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Planetary News: Cassini-Huygens (2006)Cassini RADAR Reveals Lakes on Titan At LastBy Emily Lakdawalla Saturn's moon Titan was once thought to conceal a global ocean of methane
and ethane beneath its smoggy atmosphere. However, while the landforms
seen by Cassini and Huygens show ample evidence of past modification by the
action of flowing liquids, actual bodies of present liquid have proven
elusive through more than two years of investigation. Until now. During
a July 22, 2006 flyby, Cassini's RADAR instrument has finally unveiled what
appears to be a land of lakes in Titan's northern polar regions.
"The news is lakes, lots of lakes!" said Cassini RADAR team member Rosaly Lopes. The lake-like features are "circular or kidney-shaped and very radar-dark -- the darkest things we have seen. Morphologically, they look much like lakes on Earth. There are drainage features around the sides of lakes." Seeing these features for the first time was a thrill for Lopes and the rest of the RADAR team. "I think it's been the most exciting pass so far," Lopes said. After it was pointed out that each swath seems to be described as the "most exciting," Lopes laughed. "Maybe we are just like that! But the amazing thing is how every pass is different. Every time we look at Titan with RADAR, the predominant thing on the swath is different from what it was last time. There hasn't been a single case of one of them being like, 'oh, yeah, it's just like the ones we've seen before.' So Titan's surface appears to be very diverse." That diversity hints at lots of fun and challenge ahead for Titan scientists. With the past failure to detect liquids at Titan's equatorial and temperate latitudes, the hope of Titan scientists to find lakes had already shifted to high latitudes, and in fact a single very lake-like feature had already been identified in a single Cassini camera image captured very close to the south pole from June 6, 2005. "Lakes are predicted at high latitudes because it's colder," Lopes explained. At lower latitudes, ambient temperatures are quite close to the boiling point of methane, but higher latitudes may be cool enough for liquids to persist. "The T16 swath spans a wide latitude range, and we see the lakes only above 75 degrees. The closer the swath gets to the north pole, the more lakes we see." The only previous RADAR swath to approach either pole was captured during the T7 flyby on September 7, 2005. Unfortunately, an anomaly aboard the spacecraft during that flyby prevented half of the T7 swath from being recorded. As a result, the swath only reached to 65 degrees south, apparently not far enough to expose lakes. "We did see a shoreline [in T7]," Lopes said. "But these lakes are actually darker than the shoreline we saw in T7. We are still doing the calibrations, but we think these are the darkest things we've seen with RADAR. It's pretty exciting." If they are lakes, they are presumably sitting in local topographic depressions. And the shapes of the topographic depressions that contain these putative lakes are as interesting as the presence of the lakes themselves. "A lot of the lakes seem to be in features that look very much like [volcanic] calderas," Lopes said. "We see all these types of features that are diagnostic of collapse. They are depressions that are not circular, they are often kidney-shaped, they have steep walls. They sometimes appear nested because of successive collapse. So we are pretty sure that they formed by collapse. Whether they formed by cryovolcanic activity or some general subsidence, we don't know; it can be quite difficult to tell a caldera apart from another type of collapse feature. We haven't seen anything around these calderas, like flows, that would be diagnostic of a volcanic feature. But they really, really look like calderas!" Lopes mentioned that her postdoctoral fellow Karl Mitchell was already working on examining the evidence for or against a cryovolcanic origin for some of the topographic depressions in the scene, which he will present at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society (DPS) in October. Mitchell says that although the RADAR images are "great stuff," he faces an uphill battle in trying to conclude anything very convincingly based on this data set alone. "The putative calderas are small and steep-rimmed," Mitchell said. "The problem is that we see no clear-cut secondary evidence for volcanism -- lava flows, domes, et cetera -- although it is possible that the bright halos around the caldera-like lakes could be proximal pyroclasts." Pyroclastic material is material that is spewed explosively from a volcanic vent and which may be very rough-textured, which would show up as bright to Cassini's RADAR instrument. "Alternatively," Mitchell continued, "They could be chemically-dissolved karst lakes or dolines, which would be consistent with the broad landscape." Karst topography on Earth occurs when subsurface rocks, most often limestones, are dissolved by groundwater, which eventually causes the surface above the underground caves to collapse as sinkholes. Broad plains may be covered with dozens or hundreds of such sinkholes. "But the scale would be an order of magnitude greater than anything similar on Earth, which would require some unusual chemistry -- at these temperatures, methane and ethane do not dissolve water ice easily. "Another possibility is maars." On Earth, a maar is a flat-floored crater that forms when hot magma encounters near-surface water; the sudden vaporization of the water produces a violent explosion that blasts out the hole. "But explosive interactions of rising cryomagmas and methane under Titan conditions should be very weak relative to magma-water interactions on Earth, and even the largest ones on Earth are only 8 kilometers in diameter," whereas the caldera-like features Mitchell is studying on Titan begin at around 10 kilometers (6 miles) and get larger. The discovery of these lake-like features is so exciting that the Cassini RADAR team has already changed their plans for future Titan SAR observations. Cassini's schedule of Titan flybys is firmly set, but on any given Titan flyby that includes RADAR imaging, the team can elect to point the spacecraft either to the left or to the right of its ground track as it crosses Titan's surface. Choosing left-looking or right-looking views selects a completely different set of latitudes for Cassini's RADAR instrument to study. Generally speaking, the choice of left- versus right-looking views has been planned out to maximize the amount and breadth of Titan that will be mapped by Cassini RADAR over the course of the mission, but in this case the RADAR team has now chosen to sacrifice some areal coverage in order to achieve a second look at the north polar region from a different angle. "After we saw the results yesterday, we have changed the look angle of the T19 pass to overlap [T16] and hit the high latitudes," Lopes said. "In T19, we are now going to take a swath that is just north of this one, so in fact we should see even more lakes -- or at least we should see how far north the lakes extend. It's going to get very close to the pole. It's even going to overlap a lower-latitude part of the [T16] swath. I don't know yet whether we will have overlap over one of the lakes, but the two swaths will be almost contiguous." Any areas where there is overlapping coverage will then be seen from two different look angles, which might permit some information about topography to be extracted from the data. The T19 flyby will take place on October 9 of this year, during the DPS meeting. All eyes will be on Titan to see if it gives up more evidence for methane and ethane lakes, or chooses to remain enigmatic. |
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