[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.

[Guest_Richard Trigaux_*]

Thanks BruceMoomaw for the interesting discution about Titan rain.

So Titan landscape could share similarities with Earth deserts, such a s very flat but very large alluvion fans. Likely Huygens landed on such a terrain. Not the bottomest part of the landscape.


It is interesting to infer the overall rain from the available solar energy.

But if Titan's fluids are into an aquifer, how can they evaporate into the air?

My question is: by what random the upper level of the aquifer is just the level of the ground, with a some metre accuracy, all over the "ocean" regions?

On Earth, there is such a "random" equality between the level of the ocean and the average level of the ground (on continents, mountains excepted). This equality is dynamically maintained by two opposite processes: -tectonics which raises mountains and narrows the continents -erosion which flattens the continent, but only above the ocean level (under there is very little erosion). So the erosion clips the level of the continents at just above the sea level, when mountains try to rise above.

The same process cannot play on Titan, but we have a similar equality. To explain it, there are two solutions: the aquifer level is above the ground, and we have large oceans (even very shallow) and a saturated air (which seems the case) or the aquifer level is well under the ground level and we have a dry surface and a dry air, and very few rain. To have just the intermediary solution, the aquifer level very close to the ground level, they must be linked together in some way, like on Earth.

Or the aquifer level is really under the ground level, and we must suppose some process to bring methane to the surface. Cryovolcanism for instance. Or, more likely, hydrothermalism. A source of heat underground would evaporate the methane and drive it to the surface, through some methane geysers. Much more difficult to detect than cryovolcanoes.

But this ruins your calculations, Bruce, if the sun is not the only source of methane vapour.

Or rather it may give us a mean to detect cryovolcanism and even hydrothermalism with just the rain bilan.

[Bob Shaw]

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.

Richard:

Oddly enough, there *are* drainage channels on the ocean floor, with some formed at an earlier time when ocean levels were lower (and sometimes reaching far out onto the continental shelf), while others represent erosion by detritus from rivers, and even on the abyssal plains you get channels formed by turbidity currents (more-or-less cold, wet analogues of pyroclastic flows). So there *are* a range of crossover landforms beneath the waves on Earth! None of these jump out of the Titan pictures, though!

What there is, however, on Earth's ocean floor is (as you say) an obvious and globally repeated hierarchy of features, slopes and the like, ranging from the 'shore' down to the depths. Personally, I don't see that on Titan - instead, I see dendritic drainage channels on the 'land' and a 'shore', then what looks like a flat marsh, with none of the expected sub-oceanic structures. Bruce has given us chapter and verse on the reasons for this, and barring a subtle misinterpretation due to our Terrestrial preconceptions I expect that he's quite right, and I have to agree with your description, too (well, mostly!).

The latest 'shore' features don't quite work for me, but perhaps with a bit of a clean-up (hint, hint!)...

Bob Shaw

[Guest_Richard Trigaux_*]

Richard:

Oddly enough, there *are* drainage channels on the ocean floor, with some formed at an earlier time when ocean levels were lower (and sometimes reaching far out onto the continental shelf), while others represent erosion by detritus from rivers, and even on the abyssal plains you get channels formed by turbidity currents (more-or-less cold, wet analogues of pyroclastic flows). So there *are* a range of crossover landforms beneath the waves on Earth! None of these jump out of the Titan pictures, though!
Bob Shaw

Yes this is true. But I did not spoke of this in a simplified shematics. Anyway they play little role in the overal topography.

[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.

[DFinfrock]

[COLOR=blue]As Bruce MoomMaw wrote in Post #10:

"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."

This suggests a simple experiment... well, simple by planetary exploration standards. Why not send a cannister into Titan's atmosphere, which, when it burns up on re-entry, releases a cloud of silver iodide or some other rich source of condensation nuclei. If the current model is correct, this release of nuclei should trigger a heavy methane rain. A follow-up probe, similar to the Huygens, could then descend to monitor the rainfall, and examine the expected flash flooding, erosional effects, and standing pools of liquid on the surface.

[Guest_Richard Trigaux_*]

This suggests a simple experiment... well, simple by planetary exploration standards.  Why not send a cannister into Titan's atmosphere, which, when it burns up on re-entry, releases a cloud of silver iodide or some other rich source of condensation nuclei.  If the current model is correct, this release of nuclei should trigger a heavy methane rain.  A follow-up probe, similar to the Huygens, could then descend to monitor the rainfall, and examine the expected flash flooding, erosional effects, and standing pools of liquid on the surface.

Wouaouwww! what an experiment! At least it is simple, he cannister just need to be... a cannister. The only difficulty is to have it disperse its content at the right altitude, to be sure a trigger mechanism would do, for instance at a given air pressure.

[Guest_Richard Trigaux_*]

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.

Not quite sure. On Earth there are situations with aquifers into deserts (for instance the huge aquifer which is under Sahara) and it does not evaporate.

The underground evaporation of an aquifer can happen, but only when it is close to the surface. In this case a layer of evaporites can form some metres underground (like in the Landes in France, a vast sandy alluvial plain in which we sometimes find horizontal layers of limestone or iron oxyde.)


About the watertable being close to the ground, there are several situations on Earth where this can happen, so it is seldom at random. -An alluvial plain is gained over the ocean -coral growth fills the ocean -a hollow into impermeable rock is filled with alluvions, peat, diatomite... at a pinch, if oceans were rare on Earth, they could exist only as a watertable just under a sediment plain.


To have a Titanian watertable jut some metres bellow a vast mud flat would indicate that a process is working on Titan to fill the methane ocean. Some ideas:

-chemical compounds would crystallise into the methane ocean, filling it in a similar way to coral reefs.
-Eventually this process would involve the methane itself in a crystal building, similar to the chlathrates on Earth which can form solid compounds with methane and water. Anyway both water ice and methane are very common on Titan, so that we can imagine that there would be a layer of frozen chlathrates in place of a liquid ocean.
-a "cryolife" feeding from energetic airborne chemicals would do the same.
-the whole things we see on Titan would be rafts floating on a huge methane ocean. Explaining the overall flatness.


After all, the simplest idea is that, in place of a huge methane ocean, water would make freeze it into chlatrates, as much as pectine can turn syrup into jelly, forming, not a mud flat, but an ice shield.
Eventualy this chlatrate layer would be soaked with liquid methane, or this methane would be recirculated by thermalism.


To test this idea, we even not need to go on Titan. There are on Earth many liquid methane oceans, industrial Titan simulators: all the oil industry when it deals with liquid methane in pipe lines, tankers, etc. We could ask the methane guies what happens when there is water falling into their methane. I am sure they will reply angryly with this. Water into methane will freeze, or course, but it may eventually form chlatrates, even at the very cold temperature of liquid methane.

[Guest_BruceMoomaw_*]

Yes, but in those situations water seeps into the aquifer horizontally from other surrounding locations. In the case of Earth's aquifer system as a whole, water evaporates from it back out into the atmosphere as easily as rainwater seeps down into it -- if liquid can get into the system as a whole, vapor can get back out. And I was referring to Titan's aquifer system as a whole. The difference is that on Titan -- because of the very slow rate of overall precipitation (less than one centimeter per year over the world as a whole), and the high porosity of Titan's crust (which is largely due to its high rate of cryovolcanism) -- the liquid methane gets down into the aquifer in the first place as a low-rate trickle down through the floors of muddy playas, rather than getting the chance to build up as deep surface bodies of liquid.

[Guest_Richard Trigaux_*]

Yes, but in those situations water seeps into the aquifer horizontally from other surrounding locations.  In the case of Earth's aquifer system as a whole, water evaporates from it back out into the atmosphere as easily as rainwater seeps down into it -- if liquid can get into the system as a whole, vapor can get back out.  And I was referring to Titan's aquifer system as a whole.  The difference is that on Titan -- because of the very slow rate of overall precipitation (less than one centimeter per year over the world as a whole), and the high porosity of Titan's crust (which is largely due to its high rate of cryovolcanism) -- the liquid methane gets down into the aquifer in the first place as a low-rate trickle down through the floors of muddy playas, rather than getting the chance to build up as deep surface bodies of liquid.

The difference is that on Earth, there is 70% of free surface of water, plenty enough to evaporate it, when on Titan there is near to -or exactly- zero per cent of free surface. Should it be exactly zero, with a deeply buried aquifer, there would be no methane at all in Titan atmosphere. Right on the countrary Titan atmosphere is saturated with methane (at least at certain altitudes). So we must suppose that either:

-the aquifer is close to the surface, a situation which is likely not at random
-the aquifer is deep, but cryovolcanism or thermalism recirculates the methane up to the surface, directly under the form of hot vapour, or under the form of liquid flow which can evaporate.

About the porosity of Titan crust, it is hard to make hypothesis. We can be sure it is not completelly impermeable, otherwise there would be an ocean. But is it porous, from a sandy structure, or is it fractured, from tectonics? At least there is some sand, as there are dunes (the "cat's scratches"). Not astonishing to find sand if there are valleys dug into higher lands. But the main ice body, how is it? At a moment, early into Titan formation, the water was very probably liquid. And after it got solid. If nothing affected it since, it would be still an overall solid layer, and impermeable. But likely there are today extensive fractures, like on Enceladus, or rather Europa. Eventually these fractures were consolidated, if cryovolcanoes inject liquid water in. But we can imagine many things, even an action of liquid methane dissolving water ice (from a chlathrate reaction) to form karst-like landscapes. And the mud flats themselves are likely to be porous, or likely too to be impermeable like tar.

[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.

[Guest_BruceMoomaw_*]

Note also that one of the most recent Cassini radar images ( http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=1732 ) shows strong evidence that there are indeed strong tectonic forces operating on Titan's crust, causing the course of many of its rivers to zigzag suddenly back and forth. This in itself could provide the deep crustal crevices necessary to drain surface liquid methane back into the deep subsurface of Titan.

[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.

[Bob Shaw]

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 early EOM scenario for Cassini included a flyby/rocket power/aerobrake capture by Titan... ...I wonder if that's still feasible?

[Guest_BruceMoomaw_*]

Robert Mitchell has recently told me flatly that it is not -- they simply don't have the time or fuel to make the very large number of Titan flybys necessary to aerocapture this particular spacecraft into orbit around Titan, since it was not designed for that purpose and so doesn't have a heatshield.

He told me several years earlier, however, that at that time more coverage of Titan through flybys was considered the single highest-priority goal for an extended mission. (Presumably more coverage of Enceladus has now been added to this.)

[exoplanet]

Has anyone put forward a theory that some sort of chemical interaction may be taking place when liquid methane on the surface of Titan comes into contact with the complex hydrocarbons raining down from the atmosphere. Perhaps there is some sort of organic opaque film forming on top of liquid methane and this may be the reason Cassini cannot see the specular glints from the oceans and lakes.

I posted this idea because I really am having a hard time grasping that the dark deposits are dried up mud flats from particulates being washed from the high surface into the low areas. If this would be the case - the dark particulates would be seen only in the dried up playas and intermittently filled streambeds. These dark particulates are also seen in the short channels that are most likely caused by powerful springs that are presumably still active. These channels should be light colored at the bottom and not dark due to the constant running of the methane springs.

Also, The ice pebbles surrounding the lander were very rounded, light colored and clean (without a crust of dark hydrocarbon mud) which leads me to believe that they have been eroded by liquid (wave or streambed action) very recently.

[Guest_BruceMoomaw_*]

Not necessarily. First, the only spring channels that we've seen in visual/IR wavelengths are those seen by Huygens, in one small spot on the surface, that could easily have been inactive for a long time, so that they had dark smog washed into their bottoms from the rest of the surface by the latest rainstorm, just like the rain-carved channels themselves. (After all, Cassini simply doesn't have the visible/IR resolution to more than very faintly and doubtfully sight any spring channels, light OR dark-floored, from above.)

Second, keep in mind how VERY slowly dark photochemical smog accumulates on Titan's surface -- 1 micron every 4 years for liquid ethane, a lot less for all other organics combined. Those pebbles could have been washed clean by the latest rainstorm a LONG time ago without getting any detectable crust of precipiated dark material on them since.

As for your speculation that there may be some chemically-produced solid organic substance covering Titan's surface bodies of methane/ethane: there HAS already been some speculation on that, with the suspect being "polyacetylene" (polymeric chains of acetylene) -- apparently the only possible organic that might be light enough to float on such liquid. But the evidence is mounting that we aren't seeing specular glints for the simple reason that there really is little liquid on Titan's surface; most of it has soaked underground.

[exoplanet]

Dear Bruce,

I honestly respect you opinion but I have to disagree on a few points.

"First, the only spring channels that we've seen in visual/IR wavelengths are those seen by Huygens, in one small spot on the surface, that could easily have been inactive for a long time, so that they had dark smog washed into their bottoms from the rest of the surface by the latest rainstorm, just like the rain-carved channels themselves."

This is written from my own personal experience. Years ago my husband and I lived in Arizona and we loved to explore the Huachuca mountains near Sierra Vista. We never carried water. Springs were always clearly marked on USGS maps and they were always running even in the dry season. They were never inactive - according to the locals who had lived in the area for generations (the water was of course wonderful!!!). Most springs in the Sahara Desert have been flowing for thousands of years as well. Dendric channels have not only been seen by Huygens but Cassini has imaged them on the radar swaths too. Perhaps when the next mission to Titan arrives ~ the answer to the spring cycle will then be known


"Second, keep in mind how VERY slowly dark photochemical smog accumulates on Titan's surface -- 1 micron every 4 years for liquid ethane, a lot less for all other organics combined. Those pebbles could have been washed clean by the latest rainstorm a LONG time ago without getting any detectable crust of precipiated dark material on them since."

Okay, here is my home experiment to refute this rain washing theory on Titan. Light streaks should be evident in the photo's where the grade is steep (the hydrocarbons would wash away faster). Instead the highlands are almost completely homogenous in color. Take a sputtering candle and place it underneath a film of glass - quickly you will get a micron of hydrocarbon particulate. Not only is this film very dark but it is very difficult to remove from the surface of the glass (Put it in the freezer and it gets stickier too). Titan rain should not have much force as it falls onto the surface due to lower gravity. It will fall very slowly and have only 1/6th the force as you would feel on earth. The rain would be falling so slowly you could catch each very large raindrop in your hand. Hardly the force needed to dislodge sticky hydrocarbon particulates even 1 micron thick.

"As for your speculation that there may be some chemically-produced solid organic substance covering Titan's surface bodies of methane/ethane: there HAS already been some speculation on that, with the suspect being "polyacetylene" (polymeric chains of acetylene) -- apparently the only possible organic that might be light enough to float on such liquid. But the evidence is mounting that we aren't seeing specular glints for the simple reason that there really is little liquid on Titan's surface; most of it has soaked underground."

Thank you for clarifying this for me!!! I had a hunch that acetylene would be the only hydrocarbon to be able to float on top of methane . . . what I really am beginning to wonder is this: What if the liquid that collects on the surface of Titan after periodic rains is not only pure methane but over a short period of time also begins to contain traces of ammonia, molecular water (from ice and cryovulcanism) and other dissolved minerals. How would this impact the chemistry of the slow infall of hydrocarbons onto the surface. Would this complex soup of chemicals be the possible catalyst to create an opaque layer or organic sludge onto the surface of liquids on Titan? We all know that sea water on earth is not just pure molecular water but a mixture of dissovled organics, chemicals and minerals too. What would make Titan any different than Earth in this aspect?

[Guest_BruceMoomaw_*]

"Titan rain should not have much force as it falls onto the surface due to lower gravity. It will fall very slowly and have only 1/6th the force as you would feel on earth. The rain would be falling so slowly you could catch each very large raindrop in your hand. Hardly the force needed to dislodge sticky hydrocarbon particulates even 1 micron thick."

Except that the evidence is that Titan's smog particles, by the time they hit the ground, are NOT sticky. Bar-Nun's lab simulations had indicated this even before Huygens' arrival -- the organic molecules in a Titanian smog grain cross-link with each other after just a few weeks in the air, and so become hard and non-sticky long before the particle hits the ground. For the same reason he said, correctly, that we need have no fear of Huygens' camera ports getting smeared. So the smog that hits the ground is NOT very sticky (especially at those low temperatures), and can get washed off ground "soil" (or, rather, ice) particles quite easily by Titan's rain. For exactly the same reason, accumulations of fallen smog in Titan's drier regions actually appear to get blown into dark powdery longitudinal dunes -- the famous "cat scratches", which are very common on Titan. (I originally thought these were tectonic grooves like Ganymede's -- but they all seem to aim in an east-west direction, as one would expect if they were formed by Titan's surface winds.)

[exoplanet]

"Bar-Nun's lab simulations had indicated this even before Huygens' arrival -- the organic molecules in a Titanian smog grain cross-link with each other after just a few weeks in the air, and so become hard and non-sticky long before the particle hits the ground"

Bruce,

Can you send me the URL of the latest study. The last one that I found was published in Icarus in 1998. There were also some atmosphere simulations done as far back as 1978.

Thanks!!!

[Guest_BruceMoomaw_*]

In chronological order:

(2001) http://copernicus.org/EGS/egsga/nice01/pro...cts/aac0482.pdf

(May 2004)
http://cisas.unipd.it/hasi/HASI04/bar-nun.pdf

(May 2005)
http://sci.esa.int/science-e/www/object/in...fobjectid=37610

Note one other fascinating aspect of his research: he claims that the virtually total absence of any trace primordial noble gases in Titan's atmosphere is due to the fact that they have all long since been pulled out of Titan's air by being trapped in its solid organic aerosols -- rather than because the ices out which Titan formed were too warm to contain clathrates of either the noble gases or nitrogen. That is, he doubts the now-prevailing view that Huygens has proven that Titan's nitrogen all originated as frozen ammonia. (In his 2004 paper, he says that only Kr is thus removed -- but by 2005, he's saying that Xe, Ar-36 and Ar-38 are too.) Nick Hoffman has also speculated on this possibility.

[The Messenger]

...
I posted this idea because I really am having a hard time grasping that the dark deposits are dried up mud flats from particulates being washed from the high surface into the low areas.  If this would be the case - the dark particulates would be seen only in the dried up playas and intermittently filled streambeds.  These dark particulates are also seen in the short channels that are most likely caused by powerful springs that are presumably still active.  These channels should be light colored at the bottom and not dark due to the constant running of the methane springs. 

Also, The ice pebbles surrounding the lander were very rounded, light colored and clean (without a crust of dark hydrocarbon mud) which leads me to believe that they have been eroded by liquid (wave or streambed action) very recently.

Agreed. I would expect to see chromographic banding, if dark organics are being washed by any solvent process - reverse phase or otherwise. I don't see any evidence of this.

In the Great Basin of Utah/Nevada, the "bathtub ring" caused by a lake that dried up 10-20,000 years ago is very apparent in either radar or visual imaging. In southern Utah, the millenial old uplifted ocean bed responsible for Zion, Arches, and Canyonlands national parks demonstrate little if any of the shoreline evidence obvious in 'geologically current' features.

Since the surface of Titan is relatively new, if we cannot find evidence of shoreline erosion, we should expect to find tetonic and volcanic activity.

[imran]

Just an article I found interesting.

Rivers on Titan, one of Saturn's moons, resemble those on earth

[Guest_Richard Trigaux_*]

Just an article I found interesting.

Rivers on Titan, one of Saturn's moons, resemble those on earth

I do not think that what we see on Huygens images really ressemble river beds. It is slightly different, and these differences are revealing of the conditions which created them.

If you look at river beds in temperate countries like Europe of USA (desert excepted) you see very narrow channels which would be invisible on Huygens images (assuming a replica of Huygens photographied them in the same conditions). What we see on Huygens images is hundred times wider, and rather looks like what we see in deserts like Sahara (I flew over in in aircraft, it is really visible).

This is because, in temperate climate, there is a weak but constant rain fall, which creates narrow channels, but neatly cut, what we call river beds. In deserts, there are rather large surges of rain in an however dry climate, and this does not create river beds, it creates large flow traces occupying all the valley bottom, or large alluvial fans. Sahara water traces can be several kilometre wide, in places where there is no river bed.

So we can conclude that Titan has rather rare rains, but very intense when they come. And, for some reason, these precipitations mark their passage in dark, likely some kind of tar formed by the organic materials falling from the atmosphere.

[Guest_BruceMoomaw_*]

It isn't that the dark stuff falls out of the atmosphere in large amounts in the raindrops -- it's just that it continually very slowly drifts down and coats the surface, and then gets washed off the higher areas and into the riverbeds by the rare but violent rains. Most of it gets washed into the playas, but a little bit remains on the bottoms of the arroyos as they empty and then dry out (mixed, perhaps, with ground-up water-ice "sand" accumulated on the bottoms of the arroyos).

Always keep in mind how VERY slowly the dark stuff is thought to accumulate on the surface. We're talking about 1 micron total of ALL methane radiolysis products every 25 years -- and most of that is ethane (which is liquid and trickles down into the soil) or solid acetylene (which is light-colored).

[Guest_Richard Trigaux_*]

It isn't that the dark stuff falls out of the atmosphere in large amounts in the raindrops -- it's just that it continually very slowly drifts down and coats the surface, and then gets washed off the higher areas and into the riverbeds by the rare but violent rains.  Most of it gets washed into the playas, but a little bit remains on the bottoms of the arroyos as they empty and then dry out (mixed, perhaps, with ground-up water-ice "sand" accumulated on the bottoms of the arroyos).

Always keep in mind how VERY slowly the dark stuff is thought to accumulate on the surface.  We're talking about 1 micron total of ALL methane radiolysis products every 25 years -- and most of that is ethane (which is liquid and trickles down into the soil) or solid acetylene (which is light-colored).

Very revealing are the images of the small hills just around Huygen's landing site, that we can see on the latest images. These hills look like coated in black until their very summit, when some whitish material appears, as if they were drown in a thick layer or liquid which tainted them in dark. It is difficult to believe that rains could produce a 30m thich layer of methane, even for some minutes. So I think to the dark material falling everywhere and then creeping down slope by one or several process.

[JRehling]

I do not think that what we see on Huygens images really ressemble river beds. It is slightly different, and these differences are revealing of the conditions which created them.

If you look at river beds in temperate countries like Europe of USA (desert excepted) you see very narrow channels which would be invisible on Huygens images (assuming a replica of Huygens photographied them in the same conditions). What we see on Huygens images is hundred times wider, and rather looks like what we see in deserts like Sahara (I flew over in in aircraft, it is really visible).

I think in most temperate places in Europe/USA, if you removed all plants first, and avoided areas where humans have reworked streams into artificial irrigation systems, you would see streams rather densely populating the landscape for a Huygens-like eye to perceive.

[Guest_BruceMoomaw_*]

Very revealing are the images of the small hills just around Huygen's landing site, that we can see on the latest images. These hills look like coated in black until their very summit, when some whitish material appears, as if they were drown in a thick layer or liquid which tainted them in dark. It is difficult to believe that rains could produce a 30m thich layer of methane, even for some minutes. So I think to the dark material falling everywhere and then creeping down slope by one or several process.

Do we know those hillocks ARE 30 meters high? I've been under the impression that they are just a few meters high.

[Guest_Richard Trigaux_*]

I think in most temperate places in Europe/USA, if you removed all plants first, and avoided areas where humans have reworked streams into artificial irrigation systems, you would see streams rather densely populating the landscape for a Huygens-like eye to perceive.

No, as usually the streams are very narrow, they would be under the resolution power of a Huygens-like imager. The latest would see streams only at places where they are unusualy large, such as estuaries, mangroves, etc.


My point was that flow traces on Huygens images are much larger that our usual rivers, they rather ressemble large desert flows, advocating for rare but very strong rains rather than a constant trickle.

Also the terrain around Huygens was still wet from a previous rain, but given the very low evaporation rate on Titan, this last rain could be millenias ago.

[ljk4-1]

The lighting levels are about right, supposedly Huygens did detect some lightning
(can Cassini detect lightning on Titan?), and either ignore the trees on the left or
imagine they are some kind of exotic native life forms:

http://lava.nationalgeographic.com/cgi-bin...&day=09&year=06

[JRehling]

No, as usually the streams are very narrow, they would be under the resolution power of a Huygens-like imager. The latest would see streams only at places where they are unusualy large, such as estuaries, mangroves, etc.

I missed this post until now, but Huygens-level imagers certainly could see streams on an Earthly landscape. In fact, it did:

http://www.lpl.arizona.edu/~kholso/Picture3.jpg

In the areas that haven't been reworked by agriculture, streams are easily seen even far from the nadir, and this is Arizona...

[stevesliva]

From what elevation is that? Is the convex appearance of the ground real or an imaging artifact?

[Guest_BruceMoomaw_*]

From Ralph Lorenz's very short abstract "The Surface of Titan", from this May's "Planetary Science: Discoveries and Challenges" conference in France:

"A global circulation model suggests Titan's equatorial regions should be dry, while high latitudes are saturated. The detection by Cassini RADAR of sand dunes near the equator, and an apparent coastline at high latitude, are consistent with this picture."

Of course, this doesn't necessarily mean there's much methane rain even in high latitudes -- just more than in the equatorial region. But it does mesh well with the picture of Titan that John Rehling keeps giving us.