Download this image in high resolution (1.2 megs) (Credit: H.G. Roe, Caltech)
Saturn's largest moon Titan is a wonderfully bizarre place. On Earth we have weather because conditions are right for water (H2O) to cycle between liquid, solid, and gaseous states. Temperatures on Titan are so cold that all the water is frozen solid on the surface, but the temperature and pressure conditions are right for methane (CH4) to exist as a liquid, solid, or gas. For several years we've observed methane clouds forming and raining out near Titan's south pole. When the Huygens probe descended through Titan's atmosphere and landed on the surface it showed images of numerous stream channels and what may be a pebbly dry lake bed, all suggesting that liquid methane regularly flows on the surface of Titan. About two years ago we discovered the first clouds on Titan that were not at the south pole. At that time we were unsure how these mid-latitude clouds at about 40°S latitude formed, how frequently they occurred, how often they appeared in the same locations...basically, we didn't know very much. What we did know was that these mid-latitude clouds were not formed by the same mechanism as the south polar clouds we'd been observing. (The south polar clouds are formed because it was late southern spring, now southern summer, on Titan and the south polar region received more sunlight than anywhere else on the planet, thus heating the surface in the south polar region just enough to generate long-lived convective clouds. In terms of the solar heating of the surface there was nothing special about the area around 40°S latitude and the solar heating wouldn't be enough to form clouds there at 40°S without also forming clouds all over the planet at other latitudes as well.)
Now we've observed a whole lot more of these clouds during 82 nights of observing on the W.M. Keck 10-meter and Gemini North 8-meter telescopes and have discovered that they cluster mostly over one region on Titan's surface, located near 40°S latitude, 350°W longitude. While we don't fully understand how these clouds are formed, it is clear that something funny is going on on Titan's surface in this region to generate the clouds. We think the most likely answer is that there is a source of methane venting into the atmosphere, perhaps cryovolcanoes, geysering, or cracking in the icy surface. We've published this in a paper in the October 21, 2005 issue of Science.
Here are pictures of these clouds from a few nights:
Download this image in high resolution (1.2 megs) (Credit: H.G. Roe, Caltech)
In these images taken with the W.M. Keck Observatory's 10-m telescope we are seeing Titan in a filter that probes down into the lower troposphere, where clouds are active, but not all the way to the surface. All over the planet we see the global stratospheric haze. The mid-latitude clouds are the streaky looking things a little below the center of each image. In September there was also a small cloud near Titan's south pole, at the very bottom of the image. In October Titan's south pole was swamped by a massive storm (the largest yet seen on Titan).
With the new technology of "adaptive optics" we can push large telescopes such as the W.M. Keck II 10-meter and Gemini North 8-meter to near their theoretical limit of resolution. Adaptive optics essentially allows us to "detwinkle" images of stars, planets, moons (Titan!), galaxies, and whatever else we might want to point the telescope at. Without adaptive optics Titan would be so blurred out that we would see only a big fuzzy blob. With adaptive optics we can see features on Titan as small as a few hundred kilometers across. That resolution is still not nearly as good as what the Cassini spacecraft can see when it flys by Titan, so we only see the largest clouds and storms. From these ground-based telescopes we can observe Titan much more often (sometimes every night) than Cassini can fly by Titan. So, for very high resolution details of individual clouds we should look at data from the Cassini spacecraft, but for the statistics of where/when the clouds appear, we should look to the ground-based data.
The locations of all the mid-latitude clouds we have observed to-date are shown over a map of Titan's surface:
Notice how the clouds seem to mostly appear right around 0°W longitude, with just a few clouds appearing at other longitudes. This is even more dramatically shown when we count how often a cloud appeared over each longitude range of Titan's surface:
A few snapshots of Titan's mid-latitude clouds are shown on the figure at the top of this web page. They typically are stretched out in the East-west direction. Most of the time we only take one image each night of Titan (time on large telescopes is very precious!) and we've found these mid-latitude clouds are always very short-lived. We've often taken images on three nights in a row; on night 1 we see no clouds, on night 2 we see clouds, and on night 3 the clouds are gone again. So, that says that the cloud lifetime is typically 24 hours or less.
There are several ways to form clouds on Titan. These include:
- Wind blowing across a raised surface feature (such as a mountain). These are called "orographic clouds" and are often observed here on Earth (in Hawaii and along the west coast of the U.S.).
- Heating the surface. If the surface temperature is raised a few degrees this forces slightly stronger convection in the lowest layer of the atmosphere, which can lead to methane condensation and the formation of convective clouds. This is similar to what happens on Earth in the southwest United States during late summer when the sun heats the desert floor during the day and can generate huge thunderstorms. This is the mechanism thought to form the south polar clouds on Titan that we have been observing for several years.
- Circulation patterns that generate uplift regions. In regions where surface winds converge you get uplift (all that colliding air has to go somewhere). As the air is pushed upward it cools and this can trigger cloud formation. On Earth we see this type of phenomenon near the equator.
- Injecting methane. Titan's atmosphere near the surface is not usually saturated with methane, but if there is a mechanism that injects a bunch of gaseous methane into the lower atmosphere and saturates the atmosphere with methane, then clouds will form.
We know that these mid-latitude clouds appear primarily over one small region of Titan's surface (near 40°S, 350°W), but they do not always appear in the exact same location. This rules out orographic clouds, since mountains can't pick themselves up and move around. We think that the short lifetimes of the clouds rule out a change in surface temperature as the cause, since Titan's surface has a very high thermal inertia, meaning it is very slow to change temperature. For surface temperature to cause these mid-latitude clouds would require the surface to heat up rapidly and cool back down rapidly, which is not likely.
Uplift zones due to wind circulation patterns is a very appealing possibility, especially since it is such a common and familiar mechanism here on Earth. The problem is that if there is a convergence zone at 40°S latitude, then we should see clouds all the way around Titan at all longitudes. We don't; the clouds are clustered very strongly near 350°W. Circulation may be part of the story to explain these clouds, but there has to be some other mechanism causing the clouds to appear so regularly over one region of Titan's surface. We've thought through all the atmospheric explanations for this behavior and something funky has to be going on on the surface of Titan in this region to generate these clouds.
Due to its orbit about Saturn, Titan has tides somewhat like the tides here on Earth. These Titanian tides cause the local winds in Titan's atmosphere to shift around, much like tides on Earth cause the local currents in the ocean to vary. If these tidal winds were to blame for the mid-latitude clouds then we should see a correlation between when/where the clouds appear and the position of Titan in its orbit about Saturn. We don't see any correlation, so that rules out the tidal winds as the driving force behind creating these clouds. (The cloud motions after they are formed may still be affected by these tidal winds.) We also don't see any correlation between clouds and time-of-day on Titan, which rules out some type of diurnal effect.
Release of methane from Titan's surface will generate a cloud so long as enough methane is released to saturate the atmosphere with methane in a small region of the atmosphere around the source of the methane. This region can be quite small, maybe just a few hundred meters high and a few hundreds to thousands of meters across. Saturating this small area of Titan's atmosphere will lead to condensation, which releases thermal energy, which drives convection, which causes more condensation, and before you know it there's a towering convective cloud. Winds at higher altitudes can then shear out the cloud into the elongated shapes we observe.
For years people have discussed the possibility of cryovolcanoes on Titan's surface. A cryovolcano is somewhat like a volcano here on Earth, except the lava is made out of a super-cold mixture of ammonia and water (instead of molten rock) and on Titan volcanoes probably do a lot more 'oozing' than explosively blowing their tops (Mount St. Helens). The 'lava' in a cryovolcano would also have a lot of dissolved methane, which would be released into the atmosphere as the lava oozes out of the cryovolcanic dome.
There's also the possibility that geysers (again of a super-cold mixture of water and ammonia) may spout off from Titan's surface regularly. These geysers would also release methane from the subsurface reservoirs that are thought to exist.
Another possibility is that Titan's icy surface is under stress and that cracks may occasionally be opened up that would release methane from those subsurface reservoirs. The cracks would freeze back over again in a short period of time, which would fit with the short lifetimes we observe for the mid-latitude clouds.
We don't know which of these mechanisms is actually occurring on Titan and responsible for the mid-latitude clouds, but we can say that 40°S, 350°W is the most likely place to find geologic processes currently active on Titan.
We know that something funny is going on on Titan's surface near 40°S, 350°W to generate these tightly clustered clouds. General circulation patterns cannot alone explain this behavior. We think that the most likely explanation is that there is a region of active geology, perhaps geysering, cryovolcanism, or cracking, near here that is releasing methane and triggering the formation of these clouds. Since we don't always see the clouds in the exact same location, it's probably not a single cryovolcano or geyser, but instead a whole region of geologic activity. In fact, we think there are regions of geologic activity scattered about Titan's surface and that this 40°S, 350°W region is simply the strongest and most active at the present time.
The winds on Titan blow from west to east (like on Earth), so we don't think the few scattered clouds we see at 90-150°W longitude are simply longer-lived clouds blown downwind from the source at 0°W. (If they were, then we should have also occasionally seen clouds at 160-300°W; We have seen no clouds in this region.) If you're a weaker, smaller source of methane then having a strong source of methane (such as the 40°S, 350°W region) upwind of you makes it easier to form clouds. To form clouds you need to saturate a piece of the atmosphere above you. This is harder if the atmosphere is very dry, but easier if it is more humid. The big source probably doesn't saturate the whole band around Titan at 40°S (that would take a lot more methane than we think is likely), but it would easily increase the methane in smaller parcels of air that are blown around Titan like weather fronts move here on Earth. When one of these higher-humidity fronts crosses over a weaker source of methane conditions become ripe for another cloud to form. Because Titan's winds are predominantly West-East these humid parcels stay at about the same latitude as the source that released them. Therefore we expect weaker sources to be more visible at the same latitude as the major source. If moved to a different latitude, this smaller source region (around 40°S, 100°W) would not generate cloud activity large enough for us to see from Earth. (With groundbased telescopes we only see the largest cloud events. Cassini is finding that little tiny clouds are not uncommon over much of Titan, but we are interested in the largest clouds.)
We keep observing. By observing more of these clouds we will better pin down the statistics of when and where they appear and how long they last. Also, Titan's seasons are advancing. Southern summer solstice was in October 2002 and we're now well into southern summer. The southern autumnal equinox is just a few earth years away in August 2009! If these clouds are the result of general circulation then we should see their locations move northward as the seasons progress.
In September Cassini took radar data near the region of cloud activity during a flyby. We haven't actually seen these data yet, but they may reveal what type of geographic features lie on Titan's surface in this region.
