[X] My Assistant

[nprev]

Well...I'm definitely no expert, but here's my 2 x $1.0^-2...

I think that the equatorial & temperate latitudes of Titan are, generally speaking, almost exactly at the point where liquid methane is not a permissible phase, atmospheric characteristics also considered. The air is saturated, but the surface here can't support anything but rare puddles; equilibrium has been achieved. The ground is soggy with bound methane due to undetermined processes (largely because we don't know what smust is really made of). Precipitation occurs only at the poles in the current era, where temperatures are low enough to allow methane to condense.

Simplistic, I know...but what the heck! (Just finished a round of finals, feelin' good...)

[Juramike]

Some high nerdliness on a Friday night: How much methane could theoretically be held in a type I clathrate structure in the equatorial seas of Titan?

(Someone check my math, it is not my strong point - this is why I'm a medicinal chemist)

Equatorial seas of Titan are conservatively estimated at about 10% of the surface area (25% was estimated by Barnes et al. I used a low-tech method of cutting out trace pictures of the equatorial seas, weighed them on a mg balance, and compared with the weight of a trace cutout of the global projection - polar surface will be overrepresented)

[Juramike no have cool-o image software on computer, Juramike paid to make potent enzyme inhibitors chop chop.]

Titan surface area x 10% = 8.3 x 10^6 km^2 = equatorial sand sea area.

Assume 100 m average depth, so : 0.1 km (100 m average depth) x 8.3 x 10^6 km^2= 8.3 x 10^5 km^3 equatorial sand sea volume.

Assume 10% is available so: 8.3 x 10^4 km^3 ice is available.

Assume cage filling efficiency of 10% so: 8.3 x 10^3 km^3 ice is available and filled.

8.3 x 10^3 km^3 x [1 x 10^12 cm^3 / km^3] = 8.3 x 10^15 cm^3 ice is available and filled.

Assume ice density is 1 g/ cm^3 (y'all cut me some slack here).

8.3 x 10^15 cm^3 ice = 8.3 x 10^15 g ice is available and filled.

8.3 x 10^15 g x 1 mol H2O/18 g H2O = 0.46 x 10^15 mol ice available and filled.

0.46 x 10^15 mol ice x 1:5.75 guest (methane): host (ice) ratio = 0.08 x 10^15 mol methane incorporated.

0.08 x 10^15 mol methane incorporated x 16 g methane/mol methane = 1.28 x 10 ^15 g methane incorporated.

= 1.28 x 10^12 kg methane is potentially incorporated in the equatorial sand seas.

Total atmospheric volume of Titan is 1.3 x 10^19 kg.

Titan's atmospheric volume of methane is 5% or 6.5 x 10^17 kg. The sand seas could only take up 0.00015% of this.

So if this is all correct: The equatorial sand seas could be full of methane clathrate without affecting the amount of atmospheric methane. They are not a significant methane sink.

[Juramike brain hurt now.]

-Mike

[JRehling]

Precipitation occurs only at the poles in the current era, where temperatures are low enough to allow methane to condense.

Voyager showed (dark in VIS wavelengths) that polar ring on Titan, at around 80N.

Early Cassini orbits showed (bright in IR) clouds tracing a very similarly placed ring around 80S.

I don't know if the two rings were similar or the same in nature, but they lead me to think there's something special about 80N/S as opposed to 90N/S. It could just be that a Hadley cell is dumping warm moist air into a polar collar, where the rain tends to happen.

Seen that way, things are a little different. The polar surface temperature, even stratospheric temperature is not the point, but that "warm" humid air is cooling and thus raining out, the way a cold front racing across North America (especially) or Europe tends to do, or warm coastal air at a mountain range. If this is correct, Titan's (winter?) poles take the place of the mountains.

[remcook]

"Early Cassini orbits showed (bright in IR) clouds tracing a very similarly placed ring around 80S."

i cannot remember this. or do you mean these massive tropospheric storms? I didn't think they formed a ring...the voyager collar (also observed by Keck and Hubble half a Titan year later) is stratospheric (if I remember correctly somewhere between 60 and 150 km).

"I don't know if the two rings were similar or the same in nature, but they lead me to think there's something special about 80N/S as opposed to 90N/S. It could just be that a Hadley cell is dumping warm moist air into a polar collar, where the rain tends to happen."

...but the VIMS ethane cloud and polar 'mammoth' cloud seem to go all the way to the pole.

[Webscientist]

The analysis on interactions between methane and ethane is a good basis for my thought. I fear that the seas of Titan's north pole have an ugly, dark appearance, especially if uv radiations reach in large quantities the surface. The panoramic images of the Huygens lander tend to show that the dried up rivers appear dark. So...
But, the debate is not closed, anyway!
Regarding the equatorial regions, there is a major factor which is likely to keep lakes from forming or to accelerate the evaporation process.The dominant winds which blow from the west to the east seem to be quite strong due to the way they carve the landscape ( dunes), especially close to the equator. The impression of coldness would be really terrible for a human being with this wind ( below -180 degrees celsius)!
I also think that the hadley cell movement must play a key role in forming this "mammoth cloud" or hurricane in the north pole. It seems logical.

http://www.titanexploration.com

[tedstryk]

Wow... you don't want to know how long it took me to figure out "rlorenz" - I need a vacation

[ngunn]

Precipitation occurs only at the poles in the current era, where temperatures are low enough to allow methane to condense.

At altitudes of tens of kilometres the atmosphere is cold enough to condense methane even at the equator. If (for any reason) there were a sudden, major localised release of subsurface methane somewhere near the equator I would expect it to produce a localised deluge and catastrophic flash flooding. There could be other special reasons for occasional equatorial rainstorms too, but I agree they are probably very rare. The Huygens landing site looks like such a place to me. It could have got the way it is hundreds or thousands of years ago, but not (I'm guessing) significantly longer than that or dunes would have started invading the area and covering the pebble-banks. I'm hoping that some day there will be a way to date these events.

[nprev]

Yeah...Titan is the first place we've been where measuring surface age is pretty much impossible. I'm sure the evidence is there, but we just don't know how to interpret it.

[rlorenz]

The diversity of polar phenomena is intriguing (and in some cases confusing)
The dark polar hood seen by Voyager is indeed stratospheric. HST showed it to
have disappeared by the 1990s, but a southern counterpart appeared (most prominently
in the UV) and became visible around the turn of the century

http://www.lpl.arizona.edu/~rlorenz/polarhood.pdf

It disappeared around 2003, just in time for Cassini not to see it :-(

Now, the present north polar region of Titan has all kinds of interesting stuff
going on - detached haze seems to connect with a polar haze enhancement (only
visible in transmission, as these latitudes are still in winter shadow). The
polar hood material (C4N2 etc?) likely acts as condensation nuclei for ethane
(hence the big VIMS polar cloud seen recently) and thus, perhaps, the lakes

The methane cumulus convection clouds seen around the south pole in 2000-2004
are (I think) fundamentally unconnected with the polar hood (although the incident
sunlight and the condensation nuclei may be connected with the energy budget and
microphysics of the clouds) and are just a midsummer phenomenon in the troposphere.

"Early Cassini orbits showed (bright in IR) clouds tracing a very similarly placed ring around 80S."

i cannot remember this. or do you mean these massive tropospheric storms? I didn't think they formed a ring...the voyager collar (also observed by Keck and Hubble half a Titan year later) is stratospheric (if I remember correctly somewhere between 60 and 150 km).

"I don't know if the two rings were similar or the same in nature, but they lead me to think there's something special about 80N/S as opposed to 90N/S. It could just be that a Hadley cell is dumping warm moist air into a polar collar, where the rain tends to happen."

...but the VIMS ethane cloud and polar 'mammoth' cloud seem to go all the way to the pole.

[ngunn]

It is indeed confusing (at least it has been confusing me!) and it's wonderful to have some authoritative clarification on all this. Are you suggesting that the lake-forming winter rain is actually a drizzle falling from the stratosphere, or that condensation nuclei formed in the stratosphere are seeding the upper troposphere?

I can understand (I think) why the tropospheric cumulus clouds would be a summer phenomenon. That would be when there is undersaturated air passing over the lakes and damp ground, allowing forced evaporation to start density-driven convection cells. If I recall correctly these are pretty spectacular convection cells rising to more than twice the atmospheric scale height. Is it your opinion that these clouds produce precipitation too (summer showers?) or would they be essentially dry affairs, at least at ground level?

nprev -
I have hopes for radiocarbon methods. If for example we could date individual smust particles we might be able to discover the age of the most recent ones incorporated in a particular flood residue (I won't say mudflow or people will start wanting polygonal cracks).

[nprev]

Re determining the age of the Titanian surface: Have any impact craters been positively identified above the temperate (boy, is that a relative term here... ) latitudes? Seems like the fact that we've seen at least a few in the central area argues that erosion moves slower there, although we certainly need to get a better data set for the polar regions & consider the relative surface areas being compared...

Ngunn, that does sound like the most feasible strategy for ground truth...someday, when I'm probably at or below Titan mean surface temp in the next freezer over from Walt Disney. Sure would like to get some rough ideas from Cassini, though; we need to find a way to make some global estimates.

[rlorenz]

Are you suggesting that the lake-forming winter rain is actually a drizzle falling from the stratosphere, or that condensation nuclei formed in the stratosphere are seeding the upper troposphere?

It seems (and Bob Samuelson in circa 1997 came up with this picture, with only some tentative
evidence from Voyager) that the stratospheric condensates, including ethane, may preferentially
accumulate at high latitudes because of the downwelling during polar winter. Now, if the lakes are
only ethane, that's the end of the story.

But maybe - and the fun part here is that nobody knows yet - methane plays a role too. That
the ethane etc drizzle nucleates (methane) rain in the troposphere as well, and there may be
a seasonal accumulation (and later, evaporation) of methane in the lakes. Maybe ethane-rich
and methane-rich lakes prevail at different latitudes on Titan (or even different geological
epochs, depending on how methane is introduced [episodically ? by cryovolcanism?]) into the
system... We'll just have to see if we can find evidence of seasonal change in the lakes.

I have hopes for radiocarbon methods. If for example we could date individual smust particles we might be able to discover the age of the most recent ones incorporated in a particular flood residue (I won't say mudflow or people will start wanting polygonal cracks).

Well, 14C is only good for a few tens of thousands of years, rather than millions, but I'd have to agree
that just seeing if some organic junk is radioactively hot would be a good test of whether it just
came from the stratosphere... given the production rate (and only Earth and Titan are good for
radiocarbon..) it should have an activity of thousands of counts per minute per gram - see
http://www.lpl.arizona.edu/~rlorenz/radiocarbon.pdf


(btw don't expect me too often on this site - just happening to have first full weekend at home in a
month....)

[nprev]

May you have many more free weekends, and come see us when you can! Thanks for the bounty of information, sir.

[remcook]

"The polar hood material (C4N2 etc?) likely acts as condensation nuclei for ethane
(hence the big VIMS polar cloud seen recently) and thus, perhaps, the lakes "

CIRS doesn't see C4N2 yet like for Voyager. There is a hint of HCN ice though, also from VIMS.

[ngunn]

rlorenz thanks very much - understand you're busy doing the proper stuff but hope you can give us the odd steer in the right direction from time to time. I read that radiocarbon paper a while ago and it really opened my eyes to the possibilities. I posted enthusiastically here on the subject but it didn't catch on with our leading posters at that time, who took the view that it would be difficult to find a 'stratified' site free from downward contamination. I'm sure that by analysing small enough samples there will be ways of working around that. I suspect that quite a lot happens on Titan over radiocarbon timescales and it is precisely these characteristic faster-than-geology but slower-than-weather phenomena that we will be able to study best in this way.

What's really going on in that polar atmosphere ?? In a way it's good to know that it's not just us amateurs who can't fit the whole picture together yet. We await enlightenment right behind you in the back seat!