Crater relaxation (and volatile transport?) on Ceres |
Crater relaxation (and volatile transport?) on Ceres |
Sep 12 2013, 07:52 AM
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#1
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Member Group: Members Posts: 102 Joined: 8-August 12 Member No.: 6511 |
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. |
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Guest_MichaelPoole_* |
Oct 30 2015, 12:46 AM
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I think that's less "polished ball of ice" than "dusty rubble pile that seems smooth because the photo is low-res and taken from a big distance".
Basically a big, dusty version of Itokawa. Deimos is not smooth, but it is dustier than most asteroids too. |
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Oct 30 2015, 08:00 PM
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#3
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Member Group: Members Posts: 244 Joined: 2-March 15 Member No.: 7408 |
I think that's less "polished ball of ice" than "dusty rubble pile that seems smooth because the photo is low-res and taken from a big distance". I don't think it's a lack of resolution that makes it seem so smooth. The surface is fairly well-resolved. Looking at the limb gives you a good sense of how much detail is provided in the image. It's only a few kilometers across, but according to http://www.planetary.org/blogs/emily-lakda...2/05211206.html, that image was captured from a distance of less than 5,000 km by Cassini's narrow-angle camera (angular resolution: 0.00006 radians). http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1598.pdf says the image has a resolution of 27 meters/pixel, and still shows no visible topography. Both of those sources agree that the surface of Methone is remarkably smooth, with Thomas et al. adding that it and Pallene "have likely developed shapes close to equipotential surfaces". It may still be a "dusty rubble pile"—its extremely low density estimate supports that—but it must be a very smooth one. To merge several sources' descriptive terms, 'icy fluff ball' seems appropriate. Edit: Oh, we've spilled over into page 2. Some of what I just wrote is redundant after reading the posts I missed. |
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