The Bright Spots on Ceres |
The Bright Spots on Ceres |
Mar 27 2015, 10:38 AM
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#1
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Junior Member Group: Members Posts: 33 Joined: 16-June 14 From: Sweet Home, Oregon Member No.: 7202 |
It has been suggested that the highly anomalous bright spots on Ceres represent cryovolcanic or evaporative plumes, and one of the pieces of evidence presented for this model, has been the fact that they seem to project above the rim of the crater which hosts them. However, the plume model is highly implausible, for three main reasons:
1) A plume would spread out and be diffuse, and not be concentrated in one super-bright spot.....an example would be the plumes of Enceladus, which are not even visible with the sun to the observer's back (equivalent to the orientation of Dawn when it was photographing Ceres), but rather the plumes of Enceladus are only visible when back-lit. Any plume intense enough to produce the surface brightness of the feature on Ceres, would be expected to spread out over a vast area, similar to what we see with the volcanic plumes of Jupiter's Io (which ARE visible when "fore-lit," appearing as large umbrella or parabola-shaped features rising above the limb) 2) Any plume activity vigorous enough to be visually conspicuous would result in ice crystals settling down (as "snow") on the surface, at least locally, or even globally (as is the case with Enceladus), resulting in a very high surface albedo in at least the crater hosting the bright spots. And yet there is nothing of the sort there....in general, Ceres' surface is a relatively uniform grey, even directly adjacent to the bright spots. 3) We would expect a plume to be variable, whereas the bright spot (albeit completely unresolved) was seen by Hubble years ago.....which makes the case even more strongly, to the effect that the surrounding landscape should by now have a thick layer of snow and be highly reflective, if indeed there are active plumes. As an alternative to the plume model, I would like to propose the following hypothesis: that the bright spots represent cryovolcanic spring mounds which, due to the very low surface gravity of Ceres, have grown to enormous heights....the water flows out of a fissure but quickly freezes, and then more flows out on top of that, and more on top of that....till we end up with a gigantic stalagmite-shaped structure of highly reflective ice, which may be hundreds of meters high, even perhaps exceeding a kilometer. This formative mechanism would be rather similar to that of the black and white smokers on the ocean floor of Earth where, due to the buoyancy of the water, we see an environment that simulates a very low gravity regime, and in which vertical chimneys of precipitated minerals form (which would be unstable in a high-gravity surface environment). If the outflow is liquid (not high-speed ice particles as in the case of Enceladus), then we do not face any of difficulties presented by a plume.....all the water (very quickly turning to ice) would stay in the immediate region of the vent. And while it would freeze quickly, over time it would also sublime at a substantial rate, which likely accounts for the thin water vapor atmosphere detected by Herschel. But because of the low gravity and relatively high temperature (up to minus 35 Celsius), and the comparative lack of atmosphere, this water vapor is quickly lost to space, and so does not coat the surrounding surface, except perhaps the small amount that manages to reach the poles. David Palmer |
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Apr 6 2015, 08:57 PM
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#2
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
The curvature of Ceres is enough to 'elevate' the centre of a crater to the point that it remains visible at the limb, provided the crater walls are not too high. My rough calculation yields 2.5km for the rim height required to obscure that point in this case.
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Apr 6 2015, 10:18 PM
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#3
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
The curvature of Ceres is enough to 'elevate' the centre of a crater to the point that it remains visible at the limb, provided the crater walls are not too high. My rough calculation yields 2.5km for the rim height required to obscure that point in this case. Oh, that's a very good point that I had forgotten to consider and is obvious in retrospect We don't have enough data yet to know the actual shape of the crater floor or the height of the rim with respect to it. We'll have to wait and see the stereo data. -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Apr 7 2015, 10:00 AM
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#4
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Junior Member Group: Members Posts: 33 Joined: 16-June 14 From: Sweet Home, Oregon Member No.: 7202 |
Oh, that's a very good point that I had forgotten to consider and is obvious in retrospect We don't have enough data yet to know the actual shape of the crater floor or the height of the rim with respect to it. We'll have to wait and see the stereo data. On the basis of that consideration, it sounds like the spring mound I was arguing for, will likely not need to be nearly as high. However it seems we can still rule out anything but a positive-relief feature, as a crater or sinkhole or other negative-relief feature (exposing clean subsurface ice) would disappear when seen edge-on near the limb of Ceres. |
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Apr 10 2015, 07:54 PM
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#5
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
On the basis of that consideration, it sounds like the spring mound I was arguing for, will likely not need to be nearly as high. However it seems we can still rule out anything but a positive-relief feature, as a crater or sinkhole or other negative-relief feature (exposing clean subsurface ice) would disappear when seen edge-on near the limb of Ceres. In the other thread I've posted reprojections of some of the RC2 sequence in order to show the bright spot at almost the same position. Comparing the visibility of the bright spot relative to the surrounding crater with other craters / crater rims / craters in craters when near the terminator, I can't perceive hints to a particularly high peak related to the bright spot. I'm curious what we'll learn from the new sequence. |
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Apr 12 2015, 12:41 PM
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#6
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Junior Member Group: Members Posts: 33 Joined: 16-June 14 From: Sweet Home, Oregon Member No.: 7202 |
In the other thread I've posted reprojections of some of the RC2 sequence in order to show the bright spot at almost the same position. Comparing the visibility of the bright spot relative to the surrounding crater with other craters / crater rims / craters in craters when near the terminator, I can't perceive hints to a particularly high peak related to the bright spot. I'm curious what we'll learn from the new sequence. Since the bright spots are unresolved in the pictures so far released, and thus appear to be less than one pixel wide, it seems unlikely that any associated topographic peak (or mound) would show up in such photos or projections based on them. |
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Apr 14 2015, 05:03 PM
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#7
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
Since the bright spots are unresolved in the pictures so far released, and thus appear to be less than one pixel wide, it seems unlikely that any associated topographic peak (or mound) would show up in such photos or projections based on them. ... so there is no evidence for a peak from the images. At other locations there is evidence. To make it more feasible, here a montage made from three reprojected single images: Compare the distances of last visibility of a presumed peak/rim to the terminator. The distance of the bright spot to the terminator in frame06 is about the same as the rim in frame08. The bright spot is already in the shadow under otherwise similar conditions where the rim is still visible. |
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