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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Mar 30 2015, 06:16 PM
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#2
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Junior Member Group: Members Posts: 68 Joined: 27-March 15 Member No.: 7426 |
NASA seemed to have downplayed the possibility of a mound at the location of the bright spot, in their March 2nd press briefing. They used the apparent absence of such a feature, among other reasons, to argue that a cryovolcano was an unlikely explanation for the bright spot.
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Mar 30 2015, 10:15 PM
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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 |
NASA seemed to have downplayed the possibility of a mound at the location of the bright spot, in their March 2nd press briefing. They used the apparent absence of such a feature, among other reasons, to argue that a cryovolcano was an unlikely explanation for the bright spot. That was before they observed that the bright spot was still showing when it had moved close to the linb, such that the crater rim should have hidden it if it were a low-relief feature. So the only logical explanations would seem to be that it is a tall, narrow solid structure, or else a plume. But a plume seems to be ruled out by the considerations I listed at the start of this thread. So unless we hypothesize that it is an artificially-constructed highly-reflective glass tower, the only possibility would seem to be that it is a spring mound of ice that has grown to ridiculous heights due to the low gravity (but which is narrow enough to not be conspicuous as a mound in the center of the crater....nothing more than one pixel wide at the range at which it was being photographed). And while the artificial-structure hypothesis is of course a possibility, we need to consider all natural explanations before we go down that road. |
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