Dawn's first orbit, including RC3, March 6, 2015- June 15, 2015 Options

[Ron Hobbs]

Dawn is now officially in orbit around (1) Ceres!

Congratulations, NASA. Nice images of crescent Ceres.

NASA Spacecraft Becomes First to Orbit a Dwarf Planet

[Sherbert]

I have been thinking along those lines too Phil. The infamous spot(s) 5 does seem to be in an abnormally deep crater.

If we start with the assumption that there was some sort of liquid ocean beneath the surface shell that over the lifetime of Ceres has slowly frozen, the equatorial regions being the last area to freeze, the material thrown up by an impact into a central peak will behave differently depending on the time of impact and its location. It can also be assumed that the dust/rock/organics content of the shell increases over time too, changing the properties of the shell material. As a rough generalisation the majority of the craters in the polar latitudes have identifiable central peaks and steeper, sharper rims, implying impacts to areas with a solid subsurface. Again generally, the large impacts in the equatorial belt show flat bottoms or depressions and less defined rims, suggesting slushy or liquid material was excavated by the impact. There is also much evidence of resurfacing occurring from the oldest and largest impacts, suggesting large quantities of fluid were excavated and spread across the surface when penetration to the ocean below was easier.

The spot 5 crater is at intermediate latitude, where the slowly freezing ocean below could have become a shallow, slushy, concentrated brine lake/sea at the time of the impact. Once ejected onto the floor of the crater any Water would rapidly sublimate leaving bright white salt lakes behind. Thus this phenomena is impact, location and time dependent. Other bright deposits elsewhere may also be where the same criteria have been met to varying degrees at different times and are the result of briny ejecta. It does puzzle me though, why these postulated salt flats and briny ejecta have not been buried under later deposits of darker dust and organics. Is fresh brine still seeping onto the surface? Is the salt in the form of large, semi vertical crystals which shed any dust and debris?

As the liquid below freezes into a thicker and thicker shell, so smaller craters in equatorial regions, start having central peaks as the excavated material with a higher dust/rock content, becomes more solid and able to support its own weight, eventually newer, larger craters in the equatorial regions start to have central peaks. There are sure to be exceptions and oddities within this gross generalisation caused by anything from shrinkage fissures to local elevation differences. As a "rule of thumb" place to start, it seems plausible.

Larger impacts would seem to have generated cracks and fissures in the shell away from the craters allowing for the "fire fountains" mentioned by dvandorn and the creation of scarps and gullies also evident across the surface. The large number of highly linear features made of small secondary craters intuitively implies a more fluid ejecta too, though I have insufficient knowledge to know if that is a valid interpretation. A stream of liquid in a micro gravity, vacuum environment should split into a line of spherical blobs, randomly shaped blocks of solid and semi-solid ejecta would, one would think, show greater scattering due to inflight collisions and more random starting trajectories.

I hope this brings together in a semi-cogent theory, ideas that have been suggested here and elsewhere over the last few weeks. Reality may be different. We await more evidence.