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.

[ngunn]

A bit of historical reflection is refreshing sometimes. On 'cue-ball Ceres' specifically, a glance through previous posts is evidence of plenty of healthy scepticism about that here at least.

On the bigger question of not exploring because theory predicts there's nothing to find I couldn't agree with you more - it's a bad way to proceed. It's the difference between exploration and prospecting. Prospectors are looking for some particular thing in specific places where past evidence indicates the best chance of success. Pure exploration means assuming you know very little and looking out for anything and everything that happens to be there (like the New Horizons mission).

[Guest_MichaelPoole_*]

If there was a "cue ball" moon in our solar system I would actually want to send a probe with a camera ASAP. Considering how even the oldest and least active surfaces like Callisto are very non-smooth, a cue-ball moon would be evidence of something really weird and interesting going on. Nature does not generally produce perfect spheres.

Also, I believe "cue ball moon" theory was not the prevailing one even then. The typical view was that they would be all heavily cratered.

For a little historical exercise, cue ball moon was actually the prevailing thought/mythology about our Moon before telescopes. The Moon was thought to vary in color but to be perfectly smooth, just like the Sun (Galilleo debunked that too with his observations of sunspots). So I guess people have a natural affinity for trying to find perfection.

Also, this theory must have been pre-Pioneer as while Pioneer's images of the moons were horrible, Io and Ganymede were already clearly seen as something very different from smooth white balls.

[Herobrine]

If there was a "cue ball" moon in our solar system I would actually want to send a probe with a camera ASAP.

If you just want a smooth moon, Saturn has a few tiny ones, discovered by Cassini, that might qualify, none of which have ever had whatever this JPL site I'm looking at calls a "targeted flyby".
The only one I know of with reasonably resolved surface imagery is Methone.

I think I'd pass on using that thing as a cue ball, but not because of a lack of smoothness.