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The Bright Spots on Ceres
David Palmer
post Mar 27 2015, 10:38 AM
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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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nprev
post Mar 29 2015, 04:10 PM
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Problem is, liquid water just cannot exist below an ambient pressure of about 6 mb. Based on the phase diagram I suspect that any liquid water emerging from a vent would instantly become a cloud of ice crystals.



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dudley
post Mar 29 2015, 08:25 PM
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Found both posts in this new thread interesting and informative. Am left with two impressions. First-- that an icy plume is not likely to adequately explain the bright spot, and second-- that a large ice mound is also unlikely.
Given the seeming need to account for the bright spot within a crater that has apparently fallen dark, and other observations, it appears that some phenomenon entirely new to us may be at work.
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David Palmer
post Mar 30 2015, 01:25 AM
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Actually, and contrary to popular belief, liquid water does NOT flash to vapor in a vacuum, it initially boils at the surface but then freezes over before very much volume is lost, due to the fact that the phase change is highly endothermic. In fact, it's now believed that liquid water has flowed on Vesta, even though it's never had an atmosphere (see http://www.iflscience.com/space/claims-vesta-once-had-water). And for a good laboratory demonstration of what happens to water in a vacuum, see https://www.youtube.com/watch?v=UG7nsZkVZc0
I conclude that, if liquid water were to flow from a vent or fissure on Ceres, that it would indeed form a spring mound. And it would also be easier for Ceres to have a subsurface ocean than is generally believed: if the water were saturated with salt and/or ammonia, the freezing point could be as low as minus 100 Celsius, which is not much warmer than the estimated average surface temperature of minus 106 Celsius. And even with a body as small as Ceres, we would expect the interior to be warmer than the exterior, due to radiogenic heating, release of heat due to gradual freezing of a subsurface ocean, and possibly serpentization (an exothermic reaction) if ferromagnesian silicates are in contact with liquid water.

David Palmer
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marsbug
post Mar 30 2015, 08:46 AM
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Wouldn't any significant water ice on the surface outgass, and leave a detectable signature in ceres exosphere?


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David Palmer
post Mar 30 2015, 10:18 AM
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QUOTE (marsbug @ Mar 30 2015, 01:46 AM) *
Wouldn't any significant water ice on the surface outgass, and leave a detectable signature in ceres exosphere?


Yes, and that's exactly what's been detected by the Herschel space telescope.
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marsbug
post Mar 30 2015, 02:39 PM
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Sorry, I meant by Dawn, as I recall that Herschel came up empty for water on a later observation which makes me wonder if the source is not actually intermittent.... but then I remembered it cannot sample the exosphere (at least directly).


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katodomo
post Mar 30 2015, 05:25 PM
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I seem to remember that when the plumes were discovered one calculation for a possible source was sublimation from about 0.6 kmē of surface ice near perihelion only.

Are the spots small enough for that?
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dudley
post Mar 30 2015, 06:16 PM
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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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David Palmer
post Mar 30 2015, 10:15 PM
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QUOTE (dudley @ Mar 30 2015, 10:16 AM) *
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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elakdawalla
post Mar 30 2015, 10:23 PM
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Remember that the science team isn't monolithic. The person doing the downplaying March 3 was Carol Raymond, the deputy principal investigator. The person talking about the mounts at LPSC was Andreas Nathues, Team Lead for the Framing Camera. They were both talking about the same data, but have different interpretations.


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dudley
post Mar 30 2015, 11:45 PM
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Under these circumstances, it's difficult to evaluate Dr. Nathues observations. He speaks about the probability of a mound at the site of the bright spot, but the images supporting this conclusion have apparently been withheld. A NASA spokesman has even maintained that the bright spot appearing to persist in an otherwise darkened crater is an illusion of some sort.
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TheAnt
post Mar 31 2015, 03:40 AM
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@David Palmer: Correct, a "spring mound" is possible, (though getting its energy from heat below I found the word geyser to be more to the point in describing it.)
Anyhow if the water got a temperature near freezing, it would boil briefly when the latent heat bring a minor part of the water into a gaseous phase. While the rest would simply freeze out. The question still remain if the feature is made of ice or salts.

@katodomo: The white spots might be of the right size, in fact since they're not resolved (they might still be less than one pixel in size) so the question is not if its small enough but one additional source might be needed, and as posted in the approach thread - there's two areas that show water vapour.
That the water vapour that have been detected come from sublimation of ice is still my favourite hypothesis.
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dudley
post Mar 31 2015, 07:29 PM
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I'm not really clear on why the proposed 'spring mound' would need to be tall and thin in shape. If it's 4 km across, and the crater is a couple of km deep, the mound could be three km tall, and still catch the Sun when the crater was dark, couldn't it? The strong brightening reported for the bright spot at about local noon seems to point to a fairly broad, flat feature. A tall, thin feature would present nearly minimum aspect to the Sun at this time, would it not?
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JohnVV
post Mar 31 2015, 08:26 PM
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well we will know in a bit , once the spacecraft gets close again
http://i.stack.imgur.com/Tzh3C.jpg
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