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Planetary News: Cassini-Huygens (2006)Cassini RADAR: Another Flyby, Another Completely Different View of TitanBy Emily LakdawallaMay 23, 2006
The Cassini Saturn orbiter has now completed fifteen close
passes by Saturn's enigmatic moon Titan. On five of those passes, Cassini
pointed its 2.7-meter-diameter radio dish at the moon to acquire synthetic
aperture radar (SAR) images of the surface, a technique that is unaffected
by the haze that has so frustrated scientists’ attempts to see Titan's
landscape with optical instruments. And each one of the long SAR image
strips or "swaths" has contained patches of terrain unlike those
seen in any previous image. "Titan is proving to be, if not bewilderingly
complex, then at least intriguingly complex," says Ralph Lorenz, a member
of the RADAR instrument team. The first SAR swath, acquired on October 26, 2004, contained incomprehensibly patchy terrain and a few features that appear to be cryovolcanic, such as Ganesa Macula, a flat-topped, steep-sided, 160-kilometer-diameter dome that could be a volcanic edifice. The second swath, from February 15, 2005, contained two definite impact structures, one large (Menrva) and one small (Sinlap), as well as patchy fields of dark streaks dubbed "cat scratches" by the RADAR team, and features that looked like channels draining into a rounded basin. The third swath, a south polar one captured on September 7, 2005, contained massive quantities of incised channels. The fourth, on October 28, 2005, covered vast fields of "cat scratches" and contained what looked like linear chains of mountains. And the fifth, captured on April 30 of this year, crossed rough mountains, incised with more channels. Titan's Sandy SeasThe most consistent story that has come out of the five swaths of Cassini RADAR data across Titan has been the reasonably firm identification of the "cat scratches" as longitudinal sand dunes. In a paper published on May 5 by Lorenz and a host of coauthors in Science magazine, the RADAR team explains why the cat scratches look like dunes -- and why that would make sense on Titan, previously considered a possible place for seas of liquid methane, not seas of sand.
The case was clinched by the October 28, 2005 swath, referred to as "T8" because it was captured on the Titan flyby that occurred during Cassini's orbit number 8 of Saturn. Prior to T8, Lorenz explained, "we'd seen these dark streaks, but it wasn't really clear what they were. They could have been seeps of liquid coming out of the ground and forming streaks of something, or they could have been erosive rather than depositional -- the best guess was that they were aeolian, but then when we saw similar things in T8 that had this clear topographic expression, it became really obvious what they were. And the way they deviated around and reconnected behind these little hills really showed they're longitudinal." Longitudinal sand dunes -- as opposed to transverse sand dunes -- form when winds tend to oscillate back and forth between two different directions instead of blowing constantly in one direction. Titan's wind regime is dominated by a "zonal" pattern, in which its atmosphere is blown primarily from east to west. However, superimposed on that zonal pattern is a rotating wind pattern caused by the tides raised by Titan's 16-day orbit around Saturn. "The tidal cycle gives you this fluctuating or rotating wind vector. When you mix that with the zonal wind, you get the general east-west flow; but south of the equator, the tidal winds have a northerly component, and that's the orientation that the dunes have on the large scale in T8," Lorenz explained. In other words, the general flow is always from west to east, but the tides would cause a Titanian weather vane to veer a little bit to the north of west, then a little bit to the south of west every day, pushing the piles of sand from alternate sides from morning to afternoon, and building dunes that are aligned with the average direction of the wind's flow. Nearly all of the "cat scratches" seen so far are parallel and longitudinal, but there is one spot where the dunes' flow pattern seems to have been frustrated. "It's evident that the winds are affected by topography," Lorenz said. "There is one spot where it looks like there's a big topographic obstacle, though we don't know how high it is. Not only do the dunes go around it, but it seems the flow gets straightened in this little patch of what seems to be transverse dunes before the longitudinal pattern picks up again. What I think is happening there is that the flow always has a west-to-east component that's being fluctuated north or south due to the tidal cycle, but next to that obstacle the flow's got no choice but to go through the gap, and it's always in the one direction, so you get these transverse dunes."
Lorenz is relieved to be able to speak with certainty about some aspect of Titan, even though it didn't match anyone's predictions. "With T8 we get a real sense of stuff going on on Titan that actually fits into a coherent picture, which is obviously very nice after all these years. It's of course by no means the picture we expected when we were designing Cassini. There were all those nice maps from Keck, from Antonin Bouchez and Mike Brown and gang, which had these very prominent dark areas near the equator, and of course the first thing that leaps to everyone's mind is 'seas of hydrocarbons, there they are.' …it's human instinct, I suppose, that if something's dark, you call it a 'sea' -- look at the lunar basins. But these turn out to be seas of sand." That the sand seas seem to be more concentrated in the equatorial regions of Titan, seen in the February 15, 2005 (T3) and September 7, 2005 (T8) RADAR images, is also consistent with the tidal flow story, Lorenz argued. "The diurnally averaged tidal wind generally flows equator-ward. That would tend, perhaps, to sweep the sand into these low-latitude regions where we see these dark patches. We see dark areas elsewhere on Titan that we've observed with the RADAR -- Ta had them. What are we seeing there? Maybe there's just a thin layer of sand that hasn't been swept into dunes. Maybe the wind isn't strong enough there to sweep it into dunes. Or maybe they're dark for some other reason -- they're coated with something. There's a climate model that poses that low latitudes on Titan should be desiccated -- they should dry out. Which is consistent with dunes everywhere at low latitudes. That same model predicts that high latitudes should be saturated [with methane]. If we are expecting to find lakes of liquid hydrocarbon, it will be at high latitude." Xanadu: A Different Story
The most recently acquired SAR swath, referred to as "T13," is significant because it crosses a region that has been imaged repeatedly by the ISS and VIMS instruments, permitting comparisons of the optical and radar data. The SAR images have yielded such fine details in other areas on Titan that scientists hoped the April 30 SAR data would answer some of the questions about this dark region, known as Shangri-la, and the brighter "islands" within it, including Shikoku Facula (formerly referred to as "Great Britain" because of its shape), Kerguelen Facula, and Guabonito crater. But Titan, as ever, doesn't yield secrets so easily. "I have to have to confess I was a little disappointed with T13 and Guabonito," Lorenz said. "It was known from the optical imagery that [Guabonito] was this sort of ring of bright things that looked like it might be a ring of hills poking up above some dark goop. We now know the dark goop is sand dunes, or at least equatorial dark stuff seems to be sand dunes. And so I was vaguely hoping for something looking a bit like the Aorounga structure in Chad. Aorounga is an impact crater that was seen optically first, but which is much more easily seen in radar because it's partly covered in sand dunes, and the radar sees through the dunes rather better than the optical stuff does; you can see a lot more geological structure to it. "But to a first order, at RADAR wavelengths, Guabonito looks just as it does optically," Lorenz went on. "You can see a bit more detail, a bit more streakiness to the hills, but the ring of hills is very broad. It doesn't scream 'impact crater.' But on the other hand it would be an exceptionally large volcanic structure, I don't know offhand what else would give you a 80-90 kilometer ring like that. So while it remains a good candidate impact structure, I guess I wouldn't say RADAR has made it much more conclusive than the suggestion from the optical appearance was."
The bright "islands" in Shangri-La are also puzzling. Especially puzzling is Shikoku Facula, which contains a large dark spot in the RADAR images that has no visible expression in optical images. But most other aspects of Shikoku -- its boundaries and the narrow channels that cross it -- are strongly correlated between the RADAR and optical images. The T13 swath did reveal what could be a few previously unrecognized impact craters on Titan, in the mountainous interior of the bright region known as Xanadu. One of them is clearly a circular structure with a raised rim and a flat floor, making it look "pretty much like a degraded impact structure; I don't think there's much argument with that one," Lorenz said. But another candidate crater is a little more puzzling. There is "a sort of circular basin with a central peak. And that's particularly interesting because the thing's the same size as Sinlap, in T3, yet Sinlap is flat-floored; there's no central peak." This new feature may not be a crater, but if it is, there could be something different going on under Xanadu than under the region where Sinlap formed. "That may well be telling us something about the crust or the lithospheric thickness or the rigidity of the crust over Xanadu being different from elsewhere. That would be an intriguing suggestion, but we need to do a bit more thinking about that."
While the craters are puzzling, Lorenz said, "T13 has largely met my expectations, in the sense that we had seen some of Xanadu with the scatterometer on previous flybys, and we knew it was bright at a range of incidence angles, which really suggested that it would be rough. There had been an idea that maybe it was mountainous or something. And it sure looks rough, so we were right; we're getting the hang of this now." The mountains of Xanadu lack any obvious tectonic pattern, unlike the linear chains of hills seen in the T8 swath. And they are dissected by numerous narrow channels. "The channels definitely look fluvial, they're very dendritic, they're about the longest ones we've seen [in RADAR]. This is now the third big set of channels I'd say we've seen with the RADAR -- we saw some in T7, some in T3. They're all of somewhat different character." However, Lorenz cautioned that the RADAR team has not yet developed a clear view of the diversity in channels on Titan. "New data keep getting in the way! But I guess I shouldn't be too upset," he remarked. The channels in Xanadu "look a bit like they flowed south, and I haven't figured out yet whether that makes any sense or not, whether that means they're flowing away from the center of Xanadu," which is assumed to be topographically high. In fact, "it looks a bit like they're flowing away from the edge of Xanadu toward the interior -- but it's really too early to be sure; I haven't really spent a whole lot of time with the data yet." What's NextThe next SAR swath is currently planned for the T16 flyby, which will take place in two months, on July 22. "T16 goes more or less over the north pole," Lorenz said. "I am confidently predicting a completely different Titan yet again, and maybe, maybe, the long-lost lakes. If we don't see any evidence of ponds or liquid on T16 I have to confess I will start to wonder if they're really there at all on a visible scale." Cassini is nearing the end of a long phase in which it has seen only equatorial views of Titan and the other moons, from an orbit within Saturn's ring plane. That T16 flyby on July 22, taken over Titan's pole instead of its equator, will swing Cassini out of the ring plane into a new phase of the mission. Cassini's orbit will be shifting to higher and higher inclinations, above and below the poles of Saturn and its moons. The result will be new views of not only Titan's poles, but the first-ever "bull's-eye" images of Saturn's globe neither blocking nor being blocked by its encircling rings. |
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