Ceres is fairly heavily cratered, but it has fewer large impact basins than predicted, and it's not clear why. There are three main large ones: Kerwan, Urvara, and Yalode. All three have unusual-looking interiors and associated tectonic features. Kerwan is associated with an extensive region of terrain that is smoother and less cratered than the rest of Ceres. Raymond said that the lack of large basins could indicate that larger craters have relaxed away (unlike the smaller ones), but the audience wasn't particularly satisfied with this explanation; if a crater floor rebounds over time to smooth out the globe, there is always rim topography left to indicate where the basin was in the first place.

The highly variable morphology of the craters on Ceres is another puzzle. Raymond said this was evidence for a crust whose composition and structure varies from place to place. She proposed a model to explain that: Ceres began with a water ice layer at the surface (including some fine silicate material and salt impurities) over a layer of a less-differentiated, hydrated-silica core. Impact gardening -- whereby holes of different depths and sizes are dug randomly over the surface with variable depths, tossing various proportions of ice and rock to the surface at various distances from the crater -- has churned the crust into a mix of ice, rock, salt hydrates, and frozen brines that varies from place to place.

A few talks concerned Ceres' surface composition, based on work with the VIR spectrometer and camera color filters. Before Dawn arrived, there was spectral evidence for brucite (a magnesium hydroxide) on Ceres' surface. But Carlé Pieters explained that Dawn's spectrometer can see to longer wavelengths and the spectra that Dawn is getting are no longer consistent with brucite. Instead, the Dawn team has concluded that they're looking at ammonia-bearing clay minerals, which is just weird. I've never heard of such things mentioned as the component of a planetary surface before.

The problem with ammonia on Ceres is that it's thought to need an outer solar system source. In other words, either Ceres is covered with gunk from outer solar system impactors, or Ceres itself originated in the outer solar system and was transported to its current orbital position by the same solar system kablooie that scattered most of the trans-Neptunian objects. I spent fifteen minutes at the poster session listening to Pieters and Tom McCord argue amicably about whether the ammonia really required an outer solar system source (McCord thinks it can be explained through petrology with an outer main belt origin, while Pieters thinks you can't explain ammonia being observed globally without an outer solar system origin). In his talk, Simone Marchi explored whether an outer solar system origin for Ceres -- and a late capture into the asteroid belt -- could be used to explain the relative lack of large basins, but he said even that is not sufficient to explain the low numbers of craters.

At the poster session, I asked dynamicist Bill Bottke what he thinks of a potential outer solar system origin for Ceres. It's apparently not out of the question; it is possible to start with a large body beyond Neptune and transport it inward in all the wild events that happened during solar system formation. You can even end up with a relatively circular orbit, as Ceres has. But he said it's hard to do that without capturing a lot more stuff from the outer solar system while you're at it. Which would imply that a lot of the dark objects in the asteroid belt didn't actually form in the asteroid belt. It's an interesting area for future work, he said.

There were a couple of talks and posters on the interesting geomorphology of Ceres' surface. Jennifer Scully gave Britney Schmidt's presentation on flow-like features on Ceres. They see two quite different types of flows on Ceres. One looks a lot like Martian rampart craters; they originate at crater rims, are very thin (tens of meters thick), run to 25 or 30 kilometers in length, and have "lobelettes" at floe toes. They interpret these to be localized fluidized ejecta formed during or after high-velocity impacts, "because you need high energy to mobilize material in this way, and we think they require a lot of ice." At the other end of the spectrum are flows that originate from slumps, are thicker (hundreds of meters thick), shorter (about 10 kilometers long), often have parallel furrows on their surfaces, have a distinctive, steep toe, and no lobelettes. They interpret these features to be ice-cored or ice-cemented creeping flows. These may also be initiated when impacts hit a slope and warm the subsurface, but the flow is relatively gradual. Both types of flows are not found on Vesta, so Scully suggested that Ceres' crustal material is weaker, flows more rapidly, and melts more easily. It requires about 30 to 40% for these sorts of flows to happen -- which is nicely consistent with previous talks. The flow in the 3D image below is the example of a steep-toed, likely ice-creep flow that they showed.