LPSC 2011: Day 1: Small bodies
Posted by Emily Lakdawalla
08-03-2011 12:28 CST
I am enormously grateful to astronomers Franck Marchis and Andy Rivkin for sending me the notes they took as they attended the "small bodies" oral sessions at the Lunar and Planetary Science Conference yesterday. Here are some of the noteworthy items from the morning's session on "Small Bodies: A Traverse from NEOs to TNOs" and the afternoon's session on "Asteroid Geophysics and Processes: Surfaces and Interiors." (Note: these and most other links will take you to PDF documents from the LPSC website.) The notes are largely Andy's, with some leavening from Franck's.
Amy Mainzer led off the morning with a presentation on NEOWISE, an add-on to the Wide-Field Infrared Survey Explorer's image processing pipeline that allowed the mission to detect moving objects in their survey photos, including both previously known and unknown solar system objects. NEOWISE has observed more than 157,000 objects including 123 comets, of which about 34,000 were previously undiscovered, including about 20 comets. 57% of the WISE mission data will be released to the Planetary Data System on April 15. Having completed both its primary mission and its extended "warm" mission (which followed the depletion of its cryogen on August 5, 2010), WISE was placed into hibernation on February 17. Mainzer said that while the spacecraft could plausibly be revived, it's not anticipated that it will be. Here's a cool video showing the NEOWISE asteroid discoveries, which I wanted to embed here but won't cause I can't configure it not to play automatically.
Joe Masiero presented on EOWISE's observations of main-belt asteroids. He reported a bimodal distribution of asteroid albedos -- to translate, that means that the asteroids have either bright surfaces or dark surfaces but not middling surfaces. Low-albedo objects (albedo about 0.05, black as tar) make up about 20% of the total, but make up more than 30% of the total beyond 2.3 astronomical units. The low-albedo objects have the expected power-law size-frequency distribution, meaning that there are more small ones than large ones and the dropoff in numbers with increasing size decays exponentially. But the brighter bodies (which are still pretty dark at 0.25) deviate from this expected distribution; it's not clear what's causing that, or if it's an observational artifact.
Andy Rivkin didn't send me notes on his own paper on IRTF observations of Lutetia and promised instead to tell me about it later. Andy reported differing spectral properties for Lutetia's northern and southern hemispheres.
Vishnu Reddy talked about the mineralogy of the Baptistina asteroid family. A "family" of asteroids is a grouping of objects with similar colors and orbits, suggesting that they all started out as part of a progenitor body that was smashed apart in some ancient collision; in the case of Baptistina, working the orbits backwards in time suggests that the smashing of the parent body happened 160 (+30/-20) million years ago. The reason the Baptistina family is particularly interesting is that it's been suggested (based on the previous identification of this family as having C- or X-type colors, consistent with the CM2 carbonaceous chondrite composition of a fossil meteorite found in K/T boundary rocks) that a careening Baptistina fragment is the one that crashed into Chicxulub and killed the dinosaurs. But Reddy's new observations of Baptistina-family bodies with the IRTF telescope shows that it's actually an S-class family, which is at odds with the carbonaceous chondrite composition for the fragment found on Earth. So something's wrong with the story.
That's it for the morning session notes. Now on to the afternoon:
Jim Richardson presented his work modeling how regolith is generated on the surfaces of asteroids. He found that the regolith -- the fragmented rocks and dust that cover the surfaces of airless bodies -- is mostly generated in the biggest impacts (which can reach through preexisting regolith to bedrock), while small impacts mostly lead to escape of their ejecta. So the depth to regolith on any asteroid is mostly constant in time, punctuated by sharp increases.
Ben Weiss presented his work on Rosetta VIRTIS observations of asteroid Lutetia. Franck remarked that "of course he showed E. Lakdawalla picture of asteroids." The large size of Lutetia is important; it's the first asteroid visited by a spacecraft that is large enough for the heat of its formation to be important geologically. Spectral observations suggest its surface is like that of a carbonaceous chondrite or enstatite, meaning it appears mineralogically undifferentiated; if Lutetia is undifferentiated, then it was never geologically active enough to separate into rocky mantle and metallic core. Lutetia's density was found to be about 3.5 grams per cubic centimeter (which is at the high end of normal rock-forming mineral densities, and the same as the density of Vesta). If Lutetia is chondritic in composition, that implies its porosity is near zero. But given its surface properties and past experience with other asteroids, you'd expect Lutetia to have 15 to 25% pore space. One way to reconcile these observations is if Lutetia is a partially differentiated body, with an unmelted crust of some depth (which, Andy remarked in his notes, was not formally stated, but sounded like about 20 kilometers based on Weiss' response to a later question); the crust would overlie a partially differentiated body. Weiss argued that this was potentially a common situation in the asteroid belt.
Daniel Scheeres talked about how cohesive van der Waals forces can hold together objects the size of asteroid 2008 TC3 (the few-meter-diameter object that came down over the Sudanese desert) even if they are spinning rapidly; if TC3 was made entirely of millimeter-sized particles, cohesion "goes a long way" to keep them together. These things can spin up with time, which may cause them to fission into fast-spinning tumbling objects -- and the process can repeat ad infinitum. This is a non-collisional way to create new asteroids. The work predicts that there should be a large number of small rubble-pile asteroids that are tumbling, on the road to dissipation through fission. And it suggests a fundamental question: what is the size distribution of true rocks (rather than bodies composed of much smaller particles) in the NEO/asteroid population?
James Roberts presented the idea that there could have been a transient dynamo on asteroid Vesta (work coauthored by Andy). He noted that Howard-Eucrite-Diogenite (HED) meteorites, thought to originate from Vesta, show some remanent magnetism, including one relatively young one. How long could Vesta have had a magnetic field? Dynamos require convection in a conducting fluid (in this case, a liquid metallic core), and a Vestian core would be expected to cease convecting very quickly, within a few tens of millions of years. But Vesta has that huge southern impact scar: could this have led to remelting and convection in the core? Models suggest no, that the temperature increase isn't nearly high enough. An impact that was large enough to deliver enough heat would also be more than large enough to disrupt Vesta entirely. And if it doesn't work for large, metal-rich Vesta, it seems like it can't work for any other asteroid either.
Carol Raymond presented in place of Julie Castillo-Rogez a paper (also coauthored by Andy) that asks if Ceres' surface might have undergone "mobile-lid convection," which I assumed was synonymous or at least analogous with plate tectonics, the process that dominates Earth's surface geology. About this, Andy remarks in his notes, "mobile lid convection, it was stressed, was not the same as plate tectonics, though I admit I didn't follow the distinction." Anyway, Ceres is thought to have a rocky inner core incorporating little water surrounded by a more water-rich rocky outer core, with an icy shell on top of that and a brucite/carbonate/other stuff mixture covering the surface. (Brucite is a magnesium hydroxide mineral.) The best way to get brucite to the surface of Ceres is via convection, since it would be expected to originate at the water/rock interface below the ice shell. Models of Ceres suggest that mobile lid convection can happen if its ice is impure. It wouldn't begin early in Ceres' history; in fact, it would begin quite late, about 3 billion years after Ceres formed, or 1.5 billion years ago, and it could plausibly still be going today, with the right set of conditions. In this model, ice that rose to the surface as a result of convection would carry the brucite and other hydrated minerals, and leave them behind as a lag deposit after the ice sublimes into space.