Crater relaxation (and volatile transport?) on Ceres Options

[Doug M.]

This recent paper suggests that the Ceres may be a "cue ball", because its surface will be warm enough that ice will flow -- flow very very slowly, but flow. (The technical term is "viscoplastic relaxation"). So craters and other large surface features will gradually relax and flatten out. We actually see this on Jupiter's moon Callisto; while Callisto has a lot of craters, most of them are very relaxed, and the moon's overall topography is remarkably smooth. Ceres is even warmer than Callisto, so we might reasonably expect the same effects to operate.

http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1798.pdf

A related but separate question: what about sublimation? At those temperatures, solid water and CO2 actually have a bit of vapor pressure, meaning they will very very slowly and gradually, over millions of years, sublimate into vapor. But what happens then? Well, there are two obvious possibilities: either the vapor will re-condense as frost in places that are always cold (shadowed crater bottoms, high latitudes) or, given Ceres' small size and low gravity, the vapor will simply escape into space. (Again, we seem to see this mechanism of "volatile transport" at work on the surfaces of Ganymede and Callisto. Not so much on the Saturnian moons, because they're so cold that the volatiles have pretty much zero vapor pressure.)

I e-mailed the author of the paper (who I know very slightly) with the question. He replied, yep, it's going to be one or the other but we don't yet know which. We'll have to wait until the spring of 2015.

This raises a question: what would we be able to tell? Dawn has the framing camera, VIR and GRaND. Those would give us suberb visual resolution on the surface and a pretty good idea of what the surface is made off. So, we'd see relaxation and we'd probably be able to infer volatile transport pretty clearly. On the other hand, Dawn lacks a laser altimeter, so our 3-D resolution of surface features might not be all that -- does anyone know how accurate it will be? And Dawn has no magnetometer, so we won't be able to measure whether Ceres has a magnetic field, which could certain affect how volatiles move around. (No, nobody expects Ceres to have a significant magnetic field. But Dawn won't be able to tell us for sure.) And Dawn has nothing like the NMS on LADEE or the super-sensitive Particles & Field package on MAVEN. So, no direct measurements of the Cerean exosphere.

So I guess the question is this: given the package that Dawn does have, what would we reasonably expect it to tell us about the evolution of Ceres' surface over time? And what questions would it likely leave unanswered? (And also, who thinks the cue ball theory is likely correct -- and if not, why?)

thanks in advance,



Doug M.

[Phil Stooke]

There was talk in the early stages of designing Voyager that cameras would not be useful because the moons would be relaxed into cue balls - so it could have been just a particles and fields mission.

Ceres is warmer so a very smooth relaxed surface may be more likely... but Europa and Enceladus show us what cue balls can look like - their interiors are warmer than that of Ceres probably is (as far as we know now) but its surface should be a lot warmer than theirs are.

I think Ceres will be a fascinating object. Probably quite relaxed on a larger scale except near the poles, but full of the most fascinating details at smaller scales.

Phil