[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.

[David]

How does this square with previous assertions that there is no surface liquid on Titan, other than the occasional small lake and flash flood?

[4th rock from th...]

The liquids aren't there now, but they were present in the past, I guess.
Like Mars, perhaps. Lots of evidence for liquid water but none present today.

[Jyril]

Dark area on the radar image suggests the ground must be very smooth. Such dark regions have been searched since the first Cassini radar observations of Titan (remember the "Cat" feature from the first radar swath).

In dark areas very little scatters back to the detector. So dark regions are smooth (or in some cases slopes facing away from the probe). In brighter areas (which represent rougher terrain) there are a lot of scattering and radar echo is much better.

One must remember that light and dark areas in radar images have nothing to do with visual brightness. Also, radar may penetrate the surface so some features we see may actually be underground.

[Guest_BruceMoomaw_*]

The solution to the apparent contradiction seems to be that -- in most locations on Titan where one would see seas or lakes on Earth -- we're looking instead at mudflats, like the one Huygens landed in. Rain is unquestionably MUCH rarer on Titan than on Earth -- and, on top of that, it seems to have very active cryovolcanic processes (driven by tidal heating from Jupiter, according to one startling theory) which are likely to keep pulling near-surface liquid methane and ethane further down into the subsurface and recycling it through Titan's upper crust. So we see very smooth mudflats (composed of both finely ground water ice and accumulated solid organic-smog sediment) -- but not much actual surface liquid.

Jonathan Lunine, in fact, predicted exactly this a decade ago: he said that the only model that could fit all the already-observed facts about Titan was that most of its liquid methane and ethane was in a subsurface aquifer in a highly porous surface layer, rather than in actual liquid bodies sitting directly on the surface. Bingo.

[Cugel]

Also, radar may penetrate the surface so some features we see may actually be underground.

So I guess it can also penetrate the surface of a liquid? If the dark areas are indeed shallow methane lakes, we could be seeing some features on the bottom. (which would explain the occasional speckle). I don't think this is very likely, but maybe you can't rule out such an explanation from the radar images.

[JRehling]

The solution to the apparent contradiction seems to be that -- in most locations on Titan where one would see seas or lakes on Earth -- we're looking instead at mudflats, like the one Huygens landed in.  Rain is unquestionably MUCH rarer on Titan than on Earth -- and, on top of that, it seems to have very active cryovolcanic processes (driven by tidal heating from Jupiter, according to one startling theory) which are likely to keep pulling near-surface liquid methane and ethane further down into the subsurface and recycling it through Titan's upper crust.  So we see very smooth mudflats (composed of both finely ground water ice and accumulated solid organic-smog sediment) -- but not much actual surface liquid.

The thing is, sure rain is rare, but it still has to happen sometime, and has to be flowing through those channels sometime and gathering sometime into standing liquid. Is Mezzoramia, now, such a place and time? One good reason to think so is that channels seem to link the immediate vicinity of the south pole and Mezzoramia, and the south pole is where the plurality of Titan's methane clouds appear during this season.

Look how big Mezzoramia is. If it is filled by rains originating at 80S and thereabouts, then it is getting a lot of liquid sometime to fill it from brim to brim, and probably not drying up completely very quickly.

Now the hunt for radar-specular glints (or sunshine-specular glints) over the southern dark areas has got to be a priority. Unfortunately, we're racing seasonal changes. By the time extended-mission opportunities to probe these areas come around, it might be too late. We may end up waiting for northern summer and the beginning of standing liquid there, before Cassini's instruments can put the final dot on the i with the issue of standing liquid.

[imran]

The thing is, sure rain is rare, but it still has to happen sometime, and has to be flowing through those channels sometime and gathering sometime into standing liquid. Is Mezzoramia, now, such a place and time? One good reason to think so is that channels seem to link the immediate vicinity of the south pole and Mezzoramia, and the south pole is where the plurality of Titan's methane clouds appear during this season.

Look how big Mezzoramia is. If it is filled by rains originating at 80S and thereabouts, then it is getting a lot of liquid sometime to fill it from brim to brim, and probably not drying up completely very quickly.

Now the hunt for radar-specular glints (or sunshine-specular glints) over the southern dark areas has got to be a priority. Unfortunately, we're racing seasonal changes. By the time extended-mission opportunities to probe these areas come around, it might be too late. We may end up waiting for northern summer and the beginning of standing liquid there, before Cassini's instruments can put the final dot on the i with the issue of standing liquid.

I agree with your assessment. As rare as rain may be, it is still causing widespread channels (some pretty deep and extensive) and draining all of this in what appears to be seas. Some channels we are seeing require long standing presence of liquid. Cryovolcanic processes alone cannot account for what we are seeing. Titan's surface has been significantly altered by the action of surface liquid. A major effort of Cassini's extended mission should be focused in trying to solve this mystery and the south pole area seems to be a good candidate for this.

[Guest_Richard Trigaux_*]

A strong argument in favour of rare but heavy rainfalls is the size of the drainage channels. Usually, on Earth, in regions with a moderate climate, there is much rain but spread all around the year: drainage channels (rivers) flow all the time but are narrow (and they would be invisible on Cassini and Huygens images). But in desert regions, there is much less rain but very violent when it happens, the riverbeds are much larger, we even see flows which are several kilometres wide without a definite river bed. This is very visible in aerial or satellite views of regions like Sahara, which has similarities with Titan landscape:
-Large flow marks forming valleys in mountainous places, and large series of sediment fans in plains.
-in bottom places, temporary lakes form rounded patterns outlined with vegetation (on Earth) and filled with darker dried mud or white salt.
I can just compare what can be seen when flying over Sahara and what is seen by Huygens and Cassini.

[Guest_BruceMoomaw_*]

The picture which is now forming pretty clearly of Titan IS one where rains are rare, but (when they do occur) very violent, allowing the carving of such broad arroyos. The reason is that -- paradoxically -- Titan's air is much clearer of aerosols than Earth's is. The only reason that its quite rarified mist of organic aerosols is opaque to visible light from above is simply that the combination of high air density and very low gravity on Titan causes its atmosphere, and the aerosol layer suspended in it, to tower up to an astonishing height. Earth's much thinner air layer is far more densely populated with tiny solid dust particles -- both windblown dust, and salt crystals swept by the wind out of ocean foam -- and so there are far more nuclei in it for water vapor to condense around and form liquid cloud droplets. But in the case of Titan, with its relative lack of nuclei for cloud droplets, local concentrations of methane vapor can rise to far higher levels before liquid methane droplets start to form at all in significant numbers. Once they DO form, however, the high concentration of methane vapor around them causes them to very rapidly grow to large size, and thus fall out as violent local rain -- and flash floods. (The slopes on the sides of the liquid-carved arroyos photographed by Huygens were as much as 30 degrees -- indicating very violent episodes of erosion which, however, could have been rare.)

By the same token, however, since such rains are rare, there's LOTS of time for the liquid that hits the ground to soak into a porous crust. Keep in mind that, as Ralph Lorenz notes, the total amount of rain possible on Titan has an absolute upper limit of only 0.6 to 1 cm/year -- there isn't enough incoming solar energy to evaporate more liquid methane than that off the surface and back into the air per year!

And we already know beyond any doubt whatsoever that Titan's crust IS highly porous overall -- that's the only way to mesh the fact that several hundred meters to 1 km of liquid ethane absolutely must have been manufactured in its air over the last 4 billion years with the fact that it does not have a global ocean that deep. Lunine's March 1994 "American Scientist" article emphasizes this, and adds that the transfer of liquid ethane into Titan's very deep subsurface will be more efficient over the eons if it has active cryovolcanism -- which we now know it does have, thanks to Cassini (although its exact cause is still uncertain).

Put all this together and it's simply inevitable that actual bodies of liquid on the surface of Titan -- whether lakes or rivers -- will be rare and brief. What it will have is gargantuan mudflats -- and that is apparently what we're seeing.

[Guest_BruceMoomaw_*]

I've just found an article by Lorenz in the January 7, 2005 Geophysical Research Letters ("Convective Plumes and the Scarcity of Titan's Clouds"; http://www.lpl.arizona.edu/~rlorenz/titanplumes.pdf ) which emphasizes what I'm talking about in no uncertain terms:

"A principal limiting factor in the size of extreme storms on Earth is the availability of moisture in the atmosphere [e.g., Trenberth, 1999; Allen and Ingram, 2002] and thus in a greenhouse climate, the higher vapour pressure (or specific humidity) of atmospheric water at elevated temperatures permits more violent storms to occur. However, to a first order, the overall vigour of the hydrological cycle (as measured by the convective energy flux transporting the fluid substance upwards) is unchanged, and thus the precipitation flux conveyed in the now permitted extreme events occurs at the expense of smaller precipitation events, which accordingly occur less often than before. The result is an unpleasant combination of more frequent droughts (since light rains occur less often) and of more frequent floods (since violent events become less rare.)

"Titan may represent an extreme example of this climate property. Although its hydrological cycle is weak (the global average convective flux is ~2000 times smaller than Earth’s, corresponding to ~0.5 cm of methane rainfall per Earth year [Lorenz, 2000]) Titan’s atmosphere can store more latent heat [Griffth et al., 2000].

"The column mass of methane on Titan exceeds 2000 kg per square meter (>2% of a 100,000 kg/sq m atmosphere) compared with ~100 kg/sq m of water on Earth (~1% of a 10,000 kg/sq m atmosphere.) These quantities, if they could be completely condensed, represent ~4 m and 10 cm of rainfall respectively – the meteorological turnover time for the relevant working fluids is therefore ~1 month for Earth, but a millennium for Titan. This picture seems to be supported by the supersaturation of methane in Titan’s upper troposphere [Courtin et al., 1995; Samuelson et al., 1997] which suggests that there are significant kinetic barriers to condensation. Hence, like many desert regions on Earth, a location on Titan may experience rare, but very violent, rainfall [Lorenz, 2000]. We note that the Titan thundercloud model of Tokano et al. [2001] gives column masses of methane rain and graupel equivalent to surface thicknesses of around 20 cm and 80 cm respectively – representing a substantial fraction of the total atmospheric column, and centuries-worth of precipitation. Thus, although the methane hydrological cycle is weak overall, pluvial and fluvial erosion may nonetheless be significant forces of geomorphological change on Titan."

Bang on. Very brief, violent rainfalls -- but with CENTURIES of time between them for the rain to soak down into the ground and get redistributed through the crust, with that redistribution occurring down to great depths thanks to Titan's violent cryovolcanism. And so: lots of huge mudflats, but very little surface liquid at any one time.

[Guest_BruceMoomaw_*]

Now, I HAVE found a clear reference to the fact that Cassini's radar will find shallow bodies of surface liquid -- or even solid -- hydrocarbons to be totally transparent and therefore undetectable. http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2227.pdf :

"On the other hand, the interpretation of dark areas as hydrocarbon deposits indicates local topographic depressions but no regional slopes in some areas. If the brighter spots that mottle some of these dark features are islands, relief of at
least some tens of meters over a distance of a few kilometers is implied, because the low microwave absorptivity of candidate infilling materials such as liquid and solid hydrocarbons would make them invisible to the RADAR unless they are this thick."

So, Cassini's radar images don't really provide direct evidence that SHALLOWER bodies of surface liquid don't exist -- it can't detect them even if they're there -- but it does indicate that there are few of them deeper than a few dozen meters, and we also have very solid theoretical reasons to believe the same thing. Titan is a world of deserts and marshes, NOT of seas.

[scalbers]

Haven't had a chance to do this yet with these images, though I think they could benefit from post-processing with a well chosen low-pass filter. This would remove the speckly appearance of the images that I find distracting and really allow the geologic features to pop out with greater vividness.

[David]

I don't really know that much about the science of shorelines (and particularly not on alien worlds), but it seems to me that you would get very different results from permanent, even if shallow, lakes and seas, on the one hand, and mud flats on the other. The difference is, I think, wave action, which erodes sediment into grains, shapes shorelines into smooth shapes, creates spits, headlands, and islands. If rainfall on Titan is absorbed quickly into the ground and does not pool for long enough for waves to shape the surface, I don't see how you could have any "shoreline", estuaries, or littoral features. You could get them if you have an intermittent presence of liquid (say tides), but I don't see how they could arise from merely sporadic downpours and flash floods. Perhaps someone who knows more about the hydrography of shorelines could comment on what the images from Titan suggest.

[Guest_Richard Trigaux_*]

I don't really know that much about the science of shorelines (and particularly not on alien worlds), but it seems to me that you would get very different results from permanent, even if shallow, lakes and seas, on the one hand, and mud flats on the other.  The difference is, I think, wave action, which erodes sediment into grains, shapes shorelines into smooth shapes, creates spits, headlands, and islands.  If rainfall on Titan is absorbed quickly into the ground and does not pool for long enough for waves to shape the surface, I don't see how you could have any "shoreline", estuaries, or littoral features.  You could get them if you have an intermittent presence of liquid (say tides), but I don't see how they could arise from merely sporadic downpours and flash floods.  Perhaps someone who knows more about the hydrography of shorelines could comment on what the images from Titan suggest.

If Earth could be dried up from its oceans, it would show a very distinct shoreline, a flat band much like a road built on a slope, with a detritic talus downward and a gouging upwards. And the "above" and "under" landscapes would be very different, as, of course, there are no drainage channels under the ocean when they are everywhere above it. So I think a radar scan of Earth would clearly show shorelines, as a very disting feature with a dark band often outlined by two cleared bands. It is not what we see on this Titan image, we just see a transition between a flat region and a more bumpy one.

In a general way, all the images of Titan we could see until now are very difficult to interpret. So I think that the statement "a shoreline" rather originated from the public relation team than from the scientists.