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The Planetary Society WeblogGuest Blogger: Brad ThomsonOctober 9 - 15, 2006
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After a brief stint at the Lunar and Planetary Institute in Houston, Texas, Brad Thomson is just beginning a postdoctoral fellowship at the Jet Propulsion Laboratory. Brad will be working with researcher Nathan Bridges on wind abrasion studies, to understand what the abrasion of rocks can tell us about past winds and other environmental conditions on Earth and Mars. Brad was a graduate school classmate of Emily Lakdawalla, under the thumb of the same graduate advisor. |
Greetings from the DPS Conference
Greetings constant Planetary Society blog readers! Brad Thomson here, filling in for your regularly scheduled host, Emily Lakdawalla. Those of you who have followed Emily's coverage of various planetary science conferences know that she is an insightful and voluminous commentator. Can I hold a candle to her? Not likely. But fear not, she'll return to the driver's seat soon.
A few quick things before we get started. It is an interesting experience to be attending a scientific conference as a blogger rather than as a participant. Normally, when you go to a conference where you're making an oral or poster presentation, you're so preoccupied with your own presentation that it can be difficult to fully take in everything else that is going on. Here, I have no such restrictions [*], so I get to sample sessions more freely.
Personal bias: As a geologist, I'm mostly interested in things that have solid surfaces. Gas giants, stars, protoplanetary nebulae, etc. are all interesting, but lacking a solid surface, they often fail to sustain my interest. Since the DPS meeting encompasses more astronomy than planetary geology, I'm going to swallow my prejudices and spend some time getting to know balls of gas and dust.
One of the things that annoys me about MSM coverage of conferences is that only a very tiny fraction of the cool results presented makes the news. Even very talented science writers like Dick Kerr from Science have to be choosey in what they delve into. So the great thing about the Planetary Society Blog is that it gives us science junkies a better feel for what's really going on. To this end, I just signed up for a gmail account at bradley.thomson@gmail.com. Check out the DPS schedule and see if there are any talks that really catch your eye. Send me an email with the subject "DPS Presentation Suggestion" and I'll consider focusing on that talk or poster.
*Full disclosure: Although attending this conference as a guest blogger for the Planetary Society, my name is on a poster related to a proposed mission to Saturn. This mission concept was from a Jet Propulsion Lab "Planetary Science Summer School" session held in July of 2006. I am the 19th of 20 listed co-authors.
Morning Session: Comet Schwassman-Wachmann 3
The first talk of the day in this session was from Harold (Hal) Weaver, entitled "The Amazing Apparition of 73P/Schwassman-Wachmann 3 in 2006." My hopes are high for any talk with the word "Amazing" in the title. At least there were no exclamation points. Hal's talk was an invited overview (20 minutes compared to the usual 10) that set the stage for the following Schwassman-Wachman 3 sessions (hereafter referred to as S-W 3). S-W 3 was initially detected in 1930, and in 1995 the nucleus appeared to break apart into several large pieces. It has an orbital period of 5.4 years, and it gave a poor showing in the 2000-2001 apparition (Aside: I love how comet encounters are called "apparitions"). So it was uncertain how the comet would appear in 2006, but the comet exceeded expectations. 68 fragments were discernible. The largest fragment, deemed the primary fragment, is fragment C. Fragment B is the next largest, and A (the initial detection) has not been seen again. The fragment swarm was imaged with multiple instruments, including the Spitzer Space Telescope, Hubble, and various ground-based instruments. One of the main conclusions Hal stressed in his talk is that there appears to be little compositional variations between the fragments. This means that there doesn't appear to be any depth variation in the nucleus, so it appears relatively homogenous. Hubble and Spitzer images of the broken comet look like a trail of blazing breadcrumbs and really to live up to "amazing" moniker.
 Fragments of Comet Schwassman-Wachmann 3 NASA's Spitzer Space Telescope shows "blazing bread crumbs" that are fragments of Comet 73P/Schwassman-Wachmann 3. Image was acquired May 2006. Credit: NASA/JPL-Caltech |
Two related talks by Nicolas Biver and Jacques Crovisier discussed ground-based
radio observations of the two largest fragments. Seven molecular species were
detected in both fragments, and both appear depleted relative to "typical" cometary
nuclei.
A Poor Man's Deep Impact
David Schleicher talked next about more ground-based observations of the fragmentation.
He referred to S-W 3 as "a poor man's Deep Impact." Deep Impact, of course, was the wildly successful NASA missions that continued the great scientific enterprise of smacking holes in things to see what's inside.
Production rates of various species were determined and (predictably) found to vary with heliocentric distance (i.e., distance from the Sun). This is a significant improvement over measurements made in '95, which didn't deal with multiple fragments in the observing aperture. As the previous speakers indicated, several outburst events were detected in April and in May. I am unclear on exactly what these outbursts are -- if they are additional fragmentation events or simply minor outgassing of patches of subsurface volatiles. Anyway, the various fragments analyzed show similar depletions of carbon-chain molecules, so the story continues to be that the interior exposed by the fragmentation process looks like the surface.
Morning Session Continued: Comet Schwassman-Wachmann 3
The next two talks in this session, given by Neil Dello Russo and Ronald Vervack, dealt with infrared spectroscopy of the comet fragments. Neil observed molecular line emissions and determined, among other things, the production rates of various species such as water. Although the production rates showed some variability, they were largely similar between the fragments. This is another piece of evidence suggesting that the initial parent comet nucleus was fairly homogeneous. An audience member asked a good question after this talk, which was, "How do you know these fragments aren't like 'peeling an orange' rather than probing deep into the interior?" The answer was that radar data of the two largest fragments suggests that the smaller fragment, fragment B, is a little less than half of the diameter of the larger fragment C (about 400 kilometers versus about 1000 kilometers, respectively). This is not what you'd expect if you were just removing small pieces of an outer layer.
Many speakers pointed out various lines of evidence indicating few differences between the various comet fragments, which suggests that the broken nucleus was relatively homogenous throughout its volume. The reason this topic is of high importance is because it suggests the comet interiors are undifferentiated and unlayered (at least at the ~100 meter or greater scale). As a thought experiment, imagine if you were to break up Earth (maybe with a Death Star-like laser). If you could examine the breakup from a distant vantage point, you would see lots of differences between the chunks -- some would be pieces of the surface, some would be silicate mantle rocks, some would be liquid iron from the outer core, and some from the solid, mostly-iron inner core. Comets, in contrast, appear to have a more uniform, dirty snowball-like interior.
I missed the next couple of talks to run over to the press room to type up the first part. Man, it is freezing in here! I guess they want to keep the mood right for thinking about comets.
Getting back to the S-W 3 talks, Amy Lovell talked next about radio mapping observations with the GB Telescope. (That stands for Green Bank Telescope, not Great Big Telescope, but it is 100 meters in diameter!) Radio spectra from the B and C fragments look similar, and the line widths can be used to contain the average outflow velocity (answer = 0.73 km/s. The estimated gas productions (about 1027 to 1028 molecules per second) are similar, but since fragment B is less than about half the diameter of C, that means that B must be considerably more productive per unit area of surface.
Next, Jeremie Vaubaillon talked about observations and modeling of large particles. Large in this case means particles between 100 microns to a few centimeters. Most of the blazes of light in the Spitzer telescope shown below are in this size range. Jeremie showed a neat little movie of a model of particles being ejected from the fragments and evolving into a trail of debris along the orbital pathway. I'm a sucker for a well-illustrated movie. One of the conclusions of Jeremie's analysis is that we should see a particle stream from pre-1963 orbits, but there's no sign of them in Spitzer infrared images. This means that the post-breakup orbits (after 1995) have shed over 10 times as much dust and particles than the ones prior to breakup.
Monday Press Conference: Venus and a little Mercury
At noon, Håken Svedhem was the first of two speakers giving an update of Venus Express, a mission run by the European Space Agency (this explains the ° over the a in Haken's name). Venus Express was built amazingly fast (less than 3 years) from largely re-used Mars Express components -- ESA's version of NASA's short-lived "faster, better, cheaper" phase. It is primarily an atmospheric mission, and it is currently in its primary mapping phase. Seven instruments are peering at Venus's atmosphere and magnetic environment. Science highlights to date include images of an eerie double-vortex near the south pole. You can see a movie of the vortex acquired by the VIRTIS instrument on the ESA site.
Other results to date include the detection of oxygen loss from the lower atmosphere. This is not entirely unexpected, but is not fully understood because the exact loss mechanism is not clear. Is it from disassociation of water? Some sort of surface alteration? Can you say, "grad student thesis project"?
VIRTIS Principal Investigator Pierre Drossart expanded on some of these interesting results. The double-polar vortex remains the subject of intense investigation. DPS press officer Sanjay Limaye noted that the feature had been detected 30 years ago with Mariner 10 and may be akin to an "upside-down hurricane."
Sean Solomon briefly spoke next about MESSENGER, which is an upcoming (actually en-route) mission to Mercury. There are two Venus gravity assists coming up. The first, unfortunately, occurs just before solar conjunction. This means that the Sun will be between Venus and the Earth and will impede communication for about three weeks. NASA managers have deemed taking science data during the first flyby too risky, so MESSENGER will be in low-power mode and will not make any measurements during the flyby. In the questions that followed, reporters pressed Sean to see if he had any cards up his sleeve for taking data during this period. With a wry smile, Sean explained that since the goal of this mission is to get to Mercury, safety comes first. Pitching and rolling the spacecraft to take data when you're incommunicado with Earth just isn't a good idea, especially when you won't hear from the spacecraft for three weeks. The second Venus flyby, in contrast, should have no such restrictions. This means that it is a rare opportunity to make simultaneous measurements of Venus with both MESSENGER and Venus Express.
Plenary talks: Enceladus, MER, and Venus Express
The Plenary talks are focused on mission results. First up was John Spencer talking about the exciting recent NASA/ESA Cassini mission results at Enceladus, a moon of Saturn. As Planetary Society weblog readers may recall from Emily's posts on Possible liquid water on Enceladus and her update from LPSC, the Cassini data revealed an intriguing pattern of surface cracks on this icy moon that are actively spewing liquid water into space. This discovery is notable for a number of reasons. First, this indicates that Enceladus is an active geologic body (unlike our own Moon, for example, which is largely inactive). Second, this Old-Faithful-like geyser on Enceladus means that liquid water is available in the near-surface environment. Liquid water is one of the key ingredients for life, and almost everywhere we have liquid water on Earth, we have life. So watery off-world environments are prime targets for biologic exploration.
In his talk, John mentioned that the two Voyager spacecraft, which passed through the Saturian system in 1980 and 1981, almost discovered the now-famous "tiger stripes." One of the Voyagers was set to gather high resolution images but suffered an untimely malfunction that caused the data to be lost. D'oh! The reason why Enceladus has sustained geologic activity is not entirely clear. Mimas, another moon of Saturn, lies closer in and is in a more eccentric orbit. Tidal heating is predicted to be about 10 times as great on Mimas as Enceladus, and yet we see no activity on Mimas. It's a puzzle.
Outgassing from the tiger stripe region appears to be responsible for the diffuse E-ring of Saturn. And estimates of the outgassing rate are pretty extreme. One estimate cited by Spencer is Jeff Kargel's work suggesting that, at the presently observed outgassing rate, 20% of Enceladus's mass may have been outgassed over 4 billion years. John also mentioned a new hypothesis put forward by Francis Nimmo and Bob Pappalardo that the south polar region may be explained by a single large diapir (a diapir is a blob of moving material that is akin to a teardrop-shaped bubble of molten wax in a lava lamp). It's an interesting hypothesis.
Next up was Steve Squyres, who gave a summary of the past three years of Mars Exploration Rover results with regards to aqueous activity. Starting with Spirit Rover in Gusev Crater, predictions before launch were that Spirit would be exploring a paleo-lake environment. There's a large channel that debouches into Gusev, and yet the rover found no trace of water-lain deposits in the plains. Most of the rocks encountered appear to be pieces of a basaltic lava flow. In Columbia Hills, however, the rocks tell a different story. Close-up images of rocks and outcrops reveal a mix of grain sizes, which suggests a high-energy depositional environment. This means that rather than a low-energy environment such as sediment slowly settling to the bottom of a lake, a more violent emplacement scenario such as a meteorite impact is envisioned. The geochemical signature of goethite (pronounced "grrr-tight") in rocks in the Columbia Hills suggests that water or ice may have been present in the target material when these impact(s) occurred.
The Opportunity rover, in Meridiani Planum, was sent to investigate the spectral detection of coarse-grained hematite. Hematite can form in many ways, but the science team that detected it (TES: Thermal Emission Spectrometer) favored an aqueous formation mechanism. Upon landing, Oppy immediately discovered wide-spread spherules that are informally known as blueberries. These blueberries are interpreted to be concretions, that is, grown in place as nodules from percolating groundwater. I doubt much of this is new to Planetary Society readers. What is new is that just this past week Oppy has arrived at Victoria Crater. The HiRISE camera, which just started taking super-high resolution images of Mars, took this amazing picture of Oppy near a rocky promontory called Cape Verde. You can actually make out the rover in this image, and the really crazy thing is that you can see the shadow cast by the camera mast!
 Opportunity on the move This image from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter shows the Mars Exploration Rover Opportunity near the rim of Victoria, an impact crater about 800 meters (half a mile) in diameter. Shown in the image are Duck Bay, the eroded segment of the crater rim where Opportunity first arrived at the crater; Cabo Frio, a sharp promontory to the south of Duck Bay; and Cape Verde, another promontory to the north. When viewed at the highest resolution, this image shows the rover itself, wheel tracks in the soil behind it, and the rover's shadow, including the shadow of the camera mast. After this image was taken, Opportunity moved to the very tip of Cape Verde to perform more imaging of the interior of the crater. Credit: Courtesy NASA / JPL / UA |
Tuesday Morning Sessions: More Enceladus, a Little Ceres
Monday Night's Planetary Society Event
I was unable to make it to the Planetary Society public panel discussion Monday night (I live over 30 miles from the conference location, and I need sleep!). This panel discussion was intended for a public audience. Panelists included Buzz Aldrin, Jim Bell, Chris McKay, and Bill Nye and was moderated by Planetary Society Executive Director Louis Friedman. Unfortunately, Ray Bradbury was a no-show due to illness (Ray is over 80 years old, by the way). On his behalf, Bruce Murray accepted a Mars flag that was flown on the space shuttle (STS 121). I only have second-hand information to pass along, so I can't really go into specifics, but from what I hear it was... interesting.
As another aside, many thanks to the readers who sent me ideas of presentations they are interested in. For example, Thursday morning I will definitely be going to oral sessions on Titan's surface. You can still send me suggestions or comments at bradley.thomson@gmail.com.
Tuesday Morning: An Icy "Old Faithful"
Caroline Porco led off the morning's session with a summary of Cassini's results at Enceladus. This ground was covered yesterday at John's Spencer talk, but several new nuggets were discussed here. Caroline noted that one of the interesting aspects of the terrain that contains the"tiger stripes" is that it has boundaries look like thrust faults in places, meaning that one part of the surface is being shoved against and under another part. The old view of the formation of the polar terrain (and here old means about 9 months old) is that is was due to a global-scale processes such as uniaxial compression. The team has mapped out tectonic patterns that appear to be inconsistent with uniaxial compression, and the new view is that this is a regional-scale process. Caroline mentioned the Nimmo and Pappalardo hypothesis about a single diapir (which was mentioned in John Spencer's talk yesterday), and there's another paper by the team on this topic coming out in Nature soon.
Also discussed in this Nature paper (with lead author Paul Helfenstein) is recently recognized evidence that Enceladus may have relict south polar-type terrains. One example in particular is located near the equator and has hints of faint tiger stripe-like cracks. Caroline also showed a fantastic image mosaic of the entire Saturn ring system taken while the Cassini spacecraft was in Saturn's shadow. It's really amazing -- I'm sure it's coming to a screen saver, coffee mug, and tee-shirt near you soon. Word on the street is that is will be publicly available tomorrow, so stay tuned. I'll post it as soon as I can.
Plume Models
The next speakers, Andy Ingersoll and Feng Tian, talked about models of the geysers. Andy's modeling indicated that not all of the jets point exactly normal (perpendicular) to the surface, meaning some of the jets may be spraying material off to the side rather than straight up. He also briefly mentioned an additional continuous source, which I didn't quite understand if it was separate from the plumes or if he meant that the plumes need to be continually active for his model to satisfy the observations. His favored model is a boiling liquid with bubbles driving the plumes. Feng Tian's modeling suggests that particle velocities are initially in the range of 300-500 m/s. This is an important result that points to active venting rather than simple sublimation.
I missed a talk by trying to run over to the Deep Impact session, but timing differences between the two sessions prevented me from catching much of the talk I was trying to see. Grrr. But back to Enceladus, a subsequent talk by John Cooper stressed the possible contribution of energetic particles on surface ice chemistry. Peroxide (H2O2) and free oxygen O2 may be common on all icy surfaces. This has biological implications since these may provide sources of chemical energy.
Don't Forget Poland Ceres!
Also, check out this excellent BBC article by Molly Bentley on newly revised near infrared maps of the asteroid Ceres.
Highlights from NASA Night
First of all, what is NASA Night? NASA Night is an opportunity for the planetary community as a whole to interact with the folks from NASA headquarters. The representatives from headquarters give short talks about the state of affairs at NASA and then take a few questions from the audience. There is also a NASA Night at the annual Lunar and Planetary Science Conference (LPSC) in Houston, Texas.
At the last NASA night at LPSC (March 2006), there was a quite a bit of consternation expressed by the community towards the presenters. This time around there were fewer fireworks, presumably because a) people have had a while to assimilate the funding cuts put in place for fiscal year 2007 (FY07), and b) no additional funding cuts have appeared on the horizon. I'll go over what each of the panelists said and also discuss the question and answer period. My apologies in advance for the heavy doses of acronyms that will follow, but you can't spend any time on NASA-related matters without lapsing into acro-speak.
The Panelists (Pay attention to the men and women behind the curtain!)
Colleen Hartmann, Deputy Associate Administrator of the Science Mission Directorate
Jim Green, recent Director of the Planetary Science Division
Sean Solomon, Chair of the Planetary Science Subcommittee of the NASA Advisory Council
Kevin Marvel, Executive Officer of AAS
Moderator: Dick French, Chair of DPS
Colleen Hartmann
Colleen started her presentation by discussing the structure of the NASA Advisory Council (NAC). The NAC is currently headed by Harrison Schmitt, Apollo astronaut and former US senator from New Mexico, and is one of three standing committees that report directly to the NASA Administrator, Mike Griffin (the other two are the Aerospace Safety Committee and the ISS Independent Safety Committee). Advice from the committee is circulated internally, and the administrator signs off on their final reports before they are made public.
[Now right there you can see a potential flaw in the system: if these advisory committees are supposed to be independent, then their advice shouldn't be subject to the approval of the administrator prior to their release.]
Moving on though, Colleen mentioned how planetary science has a role to play in the upcoming plans for [manned] lunar exploration. There's been talk of forming an L-DAP (Lunar Data Analysis Program) similar to the Mars Data Analysis Program, but the funding structure isn't clear yet. Colleen also dispelled a rumor that an announcement in the Discovery Program would be forthcoming this evening. Discovery Missions are lower cost missions that are competitively selected every few years. There are some 20 missions competing for a single slot, so competition is fierce. See a list of current Discovery missions. Finally, Colleen mentioned that the Mars Scout selection will occur in before the year is out.
Jim Green
Jim began his talk by discussing how he had successfully wrote 22 R&A (Research and Analysis) proposals throughout his career. I was wondering why he was sharing this information with the audience, and he next informed us that in the past year he wrote 6 proposals, of which none (0) were selected. Now, he's working at NASA headquarters. This got a big laugh from the audience, but upon further reflection I think it speaks to how extremely competitive the NASA proposal process has become as the result of recent budget cuts.
Jim mentioned how he has tasked a person working for him with looking at/documenting how the instrument development process has been working. New instruments are developed and funded in a piecemeal manner, and new instruments have to go through many, many proposal rounds before they get a chance to fly. So it's good to see that headquarters is making a longer-term assessment of how instruments have been moving through the pipeline.
The meat of Jim's talk dealt with the funding cuts to NASA's R&A programs. These are the funds that allow scientists (such as myself) to analyze the data returned by missions. These funds support people at research institutions such as NASA centers, faculty at colleges and universities, and students at all levels, so they are vitally important for the planetary community. There's been a 15% cut in R&A this year, and astrobiology took a 50% cut. Ouch. The basic message from Jim and the other panelists is that there's not a whole lot they can do about it -- their hands are tied. But there was some good news: Jim announced two new R&A initiatives. These are 1) a Jupiter Data Analysis Program that will be focused on the upcoming New Frontiers mission to Pluto's Jupiter flyby, and 2) a Lunar Science Research R&A.
Sean Solomon
Sean began his remarks by noting that he spends a lot of time in Washington these days where the phrase "Taxation Without Representation" appears on the automobile license plates. (This message is to protest that city's lack of representation in the both the US House and Senate) All of us have been taxed, Sean observed, but we do have representation. One example is the Planetary Science Subcommittee that he chairs. All of the meetings are public, and all of the results are public.
Another important avenue is the AAGs: the Analysis and Assessment Groups. Sean pointed out that these are not Advisory groups, but are more like grass-roots organizations that are intended to encapsulate the consensus priorities of the community. I think this sounds like trying to herd cats, but in practice these AAG meetings are a good opportunity to share your thoughts on where NASA's science priorities should lie.
There are 19 members of the Planetary Science Subcommittee (full list). All members of the community are welcome to attend the various AAG meetings. The Planetary Science Subcommittee (or PSS -- are you keeping all of these acronyms straight?) has only met three times since it was first convened in May, but the meetings are open and there is an opportunity at each meeting to allow the public to speak.
Kevin Marvel
Kevin couldn't make it in person this evening, but he did participate in the panel via telephone. His take home message was that "Science funding is optional, not mandatory." He noted that there have been bad decisions made regarding science in the past. The location of the Robert Byrd telescope, for example, is in the senator's home state of West Virginia despite the fact that the humid environment is "less than optimal" for the kind of astronomical observations it was intended to make. The Spitzer Space telescope was once cancelled by a House staffer who was charged with lopping $100 million from the budget. The funding was eventually restored. Other projects such as the Hubble service mission and SOFIA (an airborne infrared telescope facility) have also been cut and resurrected several times.
The point Kevin was driving at is that scientists need to take a more active role in communicating to Congress the important of a vibrant space science program. With the help of the moderator, Kevin conducted an informal poll of how many people in the audience read a DPS or AAS email (~90% of people raised their hands), how many have sent a letter to Congress (~50%), how many had visited Congress (~20%, which is pretty high, actually), and finally how many rely on government funding. Nearly everyone raised their hand in answer to the last question.
"Visit your local representatives," urged Kevin. Science funding is discretionary, not an entitlement program, he reiterated. He provided a mental checklist of things to do when visiting your elected officials, including be positive, leave talking points, and be knowledgeable about pending legislative matters related to science.
Questions, questions, questions
Note all questions and answers are paraphrased below for brevity.
Question 1: According to the NASA Administrator, NASA's new direction is whatever the President tells us to do. While this has happened before in NASA's history (for example, President Kennedy's famous vow to go to the Moon), I don't understand what the political objective is. The cost of the President's vision hasn't been made clear to either Congress or the public. What is the cost of this program?
Answer: We live in a democracy. I the 1960s, the public supported the Apollo program, and they will decide if they support NASA's mission today. NASA's vision is compelling, and that's what we're going to do.
Q2: As a postdoc, how do I involve myself in the process of talking to Congress? How do I develop a relationship with a representative if I'm always moving around?
A: The homeless lobby is particularly effective. Look at it as an opportunity to visit more Congresspeople.
Q3: We're heard that we're "self-serving scientists." And we're reluctant to go to ask Congress for funding because it seems self-serving. How to we not seem like we're asking for handouts?
A: Don't confuse the administrative structure of NASA with the legislative branch. Legislators are happy to hear from their constituents, and it is their job to serve as your advocate on the national stage.
Q4: In regard to the New Frontiers A/O, the language of the announcement was somewhat confusing. Is the outer solar system excluded?
A: The language was unclear, the outer solar system is not excluded. Some of the outer planets concepts clearly will not fit in the New Frontiers cost cap ($750 million), so an effort was made to redirect them to the Flagship class. [I'm not sure I fully understood the answer to this question]
Q5: Thanks for introducing postdoc awards.
Q6: Don't give up on FY07 -- it's not a done deal. Can you provide us with budget breakdown numbers?
A: We'll look into it. (I'm not exactly sure what the answer to this question was because I was trying to formulate my question, which I asked next, in my head)
Q7: In regards to the two new R&A opportunities announced, are they from new funding or will they squeeze the existing R&A programs (which just underwent 15% cuts)?
A: They will be from new funding.
Q7: NASA's direction, based on quotes from Griffin, seems to be towards space commerce. Is this correct?
A: Look at Mike's speeches for clarification of NASA's vision; there are no hidden agendas.
A: Presidential Science Advisor John Marburger voiced the goal of bringing space within the economic sphere of influence.
Coolest Cassini Image Ever!
I've got lots of science results to write up, but I couldn't resist posting this amazing image mosaic of Saturn released by the Cassini imaging team today. It is shows the entire ring system as seen from behind Saturn, meaning that the body of Saturn is between the spacecraft and the Sun. The high phase angle highlight very small particles of the rings. Most pictures of the rings taken in direct sunlight are dominated by coarser icy particles (meter-sized objects or greater). Particles in this image are generally micron-sized (1/1,000,000 of a meter). The wispy outer ring is the E ring now thought to be due to water plumes spewing off the surface of Enceladus.
 In Saturn's Shadow With giant Saturn hanging in the blackness and sheltering Cassini from the Sun's blinding glare, the spacecraft viewed the rings as never before, revealing previously unknown faint rings and even glimpsing its home world. This panoramic view was created by combining a total of 165 images taken by the Cassini wide-angle camera over nearly three hours on September 15, 2006. The "blue dot" on the upper left side of Saturn is Earth. Color in the view was created by digitally compositing ultraviolet, infrared and clear filter images and was then adjusted to resemble natural color. Credit: NASA / JPL / Space Science Institute |
There is more information about the image and the Cassini-Huygens mission on the Cassini-Huygens website and the Cassini imaging team homepage.
SMART-1: A Controlled Crash
Today at lunch we had another jam-packed session with press briefings on three topics. First was an update on the SMART-1 crash into the Moon. Second, an update on Spitzer results on Deep Impact. Third was a preview of a Thursday talk on the composition of the giant planets from this year's Urey Prize winner. I'll start here with SMART-1.
Crash is a scary word in most contexts because usually crashes aren't planned. Engineers prefer dry terminology like "controlled descent," but "crash" better describes the planned end of the SMART-1 mission. SMART-1 was a low-cost ESA lunar mission that tested an innovative solar-electric propulsion system and a small suite of instruments. For more, visit ESA's SMART-1 website.
At the end of the mission, low on fuel and in a decaying orbit, the mission team decided to direct the spacecraft to crash in a region that would be visible from Earth. This way ground-based telescopes could be trained at the impact point, and the effects of the crash could be monitored. Although there was considerable uncertainty in the timing of the crash due to imprecise knowledge of the vertical topographic variations of the lunar surface, Christian Veillet reported that the crash went off without a hitch on September 3, 2006.
The flash was successfully detected from Earth. Data processing proved somewhat challenging because contrast between bright and dark regions on the lunar surface is extreme. The ejecta plume was imaged as well, and showed an unusual dispersion pattern. The topography of the crash site may have defected the ejecta -- the flash shows elongation that is 15-20 degrees away from the downrange direction.
How energetic was the impact? The mass of the spacecraft at impact is estimated to be about 250 kg and was traveling at about 2 km/sec, resulting in a crater estimated to be ~10 m in diameter. In terms of the grand enterprise of studying things by poking holes in them, SMART-1 was about 50 times less energetic than Deep Impact. The crater is too small to be seen from Earth, but may be imaged by upcoming lunar missions.
I'll post my write up of the second and third speakers shortly.
And the winner is...
Three awards were given out Tuesday afternoon. Dale Cruikshank was awarded the Kuiper Prize, Gentry Lee was awarded the Masursky Award, and Jim Williams was awarded the Brouwer Award.
Kuiper prize
The annual Gerard P. Kuiper prize is given "to recognize and honor outstanding contributors to planetary science." According to the press release, Dale's award was given "in recognition of his pioneering work in the application of infrared spectroscopy to solar system bodies, his development of laboratory techniques that have become tools for interpreting the observations, and his leadership in the design of instruments for remote sensing observations from deep space planetary exploration probes."
Dale gave an interesting lecture that chronicled our increase in knowledge of small icy bodies in the solar system over the course of his career. He recalled how he and his colleagues were among the first in the 1970s to utilize the new telescopic facility atop Mauna Kea in Hawaii to examine bodies in the infrared. The elevation of the this observatory (4200 m = almost 14,000 ft!) is high enough that it lies above much of the interfering atmosphere. H2O ice was detected on many bodies, and there was an emerging picture of small bodies generally having low albedos, weak spectral bands, and reddish color (meaning that returned spectra had a positive slope going from the visible to the longer infrared wavelengths).
So why are these bodies spectrally red? Dale discussed work by others suggesting complex organic polymers might be responsible. Carl Sagan and a colleague recreated one class of such molecules in the laboratory named tholins and determined their optical properties for comparison with remotely sensed data. Understanding the origin of carbon-bearing compounds remains a topic of active research -- these compounds may be preserved from the interstellar medium (meaning they were around before our solar system formed), they could have formed in the solar nebula, and it is also possible they form in situ (Latin for "in place") on the surfaces of icy bodies.
Masursky award
The Harold Masursky award is intended "to recognize and honor individuals who have rendered outstanding service to planetary science and exploration through engineering, managerial, programmatic, or public service activities." As quoted in the press release, the award citation states, "[Gentry] Lee has set the standard for systems engineering in the complex world of robotic planetary missions, and moreover, possesses the desire to impart this knowledge to those around him, especially young engineers."
You might be asking, who is Gentry Lee? Is it unusual for an engineer to receive a scientific award? First of all, Gentry is an unusual guy. Second, the Masursky award is not just given for science. Gentry is a self-described "incurable knowledge junkie." I'm lucky enough to work at the same NASA institution as Gentry (not that I ever see him -- there are 5,000 people working at JPL), and one of the things I love about working at JPL is that it is stuffed full of bright, interesting people who all have this spark, this love for what they do. To say that Gentry's spark burns brighter than most is an understatement; the man is practically incandescent.
A few months ago, there was an interesting biographical piece on Gentry in an internal JPL newsletter written by Frank O'Donnell. It coincided with the 30th anniversary of the Mars Viking missions, which Gentry was deeply involved with. I can't post a link to it since I don't have access the JPL internal website at the moment. But one of the things that caught my eye in the article was a description of Gentry at the tender age of five. Growing up in Queens, he came to the attention of a psychologist at Columbia because he knew the batting statistics of every player on all of New York's baseball teams (which at that time were the Dodgers, Giants, and [the reviled] Yankees). At first, the researcher was impressed with young Gentry's ability to apparently memorize the daily stats. And he was astonished when he found out that Gentry didn't just memorize them, he was calculating them on the fly and keeping track of them in his head.
Gentry came to JPL at a young age and quickly sped up the ranks as his abilities became clear. He departed for a while during the "faster, better, cheaper" era of NASA, for as he mentioned in his brief remarks, "There wasn't much interest in systems engineering during that time." He has since returned to JPL, and my understanding of his position is that he is basically the chief dragon-slayer at the lab. Metaphorically speaking, they give him a sword and lock him and his team a room with a fire-breathing dragon of a problem, and a little while later he comes out (smiling, of course) with a huge platter of sliced and barbecued dragon. Occasionally, I've overheard conversations in the JPL dining hall along the lines of, "We have such-and-so problem, but don't worry, Gentry's on it."
Brouwer Award
Third in the list of awards give was Brouwer Award, which is awarded on the rather broad basis of:
(a) excellence in scientific research
(b) impact and influence in the field
(c) excellence in teaching and training of students
(d) outstanding advancement and other support of the field through administration, public service or engineering achievement.
Jim Williams was awarded the Brouwer award for his discovery of a small, liquid lunar core. In his brief comments, Jim addressed the question of whether the Moon is geologically dead (aside from the occasional impact). He used lunar laser ranging, which is a technique that measures orientation and rotation effects by bouncing a laser beam off reflectors left at the Apollo and some of the Russian Lunokhod landing sites. Two interactions at the fluid/solid boundary of the core were detected, which constrains the radius of the core to be equal to or less than 355 km. So the answer, as determined by Jim and his colleagues, is no. The Moon is not dead, but the partial melts that remain are very deep. The surface activity of its early history has faded away.
Prove Me Wrong
David Tytell from Sky and Telescope magazine did a nice write-up of a talk I wanted to see but missed. The talk was entitled, "Why Venus has no moon." Basically, the hypothesis presented was that Venus got hit twice, and the second large impact robbed it of much of its angular momentum (thus accounting for its slow rotation rate). I think it's a neat idea, and I especially love the last few sentences of the abstract for this talk, which are, "[this proposed process] ...is unlikely to have left a clear geophysical or geological trace. We have not identified a clear observational test of this model."
Saturn's Atmospheric Pearls and Weather on Titan
I tried to shed my solid-phase boundary bias and soak in some of the interesting new atmospheric results presented at the Wednesday morning press briefing. There were two presentations, one on Saturn's atmospheric dynamics, and another on weather on Titan.
Saturn's Atmospheric String of Pearls
Kevin Baines talked about new results from imaging Saturn's atmosphere in the infrared (~5 microns). In visible wavelengths, Saturn itself looks sort of bland (not considering the rings and crazy moons spewing geysers all over the place, of course). This is in contrast to Jupiter, which shows these wonderful latitudinally banded clouds and huge storms like the Great Red Spot. The reason for Saturn's apparent blandness is a layer of high-altitude haze.
But when seen in the infrared, one sees beneath this haze to the active churning clouds beneath. In particular, Kevin was reporting on the discovery of a bizarre string of pearl-like clouds at 40 degrees N latitude. These clouds are confined to a thin band, and exhibit a periodic spacing [I was too busy looking at the pretty pictures to remember to ask about their dimensions]. Nothing like this is seen on any other planet. It appears to be some sort of wave phenomena and may stretch all the way around the planet.
 String of Pearls In this image, Saturn's fascinating meteorology manifests itself in a "string of pearls" formation, spanning over 60,000 kilometers (37,000 miles). Seen in new images acquired by Cassini's visual and infrared mapping spectrometer and lit from below by Saturn's internal thermal glow, the bright "pearls" are actually clearings in Saturn's deep cloud system. More than two dozen occur at 40 degrees north latitude. Each clearing follows another at a regular spacing of some 3.5 degrees in longitude. Credit: NASA / JPL / University of Arizona |
Titan Weather: Local on the 8s
Caitlin Griffith talked next about our new and growing understanding of weather systems on the Saturnian moon, Titan. Titan is a large moon with a significant atmosphere, and was the target of the recent Huygens probe on the Cassini mission. One of the scientific goals of atmospheric folks is understanding how similar weather is on Titan to weather on Earth. Here on Earth, our weather is tied up with the cycling of water between the surface and the atmosphere. Titan appears to have a methane cycle rather than a water cycle due to its much colder temperatures.
Clouds on Titan are concentrated in the south polar region and in a thin strip near 40 degrees south latitude. These are inferred to be mostly convective clouds and generally form over regions of upwelling. A recent GCM (general circulation model) of Titan's atmospheric flow predicts upwelling in southern hemisphere and downwelling in northern hemisphere, which is consistent with the observed cloud distribution. So far, so good.
But not everything can be explained by the model. The clouds at 40 degrees S latitude cluster at a particular longitude, the cause of which is not known. Is volcanism driving these longitude distribution? Or is liquid at the surface concentrating the clouds? If so, what is sustaining the liquid? Recent radar images appear to indicate the existence of lakes in the polar regions.
How deep are these lakes? 10 m perhaps? 20 m? Something like 10 percent of the surface appears covered in the north polar region. If we assume a mean depth of the lakes of about 15 m, this is equivalent to a precipitation layer north of 70 degrees latitude ~1.5 m deep over the entire surface. By making such informed speculative guesses, we can try and tease out the relative amounts of methane sorted in the atmosphere versus on the surface. On Earth, most of the water is held on the surface rather than the atmosphere (2.7 km global average for the surface inventory versus 2.5 cm for the atmosphere). On Titan, the reverse is true. Titan's atmosphere holds a globally-averaged layer of methane about 3 m deep, while the surface has the equivalent of only about 0.7 m.
Peeking Through the Haze: Titan's Surface, part I
Charles Elachi, head of the Cassini radar team, first gave an overview of the progress to date. About 15 percent of the surface has been covered by radar measurements thus far. On each incoming pass, the radar takes three types of measurements: radiometry (where the radar just passively receives radiation), scatterometry data, and finally altimetry and SAR (synthetic aperature radar) data. It is the latter type of data that has attracted the most interest because it shows surface patterns and shapes.
One of the exciting recent results is the detection of radar-dark spots at high latitudes that may be lakes of liquid methane or ethane. Some of these lakes have intricate finger-like margins that are reminiscent of Lake Powell.
Recently, the radar has measured some of the same surface features at multiple look angles. These data are only beginning to be analyzed, but they should help us understand which effects are due to topographic configurations and which are due to composition or the physical nature of the materials. Charles showed at least one image that hinted that the radar may be actually penetrating some of the dark lake material and returning information about its interior structure. We'll have to stay tuned on that one.
Charles concluded by summarizing that Titan exhibits surprisingly Earth-like surface processes. There is evidence of fluvial modification (methane rain-fed rivers), aeolian modification (sand dunes), and tectonic processes (mountain ranges formed by crustal compression).
VIMS
R. Brown spoke next about the Cassini VIMS (Visual and Infrared Mapping Spectrometer) investigation of the surface of Titan. While the hazy atmosphere of Titan blocks most of the light, the surface can be discerned through several atmospheric windows. The large-scale albedo patterns of Titan are being mapped and named. Brown mentioned a recent paper by Jason Barnes, who will be speaking soon, identifying a possible volcanic complex named Tui Reggio. What's coming next? In two weeks, the highest optical resolution data ever obtained will be acquired.
Crater Relaxation
The next talk by Nicole Baugh considered viscoelastic relaxation of impact craters on Titan. There is an observed dearth of craters less than 20 km in diameter on the surface. Others have proposed that craters in icy targets may relax and degrade over time, and the aim of this work was to model this process on Titan. Using a finite-element code to model the behavior of ice, Nicole found that not many of these craters could be relaxed away. Only the largest crater sizes show significant relaxation on reasonable time scales, so these results suggest that other factors may be responsible for the lack of craters.
Global Spectral Diversity
Jason Barnes spoke next about the global range of surface types seen with VIMS. There are seven atmospheric windows available between 1 to 5 microns. Using these windows, it is possible to infer something about surface composition, but Tom McCord will be speaking on this topic later on. Color variations seem to be dominated by latitudinal zonation. This zonation could be haze or atmospherically driven.
Much of the equatorial regions are covered with dark brown material that contains abundant dunes evident in radar. The equatorial bright regions show channel dissection. Tui Regio, in contrast, doesn't show evidence for any channels, which suggests that it is geologically young.
Hydrocarbon Deposits on Titan: Black Tar, Texas Tea...
Oil hasn't been discovered on Titan, but Roger Clark spoke next about identifying three spectral absorption features of hydrocarbons in the dark material (a hydrocarbon, by the way, is just a 25-cent word for a compound containing carbon and hydrogen). One of these spectral features at 5.05 microns is uniquely attributable to benzene. Another feature is attributed to either methane or ethane, and there is a 5.01-micron feature that remains unidentified. Another puzzle is the non-detection of acetylene, which was expected to be more abundant than benzene. [Random fact: acetylene is a gas commonly used in welding.] The lack of acetylene may indicate that it's only transiently present on Titan.
Titan Surface Composition Using VIMS
Tom McCord talked about two methods to identify surface diversity. One method is to search for spectral diversity (method used by Jason Barnes). Another is to search for absorption bands in specific spectra windows (method used by Roger Clark). With the first method, three spectral end-members were identified on the surfaces that were spatially significant. These end members are a bright unit, a dark unit, and a blue unit, and this indicates that the surface can be successfully modeled with a few number of components.
Using method #2, for each spectral unit, Tom was able to pull out spectral end-members. Although the signal to noise ratio is low (meaning that random noise sometimes drown out the real signal), the good news for the VIMS team is that the noise appears to be Gaussian. This means it is randomly distributed and not due to instrument error. Whoo-hoo! (just kidding)
Titan's bright spots
I'd like to interrupt this narrative to announce the winner in the race to have the most accent marks appear in one's name: Máté Ádámkovics. Aside from having a linguistically superior name, Mate gave one of the last and most polished talks of this session. He spoke about ground-based infrared observations of Titan. (This was the only non-Cassini talk in this session) There was a considerable amount of work involved in the reduction of this data, but one of the key results is that the brightest regions observed on the surface in two different wavelengths don't appear to physically overlap on the surface. I'm not sure what that means, but it was a good talk.
Peeking Through the Haze: Titan's Surface, part II
Elizabeth (Zibi) Turtle -- Cassini Imaging observations of Titan's surface is obscured at visible wavelengths, but the ISS (Imaging Science Subsystem) can see the surface through the 938 nm (0.938 microns) atmospheric window. Some of the motivating science questions are what is causing the albedo variations? And are the albedo features time variable? Bright areas are about 1.5 times as bright as dark areas, but absolute albedos are difficult to determine. One very interesting result is that there is a distinct lack of specular reflections. This is important because it indicates that large bodies of liquid are not present in the equatorial regions observed thus far. ISS has yet to observe high northern latitudes where lake-like radar dark features are seen, but this will be an important test. A surface would need to be covered with 10-30 percent liquid for ISS to detect a specular reflection.
Rosaly Lopes -- Titan's Surface: Distribution of Endogenic and Exogenic Processes
Endogenic processes are processes driven by internal forces, while exogenic processes are driven by external forces. An example of the former would be volcanism, and an example of the latter would be impact. Although even this simple division can get tricky. Consider the case of Io, one of Jupiter's innermost moons. It is one of the most volcanically active bodies in the solar system, yet the ultimate driver of volcanic activity is thought to be gravitational tidal forces, an external force. Anyway, back to Titan. Rosaly showed evidence for a variety of features that may be example of cryo-volcanism. These features are best defined around a region named Ganesa. At high northern latitudes, there is also a variety of caldera-like features, and many of the lakes discovered thus far may actually reside in volcanic calderas (a caldera is a region of subsurface collapse after lava has been erupted from a volcanic source region).
Other endogenic features include tectonism in the form of mountain chains. These mountains occur mostly at lower latitudes and have a generally subdued topography (50-600 meter heights). There are only three definitive impacts discovered to date. Fluvial erosion "seems to be everywhere," and may be most recent geologic processes.
What's next? Low latitude passes should reveal more mountain chains, dunes, and plains that may be sand seas. The upcoming T23 pass over north Ganesa is a good opportunity to look for cryovolcanic features. And upcoming high latitude passes should reveal more lakes!
Randy Kirk -- Xanadu Region
Xanadu was the name given to the first continent-sized bright area imaged telescopically on Titan. The Cassini radar results confirm that it is radar-bright as well as optically bright. There is evidence for strong volume scattering, which is a different behavior than much of the rest of Titan's surface. Larry Soderblom will talk about using spectral unmixing to infer the composition of this bright material as similar to bright tholins.
The measured elevation of Xanadu is not extreme, and no steep scarps are evident at its margins. Curiously, there are branching pattern of rivers that appear to flow into center of Xanadu rather than toward margins. The majority of Xanadu is filled with structures that look like mountains. The local topography indicates that the mountains have 600 meters of maximum relief, but this estimate could be low by a factor of two (meaning the mountains could be a kilometer high).
Steve Wall -- Implications for Methane Cycle
As I mentioned in a previous post, the methane cycle on Titan is reminiscent of the water cycle on Earth. Steve's work identified three types of channels that are presumably channels carved into the water ice substrate of Titan by methane rain. The first style of channel exhibits well-developed network of long channels that appear to drain extensive areas. The second type of channel is comparatively short, doesn't seem go anywhere, and doesn't coalesce to form a coherent drainage networks. This difference in channel morphology may be the result of a different style of precipitation (for example, a constant drizzle versus a thunderstorm). A third style of channel, seen in only one location, shows a curious embayment geometry.
The liquid that carved the channels appears to have collected in radar-dark lakes. Several styles of lakes have been identified. Type 1 lakes have discrete boundaries and in a few instances, we may be seeing through the liquid and sensing the lake bottom. These lakes are likely methane of unknown purity, and the penetration characteristics of the radar depends greatly upon purity. Lake style 2, seen on Xanadu, is about 15 bB brighter and may be possibly debris-filled rather than liquid-filled. In conclusion, Steve noted that with only 15 percent surface coverage by radar, "it's like trying to analyze something looking through a venetian blind. It's hard to get context."
Karl Mitchell -- The Origin and Consequences of Steep-sided Crater Lakes on Titan
Karl began his talk by quipping that we've discovered the "lake district" on Titan. The radar-dark areas are very dark, and are therefore very smooth. 77 lakes have been cataloged in the T16 pass alone. Many have steep-sided rims, and some connections are evident between lakes. There is also evidence for drained or dried-up lakes.
The nut of Karl's talk is that some of lakes in the north polar regions appear to be filling volcanic calderas. A terrestrial analog would be Crater Lake, Oregon, which Karl points out is the deepest lake in US, and 7th or 8th deepest in world. The volcanism associated with the calderas may be ammonia-water driven. Some of these caldera have a nested geometry, implying the presence of persistent volcanism over a long period of time. Thermal modeling suggests that these potential volcanic centers could have a life expectancy of a few million years.
Charles Wood -- Calderas of Titan
Some of the potential caldera-like features show a radial texture that is consistent with volcanism, and there are also a few associated flow-like features. In the northern hemisphere, not all of the lakes are contained in calderas. Others are found within broad, shallow depressions. Some circular caldera-like features with lakes are surrounded by rings of radar-bright material that is inferred to be rough. This may represent explosive volcanism, although modeling by Karl Mitchell suggests that such explosivity may not be common. As Karl pointed out, nested calderas suggest long periods of activity. The north polar region appears to be a unique area on Titan for these calderas which are not found elsewhere.
Jani Radebaugh -- Dunes on Titan
Dunes on Titan have been found to be primarily confined to a region within 20 degrees of the equator. The first type of dunes discovered looked something like cat scratches and are confined by topographic barriers. More recently, however, vast sand seas have been detected with dunes that are not topographically confined. These dunes are interpreted to be longitudinal dunes whose crests are oriented parallel to the dominant wind direction. On Earth, most dunes are composed of quartz sand grains. Here on Titan, the sand is likely made up of hydrocarbon particles, possibly with some water ice mixed in for good measure.
Jani measured the dune lengths, and found modal lengths of about 20-30 km. Dune orientations generally indicate west-to-east wind flow. The take home message is that the equatorial region of Titan is an arid, desert-like environment.
Larry Soderblom -- Radar and VIMS Correlation
Larry began his talk with the very good rhetorical technique of presenting his principal findings first: Dark regions on Titan can be broken down into several spectral types, and dunes on the surface are the surface features that most consistently correlate with a spectral endmember. Larry was able to subdivide dark regions into a "dark brown" unit (that is everywhere associated with dunes) and "dark blue" unit that is weakly correlated with scour regions (a particle size effect?). Larry had to whip through his last few slides, but it appears that the Huygens probe did not land in a region with dunes.
Bob Nelson -- Bright Spot on Titan
The spot analyzed by Bob and colleagues is the brightest spot observed at all wavelengths. It appears to vary both in brightness and in size. It doesn't appear to be a cloud, but a fog or ground-hugging haze has not been ruled out. If it is not an atmospheric effect, one possibility is that it could be a massive, active volcano.
Radar Image of Lakes on Titan
 Credit: NASA / JPL |
This is the Cassini radar image that radar team head Charles Elachi showed
in his talk on Thursday when he made the analogy to Lake Powell.
In this image, a previously unseen style of lakes is revealed. The lakes here assume complex shapes and are among the darkest seen so far on Titan.
The lake at the left is reminiscent both in form and scale of the flooded drainage system, Lake Powell in Utah and Arizona. However, the Titan lake has been filled with liquid methane and ethane rather than water. In the lake at right, older terrain may have been deeply cut by river valleys before it was flooded by the embaying lake.
This radar image was acquired October 9, 2006 and is centered near 80 degrees north latitude, 357 degrees west longitude. It measures about 310 kilometers by 100 kilometers (190 miles by 62 miles). Smallest details in this image are about 500 meters (1,640 feet) across.
There is more information about the Cassini-Huygens mission at the mission's website.
Stardust Results in a Nutshell: The Solar Nebula was Like a Blender
Donald Brownlee reported earlier in the week on recent results from the Stardust Mission. This mission was an innovative mission to gather dust particles given off by an active comet and return those particles to Earth. Dust particles were trapped in aerogel, which is one the of the least dense (yet solid) materials in existence. The Stardust spacecraft successfully landed (softly) in the Utah desert. Unlike the Genesis mission, the entry system fully deployed. [Aside: Genesis should not be thought of as a complete failure for a significant amount of material was eventually recovered.]
Dust captured by Stardust originated in the coma of comet Wild-2 (the "W" is actually pronounced as a "V"). Don noted that these dust particles represent the third sample collected form a solar system body, with the first two being the Apollo samples and the Genesis samples. This is technically correct but slightly misleading -- we have meteorite samples from Mars, the Moon, possibly Mercury, and who knows how many asteroid parent bodies, but of course these samples were delivered rather than collected. The difference is that the Stardust samples are about as pristine as we are every likely to get. Meteorites, in contrast, get cooked on their way through the atmosphere, rained on, sometimes frozen, and are subject to terrestrial contamination while in contact with the ground.
Don showed a slide of the first comet photograph recorded in 1882 over Paris. He joked that images of the cometary dust grains collected by Stardust were a 17 order of magnitude increase in resolution. To study these tiny particles, Don and collaborators have used a sophisticated array of cutting-edge analytical tools, including nano-syms to get isotopic compositions, atomic force microscopy to attain sub-Angstrom resolution, and synchrotron micro-Xanes.
Most of the particles larger than a micron are anhydrous (dry) silicates or sulfides. They include forsterite (Mg2SiO4), an olivine end-member that is one of the first condensates from the solar nebula. The range of iron/magnesium ratios exhibited by the particles indicates that comet Wild 2 is unequilibrated.
The key finding to date is that the upwards of 10 percent of the particles measured must have formed close to the Sun. Since these particles came to us via the coma from an icy comet that formed in the Kuiper belt, this requires mixing between the innermost regions of the solar nebula and the distant Kuiper belt. There also appears to be a mix of material from the solar nebula and pre-solar materials, that is, material that formed before our solar system formed. Understanding exactly how this mixing between inner and outer parts of the solar nebula occurs remains a subject of active research. And as more particles collected by Stardust are analyzed, they should help clarify the mixing processes that occurred.
Centaurs
I caught a few interesting talks on Wednesday on Centaurs. Centaurs are a class of icy small bodies that lie roughly between the orbits of Jupiter and Neptune.
Active Centaurs -- talk by Dave Jewitt
There about 70 of these objects known, although more are continually being found. Of these, about 12 Centaurs appear to be active or losing mass. Telescopic images of these active bodies sort of resemble comets: they have a small nucleus and surrounding cloud or coma of dust and gas. Active Centaurs were found to be orbitally distinct from the rest of the population. Most are found between Jupiter and Saturn, and this relationship between orbit and activity level suggests that the activity is thermally driven. In other words, the ones closest to the Sun are the most active.
Part of the puzzle regarding these objects, according to Dave, is the exact nature of their activity. Calculations indicate that at distances greater than 5 AU (i.e., the orbit of Jupiter), the sublimation of crystalline water ice is unable to loft micron-sized dust particles. Dave suggested that one possible explanation is amorphous ice. The amorphous-to-crystalline ice transition is an exothermic reaction, and thus may be energetic enough to kick off dust trapped in the amorphous ice. However spectroscopic evidence of these objects is consistent with crystalline ice rather than an amorphous phase. Hmmm.
Motion of the active source associated with (the not-so) active Centaur 147P/Encaladus -- talk by Paul Weissman
Paul began his talk by noting the different title from his abstract. This talk was a good example of how much one's ideas can change between when you submit an abstract for a conference and the time until you actually present your results. In December, Paul reported on the discovery of an apparently active Centaur. But subsequent observations showed that the coma appears to move relative to the nucleus. It turns out there are two separate objects in the same small vicinity of the sky.
So are these objects in orbit around one another? The coma of one of the objects is evolving (as observed this past few months) and may be dissipating. Observations of the projected motion of these two bodies are not consistent with orbital motion. It appears that both objects are in similar yet distinct eccentric orbits. The distance between them was found to vary from 45,000 km at their closest approach to more than a million km.
So what the heck is the story here? Paul stated that it seems unlikely that a comet should be passing an asteroid just when we happened to be looking at it. Is this a larger object shedding pieces? The orbits don't appear consistent with a shedding hypothesis. Another mystery is why the Centaur is not active while the comet is active. Paul speculated that perhaps the activity site on the Centaur is not in sunlight.
This strange pair of objects has at least provided a double bonus for the scientists tracking it. Although it doesn't appear to be an active Centaur as initially reported, Paul was happy to announce to any IUA members present the detection of a new object: comet 174P/Choi-Weissman.
The DPS Meeting is Decadent and Depraved (with apologies to Hunter S. Thompson)
Well, intrepid readers, the week is drawing to a close. I feel like I've only been able to encapsulate a tiny fraction of the action here at the DPS meeting. There was a lot of very good science that I simply did not have time to write about. But I will say that this experience has given me a new appreciation for how science results from these conferences make it into the news. Most of what normally makes the news, not surprisingly, comes from the press conferences. And what makes it to the press conference at planetary conferences tends to be mission-driven. Usually conducted under severe time pressure, mission science is sort of a hectic grab at low-hanging scientific fruit. I'm not saying that to disparage it, but rather explain the context in which it is conducted. It's been my experience that a fuller picture often emerges later, sometimes after years of further thought and analysis. Take the geysers discovered on Enceladus, for example. Their initial discovery was trumpeted the world over, and rightfully so since it is a visually striking, unexpected discovery that has profound biologic implications. There were a few talks at this conference by folks trying to model this process, but I'd be willing to bet that some grad student is going to spend quite some time in the future coming up with a more detailed model of how these things work. Chances are when that grad student finally submits his or her thesis, they won't immediately convene a press conference.
Thanks to Emily Lakdawalla and The Planetary Society for allowing me to post some thoughts here. It's been both fun and challenging.
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