[X] My Assistant

[Palomar]

Best evidence yet

*...for a shoreline on Titan; they're calling it "dramatic." Area measures 1,060 by 106 miles. Is from Cassini radar, obtained during the latest flyby. Speculation continues regarding seepage of liquid from the ground/ground springs and/or rainfall.

[Guest_BruceMoomaw_*]

David: Nobody is saying that the methane rain that hits the surface is IMMEDIATELY soaked up by the ground. Just as in Earth's deserts, when Titan has one of its very rare but very violent rainstorms, the liquid runs along the surface, carving arroyos, until it ends up in "playas" -- temporary lakes which then slowly disappear as their liquid either evaporates back into the air or trickles deeper underground. Huygens (by very good luck) landed in exactly such a classic desert playa -- I can't think of a landing spot for it that could have given us a better understanding of what's actually happening on Titan.

Now, the liquid ETHANE that forms in Titan's air, as a result of radiation modification of some of its methane, is a different matter. It forms at such an incredibly slow rate (1/4 micron per year, at absolute most) that when the ethane mist does gradually drift down and settle on surface soil particles, it has plenty of time to slowly seep directly down into the local ground, and finally into the subsurface methane/ethane aquifer. (And, unlike the methane, the ethane will never evaporate back into the air once it's formed -- its vapor pressure in Titan's current atmosphere is only about 1/1000 that for methane.) Indeed, it's now a safe bet that the 300-1000 meters worth of liquid ethane that Titan has formed over the eons has now been widely spread throughout a very great depth of its crust by cryovolcanic cycling, explaining why we're not seeing much on the surface.

Meanwhile, the smaller amount of solid organics simultaneously formed out of Titan's methane by radiation (acetylene and tholins), after they settle down onto the surface, naturally just sit there until another violent methane rainstorm washes them into the arroyos and thence into Titan's large system of playas to form the accumulation of dark crud that we see there. (This is the one respect in which Titan is in no way an analog of Earth -- the water vapor and droplets in Earth's air do not undergo chemical reactions that turn some of them permanently into other liquid and solid substances with different physical properties, which then settle down onto the surface. Titan, by contrast, very slowly turns some of its "water" into smog.)

Richard: The evidence does indeed seem to be that the aquifer on Titan is quite deeply buried, and that it IS frequently forced back up to the surface (in liquid form) by tidal geothermal heating (strange to use that term about Titan). Remember that, while many of the channels seen by both Cassini and Huygens have fine tributaries indicating that they were carved by widespread rain, others have fewer and stubby branches suggesting that the source of the liquid in their case was from springs. And Huygens photographed a concentration of those in the same general region of the surface where it also saw what appears to be a relatively recent domelike upthrusting of the surface. Moreover, Cassini has already located two regions on the surface where there are some signs that flat-out geysers and pools of hydrothermally heated methane may be erupting into the air -- Titanian Yellowstones.

But, in any case, a buried aquifer doesn't require geothermal heating to release evaporated vapor back into the air -- if surface rain can seep down to it, vapor can rise upwards from it and out through the same surface pores much more easily.

I should add that my reason for thinking that Cassini's radar hasn't yet seen any really deep bodies of liquid is simply that one can plainly see a mottled texture within even the most radar-dark regions we've seen on it -- except, perhaps, for that very dark kidney-shaped southern patch the size of Lake Ontario, which may be a genuine deep body of liquid on the current surface. The rest of the dark regions seen so far cannot be covered by more than a few dozen meters of liquid or they'd be totally black. Indeed, since solid hydrocarbons look almost as dark to Cassini's radar, most of the dark regions seen by it so far are likely to be just mudflats with no surface liquid -- and a lot of them may even currently be DRY mudflats. A lot of that dark "sea" shown in the latest Cassini radar image is actually likely to be -- when we see it on the surface -- a dry, cracked mudflat, waiting to be remoistened by the next very rare but violent rain.