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Planetary News: Cassini-Huygens (2005)The Planetary Society Weblog:Special Coverage of the 2005 Meeting of the Division for Planetary SciencesBy Emily LakdawallaThis page collects the Weblog entries done as part of my special coverage of the 2005 Meeting of the Division for Planetary Sciences of the American Astronomical Society, Cambridge, England, from September 4-9, 2005. Sep 2, 2005 | 13:44 PDT | 20:44 UTCOn the way to DPSI'm leaving shortly for a week in sunny(?) Cambridge, England, to attend the annual Division for Planetary Sciences meeting. I should have pretty good Internet access while the meeting is going on, and will try to make a note of anything exciting I hear in this space. I've had a thorough look through the program and I'm facing some difficult choices. DPS typically runs two concurrent sessions and sometimes I want to attend both. This happens a lot at scientific meetings and can be particularly bad at the annual Lunar and Planetary Science Conference where they run three or four concurrent sessions. Each session is one or two hours long and usually contains about 5 or 10 talks by different researchers. The researchers usually get about 10 or 15 minutes to speak, and then there is supposed to be about 5 minutes of questions from the audience. Of course, researchers are career lecturers who don't often get a chance to present their work to their peers, so they typically run over their time and the Q and A doesn't always happen. Some people are worse about this than others, and I am sure they know who they are! Usually it's fairly easy to pick out the talks I want to attend most, and I jump back and forth from lecture hall to lecture hall. But the schedule I am looking at right now is packed with interesting new stuff from Mars, Deep Impact, Cassini, and telescopic observations of the outer solar system, and I am having a difficult time planning what to attend. It's worse this year because it looks like the two lecture halls are in different buildings, so I can't so easily shuttle between halls -- I will really have to commit to full sessions. Here's what's on tap for the meeting:
When I was a student I'd attend maybe half the meeting and spend the other half just hanging out with other students. But as time has gone on, and I am less directly involved with research, I find myself wanting more and more to attend the entire meeting. It's going to be a really long week though -- I don't know if I will have the stamina to make it through every session every day! I expect to be drinking lots of coffee. I'll try to take lots of notes, and post them here. I'm signing off for a couple of days -- you'll hear from me next from Cambridge! Sep 5, 2005 | 11:01 PDT | 17:01 UTCDPS: Day 1, at 7:01 p.m. Cambridge timeI can already tell that DPS is going to be overwhelming. I'm trying to be both a scientist and a member of the press, attending both the talks and the press conferences, and I think that's not going to work too well because it's going to leave little time to do anything else at all -- either to write entries here or to eat. Also, it appears that jet lag has robbed me of my ability to multitask, so today I haven't been able to update this weblog simultaneously as I listen to talks. I have 10 pages of notes and no hope of translating them all here tonight -- I will have to jot down a few key notes, and explain things in more detail later. And let me apologize right now for this write up having no illustrations. I only had an hour or so to write thus up before I was going to lose my Internet connection for the night. Hopefully I can add some in later. There were a lot of illustrations released today on JPL's Planetary Photojournal. Most of today has been about Cassini-Huygens results. Several scientists spent rather too much time explaining data that has been out there for a year rather than focusing on the new; poor Mike Bird even repeated yet again how his Doppler Wind Experiment should have worked, rather than showing us new results. They still have not extracted any wind speed data from any of the VLBI antennas that contributed to the Earth-based reception of their instrument's signal, so they still have a 26-minute gap in their data. His final slide said that Huygens drifted 3.75 degrees in longitude to the east, or 166 kilometers, from its entry into the Titan atmosphere. There was a lot of new detail in Huygens Atmospheric Science Instrument (HASI) results reported by Marcello Fulchignoni. HASI is the instrument that had the acoustic sensor that we used to create the sounds of the Huygens descent. Marcello's accelerometer data shows a lot of inversion layers in the thermosphere, at elevations of 1,020, 980, 800, 680, 600, and 510 kilometers. (If you're wondering what that means, I'm not an atmospheric scientist, so I don't know. But it was a graph that looked different from any I'd seen before so I thought it might be important.) Marcello's accelerometer data also shows a double-peak at impact, indicating an impact into "a surface that was not hard." This is not a new result, of course, but it's always really nice to see one instrument confirm a conclusion drawn from data taken from a completely different instrument. There were two little mysteries that Marcello mentioned, but didn't explain. First, his instrument included a permittivity sensor. Permittivity is one physical property you can measure for a substance, having to do with how much it resists the flow of an electric charge. Well, HASI recorded an increase in the permittivity of the Titan surface at 12 minutes after impact. They don't know what it means, and are working first to rule out anything that might be coming from within Huygens to cause that observation. Second, he mentioned that they MAY be seeing some electrical discharge events (read: lightning) in their data. Again, though, they must first rule out anything that was happening aboard the spacecraft as the cause. But to do that, they need data from all the other science teams, and individual teams are often reluctant or at least slow to share the necessary data. John Zarnecki gave a talk about the Huygens Surface Science Package results which mostly covered stuff I'd seen before. One detail I hadn't seen before was the fact that they observed a "very gradual but definite tilting" of the spacecraft as it was resting on the ground after landing, a tilt at a rate of 0.2 degrees per hour. He didn't have any explanation for that -- it was just a curious observation. He also mentioned that the part of his instrument that measured sound speed in Titan's atmosphere showed a different sound speed after landing than before. Before landing, the sound speed rose monotonically with Huygens' descent until it reached 192 meters per second at the surface. After landing, the sound speed dropped by a few meters per second, then it slowly rose again. Zarnecki said that given the Gas Chromatograph Mass Spectrometer results showing that gas was released from the surface as Huygens heated it up, the rising sound speed may be due to that gas getting into the sound speed detector. But, he emphasized, the "surface is work still in hand." Larry Soderblom talked about the Huygens DISR imaging of Titan during descent. He mentioned that his colleague Bashar Rizk thinks that the orientation of the mosaics they've released may be off of true north by 10 or 15 degrees in the counterclockwise direction. They still have not determined exactly where the landing site is. Larry showed a couple of possible locations on the VIMS image of the landing area (I will try to get a map and point those out later). But Larry confirmed that during the T8 Titan flyby on October 28 of this year, they will be able to deflect the beam to a higher latitude than originally planned and thus get RADAR SAR imaging across the landing site. Larry was pretty confident that the SAR images will allow them to pin down the exact location of the landing site with respect to the ISS imagery. (Larry is on both the DISR and the RADAR teams so he has access to all that data.) Larry also mentioned that the 3D views they've generated of the Huygens landing site have no vertical exaggeration. It is very rugged terrain, with slopes up to 30 degrees. So that was the morning session. Instead of taking a leisurely lunch, I ran to the press conference room where the first conference had already started. Radio scientist Essam Marouf was talking about some of the radio science ring occultation work they'd been doing. He was very excited about what the ring occultations were showing in the structure of the B ring, which hasn't been exposed before. He saw a lot of complex braiding structure that he called "totally new." The structure he saw in the B ring was radially symmetric, as far as he could tell, except at the outer edge. The A ring was a different story. Next, Josh Colwell talked about UVIS observations of stellar occultations by Saturn's rings. The optical depth of Saturn's rings changes depending upon the angle from which you view them. He explained that this had to do with self-gravitational clumps in the rings, clumps that are too small to be resolved with cameras. If you look perpendicular to the rings, you can see light peeking through gaps among the clumps, so they look pretty transparent. If you look at a shallower angle, though, the clumps block your view through the gaps, and the rings are more opaque. Because they have "about a dozen" measurements of optical depths in the A ring at different geometries, they can say something about the shape, size, and orientation of the clumps. The vertical height of the clumps increases as you move out in the A ring. The clumps are only 10% as vertically tall as they are wide. The spacing between them increases as you move out from Saturn. They are prominent only in the inner 2/3 of the A ring. As you go further out in the A ring, particle size drops (as seen in the radio occultations), self-gravity is not as strong, and there not any more clumps. Linda Spilker was up next and she had some very cool results from the Composite Infrared Spectrometer, or CIRS. From the Earth, she pointed out, we can only view the sunlit, day side of the rings. But Cassini orbits all the way around Saturn and can see the planet and its rings from the day side and night side, and can view the rings from both below the ring plane (where they are sunlit) and from above the ring plane (on the shadowed side). CIRS sees major temperature variations in the rings depending upon this geometry. It should be no surprise that the hottest temperatures are seen when Cassini is on the day side of Saturn and below the ring plane. From that geometry, the ring particles are brightly sunlit and have temperatures of 90 to 110 Kelvin. But when Cassini is on the night side of Saturn, still below the ring plane, it is seeing only a "crescent" view of the ring particles -- which is to say, it can't resolve any individual ring particles, but the ring particles are showing their night sides to Cassini. From that point of view, CIRS sees a 15 degree drop in temperature. So, who cares? It's actually really important, because it means that the ring particles rotate very slowly, possibly as slowly as only one rotation every revolution around Saturn. You can conclude that because the ring particles have enough time for their "night" sides to cool off when they are not being lit by the Sun, causing CIRS to see that drop in temperature on the night side. You wouldn't expect such slow rotation from most ring particle formation models. You'd expect that collisions between particles would give them a wide variety of spins, some fast, some slow, some even retrograde. But CIRS says that the ring particles -- at least the ones sending radiation to CIRS -- are slow rotators. They probably also are very very porous, which helps them to cool off very quickly. Then Carolyn Porco talked about a whole lot of ring images she released today. Three highlights. One: A new view of the D ring -- the innermost ring, fainter than the A, B, or C rings -- shows that some features have remained constant since Voyager saw them, but at least one of the D ringlets has moved toward Saturn by 200 kilometers, and has brightened by a factor of 10, in the last 25 years. Two: There is an arc in the G ring that "bears some similarity to Neptune's ring arcs" and "may be kept in place by a nearby moon which we have yet to find." Three: The faint ringlet inside of the F ring core is actually one long spiral ring that may have to do with the moonlet S/2004 S6, which could be a moon or could just be a temporary clump of matter, that crosses back and forth across the F ring somehow. I have a lot more stuff from Carolyn -- I will organize it into a coherent story and post it here later this week or next week. Finally, Kevin Baines talked about Visual and Infrared Mapping Spectrometer (VIMS) results that show clouds at depths of 50 to 100 kilometers below the cloud layers that are visible in regular camera images of Saturn. At this level, Saturn really looks like Jupiter, with stripy lanes of clouds and whirls of storms. He says it reminds him of when Voyager first saw all Saturn's rings, revealing much more detail and structure than they'd ever even imagined. He didn't have explanations for all the features, particularly the "doughnuts" visible at high northern latitudes. As with Carolyn I have a ton more stuff from Kevin, and I'll post it later when I can get a few hours to organize all the info and pictures. So then it was into the afternoon talk sessions, full of Cassini. First up was Michele Dougherty, on the Cassini Magnetometer team. She went into some detail on how MAG detected the atmosphere at Enceladus. (I want to try to talk her into letting me post one of her slides showing the three flybys of Enceladus and how the direction and strength of the magnetic field changed.) Not only did she say that there was definitely a south polar atmosphere at Enceladus -- and definitely not an isotropic atmosphere -- she dropped vague hints that there might be "an intrinsic signature of some kind" at Enceladus. Hmmm..... Ralf Srama talked about CDA results. He said that the E ring is "directly related to Enceladus." He said that in the neighborhood of Enceladus there are three distinct dust populations. There is an E ring particle background, there is secondary ejecta coming isotropically from Enceladus, kicked up by impacts on its surface, and there is the stuff coming from the south polar "hot spot." He also said that the E ring is apparently much larger than has previously been thought, but was vague as to how large that is, saying that many more measurements are needed. The E ring particle composition is almost pure water ice. Water ions cluster into large complexes of up to 12 molecules. The E ring most likely extends out to Titan's orbit, and may extend 2 Saturn radii vertically (about 120,000 kilometers) at a distance of 2 Saturn radii from Saturn. That's big. Tom Krimigis talked about MIMI measurements at Saturn. He said that the plasma environment at Saturn is dominated completely by water and its ions. This is a big contrast to Earth, which has a lot of nitrogen and oxygen ions in addition to water, and to Jupiter, where you have sulfur and oxygen ions. I don't know what that signifies but the contrast was interesting. Steve Wall spoke for Charles Elachi about RADAR. He showed a very nice image from the Earth orbiting RADAR satellite ERS-1 of "snow dunes" in Antarctica as a possible analog for the "cat scratches" seen in RADAR images of Titan. In RADAR the snow dunes look like black squiggles across a bright surface. He said that on the ground, the snow dunes have almost no vertical expression. They appear different to RADAR because of different grain sizes in the snow. Also, Wall mentioned that RADAR could be penetrating "many many 2-centimeter wavelengths," like 10 or 20 wavelengths, into the surface of Titan. Bob Brown talked about VIMS results. He showed slides of Iapetus and Enceladus, and said that they were compositionally diverse, but not as diverse as Phoebe. He also explained that VIMS image of Enceladus that was released earlier this week. The "simple hydrocarbons" that are mentioned are simply C-H bonds. The VIMS team can't yet identify what hydrocarbon molecules are there on Enceladus, but they know they have detected C-H bonds in the materials along the tiger stripes. More important, Brown said that "There is no ammonia there at all" on Enceladus. "You can cross out ammonia-water melt as the source of any volcanism." While they were switching speakers, Essam Marouf (remember, he's the Cassini radio science guy), who was sitting next to me, was fidgeting. He explained to me that there was a ring occultation happening right now but he couldn't get the wireless access to work to check on the progress. I asked him, isn't this their last one? He said "Yes, it's the last one for a year," and that he was having a tough time deciding whether to stay or to go find some Internet access so he could find out how it was going. He eventually bailed. Michael Flasar talked about CIRS results. He had lots of interesting graphs showing different wind speeds at different elevations within Saturn but they flashed by too fast for me to actually get anything useful written down about them. He then talked about the thermal inertia of Enceladus. (Thermal inertia is a measure of how fast something cools when it's no longer sunlit. Liquid water on Earth has very high thermal inertia; rocks are intermediate; and sand and dust have very low thermal inertia. That's why oceanside climates are temperate and desert climates are extreme.) He said that the thermal inertia of Enceladus was "fairly low, lower than Phoebe, and much lower than the Galilean satellites." Which means that Enceladus' surface material must be "fairly unconsolidated." The thermal inertia is lower at the equator than at the mid-southern latitudes. That sounds like (and this is my interpretation) Enceladus is covered with a material like fresh, fluffy snow, which would also be consistent with how bright it is. Then Flasar talked about temperatures in Titan's atmosphere. He went on at length about how Titan has a warm winter polar mesosphere, which doesn't mean a lot to me but which seemed important from an atmospheric science perspective. Also he mentioned that the altitude to the stratopause decreases from the north pole to the equator, then increases again slightly to the south pole, something which he found very interesting but didn't have an explanation for. Phew! That's about all I can say from today. Again, I apologize for the lack of illustrations. I will try to come back and illustrate this tomorrow, and I will definitely have pretty pictures for you in more formal news write ups later this week or early next week. Sep 6, 2005 | 06:08 PDT | 13:08 UTCDPS: Day 2, after the morning Mars sessionsThis morning started off with a bang, back-to-back talks by Steve Squyres and Gerhard Neukum. Now those of you who know those two people may be surprised that I'm excited about a Neukum talk -- he is well known as one who goes on and on in high-speed accented English about dry stuff like crater production functions. But his talk was pretty exciting this morning because of the amazing HRSC images he showed. But let me get to Steve first. I ran into him in the foyer on Sunday when I arrived to register. It's been a while since I saw him, and he is definitely skinnier and grayer than he was before the rovers landed -- it hasn't been an easy couple of years for him, for sure. He's exhausted from his book tour, he said (he has a new book out called Roving Mars, which I haven't read but which I hear is really excellent). He's been away from rover operations for more than a week and he can't wait to get home and get back to operations. So, here are my notes from his talk this morning. I know that some of this has been covered in A. J. S. Rayl's articles about the rovers but I thought it was illuminating to see what Steve thought was important and interesting to convey to his peers. He began with Spirit. "Since the last DPS it has been a long, arduous climb. As of the last meeting we were down on West Spur; as of today we stand on top of Husband Hill." He showed a graphic with the Statue of Liberty for scale compared to Husband Hill. Then he commented that since this meeting was in Britain he'd better use a different scale, and he added in Big Ben, which got a chuckle from the audience. "There are at least five geochemical classes of rocks in the Columbia Hills," Steve went on. He said that there was tremendous diversity in chemical composition, and then proceeded through the five rock classes. Each class of rocks is named for the first or type occurrence of the rock in a sample examined by Spirit. I copy here his commentary on the nature of each of the different types of rocks -- if you don't get as excited by geology as I do, you might want to skip this part.
More... on the summit, they have come across another kind of basalt, called Cherry Bomb, tentatively identified as a distinct class. In the last few days they found another exposure at a site called Irvine. It's basalt, but another different flavor. And Irvine seems to be a linear array of rocks. It might actually be a surface expression of a dike, in place! Steve then showed some pieces of the recent summit panorama. He showed the eastern one and said they will not go in that direction. Instead, they are planning to go south, toward Home Plate. "We are currently in the process of planning our traverse down." He talked about dust devils but Mark Lemmon talked about those more later so I'll reserve that. He showed one of Mark Lemmon's processed images of Phobos taken by Spirit. Mark had shown these to me yesterday, saying that they hoped to release them this week. He pointed out that "you can actually see Stickney crater" in the image and there was a great "ooh!" from the audience. "We're doing planetary geology!" Steve said. Then he went on to Opportunity. He showed the image of Burns cliff and told the crowd "This is three IMAX screens worth of data," which won him another "ooh." Then he said, "All this stuff is in the PDS now. If you want to work on it, you're welcome to it, be my guest. Because we're tired." (More laughter.) He showed a lovely stratigraphic section of Burns Cliff. Actually the fact that I got so excited about the strat column really tells me that I am, at heart, a geologist. His geologic interpretation of the environment that created the Burns Cliff outcrop is: "interdune playas fed by acidic groundwater, which evaporate to yield sulfate-dominated sand that was reworked by the wind. Very similar to White Sands, New Mexico. The groundwater comes up to the surface, pools, and ponds, and deposits sulfate-rich sand, which blows away, creating sand dunes. Conditions were habitable, but would have posed challenges to the origin of life -- an acidic, saline, only occasionally wetted environment." He talked then about getting stuck. "We were driving along using a driving technique that was basically bombing along at top speed with our eyes closed, and we got stuck." He showed pictures of the tests that were done at JPL and talked about the arduous process of figuring out how to get out of the dune. "We found that the optimal technique for getting unstuck was to put it in reverse and gun it." (More laughter.) He finished by showing a map of Opportunity's traverse, pointing out Victoria Crater, and expressing hope that they'd be able to get to it in the rover's lifetime. On to Gerhard Neukum's talk about High Resolution Stereo Camera (HSRC) images from Mars Express. As usual, he said there was "not time" to talk about any images from the Super Resolution Channel, which I found disappointing. He showed a map of the coverage they have obtained on Mars so far, I think about 25% of the planet, though I'm not sure if I heard that number correctly. He showed a gorgeous perspective view of the north polar cap, with fine layering visible. "Something that looks like volcanic activity also happened in this area," he said. "Layers of deposits disintegrate at the fringes, they are probably windblown deposits mixed in with windblown snow." "There are now so many data we cannot follow this up," he said. "We are approaching a Terabyte of data now. The first year of archives are now at the NASA PDS and are freely accessible" -- an open invitation to everybody to use the imagery to do further work on Mars morphology. He showed some lovely pictures of little volcanoes near the north pole. "We have seen fields of volcanic cones up to 600 meters high. It all appears so fresh, it seems very recent. We have not seen any craters in most cases," which would indicate a very youthful age indeed. "Of course one can be misled by erosion, so more work is needed." He cycled through many images of the largest volcanoes on Mars -- Olympus Mons, the Tharsis Montes, Alba Patera. He said that crater counting on the HRSC imagery has provided very youthful age dates for many of these volcanoes. Some ceased activity 1.5 billion years ago, others as recently as 150 or even just 5 million years. But the star images of his talk were on the recent glacial activity on Mars, work being done largely by his co-investigator and my former graduate advisor, Jim Head. He talked about recent volcanic activity at Olympus Mons emplacing flows that mobilized watery material from the subsurface, creating flank deposits consisting of ice-rich rocky debris. He acknowledged that there was some debate on this within his team: "My good friend Jim Head thinks there was precipitation due to cyclic climate variations, but I am not so sure this is true. I think the major process was mobilization of water from underground that mostly froze over, and from that, rocky glaciers developed." He surveyed more glacial materials, saying, "We don't have any age for the youngest features, because we don't have enough craters. The youngest surface we could date is 4 million years old, we can't date anything younger than that." That brought an "ooh" from the audience -- "too young to date" may not mean "today" but as far as geologists are concerned the two might as well be synonymous. To conclude, as the session chair was trying to shoo the typically long-winded Neukum off the stage, he said that "Mars was dry on a global scale by 3.5 billion years ago. But locally, it's very interesting. Over the past 500 million years until very recently, episodic fluvial and glacial activity was occurring regionally or locally, largely confined to Tharsis. The scientific community is invited to participate in the Mars Express data analysis. Data from first year in orbit have been released to the NASA PDS." Sep 6, 2005 | 10:59 PDT | 17:59 UTCDPS: Day 2: at the end, 7 p.m. local time.OK, where are we? It's the end of another long day at DPS. Today was Mars, Mars, Mars, outer planets, extrasolar planets, and planetary system formation. There were lots of interesting Mars talks going on, but I decided that the Mars stuff would probably be extensively reported elsewhere, and that the rest of the solar system is often underrepresented, so I ended up going to more Neptune, Uranus, ESP, and other sessions than Mars stuff. So, in the hour or so I have before I get kicked out of the building tonight, I'll sip my wine (I love European meetings) and try to get a few neurons marching in step long enough to make some sense of what I saw today. Mark Lemmon talked about the dust devils he's been tracking (he has a great website devoted to dust devils). He talked about how for more than one Earth year after landing they never saw a dust devil from Spirit. During that time, they devised some special techniques to catch them. The first dust devils started appearing around sol 420. ("If they had been going on before sol 420, we would have noticed it.") Now, they are so large that they no longer need to apply their special image processing techniques to find them. "We can't take a picture in the middle of the day without seeing a dust devil," he said. When they started out at sol 420, they were small, about 20 meters in diameter, and they moved perpendicular to the dust devil track trends seen from orbit. "Nowadays we are seeing a different character. We're seeing stronger, more optically thick dust devils, moving parallel to wind streaks, many significantly larger than the first ones we saw," up to 100 or even 120 meters in diameter. But Mark thinks that it's going to get even better. "We are looking forward to the strongest dust devils now. The reason I say that is that they are finally aligned with the wind streaks now." They are seeing the dust devils start to influence the amount of dust in the air at the Spirit landing site, and expect that to strengthen over time. Then I went to the outer planets talks. I must confess that I know a lot less about what's going on in outer planet science than I do about moons and Mars, so a lot of this was flying over my head, but I tried to note the kinds of things that the scientists thought were significant. I saw a talk by Glenn Orton on first results of Spitzer observations of Uranus and Neptune. His spectra are "difficult to match" with the usual library of gases you expect at these planets. The problem is that a spectrum is a single wiggly line. To figure out atmospheric compositions you try to match that wiggly line with the wiggly lines determined in the lab for pure gases. The wiggly line at Uranus and Neptune is dominated by hydrogen and methane. But once you account for hydrogen and methane, the Spitzer folks are left with several wiggles that they are having a hard time matching. The problem is, there are a lot of different ways you can match those wiggles--it's a poorly constrained problem. The best fit, Orton reported, contains a "zoo of things including acetaldehyde, acetic acid, even formic acid!" The last item is so odd a suggestion for an outer planet atmosphere that he joked "I should pitch this to astrobiologists: perhaps Neptune has atmospheric ants?" (If you don't get the joke, formic acid is used by the ants to kill their prey and to ward off attackers.) When pushed by a questioner, though, he was careful to say that this is only a model, and a weird one at that: "I am NOT going to be quoted as saying I discovered formic acid at Neptune." He said that it's just an "intriguing puzzle." Then there was a really interesting talk by Mark Hofstadter about "imaging the troposphere of Uranus at millimeter and centimeter wavelengths." This is cool because every time you can bring a different wavelength of the electromagnetic spectrum to bear on a target, you can learn something new. In the giant planets, the way it generally works is that the longer the wavelength you use, the deeper you can probe into the planet. When we look at the planets in the wavelengths we can actually see, we are only seeing the uppermost layer of the gases and clouds; these planets are fluid all the way down and must have a terribly complex structure when seen in three dimensions. There are LOTS of people writing down mathematical models for what goes on inside giant planets, but there is precious little data. Hofstadter's talk was the first time I'd seen this wavelength employed at Uranus, so it's a new window on the planet. Anyway. He showed some gorgeous images that I hope he will release publicly some day, including the first map of Uranus produced at a 7-millimeter wavelength (this is all radio astronomy). But the kicker was a map of Uranus as seen at a wavelength of 1.3 centimeters. He saw two equatorial bands across Uranus, which have never been seen before. Remember the Voyager "bull's eye" view of Uranus? All the banding was toward the summer pole, nothing at the equator. But Uranus is beginning to head toward its equinox and is beginning to get interesting in the equatorial regions. More interesting, though, the equatorial stripes that Hofstadter saw at 1.3 centimeters were not nearly as pronounced at shorter (7-millimeter) or longer (2-centimeter) wavelengths. From this he concluded that the bands must be "vertically constrained," and that they were "probably ammonia clouds." But another Uranus astronomer, Imke de Pater, asked whether they could be hydrogen sulfide clouds, and he said that that should definitely be considered. Heidi Hammel gave a talk about mid-infrared methane emission on Neptune and Uranus. She said that telescopic observations reported in 1977 by Macy and Stinton reported the detection of methane and ethane on Neptune but not Uranus. "But with Spitzer data," she said, "we have finally detected methane at Uranus." More interestingly, she said, "there is a suggestion of time variability in ethane measurements at Neptune. There's been a great deal of evolution of cloud structure on Neptune, changes in cloud patterns as a function of time." She said that Neptune's ethane emission brightened in many measurements performed between 1985 and 2003. But in 2004, she said, it had dropped down to the 1991 level. She showed more squiggly lines. In particular, she showed that the feature in the spectrum that was at the wavelength that ethane emits infrared radiation had a peculiar shape, a "notch in the spectrum." The notch was real, she said, and she suggested a couple of interesting possibilities for what could create it, including the presence of ethane ice in the Neptunian clouds. She wasn't saying she'd proven this, but she did invite the other scientists at the meeting to come up with an explanation that did a better job of explaining the "notch" in the Spitzer spectra. And this was all the morning! Over lunch, as happened yesterday, I shifted to press conferences, gobbling down half a sandwich in the five minutes between the meetings and the start of the press conferences. Sushil Atreya gave a long presentation about the current status of the understanding of methane in Mars' atmosphere. There is methane at Mars but there's controversy over how much. That story hasn't changed a whole lot since last year (see A. J. S. Rayl's report from last year's DPS meeting.) Two data sets (Mars Express PFS and one from Krasnopolsky) indicate a global average of 10 parts per billion, and a third (from Mike Mumma) indicates 250 parts per billion, an absolutely enormous difference. "If you take 250 ppb and pass it through PFS," Sushil said, "Mars would light up like a light bulb. But none of this has been seen." "Methane is quite important," Sushil said, "because methane on Earth is almost entirely from biology, only 0.2% from volcanoes. In earth’s volcanoes, sulfur emissions are about 100 to 1000 times more than methane. On Mars, sulfur dioxide has not even been detected, so a volcanic source is out." Cometary sources are also out, he said, because models indicate that only one comet hits Mars in 62 million years. Even if a comet did hit Mars recently, then the methane in the atmosphere should be globally distributed evenly. But it has regional variations. So comets are out. There are two non-biological ways to make methane on Mars, Sushil said, both of which involve serpentinization of basalt. What's that, you ask? Basalt is the most common igneous rock on Mars and it's made of olivine and pyroxene, two minerals that tend to form at relatively high temperatures and pressures. If you expose them to water, they can react and form a new mineral called serpentine. That process can liberate methane and hydrogen. But what really got Sushil going was the fact that we really don't understand the loss mechanisms of methane from the atmosphere. And one loss mechanism that hasn't been talked about much is the loss due to the presence of hydrogen peroxide. He presented a cool theory that described sand grains jumping (or "saltating") in the martian wind. Dust devils can build up huge triboelectric charges this way, but he said you don't even need dust devils to make a significant charge -- you can do it with your standard, everyday, ubiquitous martian wind. The charges can be up to 25 kilowatts per meter, and can generate hydrogen peroxide at a rate a thousand times what is produced photochemically, from the Sun interacting with Mars' upper atmosphere. The peroxide is so reactive that it doesn't last long in the atmosphere. But the mechanism generates so much peroxide that it would be everywhere, ready to eat up any chemical that's ready to be oxidized. And methane is certainly ready to be oxidized. What does that all mean? I haven't the foggiest idea. In the afternoon I wandered into some talks having to do with outer planetary satellite systems. One young guy, a graduate student named Zhang, was talking about the Neptunian satellite system. His claim was that you could determine the masses of the inner Neptunian satellites by studying their orbital dynamics. Outer planet satellite systems are dynamic places; the orbits are not stable over long time periods, especially if you've got eccentric satellites among them. But we only see their conditions at a snapshot in time. Zhang's claim was that the inclinations of the orbits of the inner satellites Larissa, Galatea, Despina, and Thalassa are a "fossil record" of their passage through orbital resonances with the largest inner satellite, Proteus. His mathematical models produced inner satellite densities that were less than that of water, which would be significant, if it could be confirmed by another observational method. But it's just a model, and models are a dime a dozen in planetary science. An aside here: I was talking with Imke de Pater in the foyer between talks -- she's a planetary astronomer who studies Uranus, Neptune, and Titan. She has data on these places, and has used her data to constrain some models of what's going on inside them. She told me that she's been approached by some extrasolar planetary scientists who are interested in her trying to apply her models to figure out the kinds of things that could possibly be happening in extrasolar planetary systems. She laughed and told me, "I say to them, 'I'd rather look at data than models!'" I think that is all that I will be able to report today. I've got a few more notes on some extrasolar planetary stuff but that is way beyond my area of expertise and I think I'll make a hash of it if I try to write any of it down, especially in my currently fatigued state. Tomorrow, we have Deep Impact and icy satellites talks to look forward to. There has been a lot of consternation surrounding Deep Impact today, because the Deep Impact press conference from the DPS meeting is scheduled for tomorrow at lunchtime, but Science magazine decided to hold a press conference tonight and lift the embargo on the Deep Impact papers that are apparently being published there this week. I am going to hold any comments on the Deep Impact results until tomorrow. And no, I have not heard a crater size yet! Funny story there -- on Sunday afternoon, when I was mixing and mingling with people during meeting registration, I ran into Deep Impact principal investigator Mike A'Hearn. The first words out of his mouth when he spotted me were, "no, I am not going to tell you a crater diameter now!" Oh well. As soon as I hear anything about that, I'll tell you. One final note -- JPL has released a science plan for the Titan-7 encounter, which includes a RADAR pass by Titan. I've seen the plan, but I am just not going to be able to post my usual preview story about it this week. I'll have to catch up with all of that next week. Sep 7, 2005 | 10:45 PDT | 17:45 UTCDPS: Day 3: Deep ImpactToday was the really busy day here at the DPS meeting in Cambridge, England, with lots of Deep Impact and Cassini results. I'll only have time to do the Deep Impact stuff tonight, I'm afraid, and that in an unillustrated way. I'll try to get to the Cassini stuff tomorrow. Mike A'Hearn showed an animation of the impact, and noted the degree to which the image appeared to "jitter." "Early on the jitter was due to trajectory correction maneuvers; later on it was due to dust hits." There were four particle hits in the last 21 seconds of approach. The first three were particles between 1 and 10 milligrams in size. The last one happened 3 seconds before the impact and was a 900 milligram particle. That's pretty big. 3 seconds before impact was after the last image had been taken and sent to the ground by the Impactor Targeting Sensor (that happened at about 4 seconds before impact). The impact happened at a shallow angle, 20 to 36 degrees from horizontal. The 36-degree number comes from the current shape model for Tempel 1, which has known problems; the shallower angles come from assuming that the elliptical "craters" visible in the images near the impact site are actually round. They have the location of the impact pinpointed very precisely, "within 10 meters." So, how about the size of the crater? "The science team is not convinced that we have seen the crater yet. It's a combination of needing to do deconvolution, coupled with the fact that there was so much dust coming out very late, and you have a lot of bright dust between you and the surface, so you have negligible contrast" to be able to pick out the crater with. A'Hearn quickly slid through several dozen attempts by the imaging team to process the images in ways that would reveal the crater, but I will verify that there was absolutely nothing in any of the images that looked convincingly like a crater (or even much of anything at all). Based on various factors, A'Hearn said that the size of the crater is probably between 50 and 200 meters, but that's all they can say right now. One interesting thing is that after all of the processing you can see one of the two circular features visible in the impact targeting sensor images, the one that was uprange of the impact (to the north). So all that ejecta was mostly scattering southward, downrange. A'Hearn said that "it is likely that we did disrupt that preexisting crater wall." That is, as the crater formed, it probably spread out as far as that crater wall. The nucleus doesn't look remotely like Borrelly or Wild 2 in terms of either shape or topographic features. In particular regards to the circular features, A'Hearn said, "there is nothing about these round features that is inconsistent with their being impact craters, including their size distribution." In other words, the shapes and range of sizes of the round things are very much like what you would expect for impact craters. This is not true on Wild 2, whose round features had very steep sides, no raised rims, and a relatively common size. But "why the craters are smooth at the bottom is unclear." The nucleus of Tempel 1 has two extremely smooth areas. They are in gravitational lows, which makes some sense, because you might expect dust to roll downhill and accumulate in a gravitational low. But A'Hearn noted that "they are mesas, bounded by a scarp 20 meters high, with a rather sharp boundary. The scarp has all the features of erosion from the side, and back-wasting. We don't understand that at all." A'Hearn said that while they'd gotten the overall size of the comet correct before they arrived, an average radius of 3 plus or minus 0.1 kilometer, he said that they had gotten the aspect ratio totally wrong and they don't know why. It's more like 7.6 by 4.9 by 4.9 instead of the 3:1 aspect ratio they'd predicted. The shape is very odd, with the face visible to Deep Impact's cameras composed of "three large, more or less flat areas." He reported that deconvolution of the high-resolution images was going well, and showed a pretty red-green-blue image with a noticeably brown tinge to it. "The color is very uniform. 'Bright white' spots have albedos of 8%. So Tempel 1 has a few black spots on a lot of ultra-black stuff." The spectrum prior to impact was "pretty typical for a comet, fairly featureless with a reddening of about 10% per 1000 angstroms." The surface temperature was 326 degrees Kelvin, "nowhere cold enough to be at the sublimation temperature of ice. So the ice is not at the surface, it's below the surface." The crater formed in a gravity-controlled regime, meaning that the impact happened into a material that had no significant strength holding it together. Which is pretty obvious from the dustiness of the ejecta. "This wasn't a surprise" to most of the team "but we're happy that we guessed right," A'Hearn said. Almost all of the ejecta that came out of the crater was in the form of particles smaller than 10 microns, which is really tiny. The total ejected mass was about 10 or 20 million kilograms, and the plume remained connected to the surface for hours (which also would be expected from a gravity-controlled crater). The trajectories traced out by the ejecta allowed them to measure the force of gravity on the comet directly, and it came out to 50 milligal, or 50 parts in a million of Earth's gravity. That, in turn, lets you back out the mass of the comet, about 7 x 1013 kilograms. That, with the shape model, lets you calculate a density, 0.6 grams per cubic centimeter, but the density has large error bars, about plus or minus 0.35. But if you believe the 0.6 g/cc number, A'Hearn said, "the porosity of Tempel 1 must be at least 75%. The comet's empty." The size of the dust particles were "mostly less than 10 microns, a mix of rocky dust and volatile solids." They have lovely spectra from both before and after impact. A'Hearn showed a graphic illustrating that the slit of their spectrometer actually mispointed off of the impact site (down-range, off the southern edge of the comet), but that was "good news" because otherwise the spectrometer would have saturated. There is a lot of information in the spectra. Water, carbon dioxide, and the C-H bond of organic molecules are obvious. Less obvious and more tentative identifications include methyl cyanide, sulfur dioxide, acetylene. "The methyl cyanide is going to be controversial, because it looks like there is a lot of it." Jim Richardson talked a little bit about the paths that the ejecta took. "Less than 10% of the ejecta escaped the comet." The material that made up the ejecta is extremely weak, with strengths measured at around 100 millipascals. But given the weak gravity at Tempel 1, this weak stuff can still hold a scarp. "You can make a 20 or 40 meter scarp on a comet, but you couldn't pile it up a millimeter high on Earth." Jessica Sunshine talked more about the post-impact spectra. They contained evidence of "literally glowing water, carbon dioxide, and organics in C-H bonds. The gases all had temperatures on the order of a couple of thousand Kelvin." Comparing pre-impact to post-impact spectra, Jessica said, they saw a 10-fold increase in the amount of water and carbon dioxide visible; but a 20-fold increase in the amount of organics. So there was a dramatic change in the abundance of organics, which they can't explain yet but has to do with what they excavated from the comet's interior. It's not clear yet whether it means that the surface is somehow depleted in organics, or maybe the organics at the surface are tied up in large grains so they only became visible when the impact dissociated all the large grains into little sub-grains. Jessica also said that they have discovered relatively recently that "almost immediately after the vapor plume passes, we identified water ice. We have very strong evidence that we have water ice near the surface." Remember, water ice can't be at the surface because the surface is too hot for it. Evidence suggests that the thermal inertia of the comet is nearly zero, meaning that it heats and cools quickly in response to sunlight, but the heating and cooling doesn't propagate into the interior. Mike Belton talked about the internal lottery that the team had going. "It was proposed to have a lottery where you could chose from 10-20 different scenarios on what might happen, to prepare oneself for the science. Unfortunately we haven't found the crater so we can’t say exactly what happened." They can say a few things though. "For example, those few of us who said the comet was going to disintegrate, they’re out of luck. I thought we’d see crust and plates and blocks. I got it wrong too. I think there isn’t any evidence for a crust at all, I think that may be one of the important discoveries in the long term. As to the rest of them, they’re still in the running, Pete [Schultz] for example. We’re going to have to find that crater and do something." Belton calculated a spin rate of 1.702 plus or minus 0.002 days. Pete Schultz reported that the observations of the impact flash, the continuous ejecta, the low velocity central plume, and several other details indicated that the comet was very porous, like the impact experiments he'd done into perlite. From the impact ejecta volume, he calculated a crater size of around 150 to 200 meters. But, he cautioned, the same volume could give you a 100-meter crater that was deep. And that the volume (and thus the crater size) could be greater for a very low surface density. He thinks that evidence suggests--but does not prove--that there is a highly porous surface layer, which is underlain by a weakly bonded upper 30 meters of the comet. Finally, Carey Lisse reported on the Spitzer observations. They have beautiful pre-impact and post-impact spectra. The post-impact one is full of features, a direct result, he said, of the fineness of the dust that the impact produced. He said that the impact must have broken apart aggregates into their component tiny grains. The spectra contain "huge silicate features, indicative of fine dust." The models of the impact process indicate that the impact produced "gentle excavation of cold ejecta" and that "the vast majority of the material was disaggregated on impact." Among the features, the silicate features from minerals like forsterite and enstatite "swamp" the other features, "so our first job will be to understand" the abundances of the silicates. They saw "a little hint of water in the first spectrum after impact but not much afterwards." In addition to the enstatite, forsterite, and PAHs, they are trying to model their spectra using a library containing dolomite (a carbonate), smectite (a clay), spinel/hibonite, pyrrhotite, iron oxide, and elemental carbon grains. Lisse remarked that "PAHs have never been seen before in a comet, but we expected them." At which point ESA scientist Bernard Foing, who was sitting behind me, muttered "no, they were" detected before. Lisse pointed out that he didn't yet have a suggestion for where all the iron and sulfur you would expect to see could be found. The carbonate and smectite that Lisse sees are the most controversial. As far as scientists understand it, it takes an aqueous (liquid water) environment to make these minerals. Yet the textural evidence suggests that Tempel 1 formed in the outer solar system. A'Hearn remarked that "The high porosity tells me that things came together at very low speeds, far from the Sun, where the Keplerian velocities are very low, and with circular orbits, because elliptical orbits lead to high-speed encounters" and less porosity and more chemical alteration of the components of the comet. It would be difficult to form carbonate and smectite in the inner solar system and migrate it out to the outer solar system and not have a more violent birth for comets. So Lisse is going to have to work hard to prove that the carbonates are there. A'Hearn said "it's premature to try to take our data, which we've only looked at 10% of in any detail, and apply it to the rest of the solar system at this point." Someone asked Mike A'Hearn, "with Deep Impact, did we learn about comets in general, or just Tempel 1?" A'Hearn's answer was that "nothing may be typical" of comets. But if the high porosity result is true, then "it's really hard to understand how you are going to attach the Rosetta lander to the surface" of its comet. "They really have to think about that seriously. How well that's going to work with this incredibly weak stuff, I'm doubtful. Uwe Keller said yesterday he thought it would sink in. But I don't think its gravity is strong enough for it to sink in." Finally, A'Hearn confirmed that an extended mission for Deep Impact is "still not funded" and that the opportunity to propose for Discovery funding to continue Deep Impact is supposed to come out in the fall. The spacecraft is in limbo. He also confirmed that the target of the future flyby would be Boethin, and that they "don't have quite enough fuel to get back to Tempel 1" even if they wanted to. Sep 8, 2005 | 06:46 PDT | 13:46 UTCDPS: Day 4: Pluto and CharonThis morning in Cambridge the day began at Pluto and Charon, with five talks, and they were pretty interesting. Leslie Young talked first about what we can expect observationally from Pluto over the next decade, between now and when New Horizons arrives. (New Horizons is the flyby mission to Pluto & Charon set to launch this January.) Pluto is now moving toward south polar winter; in the next decade its elliptical orbit will take it 2 AU farther from the Sun, and the subsolar latitude (which is a measure of the season) will change from 34 to 49 degrees latitude. According to some models for Pluto's atmosphere, Young said, the atmospheric pressure should have peaked when Pluto was at perihelion (closer to the Sun), and should have been decreasing since then, going down by a factor of 2 over the next decade. "But nature surprises us," Young said. "If you compare the atmospheric profiles from 1988 to 2002, instead of having an atmosphere that dropped precipitously after perihelion, you have an atmosphere that approximately doubled, at least at the altitude observed in stellar occultations. To some people this was a very large surprise. But if you take a look at the modeling papers by Hansen and Paige, you find a large range of predictions for behaviors of Pluto’s atmosphere. With lower thermal inertia, you can get extremely large excursions of atmospheric pressure over the change in season, and you can get peak pressure well past perihelion." One problem has been that stellar occultations at Pluto and Charon have been very rare in the past. But Young pointed out that Pluto and Charon will be traversing the Milky Way as seen from the Earth over the next decade, so "occultations should be really good. We're in the unprecedented position of getting to pick which occultations we'll study based on scientific or logistical reasons." Next, Bill McKinnon gave a talk about "Ice-eleven" in the outer solar system. Anyone who's read Vonnegut's Slaughterhouse V has heard of a fictional new crystal form of ice called "ice-nine," but ice-eleven (ice XI) is a real form of ice that is stable at very low pressures and temperatures, and McKinnon's main point was that we may already be seeing it in the outer solar system and have to think about that. The difference between ordinary ice, ice-one or ice I, and ice XI is its degree of crystal ordering. "Ice-one is a crystalline lattice of water molecules in hexagonal symmetry. In ordinary water ice the orientation of the hydrogens or protons is flipping around continuously, making a disordered structure. Solid-state physicists have long theorized that if you cooled ice enough you’d get a transition to a purely ordered solid called ice XI. In order to form it, you have to be able to reorient all the molecules. This is a well studied phenomenon. Reorientation times are 20 years to 100s of millions of years. There is a temperature range, 72 to 50 K where this works on geologically reasonable time scales." He went on to say that impurities can accelerate the process of conversion of ice I to ice XI. So ice XI can be found wherever the temperatures are right -- they must be below 72 Kelvin all the time, but not too cold or the reorientation of the water molecules can't take place. "You find appropriate temperatures in the Uranian satellites, everybody's darling Enceladus, and at Pluto and Charon, at least Pluto’s warmer spots," and Triton as well. So, why do we care? Can we see ice XI, and does its presence mean anything for the geology of icy bodies? McKinnon said that "We don’t have laboratory spectra for ice XI in near infrared wavelengths" but that you'd expect the spectrum of ice XI to look like ice I but with sharper spectral features because of the increased order of the crystal structure. The reason that ice XI is important is because it's expected to have physical properties that differ slightly from ice I, which has implications for anybody doing modeling of the geology and geophysics of these bodies. "In the lab, the thermal conductivity of ice XI is greater than in ice I by about 20%," McKinnon said, and models indicate that it may even be higher. "We are almost certainly already looking at surfaces that are mixtures of ice I and ice XI in the outer solar system. It should affect spectra and it has non-trivial geophysical effects." Next up was Mark Buie talking about Hubble observations of Pluto. "I had hoped that this would be the final word on the data set," he began, "but in fact it's going to be a progress report." He's working on data acquired with the Advanced Camera for Surveys on Hubble between July 2002 and June 2003 over 12 orbits through two filters. Previous Pluto images came from the Faint Object Camera in 1994. To make a long story short, they have a couple or three hundred images of Pluto, each only a few pixels across, and it's taken so far two years of computer time on a 20-computer parallel cluster to turn those images into maps of the albedo (brightness/darkness) across Pluto. There were "oohs" when he showed his maps though. "It is heartening to see that the general dark areas and bright areas are staying the same since 1994. The high southern latitudes are frustratingly difficult to constrain"--difficult to constrain, because they are on the limb, just barely visible, and frustrating because that's where you'd expect to see the greatest change in seasonal albedo patterns from 1994 to 2003 as the putative freezing of the atmosphere begins. Then Buie showed a color map, which really got people excited. The albedo patterns in the two color filters are similar but different enough that there are clearly different colored terrains on the surface. The color map "explains one of the longstanding mysteries about Pluto, which is: why, with such strong albedo variations, do you never see a color change" on Pluto as it rotates. Even cameras that can't resolve Pluto's disk from Earth should see its color change as it rotates and these different colored regions come into view. But as Buie showed, "the color variations are intimately intermixed on a hemispherical level, and it's hard to separate them out." His final slide was of the new maps rotating on a Pluto globe. "Frankly," he said, "I'm still at the point of just sittin' and starin' at 'em," he said of his maps, and the audience seemed to agree. I was thinking that it's amazing how much work he's having to do to get these maps to come out -- when, hopefully, New Horizons will blow them all away. But not for 10 years, by which time the season will have changed, and Buie's maps, though low in resolution, will be important for before-and-after comparisons. Finally, Bruno Sicardy talked about the stellar occultation by Charon which I wrote an article on a little while ago. He showed results from three telescopes, which got three different chords across Charon -- that is, the telescopes were in three different places across South America, so they measured the star traversing three different (but parallel) paths behind Charon. From these observations he got a very highly accurate measurement of the size of Charon: the disk was a circle 602.5 plus or minus 1 kilometer in radius. That's the first I've ever seen such a precise measurement of the diameter of Charon. From that you can calculate a density of 1.73 plus or minus 0.08 grams per cubic centimeter. Someone asked him whether they tried to fit an ellipse to it, but he said that a circle perfectly fits the three chords; they do not detect a departure from a circle. So hopefully soon they'll be able to nail down the diameter of Pluto in the same way. I have much much more from yesterday and today but now it's time to go back into Titan sessions so I'll wrap it up here. Sep 8, 2005 | 11:25 PDT | 18:25 UTCDPS: Back to day 3 stuff: icy satellitesWhere do I begin? I've got more than 20 pages of notes from today and yesterday still to write up. It's going to take me days more to finish digesting all of this stuff. It's been a great meeting, but the muscles in my fingers are fatigued from all the typing I've been doing. I guess I'd better get started on the output from the "Cassini fire hose" as Roger Clark so poetically put it. Yesterday there were some talks on general icy satellite stuff, followed by Titan; today it was largely Titan. To begin, Paul Helfenstein talked yesterday about the geological terrains on Enceladus. "Tectonic disruption is the dominant resurfacing agency. There is no evidence for extensive flood-style volcanic resurfacing." He said that the terrain on Enceladus can be broadly split into two groups: heavily cratered, and heavily fractured. Superimposed on these regional differences is "a gossamer set of fine fractures that seem to slice into almost everything, cutting the surface up into thin slabs." There are interesting patterns in where these different terrains occur. Here are those lovely Enceladus maps, which have been projected to show the whole planet in two views, one from the north pole and one from the south pole: Helfenstein pointed out that "the cratered terrain forms a band that goes from the sub-Saturn side up and over the pole to the anti-Saturn side." The sub-Saturn point is 0 degrees longitude (right side of maps), anti-Saturn is 180 degrees (left side of maps). "We see the same pattern at the south pole. But something very strange happens at 55 degrees south latitude." Encircling the south pole is a set of features with a "pinched appearance, made up of a series of ridges and mountain ranges flaring out into north-south extensional fractures" as you go north. North south fractures mean east-west extension; ridges along a line of latitude mean north-south compression. These observations indicate that "Enceladus has undergone increased flattening, perhaps caused by an increase in spin rate, but perhaps tidal deformation is also important." Jeff Moore asked Helfenstein about whether they can really rule out flooding. Torrence Johnson answered for him: "We have not seen anything that convinces us of flooding, but there might be some things you can argue about." Next, Roland Wagner talked about the geology of Dione. He described it as having "craters standing shoulder to shoulder," an image I liked a lot. "However, there are also troughs and ridges that are highly degraded, indicating expansion and contraction early on. Heavily cratered terrain is the most abundant geologic unit; but one should note that there are no old, large impact basins like the ones on Rhea, Iapetus, or Callisto. We see tectonic resurfacing, but no evidence of cryovolcanism. We do see one impact basin, which was seen by Voyager. This seems to be at least a two-ring basin. The terrain around it is less densely cratered and you also see secondary clusters and rays. Thus the largest basin on Dione is stratigraphically young," which, he pointed out, was also true on Mimas and Tethys. Crater counting gives ages of many billions of years for the surface of Dione, but depending upon your model for the cratering rate in the Saturnian system, that youngest impact basin could be anywhere from 3.2 billion to 330 million years old. You can see the basin at the lower right of this Dione image from ISS (courtesy of NASA/JPL/SSI). That was all of the talks I could go to before running over to Deep Impact yesterday, but today I managed to catch up with a couple of the other speakers and get encapsulated versions of their talks. Zibi Turtle gave a talk about the topography of Iapetus and Enceladus. I'd seen a talk last year at DPS indicating that Iapetus' topography was really difficult to model. Zibi fit an ellipsoid to it (major and minor axes 747 and 713 kilometers). The very, very strange thing about that shape is that it's noticeably oblate, that is, it's fat around the equator. This shape would be a stable one if Iapetus were spinning relatively fast, with a 17-hour period, but in fact it spins at a rate of only once per 79 days (once per orbit). It's not a gravitationally stable shape, so Iapetus has to be very very cold and very rigid to have that shape -- and it must have been that cold and rigid since it had a 17-hour rotation rate, then it "de-spun." And that's a very interesting geophysical problem. Zibi's specific work was to look at topography models for Iapetus and try to figure out "what we can say about the interior based on topography. We can do that because the topography that a planet can support is dependent upon structure and rheology." Rheology is the behavior, over time, of a material in response to stress; most materials flow more readily the higher the temperature or the longer the time you give it, but may fracture at low temperatures and short time scales under stress. Looking at Iapetus' equatorial ridge, "it's 20 kilometers high and pretty narrow, with a broad apron. If it's really cold," Zibi said, her models indicate that "even over a billion years not much happens" to relax that topography. But "if you make it warmer, say 220 Kelvin at a depth of 1.4 kilometers, you get substantial deformation and you lose a lot of topography." The ridge would literally sink inward into Iapetus, and moats would develop on either side of the ridge. Zibi said they'd be able to detect those moats if they were there. So Iapetus has likely been really cold for a really long time. Zibi also looked at Enceladus. On Enceladus, you have craters that absolutely scream that they've undergone viscous relaxation if, like me, you've had training in planetary structural geology (ahem. I know most of you readers haven't had the pleasure of that. But I can aver and affirm that they do scream it.) In other words, Enceladus' craters started out bowl-shaped, but over time and the influence of gravity the ice that the craters formed in has flowed, with the low-lying centers squirting upward and the high-standing rims squishing downward. "Enceladus needs to be pretty warm to allow that deformation," Zibi said. "Even a thin cold layer, a 5-kilometer-thick layer starting at 140 Kelvin, which is warm compared to the rest of Enceladus' surface, you don't get a lot of deformation" of craters. The reason that craters on Enceladus don't relax very easily is because Enceladus is so tiny that the gravity is extremely small, so there's not much force available to do all that squishing. Then Roger Clark gave a talk about the compositions of surfaces in the Saturn system from VIMS. My notes from this talk aren't very good, unfortunately. But I think he was talking about Iapetus when he said "There is ice everywhere" and that "we are not seeing lots of absorption bands" on the surface. He found absorption bands typical of cyanides, and thinks they are "simple 3-atom molecules like NaCN and KCN. KCN is really the best match." He also noted that they have seen HCN ice clouds on Titan. At Dione, "wherever we see dark materials we tend to see weak bands from carbon dioxide and CN" or cyanide. "In Saturn's rings, in the main rings we're only seeing ice; we haven't detected anything else. In the C ring we are seeing an iron-2+-like feature, we think. It tends to change with phase angle" or the direction of illumination, which, Clark said, doesn't make a lot of sense. They found no cyanide bands on Rhea, Hyperion, Tethys, Enceladus, or Mimas. But, he said, that may just be because they haven't gotten the spectral resolution necessary to see it there yet, which will come in later flybys. He was particularly interested in looking very closely at those dark spots on Hyperion to see if there is cyanide there. He also noted that they definitely do not see cyanide in the Enceladus tiger stripes, but that they had seen it on Callisto. "We're seeing a prominent 2.42 micron cyanide band throughout the solar system," and he suggested at the end of his talk that the exact position of the absorption band could be used as an age dating mechanism for the surfaces of icy bodies. Finally, he finished the talk with this thinker: "There is more nitrogen being found in the Saturn system than we though we would. And we thought it would be in ammonia, and we're not seeing any ammonia." I am afraid that's all I have time to write about tonight. I've still got a couple more icy satellite talks from yesterday to go through, and all of Titan, and the Kuiper Belt stuff from today. Tomorrow is a short day, mostly rings; hopefully I'll catch up a bit tomorrow but I do want to spend an hour or two just walking around Cambridge -- I've never been here before! -- before I head back to London to fly home on Saturday. I'll get it all written down eventually! Sep 9, 2005 | 05:12 PDT | 17:12 UTCDPS: Day 5: Rumors of spokes...There is a murmur of information running around the meeting that the famous spokes in Saturn's rings, so visible in Voyager images and also from Earth but never yet in Cassini images, may finally have returned. There are a couple of images of the backlit rings taken on September 5 that appear to contain what look an awful lot like spokes. I first found out about these on an online forum yesterday evening, and showed them to some rings scientists here at DPS who are not on the camera team, who carefully acknowledged that they were very exciting and indicated that they had heard some buzz indicating that the phenomenon may have been noticed already by the camera team. And this morning, after Cassini interdisciplinary rings scientist Jeff Cuzzi gave the invited talk opening the morning rings session, someone stood up and asked him: "I have heard that the spokes have reappeared in the last two days." Jeff only replied "I have heard that too, and that [reappearance] may constrain models" for how the spokes form. Not being on the camera team, Jeff is certainly not authorized to make an announcement of such a discovery, so he had to be careful in his response. So -- stay tuned to see what the camera team has to say about these images. Sep 9, 2005 | 05:56 PDT | 17:56 UTCDPS: Back to Day 3 again: finishing up asteroids & IapetusToday is the last day of DPS, and the first that has not had a press conference session at lunchtime, so I have a few moments here to do some catching up. I think I will have to wait until I get on the airplane tomorrow to be able to get to the Titan and rings stuff -- I thought I might try now to finish up with various other random items that have appeared at various moments over the last couple of days. So, back to Wednesday morning. Three long presentations were given by people who won various prizes from the Division of Planetary Sciences (DPS). The "Urey Prize" is given each year to a young researcher making important contributions to planetary science, and this year it was given to David Nesvorny, who does research on the dynamics of asteroids, among other things. He gave a really interesting talk on the long-term behavior of clusters of asteroids. Clusters of asteroids sharing an orbit, but spread out along it, are suspected to represent the remnants of a single large asteroid that experienced a catastrophic impact at some time in the past. "Unlike human families, in which all the members have different ages, all members of an asteroid 'family' have the same age," Nesvorny said. He spoke a lot about the Karin cluster, named after its largest member 832 Karin. His "simulations propagating orbits of the Karin cluster backwards in time make them all meet around 5.8 million years ago." He showed several graphs of time versus orbital longitude for these bodies, and they all crossed at very close to one time in history, indicating that all the fragments began moving from the same place 5.8 million years ago. It was really neat modeling work that had the whole audience excited. Nesvorny went on to talk about how he is attempting to figure out the size of the initial parent body and its disruptor. From his models he finds the best fit to be an initial body 33 kilometers in diameter, with a 5.75 kilometer impactor. At present, the largest body in the cluster is 832 Karin, with an estimated diameter of about 19 kilometers. Nesvorny can also use his models to figure out where in the parent body the resultant fragments came from. The cool thing about his results there is that it appears likely that 832 Karin represents the "back half" of the parent body as seen from the impactor. One side of it should be the ancient, weathered surface of the parent body, and one side should be the fresh, only 6-million-year-old surface cut through the parent body by the force of the impact shattering the body. And, in fact, Nesvorny said, "recent spectral observations indicate that Karen has two faces: a red one and a much flatter-spectrum one. It is tempting to say that the red one is the weathered surface of the preexisting body, and the blue one is the fresh surface from the interior." He said that Karin or its family members would be "a great target for a space mission, because you can use it to calibrate the crater production rate in the main asteroid belt" because the pieces are likely to have at least one face that is very fresh, only 6 million years old. Finally, I have to mention a couple more items from Iapetus from Wednesday. There were a couple of talks looking at the composition of the surface. Dale Cruikshank used VIMS spectra, and presented convincing evidence that they have seen PAHs, or Polycyclic Aromatic Hydrocarbons. ("Aromatic" refers to a molecular structure in which there are carbons bound into a ring. The simplest aromatic hydrocarbon is benzene, which has one ring. PAHs have several rings bound together.) In addition, he saw other features in the spectra characteristic of CH2 groups. CH2 (a carbon bound to two hydrogens) is a common arrangement of molecules seen in long-chain or "aliphatic" hydrocarbons (methane, ethane, propane, etc.). Cruikshank suggested that "we envision a polycyclic, aromatic structure, with aliphatic bridging units, which often terminate in CH2, NH2, and NH. This structure is very similar to kerogens found in carbonaceous meteorites. It is also very similar to interstellar dust. PAHs contain the majority of the carbon in the galaxy. The dark material on Iapetus comes from somewhere else. It's material that is also present on Phoebe, and on comets." Tillman Denk performed some color mapping of Hyperion and Iapetus. He found interesting patterns of colors on Iapetus' surface. While the dark terrain is generally on Iapetus' leading side, it does also wrap around to trailing side. He found that the leading side dark material is redder than the trailing side, which is relatively greener in color. He suggested that the greenish stuff could be "primordial," endogenous to Iapetus, while the dark red material on the leading side is exogenous. And he wondered out loud whether there may be as many as "three more or less independent processes responsible for the formation of the extreme hemispheric albedo asymmetry on Iapetus?" John Spencer used CIRS maps to study Iapetus. Because of the dark material and the very long day, Iapetus is "probably warmer than any other surface in the Saturnian system," John said. While Iapetus has a thermal inertia very similar to Phoebe's, the thermal wave from each daytime round of heating penetrates much deeper into Iapetus' skin than it does on Phoebe because of the longer day, 3 centimeters on Iapetus as opposed to 2 millimeters on Phoebe. John covered the usual arguments for why the shape of the albedo dichotomy is strange: "most simple exogenic models [that is, models in which the dark stuff comes from outside Iapetus] darken the leading hemisphere, but Iapetus' bright material extends over the poles, dark stuff extends around the equator [to the trailing side], and pole-facing slopes are bright." So John created a model in which all of Iapetus is covered in a very thin layer of typically dirty ice. The warm days on Iapetus will sublimate some of the ice, which will eventually refreeze -- preferentially so at cooler spots, like the poles. Then, he said, in his model he darkened the leading side symmetrically about the apex, which is what you would get from ballistic emplacement of exogenous material. If he runs the model forward in time, "in only about 10,000 years you start to burn off frost on the leading side. In 100 million years you start to burn stuff off on the trailing side. You can actually make this look pretty similar to the current albedo pattern. If you try to do it with a thicker frost it doesn't work." A very suggestive model, pretty cool. Back to rings sessions...more later. Sep 10, 2005 | 08:07 PDT | 15:07 UTCDPS: Day 4: Titan's atmosphereI'm on the plane back from England -- 11 hours to get caught up on my notes from the last two days of the Division of Planetary Sciences meeting in Cambridge, England. Next up: Titan's atmosphere. These talks actually began on Wednesday afternoon, Day 3, but I was only unable to attend one of the talks before I had to run off to the Deep Impact session, so I missed some potentially interesting stuff (like my friend Imke de Pater's talk on what the Keck II telescope saw at Titan during the time of Huygens' descent). But here's what I got. There were two whole sessions devoted to Titan's atmosphere, necessary to explore all the new data from Cassini and Huygens from both theoretical and experiential standpoints. Basically, any time you get a lot of new data at a planet, it tends to overturn accepted explanations for the previously available data. So first scientists have to describe all the new data, then they have to compare it to the previously existing data to see if the data disagree (suggesting time-variable behavior), and then compare it to the models that people came up with to explain the previously existing data to see whether the models still work. Huygens was primarily intended to be a probe that would precisely characterize a number of variables about Titan's atmosphere along the path of its descent. Variables of key interest to scientists include how the temperature, pressure, wind speed, composition, solar spectrum, cloudiness, etc. etc. etc. vary with elevation. Of course, they also want to know how all of these things vary with latitude, longitude, and time as well. Huygens got essentially a one-dimensional profile through the atmosphere. To understand the whole three-dimensional atmosphere and how it behaves over time, scientists will be using Cassini and Earth-based observational data for the whole planet, and will tie it to the one-time, one-dimensional path of Huygens' descent through Titan's thick atmosphere. E. Lellouch gave an invited talk on "a new look at Titan's atmospheric system." He compared the temperature and pressure profiles calculated by the Huygens Atmospheric Science Instrument (HASI) to prior models, the most commonly accepted of which is a set of models devised by scientist Roger Yelle. Lellouch found that the lower atmospheric structure in these models has been well confirmed by HASI. And that the Cassini Ultraviolet Imaging Spectrograph (UVIS) team has published a profile based on a stellar occultation that follows Yelle's model well. "But HASI gets a very different temperature structure, with no clear thermosphere. The structure of the thermosphere is dominated by waves in temperature, with large amplitude and wavelength." That is, instead of a smoothly varying temperature, HASI found many inversion layers, in which the temperature rises and falls sharply as you go upward. "This is similar to what was found from previous Earth-based stellar occultations," Lellouch said. He noted that "waves are visible in the haze structure" as seen in Cassini camera (ISS) images, and that, contrary to expectations, the Ion and Neutral Mass Spectrometer (INMS) "sees that methane and hydrogen are well mixed to high altitudes," indicating that the atmosphere is "well mixed up to 1200 kilometers. Thus the vertical structure and energy balance in the atmosphere are more complex than previously thought. We need yet to understand the origin of these waves." He mentioned that Darryl Strobel has "proposed that the waves are due to gravity tides caused by the eccentricity of Titan's orbit around Saturn." Speaking about the composition of Titan's atmosphere, Lellouch said that "no previously unknown gases have yet been observed by Cassini at Titan. However, Cassini has confirmed the presence of some rare trace gases, like benzene," which is a hydrocarbon composed of 6 carbon atoms bound together into a ring. "All photochemically produced gases have been observed to increase in abundance strongly with altitude," which is what you would expect for a photochemically produced gas -- it would be most abundant where it is produced, which is high in Titan's atmosphere, where ultraviolet rays from the Sun cause chemical reactions among nitrogen, methane, and hydrogen to produce hydrocarbons and nitriles. But, Lellouch noted, "polar enrichments of photochemical gases occur at low altitudes, because downward motion" of the atmosphere at the poles "drives a more uniform vertical profile" in atmospheric abundance. Finally, speaking of the hazes that are made by these photochemical compounds, Lellouch said that Cassini-Huygens data suggests that "haze may be coupled to atmospheric circulation. The altitude of the detached haze layer has changed dramatically between Voyager and Cassini, and this is a great mystery. Evidence suggests that the haze persists down to the surface of Titan." I missed the rest of the Wednesday afternoon talks on the composition of Titan's atmosphere. I also missed the Thursday morning talks on the same topic, because I was attending the Pluto presentations that I already wrote about in a previous entry. I came in to the session as it had shifted to discussion of clouds on Titan. Caitlin Griffith talked about mid-latitude clouds as seen from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). "Earth is kind of a mess with clouds at every latitude," she said, "but Titan is neat. It has clouds mainly at the south pole and at -40 latitude, and at -40 they exist at only about 0 degrees longitude and nearby, plus a few nearby misbehaving clouds, one of which was seen at -60 degrees." The organization of the clouds at specific geographic locations suggests what processes could be behind their origin and evolution. Griffith spoke mostly about the Tb observations, in which they took six VIMS images over a 3-hour period and tracked four different clouds. She focused specifically on the 2-micron "methane window" into Titan's atmosphere. If I had a spectrum of Titan to show you -- which I haven't unfortunately been able to find yet -- I could show you that this window in the infrared is a wide one, with sloping sides. In other words, outside the window Titan's atmosphere is very opaque. Then it gets progressively less opaque as you get closer and closer to the 2-micron wavelength. As Titan's atmosphere gets progressively less opaque, it permits the infrared light to escape from lower and lower elevations. So if you look at a set of VIMS images spanning the spectrum from where the atmosphere is opaque to where it has a window, you can see to progressively greater depths in the atmosphere. What Griffith has done with that is to look at individual clouds and see at what elevations they lie. This depends of course on her having really good knowledge of the exact depth from which infrared light can escape at each wavelength, which is still a little fuzzy, but it's improving all the time. These images show how Titan's atmosphere becomes less and less opaque to VIMS as you cross that shoulder into the 2-micron window. They are individual frames from a movie that the VIMS team released last year scrolling all the way through all their wavelengths. You can hardly even see the south polar clouds in the left-most one. As you look right, they pop into view, then you can start seeing the surface. You can't see any other clouds in this view, unfortunately. I have asked every VIMS team member I have met if they can release more sets of images like these so that you and I can see how VIMS can slice through Titan's (not to mention Saturn's) atmosphere, but I've had absolutely zero luck so far. It's too bad, it's a really cool instrument and I'd like to be able to show it off more. ISS gets all the press, but VIMS can do much more at Titan than ISS can by virtue of its ability to see all the levels in the atmosphere, color variations on the surface, possibly even topographic shading, and can achieve nearly the resolution ISS can at Titan (better, to hear the VIMS team say it). Anyway, getting back to Griffith's talk, she used her 3-hour span of observations to actually track the clouds moving vertically as well as horizontally in the atmosphere. One of them "rises 10 kilometers in about 50 minutes, then in the next hour it drops, then it actually rises another 5 kilometers. These rising motions are seen in all the clouds." She calculated the rates at which the clouds rose and dissipated. "Cloud updraft rates of 2 to 10 meters per second are consistent with convective upwelling. The cloud dissipation rate of 15 kilometers per hour is consistent for rainout with millimeter-sized drops. Therefore the data suggest that the clouds originate as compact convective cells that dissipate through rain and then evolve eastward through zonal wind transport. What powers Titan's clouds? They are primarily seen at 40 degrees south latitude, and they appear at many longitudes. They seem to be convective and governed by transport along longitude." Griffith then went through the various processes that could form clouds on Titan. "Are they marine clouds? Probably not," because there's no evidence for any oceans. "Are they orographic, caused by updrafts over mountain ranges? Probably not, because their morphology" is not consistent with that. Are they tidal? "Saturn's tides do cause strong fluctuations in pressures right at the longitudes where these clouds are observed. But I can't find any correlation between Titan's orbital position and cloud activity. Getting to the Holy Grail, volcanic outgassing can humidify the atmosphere at one latitude band. But the mass of one event needed to humidify" the full longitudinal extent of the clouds is just too much. "Circulation offers the best possibility," Griffith concluded. "Circulation differentiates latitudes. It would also explain a completely separate characteristic of Titan's atmosphere, which is that it has a polar cap that cuts off abruptly at 40 degrees south latitude. The South Pole has very little light getting to the surface," despite the summer season there, because of the thick polar haze. "So you get an upwelling branch of atmospheric circulation at minus 40 degrees latitude. But this doesn't explain the observed longitude dependence. So clouds are powered by circulation and something else" that they haven't identified yet. Emily Schaller talked about observations of Titan's south polar clouds from Earth, the Palomar, Gemini, and Keck II Adaptive Optics telescopes, and how they've changed over time. "Back in 2001 we saw a cloud near the south pole in nearly every single image, but it dramatically stopped in 2004. Before December 2004, 66 of 68 Titan observations have south polar clouds, but after that only 1 of 29 shows south polar clouds. This could be a seasonal effect. The summer solstice was October 2002. In late 2005, the south pole ceased being the maximum insolation area of Titan." One problem with Earth-based observations, though, is that for half of the Earth year Titan can't be seen from telescopes because it's in the sky in the daytime. Schaller also noted that "Cassini saw south polar cloud activity, but it could have been below our detection limit." Of particular interest to her were cloud outbursts, in which "clouds brighten by 15 times over typical levels," events they've observed a couple of times How do these massive clouds dissipate? "Raining out should not dry out clouds, but you could stop them by shutting down atmospheric convection. Large cloud events occur about once per Earth year. That can cool the atmosphere by evaporative cooling. Models suggest it would take about 9 months to reheat the atmosphere." After she finished speaking, a Cassini Composite Infrared Spectrograph (CIRS) scientist behind me asked her what the predicted amount of cooling for such an event would be. "About one degree," Schaller said. From the "hmms" of the scientists behind me, it seemed like they thought they could detect that, if they were looking for it from Cassini. Next, T Kostiuk gave a presentation about observing Titan's wind speeds during the Huygens landing from the Subaru telescope, where they had two very fancy spectrometers that could get wind speeds from the Doppler shift of Titan's eastern and western limbs. The talk was most noteworthy for his comments about how difficult it was to make the observations -- during the first of their three days of telescope time they suffered whiteout blizzard conditions, and during the second day they "had to close the telescope intermittently because of winds exceeding 50 miles per hour. The seeing wasn't so good." Although previous experiments, performed in 2003, found wind speeds matching the standard atmospheric model well, their observations during the Huygens landing "do not fit the standard atmospheric model. We couldn't find an instrumental explanation. We had two different spectrometers, and they both saw the same kind of deviations to the model. It's going to be very difficult to retrieve velocities." In conclusion, he said, "there is spatial and temporal variability in Titan's atmosphere, and I think other reports have shown similar, unexplained phenomena." Over lunch, for the benefit of the press, Sushil Atreya gave a presentation on methane on Titan. To review, there's quite a bit of methane in Titan's atmosphere. But methane is quickly destroyed by solar radiation, so there has to be some way of replenishing it, which is what led people to think that there should have been methane oceans originally. But the methane is important for other reasons, Sushil explained. "Methane is needed to maintain the nitrogen atmosphere at Titan." Methane is a powerful greenhouse gas. Without it, Titan's atmosphere would be much cooler, and the nitrogen in Titan's atmosphere would freeze out. So where is the methane coming from? "Meteorology is not responsible," because there aren't any oceans so you don't have a 'methological' cycle. "and I don't believe that biology is doing it." On Earth, atmospheric methane is almost 100% from biological activity of various kinds. Nobody's disproven life on Titan but nobody's seen it either. "So, most probably, an interior process is going on to replenish the methane." Sushil got into the details of what the Cassini and especially Huygens Gas Chromatograph Mass Spectrometer are telling us. "Nitrogen isotope ratios indicate that nitrogen has been escaping" from the atmosphere over Titan's history. "But the carbon isotope ratio turns out to be very similar to Earth. That means Titan must be replenishing methane," which contains carbon, from an internal source that is tapping a carbon reservoir. Sushil noted that the present carbon isotope ratios also argue against an identically Earth-like biologic activity being responsible for the methane. "There is a nice source for methane that works out rather beautifully," he said. Guess what it is? It's exactly the same source Sushil was arguing for methane on Mars on Tuesday, low-temperature serpentinization of basalt. "Rocks rich in hydrogen and magnesium combine with water at low temperatures, combine with hydrogen and elemental carbon or carbon dioxide, and that forms methane." This methane would slowly escape from the planet's interior by the same process that has enriched Titan's atmosphere in Argon-40, an isotope of argon that is produced by the decay of radioactive potassium. Then David Grinspoon gave a presentation provocatively titled "Possible niches for extant life on Titan." He was careful to say at the outset that he was NOT arguing for the detection of life on Titan. Rather, "We've done some speculating about the possibility of life on Titan today, and how the prospect of that has changed since the arrival of Cassini-Huygens. It's simply a question of what are the requirements for life, and how they may be met by a place on Titan. We have no biodiversity in a cosmic sense, so we really have to watch our geocentric assumptions" about where life can exist. "Before Cassini-Huygens, we already knew that there was a reducing atmosphere, like the early Earth, rich in organic chemistry, and a varied surface, which makes Titan an interesting place in the less radical sense that it's a laboratory for prebiotic conditions on Earth. After Cassini got into orbit but before Huygens, we learned that Titan had an apparently young, active surface, with things going on there, perhaps ice volcanism, tectonic activity, and erosional landscapes. Now, after Huygens, there seems to be evidence of a methane 'hydrological cycle,' active erosional features, and apparently a surface permeated with liquid methane and ethane and carbon dioxide." It's becoming more and more interesting. So what about the possibility of extant life? "We can't address that question without a discussion of what are the requirements of life, and of course we just don't know" what non-Earth life might require. "But it seems you need energy sources, probably liquids, and the possibility of complex chemistry. I would also feel that you probably need an atmosphere, and I feel that a good thing to be looking for is vibrant geologic activity continuing through the life of the planet, which you have on Earth, probably not on Mars, but you evidently do on Titan." Grinspoon suggested one place where you could have all these conditions met. "One promising location is a hot spring at the bottom of a hydrocarbon reservoir, which would have both raw materials, molten water, and higher temperatures." He showed a diagram consisting of a hydrocarbon lake at (or just below) the surface, above a solid icy crust, above an underground ammonia-water ocean. "All the features in this cartoon have been found on Titan. We have to be open-minded about [extraterrestrial] life because we don't know anything about it. But as far as we understand it today, the basic requirements of life are available on Titan." I asked him how we could test for life on Titan, and he said, "Look for very large polymers, molecules with molecular weights over 1000 -- those are the kinds of things organisms make. Chiral molecules. Certain types of structures that life makes, like membranes. Those are the kinds of things we ought to do with | ||||
