Dawn's Survey Orbit at Ceres Options

[Paolo]

daily Ceres picture from the survey orbit
http://dawn.jpl.nasa.gov/multimedia/images...tml?id=PIA19572

I started a new topic, as we are no longer in the first orbit phase

[FOV]

Polar region crater with more of the light coloured material:

[Gladstoner]

Polar region crater with more of the light coloured material:

That feature does look similar to this:



Both may be where impacts exposed subsurface intrusions of the white stuff.

[JRehling]

Both may be where impacts exposed subsurface intrusions of the white stuff.

I'm looking at photos of Ganymede, which has a lot of rayed craters that exposed bright, subsurface ice. The phenomenon on Ceres is definitely something different. Related, perhaps, but every example on Ganymede has common characteristics that none of the examples on Ceres match.

If this is a mere "ray" event (also visible on the Moon, Mercury, etc.), then there's something very different on Ceres about how the material is surfaced or how it ages.

I'd guess we're either seeing the remnants of a ray system that sputters away quite differently than the rays on Ganymede, et al, do, or these are eruptions.

If it were simply subsurface salt, I'd expect them to look like Ganymede/Moon rays and to be less selective in their occurrence.

Perhaps icy ray material on Ceres is coarse near the point of origin and sparse far from it and therefore sputters away rapidly except where it's most coarse. Eg, a 20 kg lump of ice may stick around much, much longer than lots of microgram particles.

Alternately, I'd stick with the idea of eruptions: Which may be triggered when the impact occurs, or possibly much later.

[Gladstoner]

If it were simply subsurface salt, I'd expect them to look like Ganymede/Moon rays and to be less selective in their occurrence.

This is why I suspect much of the white stuff is in the form of intrusions (veins, dikes, 'volcanic necks', etc.) in places deep beneath the surface.

A terrestrial comparison I have in mind is pegmatite, which is the solid end point of granitic magma crystallization. On Ceres, the 'granite' would be water ice, and the 'pegmatite' would be the leftover material that could not be incorporated into the ice crystals. This 'pegmatitic magma', which would be a concentrated brine containing various salt and volatiles (CO2, NH3, etc.) would either dry or freeze out in place or eventually make it to the surface as an extrusion of some kind. The entrained gases would help to drive the eruption as they do on Earth.

Spot #5 could be the best -- or only -- example where this brine breached the surface due to eruption rather than by exposure due to impact or mass wasting.

Edit 1: In many spots, this material may merely permeate existing fractures within the megabreccia (think of mineral veins on Mars). In these cases, the intrusions may resemble petroleum reserves more than they do magmatic plutons.

Edit 2: And speaking of crater rays, I would not expect exposed water ice to remain for long after an impact excavated and dispersed it like this (assuming all white material is the same stuff):