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)

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.

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

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.

Yes! In particular, to this one.

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

Bill

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.

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.

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?

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

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.

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.

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).

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.

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.

Well, what are the best models for Hadley cell formation on Titan, under current seasonal conditions? Perhaps the streak over Xanadu defines the northernmost edge of a cell which converges with other cells at the south pole?

I would expect to see clouds forming at the peripheries of such circulation cells -- perhaps this is what is happening on Titan.

-the other Doug

There is a nice illustation of methane rain on the Astronomy Picture of the Day site:

http://antwrp.gsfc.nasa.gov/apod/ap060802.html

I would lose the lightning and maybe lose the hills. The area with the lakes actually appear quite flat. I would also wouldn't call the researchers on those rain studies "astrobiologists", astrometeorologists maybe...

I have a feeling like someone different is doing APOD this week. A couple captions this week have been just ever so slightly... off. Like this one; I'm still trying to figure out what it means.

Maybe they will get progressively wacky through the week, and by Friday we'll end up with pictures of grey aliens, flying saucers and pyramids "possibly built by ancient astronauts?" ... that would be funny :-)

It is unusual to have so much light in the foreground, and still be able to see the milky way - the glow of the milky way looks like front-lit background clouds - which clearly places the stars somewhere in between.

DUCK!

Edited to add:


...I would also lose the rain streaks, that seem to indicate forground lighting. That doesn't leave much of an image

That painting is uncomfortably (perhaps intentionally?) reminiscent of primordial Earth scenes intended to depict the origin of life. But, heck, I can't blame them; if there's one thing NASA needs to learn to do better, it's marketing!

The photographer used a flashlight to paint the rocks during his long exposure. If you do it fast enough, you don't see the "trailing" of the rocks.

Sure, there are too many hills and too much lightning, compared to real Titan – and the rain clouds would probably be much, much higher in the sky (stratiform methane clouds form 21 – 30 km above the ground) but I really liked the illustration by Mark Garlick. Especially the darkness that one would experience on Titan in the middle of a methane storm and the fierceness of this “methane monsoon” is nicely accentuated. However, since raindrops would fall at a snowflake-like pace through the thick atmosphere of this low gravity moon, it would probably be some kind of “slow-motion rainstorm”.

I have read somewhere that our knowledge on Titan at the beginning of this century was in many aspects comparable with our knowledge on Mars and Venus back in the ‘50es. Think of all the great Pre-Cassini illustrations - such as this one or the ones from Don Dixon:

http://www.cosmographica.com/gallery/portf...0-TitanLake.htm

- on Titan’s lakes. Now compare them with Pre-Mariner illustrations of Mars, showing rivers, emanating from water-ice pole caps:

http://www.astrosurf.com/lombry/Images/mars-bonestell1.jpg

or plains seasonally covered with primitive vegetation or of Venus, portrayed as misty, tropical paradise. I think the Titan illustrators didn’t do such a bad job. (Or Titan did a fairly good job in - at least partly - providing what we originally intended to find).

Edit: Couldn’t help posting this great illustration – again from the unforgettable, unrivaled Chesley Bonestell (1888 – 1986). Note the dust devils und barchan dunes to the left and the Martian vegetation to the right.

Sorry to revive a dormant thread, but I couldn't decide where to place the link to this newly published paper in JGR-Planets:

Voss, L. F.; Henson, B. F.; Robinson, J. M.
Methane thermodynamics in nanoporous ice: A new methane reservoir on Titan
J. Geophys. Res., Vol. 112, No. E5, E05002
10.1029/2006JE002768
04 May 2007
Abstract

Now that is very interesting. I know we have discussed the possibility of Titan's surface materials having a nanoporous structure on this forum a number of times over quite a long period. Is this the first professional paper to look at the possibility? I do not have access to the full text. Do they mention the possible implications of such material structures for their IR spectra?

My experience, with chemicals that have highly polar sites but high melting points says this is both possible and unlikely. I am talking about powders that, due to high polarity, are slightly soluble in water. In this very low temperature analog, flaky water-ice would be the powder, and methane a hydrophobic 'liquid' which travels through the snowball matrix.

The problem, is the that even substances with high melting points, but also high polarity, fuse into solids when there is any significant pressure placed upon them at room temperature. The bouyancy provided by methane might slow the process, but one should not expect a crust made out of frozen water-ice to remain porus to any significant depth for significant periods of time - water under pressure will more likely form 'obsidian' than 'pumice'.

Basically the paper is modifying the initial Titan climate model put forward by Lorenz et al (yup, our own rlorenz) by including nanoporous ice.

The idea is that some of the water-ice making up the surface-accessible volume of Titan has little tiny pores in it. These pores make the water-ice behave like a sponge to suck up methane.

WARNING: Chem nerd digression:
[This is different from a clathrate. A clatharate can be thought of as a single molecule fitting teeny tiny pore, just big enough to fit a methane molecule, only a few Angstroms across.

Thier definition of nanoporous water ice really big pores on a scale of 4 nm. Small to us to huuuge to a small molecule (and methane is a very small molecule) . Molecules wander into the pore space, and bump around quite a bit, getting lost, before eventually finding their way back outside of the material.

Biologists use nanoporous material to purify proteins using size-exclusion gel chromatography. The way this works is that big proteins go whooshing down the column, while little molecules stop to wander into the pore and poke around in every nook and cranny. Big proteins come off the column quickly, but little molecules seem to take thier own sweet time.

http://en.wikipedia.org/wiki/Size_exclusion_chromatography

Think of a clathrate as a car (methane molecule) parked in a single car garage, and a nanopore as a huuuge mulitlevel parking deck with lotsa cars driving around trying to find their way out. ]



The experimenters modelled ice formation at low temperature and found that it does indeed make nanoporous structures. (Distribution peaks at 4 nm, with most less than 20 nm). Perhaps this happened during water redeposition at low temperature followign impacts (their claim) or maybe it happens during cryovolcanic action (airfall of ice pumice - my opinion).


As the methane is sucked up, the nanopores fill up, smaller ones first, then the larger. This potentiates the amount of methane in the atmosphere. This buffers any early greenhouse effect of methane. They claim that in the past, methane was in a suface ocean, then was slowly sucked down into the pore structure of the regolith. [Maybe this happens as a cycle?]

According to the authors the nanoporous ices give "a reduction of the dependence of methane vapor pressure on surface temperature". This also means that "the system is stable to perturbations in methane vapor pressure" More methane pressure, more methane gets sucked up into the larger pore spaces. Methane pressure drops, and methane comes out of the larger pore spaces first, then out of the smaller ones next.

So the regolith acts as a methane sponge to buffer atmospheric methane concentration.

How much? The authors base their calculations on assuming that 1.5% of the 10 km deep regolith is nanoporous. (1.5% of 8.3E20 kg regolith ice)

This makes a better idea (IMHO) for a large methane reservior than clathrates or interparticle spaces. I'm still obsessed on the idea that the Equatorial Sand Seas are a methane reservoir. I'd need to crunch through the numbers, but I'd guess that while clathrates accessibility would be small for the Equatorial Sand Sea volume, that the interporous space amounts might match nicely with interparticle space available in the Equatorial Sand Sea volume. Plus, the nanopores help explain why eventually the methane doesn't all flash to vapor and escape, kinetically the molecules get stuck in the pore spaces.

-Mike

Sadly, I would guess that like for methane-ice clathrates, the IR spectra would still look like methane+water.

I have no idea if there is a way to easily spectroscopically determine pore sizes. The authors of this paper measured pore size based on experimental absorption of liquid Ar into vacuum deposited water ice.

-Mike

I've got no problem with the pebbles actually being water ice, especially since now we have the suggestion that the bright highlands have a non-water ice signature because they are dusted with as-yet-undarkened bright 'smust'. They could still be water ice underneath and be the source of the ice (or ice-pumice) pebbles.

As a separate issue, I'm sure you are right in your statement above as long as the whole spectrum is available for inspection. However I do think that when only a few frequencies are available and the study boils down to ratios between the intensities at these frequencies the conclusions could be vulnerable to enhancement or suppression of certain wavelengths due to an echelon effect when the grain (or bubble) sizes correspond with one of the observed wavelengths. Some foams have remarkably uniform bubble sizes and I'm sure the same is true of some particle deposits.

Separate again: I like the idea of materials that can soak up a lot of liquid before getting 'wet'. I proposed this for the smust, now the nanopores paper proposes it for the ice. Perhaps it is true for both. That would make for a very interesting methane cycle indeed.

On porous ices not remaining that way for long due to pressure fusing in polar solids even far below the melting point - I take the point and I'm glad to know it, but I'm really not sure if it necessarily applies under Titan surface conditions. I'll watch out for that one though.

More cloud activity on Titan .....

"These cloud streaks are near the same latitude as similar clouds observed above different longitudes on Titan. "

http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2645

Nice ....

Craig

I *FINALLY* got photoshop back on a computer at work, so I can load a picture, export a ".raw" file and load it into our industrial 32 bit image processing system.

Here's what I've pulled out of the two recent titan-with-clouds images... lucky this new one showed up today.

I'm doing a "band-pass" filtering where contrast at different spatial frequencies are more or less equalized. Often very effective.

Missed one... the really GOOD shot of the north-polar sea.

cool north-polar clouds! never seen them so clearly
which flybys were these pictures?

It's amazing what you've managed to do with those, edstrick. I presume the last one is prepared from the 'Seeing Further North' image.

I'd have to trace down the PIA#### numbers on Planetary Photojournal and read the captions. I was in a hurry with limited time and just searched "titan" or "Cassini" and grabbed likely candidates.

Tonight or tomorrow, I'll post a few of the northern clouds that processed-up nicely... and the new Venus pic from Messenger.

It looks like the processed image formed by differencing raws at different wavelengths that was featured on CICLOPS fairly recently. I'd be amazed if you had pulled all that detail from a single raw image. (I'm amazed enough anyway!)

I'm not using "raw" images.. I can get a lot out of them, but the jpeg-type artifacts are hideous. I grabbed the "tif" format images from the photojournal. Since these are pre-processed press release images, the've at least had a lot of image crud removed, even if they aren't enhanced. Some are, more than others.

here's the jpg (I made it from the tif to save space, rather than go back to the photojournal for the official jpg version)

Here's 3 pairs of jpgged tifs and the enhancements made from the tifs. different view of the north polar hazes. First one's bigger than the other 2 pairs.

This is the difference image I was talking about:
http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2587

You mean this one? (enhancing it flattens that brightness hump in the polar collar area)
And they posted a new higher resolution pic on the Planetary Photojournal today (still Friday in Tx)

My eyes keep wanting to organize this feature into a ring... degraded breached crater anyone? (very speculative ID)

...WOW, Ed!!! The "Caspian Sea" is very clear in your left image!!! Puzzled why there seem to be high northern latitude clouds in the same image, though, and no evidence of the southern-lat clouds as seen in the right pic...are these from the same rev? (Sorry; gotta ask.)

EDIT: Disregard; for some reason, the board hadn't refreshed for me beyond your post #30. This is groundbreaking stuff, Ed; congrats! The North Polar Sea looks almost like a stylized, rounded, softened Chinese ideogram, such incredible complexity, doesn't even look fractal...

FYI: looked up the flyby numbers
post 30: left: T28
right: T30
post 31: T25
post 39: left: T27

" looked up the flyby numbers"

Thanks, much obliged.

New Titan 1 km/pixel ISS image posted a few days ago.. northwestern Shangri-la, taken May 13
Here's the enhancement