[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_*]

Actually, I believe you're right -- on thinking more about it, I believe that I have more provided an explanation for why Titan MAY be the way it so far appears to be to Cassini, rather than an explanation for why it MUST be that way.

The fact that both the evaporation and precipitation rates of liquid methane on Titan must be very slow does not in itself provide any reason to think that there couldn't be large bodies of liquid methane actually on the surface. And -- aside from the radar maps so far -- the only evidence we have that there are not large surface seas of liquid methane is the fact that Titan's orbit is eccentric, which (as Lunine says in that "American Scientist" article) would seem to mean that its liquids must be in a subsurface aquifer that greatly reduces their ability to flow from one place to another in response to tides, since their free tidal flow on the surface would otherwise generate enough friction to have long since circularized Titan's orbit. But that conclusion has always been based on the belief that there is nothing else actively working to maintain the initial eccentricity of Titan's orbit -- and we now know that there is some force that does so, since that force must be providing the continuing influx of energy that drives Titan's continuing intense cryovolcanism. (The best theory I've seen is that the driving force is resonant tugs from Jupiter during its periodic movements past Saturn, because these are roughly synchronized with the periods of Titan's orbit around Saturn.)

So: I'm going to have to retract much of what I've been saying. So far, I've provided an explanation for why all of Titan COULD easily be the way the sample of it that we've seen so far appears to be: that is, almost devoid of surface liquid. But it remains entirely possible that there are fairly large bodies of surface liquid on some of the parts of Titan that we have not yet clearly viewed in any way.

[JRehling]

But it remains entirely possible that there are fairly large bodies of surface liquid on some of the parts of Titan that we have not yet clearly viewed in any way.

Note that Mezzoramia, the large dark feature in this "shoreline" RADAR image is much bigger than what the image shows, and in infrared imagery, shows darker regions farther to the south than T7 revealed. (And that would not have been corrected if the full swath had been recorded.)

A simple (and perhaps even true) interpretation would be that the darker areas are either the only areas with standing liquid, or host deeper liquid than the somewhat-dark areas, and that we have yet to see them with RADAR.

A huge question, and one that will not be answered for years is: What is the annual cloud cycle on Titan like? We know that the summer pole hosts frequent cloud patterns in a ring/patch around 80-90 degrees latitude. How does this transition through the equinoxes? Does the cloud "ring" slide across all latitudes as the subsolar latitude changes? Does it disappear at one pole and reappear at the other? How many of the 15 Earth years in a half Titan year does it last? Finally, is it even symmetical between north and south (I suspect yes).

The reason why this is relevant is that the clouds are the source of any occasion rains, and my take on Mezzoramia is that it is one of the respositories of the south summer rains (there may be other smaller ones, and perhaps another large one about 90 degrees of longitude to Mezzoramia's east). If we allow that a centimeter of rain can fall in a Titan year, but all of that ends up pouring into two basins the size of Mezzoramia, we would be talking about several meters of depth even if Mezzoramia is bone dry come springtime. Of course, it is also possible that it is full of liquid year round, and is merely replenished in summer.

A single well-designed flyby could align so as to place the specular point for sunlight into the middle of Mezzoramia's darkest region and image that place with ISS and/or VIMS. That is an easy conclusive test -- for the moment that such an image is taken. We won't be able to rule out that a place wet in 2005 may have become dry in 2009.

Assuming that Cassini lives way past its nominal mission, and that the northern seasons aren't too different from the southern, there'll be less of a rush to explore the issue in the north during its summer, 2010-2025. Some RADAR mapping could already be in hand by the equinox, and in any case, ISS should show us the whole north at modest resolution as soon as the geometry allows. I would not design an extended mission that didn't have contingencies for retargeting as the need to seek out specular-glint opportunities allowed. It would be unfortunate to see a northern Mezzoramia in 2011 and have to wait until 2014 to set up the right investigation.

One more thing to watch is for seasonal changes in IR bands. If darkest areas indicate liquid, then we can see if the southern dark areas shrink while the northern dark areas grow as the season shifts.