Simone Marchi made an intriguing observation about Ceres' craters. There are quite a lot of them; the Dawn team's map now includes 544 craters larger than 20 kilometers in diameter, and many smaller ones. For craters between 20 and 80 kilometers in diameter, Ceres is "saturated," meaning that each new crater obliterates, on average, one older crater, so that the number of craters stays constant with time. But at larger diameters, craters are missing -- there should be more craters of sizes greater than 100 kilometers than are observed. Models suggest that there should be 10 to 12 craters larger than 400 kilometers in diameter; Ceres has none. There ought to have been 180 craters larger than 100 kilometers, of which 40 would be observable today after all that battering; yet only 16 are observed.

Maybe, Marchi suggested, the big craters are just hard to spot: "let's try to squeeze the data and see if we can find something." Using the global topographic data set, he tentatively identified a few large-scale depressions; he was confident about one of them, less so about the other two. These may be the missing biggest basins, but he still can't find the mid-sized ones. Marchi explored a variety of hypotheses relating to collisional or orbital history to explain the missing mid-sized craters on Ceres, but couldn't make any of them work. "The answer for this conundrum needs to be some internal process," he said. "Internal evolution of Ceres is key to understanding its cratering record."

Some clues to Ceres' internal evolution can be found in the shapes of its craters (which are often polygonal) and in its global sets of fractures. Katharina Otto, Debra Buczkowski, and Jennifer Scully all reported on their efforts to map these features, but the work was fairly preliminary; they have made maps, but have much more work yet to do to interpret those maps and compare them with each other. Scully did report that one of Ceres' two global fracture sets has a geometry that points back to one of Marchi's basins as a possible source of the stresses that caused the fracturing. But the other set of fractures that she mapped didn't align with any obvious geological feature.

In general, the talks at LPSC continued to reinforce the conclusion, made at DPS, that Ceres' outer shell is not pure ice but rather a mixture of mostly rock and less ice. This rock-ice mixture may explain some of the interesting transitional geology found in Ceres' craters, as Paul Schenk described them. Under ordinary conditions, Ceres' crust is strong like rock. But when smashed in an impact, it behaves like ice. Impacts into this material produce flows that look like lunar impact melts, though on Ceres it would have been "more like an impact mud than an impact melt." Tim Bowling showed some geophysical modeling work in which he applied rock-like behavior for some physical conditions, and ice-like behavior for others. He was able to match the shape of Occator crater -- including its central pit -- with a model that used a rock-like strength for Ceres' crust, but where the crustal material responded to the shock waves of the impact to acoustically fluidize as ice does. The conclusion he drew is that the rocky component dominates the strength of Ceres' crust, allowing it to hold up topography; but when the crust is being stretched, the weakness of the ice allows it to be pulled apart. This work, like much of what I saw in the sessions, was preliminary, as Bowling acknowledged to chuckles from the audience: "My thinking on this has actually changed in the last 24 hours after a conversation with Gareth Collins and some hasty reanalysis."

Julie Castillo-Rogez presented some early work on the puzzle of how an ice-rich Ceres can lack a pure ice shell. If you start out with a mixture of ice and rock in a body as massive as Ceres, and melt any of the water, gravity really ought to force the rock to drop to the center and the water to float to the top, stratifying it into an ice mantle over a rocky core. As the water freezes, it even excludes salt, forming a pure ice layer at the very top, as with the icy moons of the outer solar system. But that's not what we see at Ceres. Instead, Castillo-Rogez said, we see this mixture of ice and rock, as well as spectral evidence for minerals that formed at higher pressures. Is it possible that Ceres once did have a pure ice shell, but has since lost it? She suggested that impact-induced sublimation could have caused Ceres to lose up to 50 kilometers' thickness of ice shell in 100 to 200 million years of bombardment.

All in all, three days of Ceres was a lot to swallow, and it's taken me another three days to try to absorb it all and summarize the presentations in this post. We'll continue to drown in data from Dawn for as much as another year, and then the mission will be over. I fully expect it to take decades for scientists to get everything that they can out of this rich data set.