[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

[Gladstoner]

As we've heard, something noteworthy was spotted when the bright-spot area was near the limb (or at least that's been the impression). It's interesting that no images of this have yet been released -- including the missing frames in the gibbous-Ceres rotation sequence. Other unreleased images of the bright spot(s) at the limb likely include the half- and crescent-Ceres images, as well as any unscheduled imagery (which very well could have been taken if something extraordinary was spotted).

Plus, if there are geysers/spring mounds present, they could consist of a field of structures rather than a single edifice:



On Earth, fields of small cinder cones form when magma ascent becomes insufficient to form a single volcano. Compare Mauna Loa with the volcanic field around Flagstaff, Arizona. Any activity on Ceres could be meager, so I'd think a field of vents would be more likely.

Plus, visible plumes may not necessarily be the same thing as the bright spots as we've seen them. As the area moves close to the limb while Ceres is in a crescent phase, any otherwise faint plumes could become visible when back lit, even if the bright terrain becomes less conspicuous:



Again, all of this considers the possibility of venting activity. The truth could hold something completely different in store.