[X] My Assistant

[David Palmer]

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

[dudley]

The presence of a spring mound on the surface of Ceres appears to be contingent on a number of factors, each of which must occur in a particular way, before this could happen.
First there must be a good deal of water in Ceres' interior.
Then, the water must be kept liquid either by sufficient radiogenic heat and/or the inclusion of the necessary amount of minerals, probably salts.
Then, there must be sufficient pressure on the water, from overlying layers of material to force the water to quite near the surface.
Then, There must be a crater deep enough to reach the underground water.
Then, the coating of salt on the spring mound, left after the top layer of ice has sublimed away, must not act to retain enough heat to melt the underlying ice, which would presumably destroy the spring mound.
It has been established that halite, mineral rock salt, is essentially transparent to infrared radiation. It could act like a the glass of a greenhouse, allowing heat to build up beneath it.
When a number of contingencies must all cooperate to produce a phenomenon, the more of these there are, the less likely is the phenomenon to occur.

[Gerald]

The presence of a spring mound on the surface of Ceres appears to be contingent on a number of factors, each of which must occur in a particular way, before this could happen.
First there must be a good deal of water in Ceres' interior.
Then, the water must be kept liquid either by sufficient radiogenic heat and/or the inclusion of the necessary amount of minerals, probably salts.
...

Up to this point, I'd say that's within reach, here a back-of-an-envelope calculation.
But then it would be necessary to show, that water can stay liquid while being transported through kilometers of ice.
Might be fast, short-term (explosive) eruptions could be possible, maybe driven by steam from water suddenly exposed to vacuum after being released through fractures by some tectonics (e.g. due to thermal stress, or due to phase transitions between modifications of water ice), freezing down to snow or frost, together with salty dust. The question is, why is the bright spot so local? If it's the result of an eruption, shouldn't the bright material be more dispersed in low gravity?
Maybe it's a weathering-resistant remnant of an old (cryo?)volcano (volcanic pipe / magma cone analog).
There are certainly many other options, like sinkholes due to comet-like sublimation activity, or impacts, exposing fresh material.
I'm sure Dawn will narrow down the options within the next few months. One or a few a-priori maybe unlikely or unconsidered options may emerge as likely with new data.