Thanks to Ted Stryk for contributing the following notes on Wednesday's sessions from the Lunar and Planetary Science Conference on small bodies and Mercury. --ESL
I began the day in the small bodies session. Mark Burchell presented an analysis of cratering on Steins that indicate that the size distribution and morphology is similar to other small bodies. Earlier in the session, Britney Schmidt presented Hubble results from Pallas that make it seem almost like a mini-Ceres. [Pallas is the third largest member of the asteroid belt, and if it's a mini-Ceres, that makes it very different from Vesta, the second largest asteroid. --ESL] Some fascinating Adaptive Optics asteroid images were presented by Al Conrad. [That abstract is pretty darn cool -- with Adaptive Optics they were able to resolve as separate light sources the two components of a couple of binary systems, actually discovering a moon of asteroid (41) Daphne. It's worth downloading to see the images. --ESL]
Moving on to the Mercury session:
The Raditladi basin near Caloris is apparently younger than the other basins. Its crater density is 10 times less than the density around Caloris. Louise Prockter proposed that it is less than one billion years old. The distribution of plains is controlled by topography, but there is no clear embayment of the topography by plains. Ejecta from the basin forms a continuous blanket to about one diameter out. It has well-developed secondary crater chains. But radial troughs observed in the basin's center seem to contradict the interpretation of a young age. Another crater, Rembrandt, also has internal radial structures, but there's nothing like this anywhere else in the solar system. The radial structures could due to forces of volcanic intrusions, but this would require relatively recent volcanism. So perhaps all the apparently volcanic features are actually impact melt. This begs the question of why this rebound-impact melt features don't appear more often. Perhaps older, similar features in other basins were obliterated by volcanism or impact erosion from later impacts. Perhaps subsidence is possible.
im Head spoke about the overall paradigm for Mercurian geology after flyby one. It wasn't clear from Mariner 10 if smooth intercrater plains were volcanic or impact melt based on lunar analogs (Apollo 16 data from the highlands). Now, MESSENGER has identified shield volcanoes including one four to five times larger than their lunar counterparts. In addition, multispectral data has shown spectrally contrasting plains that virtually confirm volcanic origin. Many of these are not associated with impact basins, further supporting volcanism.
Other interpretations have also changed. After MESSENGER, it appears that the flooding of the Caloris Basin is more on the scale of the lunar Imbrium basin than the Orientale basin (which is not very flooded). Previously, the opposite was thought true. Individual plains on Mercury were created over a long history through several episodes coming from both inside and outside the basins; lunar basins don't have this flooding from external sources. Also MESSENGER produced evidence of graben formations (most notably the Caloris 'spider') and other intrusive processes elsewhere. Hence, Mercury has a much more dynamic and volcanic history than once thought.
In the afternoon Mercury session, Tom Waters spoke about a large new basin discovered in MESSENGER's flyby, though (in retrospect) its east rim was visible in the first flyby. The east rim was in shadow during flyby 2; assembling images from both flybys shows the full basin, which has been named Rembrandt. It is younger than Caloris, Tolstoj, and Dostoyevsky. Smooth plains from outside breached the rim and flooded the floor. Smooth plains may be as deep as two kilometers thick at the center of the basin. This represents an intermediate degree of filling between Mare Imbrium and Caloris (which are very filled) and Mare Orientale (which is not very filled). It has radial grabens and ridges as well as concentric ridges. The radial grabens cross-cut wrinkle ridges, indicating several episodes of contraction and uplift. A huge thrust fault 600 kilometers in length -- the longest known on Mercury -- cuts through Rembrandt.