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Planetary News: Asteroids and Comets (2006)News from the 37th Lunar and Planetary Science ConferenceSpecial Coverage from The Planetary Society WeblogMarch 13-20, 2006The Planetary Society Weblog is written by the Society's Science and Technology Coordinator, Emily Lakdawalla. She traveled to Houston, Texas in March, 2006 to file these special reports on the 37th Lunar and Planetary Science Conference, held annually by the Lunar and Planetary Institute near Houston, Texas in March. Mar. 13, 2006 | 09:00 PST | 17:00 UTC I'm at LPSC"LPSC" is the Lunar and Planetary Science Conference, held annually by the Lunar and Planetary Institute near Houston, Texas in March. I've been to this conference every year since 2001, and this year I am staying for the whole week. It should be exciting, with all the new news from Stardust, Cassini, the rovers, Mars orbiters, Hayabusa, and ongoing research projects. But the first item of business on most people's agendas for a meeting such as this is to meet and greet old friends and new ones. I enjoyed a convivial evening last night at registration, seeing all my friends from graduate school, commenting on weight loss and hair loss, new homes and additions to families, and then went out for dinner with several old friends and one new one, pictured below. This is Phil Plait, known to most as the host of Bad Astronomy, a long-standing Internet source for skepticism and the debunking of the nonsensical "theories" that seem to be an unavoidable by-product of the public interest in space. Phil's also a working astronomer as well as a blogger, and just a fun and witty guy, quick with remarks that got the whole table laughing over our enchiladas and margaritas.
Mar. 13, 2006 | 14:20 PST | 22:20 UTC Monday: Notes from the Stardust sessionMy choice among the morning's four sessions was to go learn about the first results from Stardust. To do that, I had to skip hearing the talks about new results from Mars Express MARSIS, which was kind of a bummer, but this conference will be full of such difficult choices, and I was hoping that there would be exciting things reported from the Stardust team's first look at their samples. I wasn't disappointed. Before I dive into the more technical details, let me report something that probably a large number of you are interested in, and that's what the team had to say about the status of the Stardust@home project. In brief, a couple of the members of the team have said that they have just not had time to even look at the tray containing the interstellar samples -- it's still sitting in nitrogen storage. But they repeatedly said that next week or the week after they plan to start doing the microscopic scanning that will produce the "movies" slicing through the aerogel samples, which will produce the data set that Stardust@home participants will analyze. Mike Zolensky remarked during his talk that he had originally planned to schedule the tray scanning for April 1, but thought better of it, and is now saying April 2. (For those of you who don't understand the significance of April 1, that date is known here in the U.S. as April Fool's day and it's traditional to play practical jokes on people on that date.) So, moving along into the technical talks, Principal Investigator Don Brownlee gave the first one, an overview of the samples (here's the abstract. This and all other links to abstracts will get you to PDF files containing 1- or 2-page summaries written by the scientists about their work.) He asserted that he believes that the particles that Stardust picked up came from subterranean ice-rich regions of the comet and were only two hours old when they were captured, so they should be quite pristine. (Of course the capture process vaporizes any volatile minerals in the samples, so they're not the same as they were when they came out of the comet -- more on that later.) Brownlee got a laugh from the audience when he showed a slide that he titled "Progess in our view of comets." One side of the slide was a 19th century engraving of a comet stretching across the sky of Paris. The other side was an atomic level view of an olivine crystal from the comet. He said that it was a view 17 orders of magnitude more detailed! Brownlee went on to describe the particles they are seeing, and they are big. Some he called "rocks" because they are larger than 10 micrometers in size. (For context, a human hair is roughly 100 micrometers in diameter.) "We could have done everything right and the comet could have given us stuff that we couldn't collect and we couldn't work with" because it was too small, he said. "Instead, we have what we consider huge rocks -- we don't really know how to deal with 10- to 15-micron grains, the slicing techniques we use tend to chatter them" or produce striations on their surfaces when they are cut. But the most astonishing thing was not the size of the grains, but what they were made of. Many of these largest grains were what mineralogists call "refractory," which means that they formed at high temperatures, temperatures up to 1400 Kelvin or so. Minerals like olivine and pyroxene that we on Earth are familiar with as being the constituents of basalt, the highest-temperature lava that erupts on the surface of the Earth. But remember, these grains were in a comet, which must have formed in the outermost, coldest part of the solar system in order for it to retain its primordial ice. "We have hot minerals in the coldest place in the solar system. Where did they come from? They didn't come from there. They either came from the inner regions of the solar system" meaning the location of the terrestrial planets like Earth and Venus "or from other stars. If this was astronomy, we would stop there, with that question. But we have samples in hand. We will solve the mystery. Stay tuned; and I encourage you to join in the analysis," he concluded.
Next up was Peter Flynn, who talked about the preliminary chemical analysis of some of the first grains that were removed from the aerogel cells (here's the abstract). Rather than diving right in to the chemistry, he opened by explaining the context of their work. Basically, their first interest is to figure out just how many individual grains they were going to have to analyze to get a good representative sampling of what's in the comet. One prediction is that the Stardust particles would look like interstellar dust particles (often abbreviated IDPs), which are pretty well mixed up at a scale of 10 micrometers or so. If that were the case, Flynn said, they'd need to look at 30 grains to get a good bulk composition for Wild-2. But the more they look at the grains, the more variety they see. Furthermore, not only is there variation from one grain to another, but when they analyze all the little bits of grains that got deposited along those carrot- or turnip-shaped tracks made by the big particles, they find varying compositions along the tracks. For example, Flynn showed one track where they saw iron and nickel deposited pretty much homogeneously along the whole track, whereas zinc was there only along one side of the track, and chromium was there only at the end of the track. Some individual grains have strangely elevated abundances of one element or another. All in all, the composition of the grains is going to be a big, big, long-term analysis project. Some analysis will have to wait for the development of new technology. Flynn talked about one technique they were using that is able to analyze samples only 200 nanometers across; that's smaller than individual light waves! But even that tool is too coarse to analyze some of the particles. That's all I've got time for now -- I have to run to "NASA night," where NASA headquarters gives their spiel about their future plans. In light of the implications of their FY 2007 budget, it is probably going to be a bit of a bloodbath. I have to go early to try to get a seat from which I'll be able to get to a microphone. More later… Mar. 14, 2006 | 08:38 PST | 16:38 UTC Stardust, Monday morning, continuedIt's only a day into the Lunar and Planetary Science Conference and already I'm nearly a day behind! --but there was a lot to say about Stardust, and fewer things today that I was so interested in, so I wanted to take my time to cover Stardust completely. Yesterday I described the first three talks of the Stardust session. I have a few more to write about. Mike Zolensky, who is the curator at Johnson Space Center for the samples, spoke about the mineralogy and petrology of the retrieved samples (here's the abstract). This is a separate issue from the elemental composition, which Flynn spoke about yesterday. There are fewer than a hundred naturally occurring chemical elements, and only a dozen or so are truly common. But depending on the conditions of pressure and temperature when stuff was forming, you can get a wild diversity of thousands of different types of minerals made up of those elements combining in different crystal patterns, and those thousands of minerals can combine in an equally mind-boggling array of rock types, and that's what Zolensky is investigating -- what the minerals and rock types present in the Stardust samples can tell us about the ambient conditions when that stuff was forming, during the birth of the solar system. The biggest challenge Zolensky faces is figuring out to what extent the Stardust samples were altered by their capture. During capture, they were decelerated from 6.1 kilometers per second to zero over a distance of less than 3 centimeters, an experience that would tend to release heat and thus cause alteration of the comet fragments. "They are fractured and mixed with compacted or melted aerogel, sometimes at a very intimate scale," Zolensky said, meaning that aerogel and sample bits are mixed with each other microscopically. "In some samples, the mineralogy has been altered, but in the majority of samples the mineralogy has survived. We are seeing magnesium-rich olivine, both low-calcium and high-calcium pyroxene -- a wide range of olivine and pyroxene compositions -- and plagioclase." These are all crystalline, but they are also seeing glass, which may have the same elemental composition but not the crystal structure of these other minerals. "We're not sure yet whether the glass is produced by an impact process" (such as melting) "or is original." "Sulfides are important," Zolensky continued. "Sulfide structures are very sensitive to temperature." That's important because if sulfide minerals are found in the samples, then that means the samples could not have been heated too much during their capture. "I think we can establish a sulfur temperature scale. Most of the samples have lost sulfur, so I think that is one way of getting at the individual heating that each of the grains has seen at capture." He reviewed the kinds of mineral grains that Brownlee, Tsou, and Flynn had showed earlier, and mentioned that "the presence of refractory grains" like olivine and pyroxene "is a prediction of the X-wind model" of solar formation. Zolensky was not the only person who mentioned this "X-wind model." I'm afraid I'm not familiar with it but judging by the diagrams they showed it seems to predict that as the solar system formed there were jets of some sort tossing material out from the innermost solar system. If there is anyone reading this who can tell me (and the other readers) more about the X-wind model, please feel free to share an explanation! Zolensky continued by saying "we are not seeing phyllosilicates or carbonates" but that they are seeing some exotic minerals like vanadium-bearing osbornite, which he joked should be christened "brownleeite" after the principal investigator. There's certainly precedent for this type of mineral naming -- in college I was taught about a mineral called jimthompsonite and a related mineral called clinojimthompsonite -- but I don't know whether Zolensky was serious. Finally, he said that as soon as the preliminary analysis period ends around September or so, "we'll be able to begin allocating samples to anyone who's qualified" to study them. The next talk was by Lindsay Keller on UV/VIS and Raman spectroscopy, but my notes are extremely slim from his talk for some reason. I think I needed to go get some coffee at that point. (here's the abstract.) Next up was Scott Sandford, who discussed the preliminary analysis of organics present in the samples, a topic of great interest to a lot of people (here's the abstract). Organics are definitely found in comets -- they are easy to spot in Earth-based spectral observations -- but it was not at all clear whether such materials could survive the collection process. For organics, "contamination control is important," Sandford began. "We could have had contamination from the spacecraft, from the landing site, from the aerogel…these concerns are being addressed by a whole series of studies" being conducted in parallel with the studies of the samples. "The most problematic source of contamination for us is that the aerogel contains some organics in it."
So, keeping in mind all of the possible sources of contamination, Sandford went on to present some preliminary results. "Carbon is very heterogeneously distributed within individual particles. This is not what I would suspect from a contaminant -- but who knows?" To address the problem of carbon being present in the aerogel, he performed an analysis where they took a profile perpendicular to the track, measuring spectra of points completely within the aerogel, on the edges of the track, in the center of the track, and back out the other side. What he found was that while there were definite differences in the carbon abundance within the track and completely outside the track, in unaltered aerogel, the boundary between the two was not sharp but rather gradational. That means "either the organics from the original particle diffused into the aerogel during the impact, or the heat of entry alters the carbon in the aerogel. Preliminary analysis is suggesting the former." In terms of the carbon chemistry, "we see aliphatic hydrocarbons, not as much evidence for aromatic, but then we are not as sensitive to those." He closed by emphasizing "we have yet to verify that the organics are actually of cometary origin, but there are encouraging signs."
So that's it for the Stardust session. I watched the press conference too, via NASA TV, where much of this information was repeated; I plan to try to pull together a coherent story about all of this later. Mar. 14, 2006 | 12:46 PST | 20:46 UTC Monday afternoon and "NASA night"After the Stardust sessions in the morning, I went out for a fine Indian lunch with several people and had a nice long conversation with Olivier Barnouin-Jha, who is best known as an impact experiment person but who has spent the past year or so working on the LIDAR team on Hayabusa in Japan. I know Olivier from graduate school -- he had already graduated and left Brown by the time I got there, but I met him through other Brown students at conferences like this, and I have always looked forward to talking with him because he is so good at explaining what he's working on (and also because he's just a nice guy). Olivier talked about how Hayabusa was really a mission driven by engineering -- that is, it was intended more as a test of technology than a science mission. But he said the engineers learned that they needed analysis by scientists to help them to understand exactly where they were with respect to Itokawa. (Unlike giant planets and even medium-sized asteroids, tiny Itokawa has such an insignificant gravitational field that it's very difficult for navigators to be able to predict the spacecraft's position and course near it with any accuracy.) Olivier, who is working with the Mercury Laser Altimeter team on MESSENGER, went over there to try to understand what the Hayabusa LIDAR data could tell the mission controllers about where Hayabusa actually was. With regard to Itokawa science, Olivier was pretty excited about what he saw as the inescapable conclusion that Itokawa truly is an unconsolidated rubble pile, made of rocks only very loosely clumped with a porosity somewhere in the neighborhood of 60%. We talked about more stuff but there is a full morning's session coming up on Hayabusa on Friday, which will cover all of these science results in more detail and with pictures, so I think I will wait until then to talk more about it. Olivier mentioned that some of the pictures that Hayabusa returned have resolutions as high as mere centimeters per pixel. I can't wait to see those. The next item on my agenda yesterday was NASA night. This is a one-hour meeting that happens every year at LPSC, where some folks from NASA Headquarters present the future plans of NASA to the audience. I was not looking forward to it this year, because I anticipated that it would be ugly due to the terrible cuts to science present in NASA's fiscal year 2007 budget. But of course I had to go to see what the NASA folks would say, and how the scientists would react. The room was absolutely packed with several hundred scientists as the presentation began. Mary Cleave, the Associate Administrator for the Science Mission Directorate, was the main presenter, and there were also contributions by Andy Dantzler, who is the director of the planetary science division. I knew as soon as Cleave began speaking that it would indeed be ugly. She had chosen to present NASA's future as being bright and rosy. Perhaps there are great things in NASA's future, but I think she would have done better to acknowledge up front the fact that there are many fewer great things to look forward to than the folks in the room had been expecting. Of course, it is worth acknowledging that NASA has indeed had a fantastic year. Continued success of the rovers and Cassini, Deep Impact, Voyager traversing the heliopause, the successful launch of Mars Reconnaissance Orbiter, and on and on. And they should toot their own horns about that. But all the successes of the past couple of years, which have kept us all very busy, may well yield to a period in space exploration that will be far less exciting, with few launches and few events. After talking about the successes, Cleave talked about various reorganizations of NASA (they talk about reorganizing NASA at every NASA night, so I'm afraid I never pay any attention, because I figure it's not worth learning something that will be different next year). Then she got to the budget. Immediately her language became more hesitant. I tried to write down what she was saying as verbatim as I could, but it may not be perfect: "What we did in trying to build this budget, we had unexpected budget liens in the shuttle program. And those liens needed to be covered. So there was no money left in aeronautics, so we were the only ones left. So we are having reductions in our growth to cover liens in the shuttle program. Compared to other agencies on the discretionary budget side, we are still growing, so we are happy. So the total decrease to our budget is 3.1 billion dollars from the FY06 budget runout." In conversations afterward with other folks from NASA Headquarters, I heard that Cleave and Dantzler and others fought tooth and nail for science at NASA, and the budget situation was the best that they could do. I guess that politics forced Cleave to say "we are happy" about this budget, but there was a noticeable undercurrent of grumbling at that remark of hers. The mood in the room was rapidly declining. I could go on with details, but the bottom line is that Cleave and Dantzler attempted to tell the scientists in the room that they should be happy about the 2007 budget, and the scientists were not in any mood to hear that message. They are angry about the cuts to missions like the Europa mission and Dawn, and to the research and analysis funding that is forcing them to cut postdocs and graduate students, and they wanted to hear Cleave and Dantzler, their advocates in Washington, acknowledge that it is a bad time for science. But during the question and answer session Cleave responded to one vituperatively angry scientist by saying "I don't know why you're so angry." It was kind of a bad scene. What were people angry about? The people who were most angry were angry about the cancellation of specific programs, like Dawn, Europa, and the endlessly put-off Mars Sample Return, and they were angry about having to turn away students that they had already brought on board. Another class of angry people was Europeans. Gerhard Neukum, who is a very senior German scientist at DLR, stood up and chastised the NASA representatives for just canceling programs with European contributions without any consultation of the European partners, remarking that NASA is increasingly being seen as an unreliable partner by Europe. His views were echoed by two other speakers from Europe. Scientists were also angry that the budget did not seem to reflect the priorities set by the science community. The priority argument was primarily cited by the advocates of the Europa mission. Bob Pappalardo would not sit down until he got Cleave to acknowledge that Europa is the consensus highest priority of the planetary science community. It was ugly enough that some scientists clearly felt that balance was necessary -- not to defend NASA, but to try to do something more than rant. One scientist stood up and said that he thought that one reason people may be so angry is that NASA failed to involve the science community in the incredibly painful decisions necessary this year in the budget process, and Dantzler acknowledged that and suggested some ways that communication may improve. There's been a lot of hallway discussion surrounding the NASA night discussion and the budget situation in general. This is the largest Lunar and Planetary Science Conference ever. The amount of work being done in planetary science is increasing continuously, and the public is growing more aware of what's going on. The public is also increasingly aware that it's the robotic missions, not the manned program, which is producing the headlines. That's not intended to be a manned-versus-unmanned argument, just an acknowledgment of the fact that this year, it's the unmanned program that's producing the good news and the exciting results. There is a whole generation of new Mars scientists that is growing up with the incredible activity of six operational spacecraft at Mars. And with this budget, what do those people have to look forward to? It's a very scary future for scientists to look forward to. Sure, some years will be better than others; I think that most scientists would acknowledge that. But the scientists are looking at going from feast to famine in just a few short years; they're anticipating a drought, and are beginning to look around them and wonder who will survive. The senior scientists will probably all survive, but outer planets people may have to give up on their field, and without sufficient funding for students, everybody's research productivity will dwindle. My job at NASA night was to step up to the microphone and say that The Planetary Society is trying to do something to make this future less bleak, by asking our members and supporters to make it known to Congress that they want to stop the drastic cuts to science. I called on the scientists to be involved just as we've already called on members. I said my piece and sat down, and Cleave asked "What was the question?" I guess I cheated in the question-and-answer session -- I had no question, but hopefully we have some piece of an answer for those angry scientists. Mar. 14, 2006 | 13:12 PST | 21:12 UTC Other bloggers and writers at this conferenceI'm not the only one here trying to keep people up on what's happening at LPSC. I was pleased to run into Mark Peplow from nature.com, who is also blogging away (and whose comrade Oliver Morton was kind enough to post a link from his website here, so I am returning the favor!) There are also plenty of journalists here of course. I've seen Dick Kerr from Science, Kelly Beatty from Sky & Telescope, Leonard David from space.com, and David Chandler and Maggie McKee from New Scientist, and some others I'm sure I'm forgetting (for which I apologize). All of us who are attempting to present this conference to people not attending it have to try to figure out in advance which sessions are going to be interesting. Despite the length of the typical LPSC abstract -- the short paper that a scientist submits in order to have his or her work selected for a talk or a poster presentation -- it's not always easy. Stardust was definitely interesting yesterday. This afternoon, I went to the Genesis session. Their abstracts indicated that despite their crash two years ago they are now actually getting some results on their analysis of the composition of the solar wind from the samples that Genesis returned. However, I am very sorry to report that I wasn't able to understand what they were talking about well enough to report on it. With Stardust, although I may not have recognized all the mineral names or the analysis techniques, I was still able to (mostly) comprehend the big-picture significance of each presenter's results, but with the Genesis talks, I couldn't see the big picture. So, I don't have much to say about that. Tonight will be the first of two poster sessions. Not every scientist who presents at LPSC does so in talk format; only a fraction are selected for talks, while many more present their work as a poster, a format that should be familiar to anyone who ever participated in a "Science Fair" in grade school. I always find it very difficult to get much out of posters, because there tends to be so much text on so many posters to wade through, and it doesn't help that the poster sessions are (like science fairs) often held in a gymnasium, complete with the lousy accoustics and the pervading odor of sweaty socks. At LPSC, that odor is overprinted with the smells of cheap beer and popcorn. It's kind of distracting. But I will go this evening and try to pick a couple of interesting ones to talk about. Mar. 15, 2006 | 15:36 PST | 23:36 UTC Courtney Dressing at LPSCI've spent a full day at the sessions -- Titan in the morning, and the rovers, Enceladus, and Deep Impact in the afternoon, with a lunch meeting on the fate of SMART-1 in the middle. This is after last night's poster sessions. I've been too much in meetings to write up any of my notes -- I will get to it when I can! But I thought I would drop a quick post with this snapshot from last night's poster session. The lady I'm standing with is former Red Rover Goes to Mars Student Astronaut Courtney Dressing, who's now a high school senior, and was at the conference presenting a poster on research she's performed with a couple of classmates under the tutelage of Mars geologist Jim Zimbelman. We're very proud of Courtney!
Mar. 15, 2006 | 18:36 PST | Mar. 16 02:36 UTC Wednesday morning: TitanThis morning at the Lunar and Planetary Science Conference began with Titan, and then later in the morning I had to choose between skipping Titan and going over to rover sessions, or staying with Titan. I elected to stay with Titan because there was a suite of interesting-sounding VIMS talks up against the rover talks, but I'm not at all sure I made the right choice. In fact, there was not a whole lot in the Titan session that struck me as being very new or a deeper understanding of ideas I'd previously heard before. Ralph Lorenz gave a talk about the dunes on Titan (here's the abstract). Since much of the material from his talk is in a paper that is apparently in press in Science, it was a little better developed than I'd heard before. For example, in the past I have heard Ralph cite examples of dune-forms developed in snow in Antarctica where the dunes have almost no topographic expression but are visible to imaging techniques that are sensitive to ice grain size. Now, however, Ralph was reporting measurements of the heights of the Titan dunes -- they average 150 meters high, with a 2-kilometer spacing from crest to crest. He's now citing examples from the Namib desert. He showed some really beautiful Space Shuttle photography of those features. Chuck Wood gave a talk about craters on Titan, but he had little new to say because after nearly two years in Saturn orbit Cassini has seen only two unambiguous craters on Titan (here's the abstract). He pointed out some very small features in the radar data that he said looked much like craters, but they were so small (5 to 10 kilometers diameter) as to be under the lower size limit that is imposed by the present density of the atmosphere (which would shield Titan from smaller impacts). He said that this could be evidence that either we aren't modeling the impactors coming in to the atmosphere correctly or maybe the current pressure of the atmosphere is transient, but I don't think anybody thought that the "craters" were compelling enough to force such a reevaluation. Jani Radebaugh talked about the mountains on Titan observed by Cassini radar (here's the abstract). She performed radarclinometry to try to estimate the heights of the mountains and found them to have mean slopes around 8 degrees and heights around 300 meters, none (of 50 she measured) over 600 meters, over a mean basal diameter of 5 to 25 kilometers. So far this is just sort of basic fact information, not Earth-shattering, but of course it's significant to get any topographic information at all from Titan because the highly scattering atmosphere prevents us from using shape-from-shadow to figure out topography as we can do on all other solid surfaces in the solar system except Venus. I expect that the story of topographic information that you can get from radar data will get much more interesting as they get more overlapping data between radar and the imaging and spectrometers -- and as that data enters the public domain so that the different instrument teams can start looking at each other's data. (I thought it was very interesting that Radebaugh's was the only one of the radar talks that employed Cassini imaging data as context for radar data -- everyone else was using the crude Hubble albedo map as their base map. Cassini folks don't share their data as much as rover folks do!) Guiseppe Mitri presented an interesting modeling study where he asked the question: Are the observations of atmospheric methane relative humidity and thunderstorms/cloud frequency consistent with a desert planet containing tiny fractional lake coverage? (here's the abstract.) According to his calculations, he said, a 50% relative humidity of methane in Titan's atmosphere could result from lakes covering only a small fraction, 0.2 to 4 percent, of the surface. (This was assuming "tropospheric overturning scales of 10 to 100 years" but I don't know what that means.) I also noted that his calculations implied that if such lakes exist, they evaporate at a rate of 3 to 10 meters of elevation per year. One talk later in the Titan session that I thought was pretty interesting was the one given by S. Rodriguez (I did not catch his first name), who was attempting to use Huygens DISR spectral information to correct for the atmospheric scattering that is giving the VIMS team such difficulties in trying to pull compositional information out of their Titan data (here's the abstract). According to his talk, he claimed to have some success with this approach. Here's the deal: typically, when you want to find out information of the spectral properties of a surface -- how its reflectivity changes as you go from one wavelength to another -- you calculate a ratio image, where you divide an image taken in one wavelength from an image in another wavelength. You can do this because digital images actually are really just grids of numbers, each pixel represented by a number. What's cool about calculating a ratio is that the ratio usually removes the effects of any process that changes the brightness but not the color of a surface. For example, when the Sun illuminates a surface, it makes some parts look brighter and some parts look darker because of shadowing, across all wavelengths. When you calculate a ratio, you divide out this effect of light and shadow, and you're left with color differences. However, when you calculate ratios for Titan using VIMS images, the ratio images look awfully similar -- with the same bright and dark patterns -- to the original images. That indicates that there is something going on that can't be canceled out by the ratio. Rodriguez argued that what's going on is atmospheric scattering that is adding brightness at all wavelengths, and you can't calculate out this additive component with a ratio. He was able to use the Huygens data to get an estimate of what this additive component might be, and when he subtracted that component out of the VIMS image of the Huygens landing site and then calculated VIMS image ratios, suddenly those bright and dark patterns disappeared, and instead he started seeing different spectral units pop out (that is, regions that have different relative brightnesses in different ratio images). It seemed pretty impressive. In one of the units that he mapped, he argued that he identified the spectral signature of water ice. I honestly don't know if other people in the room agreed with or disagreed with his methods and conclusions, but it was an interesting presentation anyway. So, so much for Titan. All in all, there wasn't a lot that was new, either new data or new insight. I wish now that I'd skipped the latter part of the Titan session and gone to the rovers, but it was too late for that. The radar story will get more interesting, because after a long hiatus in the acquisition of radar data they are going to be getting a lot more radar passes beginning with the T13 flyby on April 30, so there is much to look forward to there. Mar. 15, 2006 | 18:56 PST | Mar. 16 02:56 UTC Wednesday lunch: Planning for the crash of SMART-1Over lunch they had scheduled a special session to acquaint the community with the plans for the end of ESA's SMART-1 mission to the Moon. I ran and grabbed a pretty awful sandwich from the hotel lobby and sat down to listen to Project Scientist Bernard Foing talk about the status of SMART-1. Originally, their orbit would have had them crash on August 17 of this year on the far side of the Moon, where it wouldn't be visible from Earth. They've exhausted their xenon propellant (which is what they use in their ion engine), but they still have a couple of kilograms of hydrazine fuel left to try to change that.
OK, so why don't they know the impact time to an accuracy of less than 7 hours? The problem is that when SMART-1 crashes, it will do so at a tiny, tiny, glancing angle -- it will be coming in at an angle of 1 degree over a landscape that has local slopes of up to around 10 degrees. Unfortunately, the topography of the Moon is only mapped at a horizontal resolution of about a kilometer. What that means is that they can't know the characteristics of the local topography underneath SMART-1's course well enough ahead of time to predict on which of three orbits SMART-1 will impact the surface. In other words, if there's a high hill in SMART-1's path, it will hit one orbit before the one they predict; if SMART-1 happens to slide through gaps between hills, it could hit one orbit after the one they predict. The orbits have about a 5-hour period. Thus the uncertainty. As a result, Foing has to go to a lot of telescopes on Earth and ask them to be ready to photograph a flash that they may end up not being able to see. One mind-boggling fact he mentioned was that on the orbit before the crash, they could well be sailing along only 400 meters above the lunar surface. Yikes. He offered some information about what kinds of effects the impact might produce. The spacecraft is about a meter cube and weighs 285 kilograms. Of that, 200 kg is aluminum, 3 kg will be leftover hydrazine, 0.26 kg is xenon, and then there are the 14-meter-wide carbon-fiber arrays. Based on the experience with the Hiten spacecraft, which crashed onto the Moon carrying only a kilogram of hydrazine, the flash from the hydrazine alone should be visible from Earth, at least in infrared wavelengths (around 2 microns). Another interesting possible effect is that although the impact will happen in the dark, if it manages to send off any ejecta with vertical velocities greater than 200 meters per second, that ejecta will reach an altitude of 24 kilometers, where it will be in sunlight, and should become visible. He estimated that the resultant crater should be 3 to 10 meters in diameter and be very elongated, and expressed hope that some of the future lunar orbiters planned by the US, India, and China might be able to spot it. One thing that I thought was amusing about this part of his discussion was that he kept on looking at Deep Impact Principal Investigator Mike A'Hearn as he was going through all these numbers. A'Hearn was sitting in the front row and nodding occasionally, so it seems he's been consulted on these predictions, and he should know about artificial impacts! There was a brief question and answer period, and Chuck Wood stood up and asked when the imaging data from SMART-1 was going to be released. (He was sitting next to me earlier and had mentioned with some disgruntlement that they've only released 11 images so far.) Foing said that they are working very hard to clean up the data and release it on ESA's servers around the time of the impact. However, he said, if anyone in the room was interested in collaborating with the SMART-1 team in its waning months to work on the impact simulations or any other lunar science project, that they would certainly share the raw imaging data with those collaborating scientists. I still have notes to write up from the afternoon, but I'm pretty weary; I'll have to have a go at it tomorrow, in between talks on Mars and the Galilean satellites, and before the second poster session. And I still have notes from Tuesday night's poster session to write up too. I may be approaching the end of my stamina for attending marathon talk sessions; I might just have to select a couple specific talks to go to tomorrow, and conserve my strength for the poster session in the evening. And I need to save a little for Hayabusa on Friday morning. But the Mars session opens tomorrow morning with someone arguing that the rover folks have got their interpretation of Meridiani Planum all wrong, so it seems like I'd better go to that just to see how it's received. Mar. 16, 2006 | 15:10 PST | 23:10 UTC Wednesday and Thursday: A few Mars Exploration Rover-related talksToday has been an interesting one for the discussion of the future of outer planets exploration, but I don't want to get ahead of myself; I need to continue catching up with the talks I went to yesterday. In the afternoon, I was faced with an even tougher choice than I had in the morning: there were concurrent sessions on Cassini's studies of the icy satellites and rings; the rovers; and results from Deep Impact. After last Friday's Enceladus hubbub I thought that Cassini would be the shoo-in, but as I read the abstracts I decided that several of them sounded awfully similar to results I'd seen at the DPS meeting last September. As it developed, I ended up going to a couple of rover talks, then went to Saturn, and then took a much-needed break from sitting in the dark and typing, and finally finished in Deep Impact. I only went to two rover talks but Steve Squyres and Matt Golombek talk so danged fast that I took almost as many notes as I had for the Titan session. Steve's job was to give an overview of the most recent 150 sols of the Spirit mission, which I think pretty much took people almost but not quite back to DPS (here's the abstract). "Happy sol 781, everybody," he began. "My talk will feature exclusively stuff after Haskin Ridge and heading down into the inner basin." He said he'd talk about four areas: (1) what they call "the Land of Olivine" up to (2) the sand dunes of El Dorado and then (3) Arad, a crazy soil location, and finally, (4) Home Plate. The Land of Olivine: (I put this in quotes because it's impossible to write down notes from what Steve says without doing it in his voice, but as usual I warn that this is more accurately a paraphrase than a quote:) "As we came down off of Haskin ridge, we went over lots of exposed terraces. We went over a class of material we called Seminole. Seminole, Algonquin, Comanche, all of these rocks, and indeed this whole area very very mafic in composition. High in magnesium, low in iron. If you do normative mineralogy you get up to 50% olivine; up to 70% of the iron in these rocks is in olivine." The "normative mineralogy" bit means that he isn't saying the rocks are made of 50% olivine, it's just that when you count up all the atoms of all the elements they detect with APXS and then start stuffing them into chemical formulae according to the order they'd crystallize if the rock started as a melt, you'd get that much olivine. It's a standard way of comparing the elemental composition of dissimilar rocks to each other -- it's sort of melting them by thought experiment.
"Arad was a surprise. We were driving along minding our own business and the wheels churn stuff up and there's this white bright-toned soil exposed. Not like Paso Robles. 38% SiO2, 35% SO3, 19% FeO, 4% MgO, not a lot of anything else. Remarkably high in ferric sulfates. Very intense salt content; unlike Paso Robles, no significant phosphate. "Home Plate has occupied most of our energy and attention for the last month or so. It is a spectacular plateau a couple of meters high, 80-90 meters across. It has several distinct units. It has a lower unit, that is very coarse in nature. Poorly sorted, chock-full of coarse granules up to several mm in size. Somewhat rounded, prominent throughout lower reaches. Laterally, you get to one of the most interesting things we have seen, a probable bomb sag in this deposit. A rock has fallen from the sky and has deformed these layers. This is the only one of these we have found anywhere and we have looked hard. The upper unit is quite fine grained, much more well-sorted, very, very finely layered. You can get some hints of low angle cross-stratification. We drove around to the side of Home Plate and we saw some beautiful cross stratification, lovely crossbed sets, some of the most spectacular we've seen in the entire unit and indeed on Mars. Upper unit is distinctly different from lower unit. Here you see extremely well-sorted, extremely well-rounded grains. In searching through our images, the best soil analog we can find anywhere is El Dorado!" Steve got a laugh from the audience when he pointed mentioned that one image from his talk, showing outcrops at Home Plate, was referred to by the team as "Rock Monster" because of you could see the eyes, nose, and mouth…
"We are now in a drive-or-die situation. We are down to about 350 Watt-hours. The vehicle is very dusty and we are getting into winter, we need to get to north facing slopes 120 meters away in order to continue doing science during the winter. We need to drive like hell and get to safe winter haven. Our focus right now is on Foget, Korolev, and Oberth." These are all names for spots on McCool Hill that the drivers have identified as possible winter parking spots for the rover. "Spirit has just completed a drive in the direction of Oberth and Korolev, we hope in a week or so to be on one of those nice toasty slopes that will allow us to survive another winter at Mars." So that was all one fifteen-minute talk. I should add, in this context, that I asked one of the people I know from the rover mission later about the problems that have been cropping up again in Spirit's balky wheel. He confirmed that there were problems but that they don't have the luxury of stopping for two months and figuring out the problem and an optimal solution; they have to get Spirit to a north facing slope, or it'll die, so it's damn the wheel problem and full speed ahead for the team. I took nearly as many notes for the following talk, by Matt Golombek, but they are much less coherent (here's the abstract). Really, his talk was intended to unmistakably drive home two points. The first point is that there is no evidence at all in Gusev crater for the paleolake deposits they had hoped to find there. "The cratered plains of Gusev are dominated by aeolian processes. THe rock distributions can be related directly to the impacts that occurred here. Liquid water has not shown its face ANYwhere except in alteration rinds; this has been dry and dessicated since the lava flows formed in the Hesperian. If you go to any place on Mars where the rock crystallization age is equal to the crater age, this is what it will look like." However, in regards to that crater age, Matt had another point that he wanted to drive home. To paraphrase, the craters that Spirit has seen, at every scale from Bonneville to the tiniest "hollow" only 40 centimeters across, is very shallow, with a depth-to-diameter ratio of less than 1 to 10. Though some are certainly filled with sediment, the freshest look reasonably fresh, with rocky bottoms. The shallowness, Matt argued, strongly suggested that every crater that Spirit has seen to date is a secondary. (That means that they didn't form from an original meteorite hit; they formed when the spray of debris from a single large meteorite hit fanned out and splatted into the ground. The lower speeds at which secondary craters form result in the shallower depths.) This observation "has very important implications for age dating using very small craters," Matt finished. What he's implying is that people may be significantly over-counting craters on Martian surfaces similar to the one that Spirit's sitting on. At that point I skipped over into the Cassini talks. Since I'm a day behind now though, I think I'll jump forward in time to one more rover related talk I attended this morning, given by L. Paul Knauth, titled "Impact Surge as the Simplest of the Proposed Hypotheses for the Origin of Sediments at the Opportunity Landing Site on Mars." (here's the abstract.) If you didn't catch the implication of his talk title, let's just say he may as well have subtitled it "The MER Team Has Got It All Wrong." So of course I had to go and see what he had to say, which was basically that you can explain the Opportunity landing site rocks with impact-related deposits, and you don't need liquid water sitting around as the rover team has argued. Knauth puts together a good presentation, and he opened with a slide showing a nuclear bomb explosion, which I have to say I wasn't mentally prepared to see first thing in the morning. But his point was "Here's a nuclear explosion that shows a surge deposit that flows out from the base of the stem. These things produce sedimentary rocks. All impacts produce some kind of basal surge." He showed lots of extremely lovely slides of Earth rocks formed by impact or volcanic surges, with selected inset images from Opportunity that did look strikingly similar. To address the subject of the hematite blueberries at the Opportunity landing site, he pointed out similarities to small, spherical structures in surge deposits called accretionary lapilli. "Accretionary lapilli form like hailstones in the cloud as it goes along, and they rain out. They are made of target material particles. So the question is, could we get a lot of iron oxide in the lapilli? On Mars, there are very high-iron lava flows. In magma chambers, sulfides separate as immiscible drops, and you can get cumulates at the bottom, and disseminated particles in the magma as it cools. This happens to some extent in any magma. If, at Meridiani, you had some of these, you could have had an impact onto it -- it could have hit a huge cumulate. You could have had multiple impacts with many surges, or one enormous one. It could also be that Meridiani was hit by a large iron meteorite. The important thing is that basalt grains, accretionary lapilli, salts, ice, sulfides, and brine are mechanically emplaced in base surges. You don't need an acid lake or an acid aquifer to make jarosite, all you need is sulfides and water vapor." He went on to consider the festoon structures observed by the MER team. At this point, he let contempt creep into his statements, and he began to lose my interest; when scientists waste argumentative effort being contemptuous, I always begin to feel less inclined to believe their stories, no matter how logical they seemed to that point. He argued that the festoons identified by John Grotzinger are "nothing but topography" and anyway it "turns out it doesn't matter because you do see festoons in base surges." He went on to say that "there are also some problems with the MER team in geochemistry. You can't maintain an acid aquifer in the presence of basaltic minerals." As you might expect, as soon as he finished talking about six people jumped up and beelined for the microphones. John Grotzinger got there first, and unleashed a little contempt of his own, saying "We appreciate what you're trying to do but while you were trying to download slides yesterday you obviously missed my talk." He said that the scales of Knauth's examples did not match the scales of things on Mars. (Personally I'm a little bit leery of arguments based only on scale, because the different force of gravity and other stuff on Mars tends to make some Earth processes scale strangely.) Grotzinger continued, "The most amazing thing about stratified rocks is that they're stratified. Details matter. All the data suggests that the evaporite playa is the best model." Knauth responded: "You're using a eutectic brine," meaning that Grotzinger's models relied on a water that was utterly and completely saturated with salt. "Do you know what the viscosity of brine is? That flow regime diagram you're using doesn't apply…." Grotzinger replied that the diagrams are nondimensional and that changes in viscosity have a negligible effect. Knauth asserted viscosity is not negligible, and he'd do the experiment to test it. After this exchange there was no time for further questions, so the other five questioners were told to sit down. One of them, Ray Arvidson, was sitting next to me, and I asked him what he had wanted to say. He said that "the regional geology doesn't work" in Knauth's scenario. "Most of the sediments are sitting on the craters," meaning that they could not have formed as a result of the impact craters themselves. I haven't read the rover team's papers closely enough, or indeed Knauth's for that matter, for me to be able to independently evaluate the relative veracity of the two very opposing viewpoints. I can tell you, however, from having been inside MER mission operations during the first couple of months that (a) the MER team is a very large one with a very diverse group of scientists and (b) that one of the central themes driving their choices of observations to make with the rovers' finite time on Mars is to form multiple hypotheses and come up with observations to test those hypotheses. I know that when they first saw those blueberries they had a lot of hypotheses, which included concretions, accretionary lapilli, and other possibilities, and that they designed tests for all those hypotheses, and therefore their results are the results of not only observations but careful tests. So I'm generally inclined to believe the MER team's story unless there are a lot more than one guy objecting. However, people like Knauth are very, very important to the advancement of our understanding of the solar system. By raising objections, they force other scientists to be methodical and meticulous, and to develop tests and counter-arguments to every one of the objector's arguments. It improves the quality of the science overall, by suggesting new tests and new experiments to perform, and forcing scientists to address weak points in their theories. And, every once in a while, they are right, and the new line of investigation yields a whole new viewpoint. This give-and-take is central to the scientific process. By the way, here is the latest news we have on our site from the rovers, and I know that A. J. S. Rayl is working on the next update, which I'm guessing you'll see on this site in the next week or two. Mar. 17, 2006 | 08:39 PST | 16:39 UTC Wednesday afternoon: Cassini at EnceladusSo after those two rover talks I skipped over to the other large room to listen to what the Cassini science teams had to say about Enceladus. I had missed the first two talks, which covered the discovery of the atmosphere by the magnetometer team, and the fracture/tectonics patterns surrounding the south pole, which I had already read about in the Science paper. But I came in to check out John Spencer's talk on CIRS (here's the abstract). I'd talked to John when that Enceladus news broke last week, and he primarily covered the same material. There were a few interesting additional facts though. While CIRS does see a noticeable correlation between their hottest temperature detections and the locations of tiger stripe cracks, they also have footprints that don't cross tiger stripes but still have significantly higher temperatures than the Enceladus background. In other words, not all the heat is coming from the cracks. John did a calculation to figure out exactly how much power is being radiated from the south pole, and found it to be a little more than 3 Gigawatts, or just a tiny fraction (6 · 10-5) of Io's. This amount of power "is plausible for tidal heating in the current orbit if the Q is very low." "Q" is the tidal dissipation, essentially a measure of how squishy the interior is, how efficiently it turns tidal energy into heat. Very stiff bodies have higher Q. But, he continued, "the puzzle has always been that Mimas, because it has a much more eccentric orbit, should have a tidal heating rate ten times Enceladus. So Enceladus is in a self-maintaining warm state; the only trick is to kick-start Enceladus into a warm state." There were several comments on John's talk. Carolyn Porco stood up and said that there must be temperatures higher than he is detecting in order to support liquid water reservoirs close to the surface for geysers. Jeff Kargel stood up and pointed out that folded mountains on Earth require rheologically layered materials -- that is, stacked layers of materials that have different mechanical properties (some more stiff, some more prone to bending or faulting). He said "that kind of structure could make tidal heating much more effective." Bill McKinnon stood up and said "There is all too much hand-wringing about Mimas; the answer for Enceladus is tidal heating, period." Next, Dennis Matson stood up and talked about the hydrothermal geochemistry of the geysers (here's the abstract). First he showed some models where he tried to figure out what initial conditions would be necessary to keep Enceladus hot down to the present time, and he concluded that Enceladus needed to form early, as early as Iapetus, in order to retain enough short-lived radioactive isotopes in its interior to get it up to an initially hot state. This model gave a compositionally stratified Enceladus with a molten interior. One interesting element of his models is that a body as small as Enceladus could have been porous initially, but heating it up can make that pore space collapse, shrinking the whole body. If you're not sure what I mean by "models," building a geophysical model of a planet basically involves writing down a few differential equations that relate the bulk density, pressure, temperature, and viscosity of a body's materials and that can incorporate heat flow by either conduction or convection. These kinds of models can have multiple layers. They are usually defined on paper but then plugged into a computer that runs the equations forward in time to predict how all of those physical properties will evolve. I did a little of this kind of work in graduate school; it's actually surprisingly powerful at predicting the global-scale behavior of planet-sized bodies, if you choose your parameters right. I should also add in here a conversation I had with Dennis at the poster session on Tuesday. I had told him I was looking forward to the Enceladus talks, and he told me, "Tell us what acetylene means on Enceladus and you win a prize." Apparently one of the instruments (I would guess INMS, I have to finish reading the Science papers to find out for sure) has detected acetylene, among other interesting things, in Enceladus' plume. Acetylene is C2H2, and those two carbons are joined by a triple bond. "Forming acetylene requires the breakdown of long-chain hydrocarbons or temperatures of 1,770 Kelvin, which you don't have," Dennis said. "So we think we must have catalytic chemistry down there, which could mean there's all kinds of interesting things we're not detecting" so far in the chemistry of Enceladus' geysers. Cool. Next, there was a talk by Bob Pappalardo called "Diapir-induced reorientation of Enceladus." (here's the abstract.) Basically, Bob's thesis is that if you make a plume, or diapir, of hot material inside Enceladus through internal heating and convection, the density contrast between that plume and the rest of the moon could have caused the whole thing to reorient, putting the plume at the south pole. (These kinds of hot plumes are theorized to exist on Earth beneath Hawaii, Yellowstone, Iceland, etc.) This idea sounds a little crazy but it's pretty much accepted that the gigantic volcanic deposits of the Tharsis rise on Mars have caused that planet to reorient to put Tharsis at the equator; this is called "true polar wander." Tharsis is at the equator on Mars because there is extra mass there, and Mars' rotation is most stable with that mass anomaly located at its equator. By contrast, Bob outlined, a plume on Enceladus could produce a negative mass anomaly, which would be most rotationally stable sitting at one of the two poles. Bob considered both the possibilities of a silicate (that is, rock) diapir within Enceladus' core and an ice diapir within Enceladus' mantle. In his models he found that a silicate diapir was more efficient than an ice diapir in reorienting Enceladus, and that it was most efficient if a silicate diapir was coupled to an ice diapir. However, he pointed out an interesting problem: in order for a silicate diapir to reorient the whole of Enceladus, then there must be no global ocean, because if there was a global ocean then the solid icy crust of Enceladus would be totally mechanically decoupled from the rocky core, and the core could reorient completely independently of the crust. That's a pretty important prediction for the history of Enceladus. Bob pointed out some tests that could be done for reorientation. One is that the crust should show more craters on the leading side than the trailing side of the moon; if the crater patterns don't match that, then there's a good case for reorientation. Also, he said, there should be a large gravity anomaly at the pole, so "we urge the Cassini project to consider gravity-only tracking paths across the south pole" in the extended mission. After that I took a break for a little while, then I moved on to Deep Impact. But right at this moment, I want to run back to check out some more of this morning's Hayabusa talks -- and then I'll have to run off to the airport. I'll probably write up some more stuff on the plane and post it tonight or tomorrow. Mar. 17, 2006 | 16:03 PST | Mar. 18 00:03 UTC Wednesday afternoon: Deep Impact resultsMy longest day at LPSC ended with a few talks on the composition and structure of Tempel 1 from the Deep Impact mission. Before I begin to summarize, I wanted to mention that I've been getting some mail complaining about the outcome of the Great Comet Crater Contest, in which we were forced to pick winners from among the large number of entrants who guessed a crater diameter between 100 and 250 meters. One commenter even suggested that the lack of knowledge of the crater size was "bad science" and that "based on the contest results, it was probably pure fortune rather than scientific and engineering foresight" that Deep Impact even managed to hit Tempel 1 at all! Well, we're sorry to those of you who were so disappointed, but the fact is that if the results of planetary missions were perfectly predictable, there would be no point in sending the mission at all. True, Deep Impact's inability to see the crater was, in a sense, a failure of the mission, but that failure occurred in part because so little is known about the surfaces of comets -- the very point of launching Deep Impact in the first place! We certainly know a lot more about comets than we did before; and Deep Impact did accomplish its other objectives. This sort of partial success is hardly uncommon. When I get to it, I'll be writing up the first science results from Hayabusa -- a mission for which it will be nearly a miracle if it succeeds in its goal to bring back a sample from Itokawa. But Hayabusa has already returned on JAXA's investment by providing an incredible and unprecedented view of a tiny Earth-crossing asteroid. Enough ranting, and back to the Deep Impact session. I came into the room as Seiji Sugita was in the middle of his presentation on mid-infrared wavelength observations of the impact from the Subaru telescope (here's the abstract). I wasn't in time to see his data but I jotted down a few notes from the end of his talk -- that he saw evidence for the dust that came out of the impact being accelerated by gas, which I think means that the vaporization of volatiles from inside the comet caused the dust to expand more rapidly than it would have otherwise; and that their observations suggest that the growth of the crater was gravity-controlled, rather than strength- or compression-controlled. Next up was Casey Lisse, who presented spectrometric measurements from Spitzer (here's the abstract). His presentation wasn't all that different from his talk at DPS: apparently the dust that was ejected from the impact never got particularly hot, so very little of it could have melted and recrystallized. Instead, the impact dis-aggregated large particle aggregates into constituent crystalline particles "without doing chemistry to them." This is based on the observation that prior to the impact, Tempel 1's coma had an almost featureless spectrum; afterward, "we saw a huge increase in the silicate emission feature" in the spectrum. Lisse went on to point out a "hint of a PAH [polycyclic aromatic hydrocarbon, an organic compound] feature, lots of crystalline olivine and pyroxene, and 5-8 percent carbonates." Unfortunately, Lisse left no time for questions, so I'm unable to say what the audience thought of his model fits to the Spitzer spectrum.
But temperature remains a problem. "If we had pure ice on the surface, we would have to have a temperature of 200 Kelvin," the freezing temperature of water in a vacuum. "What we have is 280. So we can't be seeing large amounts of ice on the scale of the pixels. In other words, whatever ice is there, is thermally decoupled from whatever else we're measuring the temperature of." She attempted to model the spectrometer data with mixtures of ice particles of different sizes and found that about a 4% component of 20-micron-diameter ice particles best fit the data. Jessica then moved on to infrared spectral imaging of the plume, and showed something else interesting. Looking at the plume in a wavelength that emphasizes dust, you see a broad, fan-shaped plume, just like in the visible images. But if you look at images that are processed to emphasize the presence of water, ice, you see something very different: "it is very collimated and is not broadening the same way that the dust was. Also, the strength of the water emission does not change very much" with distance from the impact site, "suggesting that the water is not sublimating very quickly." Again, Jessica used a model to constrain the particle sizes, and found that 5 to 10-micron-diameter particles were the best fit. "Compare that to the much larger ones we found on the surface," she pointed out. (Note that the particles in the plume are 1/4 to 1/2 the diameter of the surface ones -- which gives only 1/32 to 1/8 the mass. "The change in particle size strongly suggests disaggregation on impact," just as Lisse was saying. Concluding, Jessica said "there must be extensive subsurface water sources, near, but not at, the surface. This suggests that the dormancy of comets is not due to volatile loss" but rather due to the development of a refractory crust. Next was a talk by Lori Feaga, employing the same sorts of spectral observations Jessica had, but at a greater distance and before the impact on the coma of Tempel 1 (here's the abstract). She found some striking asymmetries in the distribution of carbon dioxide and water in the coma. There was more water in the sunward direction and in a northern jet; more carbon dioxide in the southern direction, particularly in the southern, anti-Sun direction. I'm not sure if this asymmetry implies anything more profound than the fact that the comet is inhomogeneous. I should note here that apparently, earlier in the session, someone (I'm assuming Mike A'Hearn) apparently showed some newly enhanced and processed images that reveal a pronounced jet or several jets visible on the northern side of the comet, just off the limb. Others showed those images and referred to them in passing. I'm looking forward to being able to get a closer look at them. Finally, Kevin Housen delivered a talk that was intended to be cautionary to those who thought they might understand the impact mechanics (here's the abstract). Most people appear to be behind the conclusion that the crater growth was gravity controlled, meaning that the particles composing the surface of the comet were essentially strengthless, powdery and cohesionless like copier toner rather than having some stickiness or strength like mud, ice, glass, or rock. Strength is measured in units of pressure, Pascal; the current estimates for the strength of Tempel 1's surface is a few tens of Pascal, a tiny quantity. "There are two features that are used to argue for gravity scaling," Housen said: "that the plume remained attached to the surface, and the mass of the plume." The "plume attached" argument goes like this: particles near the center of the impact site are ejected with some high velocity. As you go away from the center of the impact site, the force of the impact attenuates, and the ejection velocity decreases. If the surface has any strength or cohesion, at some point before the ejection velocity goes to zero, the force is not enough to overcome the material's strength. The last stuff to be ejected is ejected with nonzero velocity, and at that point the expanding ejecta curtain detaches from the surface. With no strength, stuff keeps being ejected until the ejection velocity goes to zero, at which point the ejecta essentially hits the ground at the same moment that it rises; the curtain remains attached to the ground. But Housen argued that laboratory experiments have shown that gravity-controlled growth is not required for the plume to remain attached to the surface, and he showed some experiments that got a murmur from the audience. "The strength of the surface could be as much as 10 kilopascal for the plume to still remain attached to the surface," Housen said; that's more than a thousand times the current estimate for Tempel 1. Secondly, there's the issue of the mass in the plume. He showed some graphs that showed that although there is, indeed, more mass ejected initially from a gravity-controlled crater, that hours later after the ejecta has started falling back in to the surface there's no difference between a strength controlled and a gravity controlled crater. In point of fact, he said, neither strength-controlled or gravity-controlled cratering are consistent with accepted models of crater formation. "We have a bit of a problem," he said. The amount of mass that's been estimated by several observers -- more than a million kilograms -- requires some new process: "either an acceleration to allow more ejecta to escape but which is still consistent with the observed rate of growth of the base of the plume, or some nonstandard cratering mechanism." It's fun when something crops up that nobody can explain! Scientists will be working on this problem for a while. Phew! That's finally it for Wednesday. Next up: the poster sessions from Tuesday and Thursday evenings. Mar. 17, 2006 | 22:07 PST | Mar. 18 06:07 UTC Thursday: The Moons of Jupiter and the future of Outer Planet ExplorationI said earlier I was going to cover the poster sessions next, and there are some cool things that I want to write about, but I thought I'd better get to something a bit more topical a bit sooner: Europa and the other Galilean satellites, and when (if!?) we'll be exploring them again. After checking out Knauth's talk in the Mars session I went over to the one session on the Galilean satellites of Jupiter. I took some notes, but frankly I'm not sure they're of too much interest to most people. It's not because Io, Europa, Ganymede, and Callisto are boring -- far from it. It's just that Galilean satellite science has matured to the point where it's less about discovery and more about models, which are a little more difficult to explain. It seems to me that the outer planets community has basically finished cataloguing the types of things that you can see in the Galileo images. We know how many craters there are, and their sizes and distributions. We've described and organized all the types of ridges and cracks on Europa and Ganymede into classification schemes, and mapped how they're oriented. We've figured out the temperatures of the eruptions on Io. A lot has been done. So what's left to do? Well, we're still far from being able to explain most of that stuff. And even if there are explanations that seem to work to explain a feature generally, when you get into specifics or variations you have to develop explanations for why this or that feature is locally different. So there were talks on trying to figure out just how fast the volcano Pillan on Io erupted (Ashley Davies concluded that when Galileo saw Pillan erupt, that was probably the biggest volcanic eruption ever witnessed in recorded history). There were talks attempting to explain the patterns formed by the cycloidal cracks on Europa (obliquity could have an interesting effect, but questioners pointed out that true polar wander was probably more important). Several people were looking at how stresses in Galilean satellite ices might change the grain size, and potentially the viscosity, of the ice, which could answer questions about just how vigorous a process convection is inside these bodies. I listened to some of it, but didn't find much that I could explain very readily, so I decided to take a break, post a blog entry, go buy some tacos, and return for a lunch meeting on the future of outer planet exploration. The meeting was organized by Torrence Johnson, Bill McKinnon, and Bob Pappalardo, who loosely represent three professional generations of outer planet scientists. Torrence opened the session, which was attended by maybe 75 or so people: "This is a kickoff meeting to see where we are at the moment." He showed a graphic with time on one axis and missions on another, with each mission represented by a green and a red bar. The green bar began when the mission first started being talked about, and had an asterisk in it where NASA actually funded the mission start. The green bar changed to red at launch, and projected out until the end of the mission, with markers for planetary encounters. There's a short but distinguished list of past or current outer planets missions: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Cassini-Huygens, and New Horizons. (I would very much like to reproduce this graphic but don't have the time to hunt down the "first started talking about it" and "NASA started funding" dates -- I'd love help.) Pointing to the diagram, Torrence said, "It's very easy to concentrate on minimizing trip time. In fact, if you look at this graph, a good half of the amount of time that a mission is being considered by a community, it isn't in flight at all." Indeed, the green bars and red bars show a strong tendency to be of roughly equal length. "It takes a long time to get unity in the community on these," by which Torrence meant goals, instrument suites, mission profiles, etc. "If you want to get something going, you have to get it started, talk about it, work with it, and persist. That's been reasonably successful for the last 30 years. Because we've leapfrogged things, we've had a staggered approach that's taken into account long trip times. The result is that if you draw a line down in any given year, there has usually been a mission in flight, with data coming down, or about to come down at any time." Indeed, the green bars and red bars have overlapped quite systematically for the last 30 years. In flight doesn't necessarily mean that a mission is returning data, but even when a mission is just in the cruise phase it's generating a lot of activity among scientists as they prepare their instruments and their theories for a new encounter with an outer planet. Of course, Torrence was leading up to a point that you probably predicted already, if you've been paying attention to what's been going on at NASA lately. "At this point, we would normally have started" the next mission. Torrence had a couple of abortive bars on the graph representing future missions. There's JIMO, which has a short, dead bar. There's "Europa Orbiter," which has a slightly longer bar but is also dead. There is Juno, which is moving forward. But clearly there is a gap, where we should have started the next big mission, and we just haven't. "We've got nothing beyond those missions, and we've got nothing building on the Galileo and Cassini discoveries at Saturn and Titan." Torrence went on to refer to something that had happened earlier in the week that has really stirred up the outer planets community. Titan researcher Jonathan Lunine gave an invited talk after lunch on Monday (when I and apparently every space journalist except Leonard David was watching the Stardust press conference) in which he, too, discussed the future of outer planets exploration, and seemed to advocate the consideration of a mission to Titan and Enceladus next rather than a mission to Europa. While such a suggestion should of course be at least spoken about in light of Cassini's discoveries there, in the context of the slashing of science funding and the cancellation of the Europa mission by NASA, many in the outer planets community fear that by making such a suggestion in a public forum, Jonathan ran the risk of giving the budget wonks at NASA the ammunition they need to say, "look, you guys aren't ready to start a flagship mission, you still can't agree where you want to go." As a result, it seemed this week that conflict might open up between Europa and Titan advocates. I'll be returning to this later, but let me continue with Torrence's point about the lack of current planning for a future outer planets mission. "It's what I call the CEV gap," Torrence continued. "Budgets back at NASA are frozen until we get the CEV going. There isn't much hope for new starts in this time period. So even if you are optimistic, you are talking about no launches until 2020." In the spirit of optimism, Torrence discussed what is being done to look forward to future outer planet missions. In particular, he highlighted the activity of the Outer Planets Assessment Group, or OPAG, which is a body that holds meetings open to the community (there are equivalent groups for the Moon, Mars, and Venus, called ILEWG, MEPAG, and VEXAG), and NASA supports these. He asked Curt Niebur of NASA, who's in charge of OPAG, to stand up and talk about that. Niebur began with a clear reference to the conflict raised by Jonathan Lunine's talk. "We will all either hang together or we will, assuredly, hang separately," he said, and there was murmured agreement. "Fran Bagenal [who runs OPAG with Niebur] has started a Europa focus group chaired by Ron Greeley [an outer planets researcher of Torrence's generation]. That group works directly with mission designers at JPL. We spent about $500,000 of HQ money on a very simple study, which was: how much mass could we throw into Europa orbit? And the answer was, a lot of mass. So what this group has been doing is going through what kind of goals we could achieve with that mass." He finished by asking the room for further input, saying, "What studies should we be doing? What technology should we be developing?" Torrence then gave the floor to Bob Pappalardo to talk about another group that is planning for the future of outer planets exploration, called the "Europa Focus Group" (not to be confused with the Europa focus group within OPAG). "This is a child of the NASA Astrobiology Institute," Bob began -- I'll remind you here that astrobiology funding has been slashed 50% in the fiscal year 2007 NASA budget. Bob showed several slides detailing an overarching goal, and the main science objectives that OPAG has developed for a Europa Orbiter mission. The goal is, "Explore Europa and Determine its Potential for Life," with the following detailed science objectives:
Establishing objectives such as these is a necessary first step in organizing a mission; along with the mass and time constraints, it helps to organize what suite of instruments and what mission profile will be necessary. Bob also mentioned that in addition to OPAG and the Europa Focus Group, there are also discussions taking place between NASA and ESA scientists. "The Europeans have an opportunity to propose including outer planets in their Cosmic Vision, with a launch somewhere in the 20-teens," Bob said. He then gave the floor over to Bill McKinnon, who was representing the Division of Planetary Sciences of the American Astronomical Society (DPS). "The DPS committee was faced with some very distressing budget news recently," Bill said. "Our overall strategy is to support planetary science in general, and we are faced with rather catastrophic cuts to research and analysis. My personal opinion is that I think that there is a lack of appreciation at a high level for us." Basically, he said, NASA HQ (or perhaps he was referring to the Administration, I am not sure) seems to see scientists as no different from skilled workers in other fields. In the last couple of decades it's become common to lay off skilled workers when times are bad, and then re-hire them all a couple of years later when the economy approves. Bill said that, by contrast, scientists are generally "in it for life, or not at all." However, he added, "I think that working with the good people at Headquarters, and there are good people at Headquarters, this problem is on the road to being solved. Our overall strategy is that we have to save the field itself, not just a particular mission." He mentioned the testimony given by Wes Huntress to the House Science Committee, which -- when forced to consider the NASA science budget as a zero-sum game -- ranked research and analysis absolutely first in importance (to maintain the science community), then the least expensive missions (Explorers and Discoveries), then medium missions (New Frontiers), and then, sadly, lastly, flagship missions. This was not to say that flagship missions aren't being advocated for, Bill said: "We are very supportive of trying to get a start for Europa. We are pursuing multiple strategies." Then it was my turn, as the representative of The Planetary Society and the public, to stand up. I reminded the room that Wes is also our President, and that his and others' advice is guiding our own activities. I told them about our campaign that began just as saving the Europa mission and is now broadening into saving science at NASA in general. I told them that New Horizons has demonstrated that the public can be a very important voice in helping to overcome obstacles to missions, if they want a mission to happen; and I invited everyone in the room to talk with us and participate in our advocacy activities in general and in our current campaigns to advocate for restoring science funding to the NASA budget in particular. At that point, the floor was open to questions. Bill asked me whether the public wants a Europa mission. I answered, definitely. When you have such a young world concealing a dynamic environment of briny oceans and the potential for life, the public is definitely interested -- they've even seen an IMAX movie about it. Jeff Moore asked if the pain of a flagship mission could be reduced by giving it a longer but flatter funding profile that might hit NASA for only, say, a hundred million dollars a year. Torrence answered that that just wouldn't work; in the end it would cost more, and that no matter what, a flagship-sized mission would need to ramp up to 300 or 400 million for the year or two before its launch. | |||||||||||||
