DPS: Back to Day 3 again: finishing up asteroids & Iapetus
Today is the last day of DPS, and the first that has not had a press conference session at lunchtime, so I have a few moments here to do some catching up. I think I will have to wait until I get on the airplane tomorrow to be able to get to the Titan and rings stuff -- I thought I might try now to finish up with various other random items that have appeared at various moments over the last couple of days.
So, back to Wednesday morning. Three long presentations were given by people who won various prizes from the Division of Planetary Sciences (DPS). The "Urey Prize" is given each year to a young researcher making important contributions to planetary science, and this year it was given to David Nesvorny, who does research on the dynamics of asteroids, among other things. He gave a really interesting talk on the long-term behavior of clusters of asteroids. Clusters of asteroids sharing an orbit, but spread out along it, are suspected to represent the remnants of a single large asteroid that experienced a catastrophic impact at some time in the past. "Unlike human families, in which all the members have different ages, all members of an asteroid 'family' have the same age," Nesvorny said. He spoke a lot about the Karin cluster, named after its largest member 832 Karin. His "simulations propagating orbits of the Karin cluster backwards in time make them all meet around 5.8 million years ago." He showed several graphs of time versus orbital longitude for these bodies, and they all crossed at very close to one time in history, indicating that all the fragments began moving from the same place 5.8 million years ago. It was really neat modeling work that had the whole audience excited.
Nesvorny went on to talk about how he is attempting to figure out the size of the initial parent body and its disruptor. From his models he finds the best fit to be an initial body 33 kilometers in diameter, with a 5.75 kilometer impactor. At present, the largest body in the cluster is 832 Karin, with an estimated diameter of about 19 kilometers. Nesvorny can also use his models to figure out where in the parent body the resultant fragments came from. The cool thing about his results there is that it appears likely that 832 Karin represents the "back half" of the parent body as seen from the impactor. One side of it should be the ancient, weathered surface of the parent body, and one side should be the fresh, only 6-million-year-old surface cut through the parent body by the force of the impact shattering the body. And, in fact, Nesvorny said, "recent spectral observations indicate that Karen has two faces: a red one and a much flatter-spectrum one. It is tempting to say that the red one is the weathered surface of the preexisting body, and the blue one is the fresh surface from the interior." He said that Karin or its family members would be "a great target for a space mission, because you can use it to calibrate the crater production rate in the main asteroid belt" because the pieces are likely to have at least one face that is very fresh, only 6 million years old.
Finally, I have to mention a couple more items from Iapetus from Wednesday. There were a couple of talks looking at the composition of the surface. Dale Cruikshank used VIMS spectra, and presented convincing evidence that they have seen PAHs, or Polycyclic Aromatic Hydrocarbons. ("Aromatic" refers to a molecular structure in which there are carbons bound into a ring. The simplest aromatic hydrocarbon is benzene, which has one ring. PAHs have several rings bound together.) In addition, he saw other features in the spectra characteristic of CH2 groups. CH2 (a carbon bound to two hydrogens) is a common arrangement of molecules seen in long-chain or "aliphatic" hydrocarbons (methane, ethane, propane, etc.). Cruikshank suggested that "we envision a polycyclic, aromatic structure, with aliphatic bridging units, which often terminate in CH2, NH2, and NH. This structure is very similar to kerogens found in carbonaceous meteorites. It is also very similar to interstellar dust. PAHs contain the majority of the carbon in the galaxy. The dark material on Iapetus comes from somewhere else. It's material that is also present on Phoebe, and on comets."
Tillman Denk performed some color mapping of Hyperion and Iapetus. He found interesting patterns of colors on Iapetus' surface. While the dark terrain is generally on Iapetus' leading side, it does also wrap around to trailing side. He found that the leading side dark material is redder than the trailing side, which is relatively greener in color. He suggested that the greenish stuff could be "primordial," endogenous to Iapetus, while the dark red material on the leading side is exogenous. And he wondered out loud whether there may be as many as "three more or less independent processes responsible for the formation of the extreme hemispheric albedo asymmetry on Iapetus?"
John Spencer used CIRS maps to study Iapetus. Because of the dark material and the very long day, Iapetus is "probably warmer than any other surface in the Saturnian system," John said. While Iapetus has a thermal inertia very similar to Phoebe's, the thermal wave from each daytime round of heating penetrates much deeper into Iapetus' skin than it does on Phoebe because of the longer day, 3 centimeters on Iapetus as opposed to 2 millimeters on Phoebe. John covered the usual arguments for why the shape of the albedo dichotomy is strange: "most simple exogenic models [that is, models in which the dark stuff comes from outside Iapetus] darken the leading hemisphere, but Iapetus' bright material extends over the poles, dark stuff extends around the equator [to the trailing side], and pole-facing slopes are bright." So John created a model in which all of Iapetus is covered in a very thin layer of typically dirty ice. The warm days on Iapetus will sublimate some of the ice, which will eventually refreeze -- preferentially so at cooler spots, like the poles. Then, he said, in his model he darkened the leading side symmetrically about the apex, which is what you would get from ballistic emplacement of exogenous material. If he runs the model forward in time, "in only about 10,000 years you start to burn off frost on the leading side. In 100 million years you start to burn stuff off on the trailing side. You can actually make this look pretty similar to the current albedo pattern. If you try to do it with a thicker frost it doesn't work." A very suggestive model, pretty cool.