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 1 2015, 03:33 PM
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#2
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Junior Member Group: Members Posts: 68 Joined: 27-March 15 Member No.: 7426 |
It's been reported that thermal measurements of the bright spots reveal that they have the same temperature as their surroundings. This was considered surprising since light-colored areas should reflect more light, and, so, remain cooler than dark ones.
If there is continual freezing of water emerging from the interior, this would release a certain amount of heat. It seems a remarkable coincidence that this added heat would affect the bright spots' heat budget to just the right degree, so as to make it very closely match that of their darker surroundings. |
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Apr 2 2015, 05:35 AM
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#3
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Junior Member Group: Members Posts: 33 Joined: 16-June 14 From: Sweet Home, Oregon Member No.: 7202 |
[quote name='dudley' date='Apr 1 2015, 08:33 AM' post='219247']
It's been reported that thermal measurements of the bright spots reveal that they have the same temperature as their surroundings. This was considered surprising since light-colored areas should reflect more light, and, so, remain cooler than dark ones. Does anyone know what instrument on Dawn measures the surface temperature, and does it have the same resolution as the Framing Camera's visible-light photos? If so, I'm not sure we're in a position yet to tell if the temperature of the bright spot is the same as its surroundings, as we still haven't resolved the bright spot with the Framing Camera. |
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