earth-based telescopes.

--Emily

[Mariner9]

I wasn't expecting Cassini to actually see it rain on Titan, anymore than I think Earth orbiting sattelites see it rain on Earth. I was just wondering if the lakes on Titan appear to fill and then drain over time, if Cassini's mission might last long enough that it would possilby pass over another area which was still 'wet' after a storm.

I suppose as you say the best evidence would be a before and after shot with the radar, but from the diagrams I've seen it doesn't seem that many of the swaths overlap. So that seems very unlikely to happen unless such a thing was deliberately planned during the mission extension.

And speaking of deliberately planning to refly over a specific area, I can already guess where a lot of people are going to want to get more radar maps during the mission extension.

[ugordan]

Cassini has seen clouds that were gone later;

Speaking of seeing the clouds, here's an image Cassini took in October 2004, showing the south polar clouds are pretty well visible even in the RGB images. The image on the right was enhanced. Taken from a distance of 5.4 million km.
There are also some slight hints of the surface features at mid latitudes -- they appear slightly greenish in the visible images.

[volcanopele]

That's the giant cloud burst from early October 2004. A good chunk of the south polar region was covered in this one giant cloud field.

There are two additional clouds in the CB3 image from that set. One in a usual location and the other in a most unusual one (the problem being that we only saw it once so have no info on speed, altitude, and how that cloud evolved).

[ugordan]

There are two additional clouds in the CB3 image from that set.

I haven't even realized there was a CB3 frame. I assume you're talking about two streaky clouds, one near the terminator and one closer to equator, above western Xanadu if I'm not mistaken. The polar cloud is really huge.

[volcanopele]

hehe, that's the one. Almost every flyby I've been looking out for another cloud in that area or an cloud in that latitude range, but we have not seen it repeat. It has never been seen by ground-based observers. So there really isn't that much one can do with this. Maybe the cloud field and and the streak over Xanadu are related.