Dawn approaches Ceres, From opnav images to first orbit Options

[Phil Stooke]

On Tuesday (two days from now, for visitors from the future), the first optical navigation image will be taken... hopefully we'll have it in our hands soon after that. So it's time for a new topic. Over the next few months we'll have progressively closer images and full orbit characterization sequences, no doubt including multispectral image sets.

A new world...

This is a bit of reprocessing I have been doing with the Hubble images from a few years ago.




Phil

[dvandorn]

I'll also point out that variable reflection may indicate an asymmetry in the albedo of an ice-enriched central peak.

As a gedankenexperiment, let's imagine that the crater within which the Ceres White Spot ("feature number five") resides has a pronounced central peak. Even without any cryovolcanism, the central peak rebound will pull up the deepest levels of the impact target -- so let's say that, in the case of this crater, what was pushed up as a central peak shows where the impact punched through to a water ice layer, and as such is highly enriched in water ice as compared to the crater's rim and floor.

Now, as time goes along, the crater rim and floor will darken with ejecta from nearby impacts, and perhaps weren't as enriched in ice as the central peak. The central peak, however, as we have seen in lunar central peaks, has a tendency to cleave down as impacts happen onto it, leaving huge blocks of itself at the base of the peak.

Over time, the central peak will darken such that it's still brighter than the floor or walls of the crater, but not tremendously so. Then a decent-sized small impactor hits the central peak right near its top. All sides of the peak shed off accumulated dark covering debris, and if it's got a really high ice enrichment, it might leave nice, flat cleaved faces.

In other words, a mountain of ice with steep sides can get its darkening cover blasted off repeatedly over time, and each time it happens, the sides of the peak become highly reflective. But they don't necessarily reflect in all directions the same, with some slopes pointing more directly at a given observer than at another observer looking from a different direction. And, at least for a while after such an impact, the resulting sudden mass wasting that occurs (think avalanche or landslide) also piles layers of bright water ice at the foot of the peak.

So, at dawn, the sunlight strikes a (relatively) reflective ice surface and reflects the solar image brightly. As the day progresses, and the feature moves from left to right (as we've been looking at it), the reflection shifts from one bright facet of the peak to the next. But as we approach the right terminator, the light dims because the side of the peak we're looking at is rougher and darker, or just not "aimed" as directly back to the observation point.

This thought experiment derives a possible configuration of a crater that fits with what we know about crater formation, what (little) we know about the composition of Ceres, and what kinds of effects could cause variable brightness in reflections from potentially icy surfaces.

I surely don't insist this is the correct theory, but I think it fits all the facts we have at present. Personally, I'd prefer to see a variably-deposited snowfield surrounding a geysering central peak, but since that's what others are looking at, I figured I'd see if there are other ways to explain what we're seeing, and I think I found one. It's just not the one I want to see...

-the other Doug