More excerpts from Emily Lakdawalla's report from the DPS. Hopefully most of us will read the whole thing at her website.
==excerpts from Emily
http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html ==
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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.
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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. ..
... 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% [ice?]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.
[see the photograph in original]
Jennifer Scully and Debra Buczkowski presented adjacent posters on preliminary geologic mapping of Ceres. I talked to them for a bit about how
strange Ahuna mons is and why aren't there any other features like that on Ceres? Scully told me there's another candidate feature similar to it close to the north pole, but it's hard to see because the illumination is poor.
Hanna Sizemore had a poster looking at smaller mounds found all over Ceres, and showed they likely had a variety of origins -- some are just preexisting topography embayed by impact melt (like you see in mounds on the Moon), but others could have a
pingo-like origin, and some could conceivably be volcanic. ...
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By now I know several of you are probably asking: what about the bright spots? They haven't been easy for Dawn to investigate, because Ceres is so very dark; exposure settings that allow the visible-light instruments to see most of Ceres' surface were too long for the bright spots, and they saturated the detectors, making compositional data useless. Pieters said they had to get special data in order to get the bright spots onto the scale. Even so, it's still not obvious what they are. ...
...It could be salt, but it's not a lot of evidence to go on. Paul Hayne showed that although water ice isn't stable on the surface of Ceres over geologic time, it can last tens of thousands of years, so in fact Ceres has less exposed ice from recent impact craters than he expected. This is yet another argument for a rock-rich crust for Ceres.
I think Ceres is an intriguing little world, and it's fun that it's generating puzzles for so many different kinds of scientists: cratering people, dynamicists, structural geologists, petrologists, spectroscopists. This is going to be a very productive mission, and they're just getting started!
==endquote==
