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Crater relaxation (and volatile transport?) on Ceres
Doug M.
post Sep 12 2013, 07:52 AM
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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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TheAnt
post Sep 12 2013, 03:10 PM
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I never had thought on how ice would behave on Ceres. Even though I happen to know a bit on how glaciers behave.
But now you brought it up I must say that the cue ball hypothesis might very well have quite some in favour for it.
The daytime high temperature on Ceres might reach -35 degrees Celsius. Yet with an average surface temperature of -106 ice would not exactly flow fast.
Even so, at this temperatures range ice is still somewhat flexible, and over millions of years crater walls would indeed get softer edges, and over a large fraction of the solar system they might vanish completely.

As for your related question, that is entirely correct. When I wondered what conditions on Ceres surface might be like I did indeed come upon the idea of sublimation as one explanation for those reports of polar caps at Ceres and a thin hydroxyl atmosphere/exosphere Water ice might sublimate near the equator and part of it could freeze out at more northern localities at night and so migrate toward the poles. Yet even there water ice might not be safe against sublimation.

Yet when the ice is protected by interplanetary dust and what minerals that might have been brought there from the collision of smaller asteroids - it might be kept colder and the sublimation might be relatively slow. And so we might not see that much ice on Ceres but a dusty grey surface - except in faults, surface fractures and more recent craters.

Ha! While looking for a reference before I posted I found a NASA summary that said almost the same as I just did - so much for that. smile.gif
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Holder of the Tw...
post Sep 12 2013, 05:50 PM
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QUOTE
We generally consider a water ice half-space for the ductile rheology, assuming the non-volatile surface veneer is too thin to affect appreciably the material strength profile (italics added).

First question that popped into my mind was why then that the surface crust would spectrally appear to be solid chondritic material. This appears to be their answer, that the equatorial surface is mostly everywhere dried out to a shallow depth.

QUOTE
its low bulk density suggests a high volatile content, presumably dominated by water ice.

How sure can we be that the upper surface is simply not more porous than is being assumed here?
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Phil Stooke
post Sep 12 2013, 07:54 PM
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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



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ngunn
post Sep 12 2013, 09:30 PM
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Vesta is anything but 'relaxed' so I'm sceptical about the the idea that Ceres will be a smooth sphere. Its gravity is after all very slight. As for evidence of redisribution of ice by sublimation, I'm expecting that for sure, but not to the extent that we see on Iapetus, or Hubble would already have observed it.
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Phil Stooke
post Sep 12 2013, 11:28 PM
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Enceladus is much smaller and is quite smooth with many of its craters largely flattened, so Ceres could be too, especially since its surface will be much warmer.

Phil



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Holder of the Tw...
post Sep 13 2013, 10:06 PM
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We know that Enceladus does have a surface of almost pure water ice, while we still don't know if that's the case for Ceres. Also, there is ongoing geothermal activity on that moon with active eruptions resurfacing the landscape and erasing the craters, the result of active tidal heating from Saturn. So not all of it is flattening, some of it is filling and erasing. Even with all that, Encleladus still sports a few good craters in parts of the northern hemisphere.

The only likely significant heating for Ceres after its formation would be radiogenic, which would have mostly died out billions of years ago. So it will lack any help from that direction. For all we know it could have had an icy layer that was eroded off and lost by bombardment. Like Vesta, Ceres lives in a rough neighborhood.

But still not a bad point that Enceladus is both low profile (which very few people would have expected) and much smaller than Ceres. At this stage, before we really know, anything is possible.
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Doug M.
post Sep 14 2013, 03:27 PM
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QUOTE (Holder of the Two Leashes @ Sep 14 2013, 12:06 AM) *
The only likely significant heating for Ceres after its formation would be radiogenic, which would have mostly died out billions of years ago. So it will lack any help from that direction.


Well, theory says that serpentinization could be a significant source of heat. That still hasn't been confirmed, but if the theory is correct then the amounts of heat involved could be quite large, and could endure over geological time.


Doug M.
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MichaelPoole
post Oct 28 2015, 09:31 PM
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No disrespect, but this just shows how theories and speculations can end up totally wrong. I think even back before the Dawn orbit it was established that Ceres is a rocky, C-type asteroid with ice underneath the dusty rocky regolith, not a Callisto like body. Ceres is actually inside the ice-line, so exposed water ice is unstable. Detection of water does not make an iceball. Ceres looks like a small version of the Moon without maria but with white spots.

As for the pre-Voyager hypothesis, it seems really stupid to me to just close your eyes and pretend there is nothing to see anyways, which is what people with the "cue ball theory" wanted to do. It would be a big shame if Voyager had no cameras and we missed all the wonderful outer solar system moons - none is like the other. Hell, even "dead" Callisto has cool impact rings and formations and spires of sublimated ice. Look at how Pluto totally surprised people. Theories have a value in science but we should never cease observing just because we think our theories are right.
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ngunn
post Oct 28 2015, 09:57 PM
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A bit of historical reflection is refreshing sometimes. smile.gif 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).
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MichaelPoole
post Oct 28 2015, 10:58 PM
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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.
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ngunn
post Oct 28 2015, 11:22 PM
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I don't understand why you think old surfaces (like Callisto) should be smoother than young ones. I would expect the opposite where volatile materials are involved.
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MichaelPoole
post Oct 28 2015, 11:43 PM
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QUOTE (ngunn @ Oct 29 2015, 12:22 AM) *
I don't understand why you think old surfaces (like Callisto) should be smoother than young ones. I would expect the opposite where volatile materials are involved.


I don't think that, this theory presumed that wink.gif . It assumed all the craters would just slump away. Now we know better. I mentioned that a q-ball moon would be an evidence of something very different than an old, inactive body.
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Herobrine
post Oct 29 2015, 06:17 PM
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QUOTE (MichaelPoole @ Oct 28 2015, 05:58 PM) *
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.
Attached Image
I think I'd pass on using that thing as a cue ball, but not because of a lack of smoothness. tongue.gif
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MichaelPoole
post 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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