The color images taken by the Huygens probe during its descent reveal a major contrast between a dark area apparently made up of wet sand and white and bright elevated terrain composed of multiple channels. I have the feeling that those white and bright hills consist of ice.What kind of ice? perhaps water ice, or a mixture of hydrocarbon molecules, methane or ethane for instance.
And if it's really composed of ice, it's not forbidden to think that the channels on its edges are fractures rather than rivers, especially if those ice blocks or "icebergs" are moving on the wet sand.
Regarding Titan meteorology, if the hypothesis is correct, it's likely that this supposed ice can evaporate and form clouds.And then, it can rain or snow depending on the environmental temperature.

http://www.titanexploration.com

I remember that all hydrocarbons are soluble in each other. And one hydrocarbon will float above another.
On Titan, the first three alkanes (Methane, Ethane & Propane) are liquids and all others are solids. And all other hydrocarbons are solids on Titan too. So I could see icebergs on Titan.

ie blocks of Butane floating on Ethane lakes.

It could be possible that bits of liquid hydrocarbons could be encased in solid hydrocarbon blocks.

However, despite lots of searches, no liquid lakes have been detected yet. So, from this, no icebergs have
been detected so far.

Methane: MP -182.5 Deg C, BP -161.0 Deg C
Ethane: MP -185.5 Deg C, BP -88.6 Deg C
Propane: MP -187.7 Deg C, BP -42.1 Deg C
Butane: MP -138.5 Deg C, BP -0.5 Deg C
Ethlene: MP -169 Deg C, BP -103 Deg C
Ethyne: MP -84 Deg C, BP -81 Deg C

All MP's and BP's are at 1 atm of pressure.

Alas, solid butane is more dense than liquid ethane. Indeed, most all solid hydrocarbons are more
dense than liquid ethane. No butaneburgs.

-Kevin

And if faculae are composed of water ice, presumably, then they couldn't possibly form icebergs because water ice is denser than liquid ethane/methane.

one point that Webscientist brings up that I do think is useful to consider is the possibility that some of the "channel-like" features we see are actually fractures (though liquids could use fractures as channels...). Doesn't mean icebergs, but it could tell us something about how the crust has been modified. Good examples of this kind of structural control include the "landing strip" channel in the huygens images and a channel seen by RADAR in the T7 as seen in the upper right of PIA03564.

Also, there's no way that water ice is ever going to evaporate in Titan's supercold air, any more than granite gas is a component of Earth's air. There IS a "hydrological" cycle on Titan, but it involves methane rather than water.

Regarding icebergs, there is one long-shot possibility -- one abstract (I'll have to track THAT down, too) says that some of the solid polymeric compounds made out of acetylene can take on a very lightweight foamy texture which is so low-density that it might be able to float on a predominantly liquid-ethane sea. But the odds against this seem extremely long.

More generally, since solid hydrocarbons are denser than liquid ones, it seems that hydrocarbon seas would freeze from the bottom, meaning they'd probably freeze once and stay frozen except for a thin layer of liquid on top. I remember reading about this in Poul Anderson's 1963 book "Is There Life on Other Worlds," but it seems just as true today.

I'm sure this has already been carefully thought through, but has anyone proposed a connection between this and the fact that we seem to see evidence of fluid erosion on Titan but no fluids? Perhaps there is a "rainy season" when something (somehow) melts just the top meter of "ice," but the rest of the time, the oceans are 100% frozen.

Apologies if I'm just being naive here, but I've been watching for someone to make a speculation in this direction, and I haven't seen anything. This seems to me to be Titan's #1 mystery though.

--Greg

The most common theory is that the overal crust of Titan would be water ice, floating on a deep ocean of water (or ammonia-water mixture). But it seems that this crust don't experience anything similar to our plate tectonics. Perhaps the mountains and other features we see are a result of tides, but it is not proved that this ice crust is free to move relative to the inner rock core. So we cannot say that Titan's "continents" would be icebergs, as the "seas" are just places where the ice crust is thinner.

To have ice floating on hydrocarbons would require an hydrocarbon mixture more dense that 0.9. As far as I know there are only some kind of oils to match this criteria, and nobody expect to find such an oil ocean inside Titan.

Even solid hydrocarbons into Titan ice crust would tend to migrate to the surface (like salt diapirs) provided the ice is not too cold and too rigid.

On the surface, only the three hydrocarbons quoted above could be liquid. Deeper into the ice crust, others could be liquid, and climb up, to form "volcanoes", or dykes of solid hydrocarbons, when approaching the surface. But there is no indication that this really exists. Especially if the hydrocarbons are formed above the surface.

Actually, when I say "ice" here I mean frozen hydrocarbons, not frozen water. In the context of Titan, I think of water as a rock.

The question I have, then, is whether a contributing factor to us seeing no oceans of methane or ethane is the fact that frozen methane or ethane would sink, not float, meaning the oceans would be solid on the bottom all the time. I'll admit I don't have a clear mechanism for what would cause the relatively-thin liquid layer to be transient, but I keep looking for someone to propose one. So far I haven't seen it, though.

--Greg

Greg,

there was already discutions on other Titan threads which may reply your question.

First, methane, believed by most theories to be the source of all Titan hydrocarbons, is not stable: being present under the form of vapour, it reaches the upper atmosphere and is destroyed by UV light into larger hydrocarbons and perhaps into a kind of tar which forms the orange haze into Titan atmosphere. Then this haze drops slowly onto the ground, and it is perhaps at the origin of the dark material seen into the lower regions. All the methane should be destroyed in a relatively short time (millions of years) and this arises the question of the mecanism (continuous or discontinuous?) which provides such methane (cryovolcanism?)) and what produces it (internal methane reservoir - life process?) a difficult question as any internal reservoir is expected to contain ammonia too, of which we see no trace into the atmosphere.

Second, methane is not in large quantities. If all was to drop on the surface, it would just form some lakes, not an ocean. It seems obvious that there are methane rains on Titan, to form the many flow traces observed (although there are some alternative theories). So there must be some methane cycle: vapour, clouds, rain, methane flows, methane lakes or watertable, evaporation... Clouds are often observed into some places, but no actual rains, flows or lakes. This could be explained by the factt hat the total sun energy available on Titan ground is very scarce, so that this cycle would work, as on earth, but much slowlier. The favourite picture is of large storms, providing violent rain falls and methane flows, but so rare that a given surface spot would receive rain only once over thousands of years. Conversely, once wet, the ground would require thousands of years to dry. Eventually some clear traces (like the "smile") could be "fresh" snow, which would need centuries to melt. A global evidence is available for the existence of rain: the lower layers of the atmosphere, which is reached by clouds, is relatively free of haze. This would result of the rains dragging the haze particules to the ground.

While there's still a knock-down fight over why we aren't seeing much liquid on Titan's surface, there's no chance that it's because the surface is cold enough to freeze methane or ethane -- it isn't. One parricularly convincing new model comes from Mitri and Lunine ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1962.pdf ). This is simply that Titan, in terms of surface liquid methane, is a desert world similar to Earth's deserts, with only small lakes of methane scattered across the surface -- despite the fact that its atmospheric humidity of methane is quite high.

The reason for the paradox is that the difficulty we have seeing Titan's surface from above through the haze is deceptive -- Titan's atmosphere is actually much cleaner particle-wise than Earth's is. Its haze is very rarified; the only reason it blocks our view of Titan's surface features is that Titan's atmosphere, and thus the haze in it, towers up to such a huge height above the surface. Earth's lower atmosphere, by contrast, has a far denser accumulation of solid aerosol particles -- not just windblown continental dust, but vast quantities of microscopic salt crystals whipped off the froth of its ocean waves.

And so, with far fewer solid particles in its air to serve as nuclei around which its atmospheric humidity (water for Earth, methane for Titan) can condense into liquid cloud droplets, Titan -- despite its high methane humidity -- has very few clouds. And on those infrequent occasions when cloud droplets do start to condense in its air, the initial tiny liquid droplets serve as condensation nuclei around which very large additional amounts of methane vapor suddenly condense, so that the cloud droplets very quickly grow in size to methane raindrops and thunder down copiously on the local surface, producing brief local flash floods and arroyos. However, there is never very much liquid methane on Titan's surface at any one time, any more than there is in Earth's deserts despite their own occasional local flash floods. "We have demonstrated that the high relative humidity of ~50% on the surface of Titan can result from lakes evaporation covering only 0.004 – 0.04 of the whole surface. Even if only a small value of the surface of Titan is covered by hydrocarbon lakes, the atmosphere is nearly saturated and, therefore, can generate thunderstorms." (Ralph Lorenz also saw this possibility coming in his book "Lifting Titan's Veil".)

As for the ethane: the theory has been that most of it sinks down through Titan's porous upper surface to form an underground "aquifer" -- and ethane virtually does not re-evaporate at all in Titan's air. Moreover, Sushil Atreya et al conclude in a new paper in "Planetary and Space Science" ("Titan's Methane Cycle": http://www-personal.umich.edu/~atreya/Arti...tan_Methane.pdf ) that the total amount of liquid ethane smog that has been slowly manufactured in Titan's atmosphere over the eons has been seriously overestimated, and we are probably talking about a total global layer only about 200 meters thick manufactured over Titan's lifetime, making it much easier for most of its ethane to permanently settle underground.

Great article! Thanks for the link.

I gather we still haven't actually seen anything that looked like a storm (producing rainfall) in progress. If I read it correctly, the article appeared to suggest that storms might be seasonal and that we might have a better chance of seeing one as Saturn approaches its equinox. I like that idea, since it's hard to believe that so much erosion can be accounted for by small local storms that are so rare there may not be one on the whole planet (er, moon) in a year. Do we at least seem to be seeing more clouds on Titan as time goes by?

The paper's model for replenishing the methane left me with one question: where does the hydrogen go? The photolysis from CH4 to C2H6 loses H2. It would seem that it would be lost to space, yet the article describes the system as closed, and it concerns itself only with how the Ethane gets turned back into CH4. Did I miss something?

Finally, since the surface of Titan is close to the triple-point of Methane, why does it seem so unlikely that a deep pool of it might freeze at the bottom? It should be both colder and under more pressure.

--Greg

The H2 does indeed escape into space -- if the article did refer to the system was "closed" in that respect, it was wrong. (H2 actually is the third biggest component of Titan's atmosphere, although it's still -- I believe -- just 0.2%.)

Many clouds were seen, swirling around the south pole, or isolated clouds at mid latitude. They seem of the cumulus family, and they evolve quickly, in a matter of one hour. But we cannot say if they just dissipate or if they gave rain falls. After what Bruce explains, they should give heavy rains, but we have no evidence of this.




again, no evidence of this. But I guess that some large zones with a high reflectance like the smile could be snow.
As to frozen methane lakes, it would be as on Mars with frozen water lakes: the surface sublimates and gather dust, so that we would not recognize it from the surrounding ground. Even a liquid lake would not appear clearly, especially if it is shallow. This is the reason why they are looking for surface reflections, but we would need a lot of chance to catch one, if there are.

We seem to be seeing exactly the opposite -- fewer and fewer clouds. Titan's southern hemisphere is getting less sunlight each day (AFAIK, it's currently late summer there) and that's where the majority of the clouds were detected (apart from sporadic streak-like clouds at southern mid-latitudes). Polar regions are obviously regions where most of the precipitation occurs, for some reason. Coincidentally, that's the most likely place the scientists expect to find liquid methane lakes. It's reasonable to expect very little further cloud activity up to the beginning of northern summer, which is still some years away.
Looking at images from the past few Cassini flybys, no significant clouds could be seen, though admittedly the flyby geometry wasn't optimal for imaging the south pole. It appears Cassini arrived at Saturn at the very end of the southern cloud season on Titan.

If the clouds are disappearing as the southern hemisphere progresses towards winter, this argues against a volcanic or internal source of energy for cloud formation. All the more puzzling, because it would seem like we should expect clouds to appear in the horse latitudes. I have tried to imagine a scenario that would produce clouds the way the ozone hole progresses, centered at the poles. Since ethane is easily broken down by the sun's uv - could ethane be emerging in the heated regions near the equator, but surviving UV breakdown only if it escapes to the poles?

Considering the rate of ethane production - 1 km for 4 billion years, the clouds area - about 1% of total and average cloud lifetime - about few hours, the clouds should hold only few tens nanometers of precipitates, which is too small, I think
However, there was an information after first flybys in 2004 that the particles of these clouds are very large to be methane. Have something changed since then? In theories, they are considered to be methane on default, but what if that's wrong?

The clouds are not the smog of other organic compounds (including ethane) which is made (extremely slowly) out of Titan's methane by solar and Saturnian radiation -- they are separate, infrequent, and small clouds of genuine condensed liquid methane droplets, with those droplets being vastly bigger than the smog particles. Never confuse Titan's omnipresent but rarified non-methane smog haze with its small but concentrated liquid methane clouds.

And it is now considered a safe conclusion that those clouds are indeed methane, as was originally thought. Remember that -- because of the ironically rarified nature of Titan's smog as compared to the number of solid particles in Earth's lower atmosphere -- Titan's high atmospheric concentration of methane vapor has difficulty condensing into liquid droplets at all because of the shortage of smog particles to act as condensation nuclei. And so -- on those rare occasions when methane droplets do start to condense in some local area -- they themselves serve as nuclei for more methane to quickly condense around them; and so the cloud droplets very rapidly grow to large size and thus tend to rain quickly and forcefully out of the atmosphere, accounting for Titan's rare but violent rainstorms. So, whenever you do see a methane cloud on Titan, its droplets are likely to be unusually large in size -- and they are also not likely to remain suspended in the air very long.

Finally, to answer "Messenger's" question: ethane is extremely reluctant to vaporize at all at Titan's surface temperatures -- its vapor pressure there is (I believe) about 1/1000 that of methane, even in Titan's warmest equatorial region. So it is simply impossible that the clouds should contain any significant trace of liquid ethane mixed in with their liquid methane.

Bruce, with your theory of rare condensation nuclei we could imagine the following scenario:
-lower Titan atmosphere is relatively free of haze. So methane "humidity" could accumulate there.
-this lower "damp" layer is unstable, as methane vapour is lighter than nitrogen (16 to 28 molecular mass). So upwelling and convexion can occur (On Earth too damp air rises like hot air, and sometimes more strongly than hot air).
-When convexion towers up to the higher altitude haze layer, it finds condensation nuclei and rain falls.
This mechanism could govern the formation of small but violent rains. Of course solar warming could act too, favouring the summer hemisphere and day side. But basically the rythm of the formation of rain would be governed by the rythm metane vapours raises into the hazy upper atmosphere.

This process would clean the lower atmosphere of haze. But this haze could continue falling from the upper atmosphere, or mix with the lower atmosphere, driven by convexion (most of the solar energy being absorbed into the upper haze layer, we can expect that this layer has a convexion pattern, explaining the winds at high altitude).

Greetings,

In addition to the idea about condensation nuclei being rare, I think the small amount of solar energy might be an important factor in having little total rainfall on Titan. I would surmise that the balance between the two would help determine the relative humidity. So, more specifically, if condensation nuclei weren't rare, how much more methane rainfall would there be, in this scenario where solar energy input would more clearly be the limiting precipitation factor?

Not much more, I think. If methane arises from the ground, it raises until if finds condensation nuclei.

The two extreme senario are:
-there is no condensation at all (no rain), the methane being in equilibrium with the ground, and the ground emitting in infra red the solar energy received.
-all the sun energy is used to evaporate methane, which re-falls quickly on the ground. In this case the rain rate is determined by the sun energy available to evaporate methane.

We are somewhere in between. Certainly not in the first extreme, as we see clouds which evolve quickly, likely to give rain. At rough guess we are closer of the other extreme, but a more accurate answer would require a thermal model of Titan atmosphere (radiation budget, air movements, and methane cycle).

Actually, Mitri and Lunine -- and Lorenz before them, in his book -- all point out that the low input of solar energy capable of evaporating surface liquid methane back into the air must place very severe restrictions on the total yearly precipitation rate for Titan as a whole. But it seems to be the shortage of condensation nuclei which assures that such rain, instead of being very sparse but widespread, is instead very localized and violent, thus creating the runoff arroyos.




I'll have to review this question before I'm qualified to even begin to write a reply.

I wonder if we should be looking at Titan like an atmosphere laid on its side, with much greater temperature differentials pole to pole than planet surface to the extended atmospheric limit. This would naturally lead to more condensation near the poles, where solubility limits are most likely to be exceeded.

Yes this is true for the south pole, on the summer side, where an active circle of clouds was seen. But what happens on the north pole, which is now in winter? Logicaly there must be a temperature difference, and condensation (snow?) should occur here. A global heat transfer from the south hemisphere to the north hemisphere, perhaps ending near the north pole. But we don't see what happens in the north pole. What is striking is that, in the outer atmosphere, the upper layers of haze seem to actively raise from the north pole, so fast that their movement could be caught in one Cassini pass.

There is some good news about Titan's darkened North Pole. On July 22, 2006, Cassini will fly right over the moons North Pole. There will be RADAR imaging on this flyby. So we will see what is happening up there. I guess the other instruments will contribute as will.

I prefer the Dunes
http://uanews.org/cgi-bin/WebObjects/UANew...ArticleID=12614