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originally posted by: Ross 54
originally posted by: cmdrkeenkid
a reply to: weirdguy
The albedo, or ability to reflect light, of Ceres is about 9%. This means that of all the light that teaches Ceres only 9% is reflected back. This is pretty low, due to the dark soil and dust that compromises most of the surface. So when you have an area of a significantly higher albedo in these dark areas, the contrast makes it go all wonky and creates the bright spots we're seeing in the photographs.
For comparisons: The albedo of ice is around 35%, while snow can reach up to the mid-80s% if it's really fresh. I don't know what salt is offhand, but 20-40% comes to mind got some reason.
So it could be salt, at least until salt is officially ruled out.
The figure I found for fresh salt was a maximum of about 0.4 albedo (40 percent reflectivity). The current figure given for the bright spots is ~ 50 percent. I'm seriously inclined to doubt it's salt. So is NASA, I expect. Still, they're being careful and thorough; checking the most likely things first.
It might be some other light colored minerals, plowed up by asteroid impacts, but NASA has already noted that the distribution of the spots isn't like that of normal cratering.
Maybe some gas trapped below the surface, blowing up surface dust. Not at all likely, especially on this large scale, and on a continuing basis, but it can't be entirely ruled out yet.
Cryovolcanoes, or geysers--probably not, they should have been able to detect the water vapor, but didn't. Besides that, Ceres isn't tidally squeezed, as Enceladus is by nearby Saturn. Where's the heat to drive them?
Another explanation is cryovolcanism, in which ice and water are forced out of the surface by processes similar to those that drive magma volcanoes on Earth. But according to a second model presented at the LPSC by David O’Brien of the Planetary Science Institute in Tucson, Arizona, Ceres doesn’t have enough muscle to drive these eruptions.
Water down deep
The idea is that Ceres has a subsurface ocean covered by an icy shell. As the bottom of the shell freezes, it expands, putting pressure on the ocean and the shell itself. In order to create a cryovolcano, says O’Brien, the water pressure needs to build up enough to launch up through the shell before the ice cracks and relieves the pressure.
We don’t know exactly how deep the ice is on Ceres, so O’Brien tried a range of plausible depths. None produced the conditions for spewing cryovolcanoes – the ice always cracked before enough pressure built up. The best case scenario was water reaching about 90 per cent of the way to the surface.
Intriguingly, that means water could potentially reach the surface from a deep crater, where there was less ice to get through – perhaps even from a crater like the one where Dawn saw the bright spot. That doesn’t mean there is a cryovolcano producing a massive plume, but it could be just enough to replenish the ice on the surface, countering the instability that Titus discovered.
These bright features have an albedo of about 40%...
In this case, the feature is very reflective... Now, we don't know what size it is, so we can't tell if the albedo is 40 percent, 60 percent, 80 percent or 100 percent, but it's probably in that range someplace.