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

[scalbers]

Here is an updated map, adding 3 recent higher resolution images that include a couple of the white spot areas:



Full 4K resolution and polar views are here.

[Gladstoner]

Indeed, yet smaller craters do seem to often have that central peak we've learned to expect. Even in that image there's a few such craters at lower right.
So was that central pit a bright area similar to spot 5 in the past? If that turn out to be correct, the case for ice grow stronger, since a pit would suggest something have been removed/evaporated, salts could remain though leaving the pit bright so have they dusted over or darkened by some other mechanism.....?

That's the process that comes to mind for me as well. And perhaps smaller craters having normal central peaks indicates that larger impacts are more likely to 'tap into' icier material deeper down.

[Gladstoner]

The Piazzi dark feature, and its proximity to the nearby impact structure, reminds me of the dark pyroclastic halo on the southern rim of Mare Orientale (albeit far more diffuse):

[attachment=36084:mo2.jpg]

I hadn't heard about the Orientale halo. Fascinating.

It is interesting how the Piazzi 'halo' seems to be associated with the cluster of tiny bright spots.

[Phil Stooke]

Central pit craters are common on Ganymede and Callisto, and in certain areas of Mars, and have always been interpreted as if related to ice in the subsurface. The rebounding central peak is made of or rich in ice, which then sublimes away (or melts and flows, etc.) to leave a pit.

Phil

[MarkG]

Again, it helps to consider the profound ancient-ness of the surface we are seeing.

[Greenish]

Here is an updated map ...

And here is your map, re-projected (azimuthal equal-area) to 4 hemispheres with grid lines. Mostly for my own use since I keep getting lost, now that the whole disk isn't visible.



(re-posted w. proper credits)

[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.

[alan]

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?

Could the old salt flats produced by this process have been buried in the past and then been re-exposed when more recent impacts occur in the right place?

[Phil Stooke]

Greenish - fantastic maps, thanks for doing that.

Phil

[nprev]

Quick note: Please post work from & comments related to the survey orbit phase here.

This thread will remain active for discussion of data & events prior to 15 Jun 2015.

Thanks!

[Greenish]

Phil - Glad they are of use.... all credit to scalbers for making the basemap & the folks who make G.Projector ... all I did was push some buttons in that very helpful program, and mosaic the result.