[X] My Assistant

[dvandorn]

I've been looking at this image of Oppy's immediate destination:

http://www.msss.com/moc_gallery/r10_r15/me...15/R1500822.jpg

Staring at it without my glasses, slightly off-focus (a good technique, sometimes) I see that the etched terrain starts to resemble the splash-pattern ejecta blanket structure common to a lot of Martian craters.

There is a cluster of old, degraded craters just west of Victoria, of which Albert is one of the more distinct. The etched terrain seems to be patterned like splashed, semi-fluidized ejecta from these craters, which has been heavily eroded in the same manner and by the same processes that have eroded Albert and the other similar craters in the cluster.

I'd guess the timeline on the formation of the Victoria/Albert area is:

- Shallow seas form and evaporate over a few million years, laying down layers of sandstone and evaporite rocks.

- A cratering event makes a cluster of craters, including Albert and its neighbors. The target rock was either covered with ice or water, or had significant groundwater, resulting in the characteristic Martian splash-effect ejecta pattern.

- Seas continue to flood the area periodically for another few million years, eroding Albert and its neighbors, laying new layers of sandstone and evaporite on top of and within low spots within the craters and the ejecta blanket. This process comes close to smoothing the craters and the ejecta blankets out to a smooth plain, but remnants of the ridges in the ejecta and the crater rims are preserved.

- Mars gets really cold and dry for one or two billion years, and the blueberries (plus other dark minerals) erode out of the upper layers of the evaporites that cover this area. The landscape of exposed evaporite rock is slowly covered by a dark regolith made up of eroded blueberries and sand/dust imported via dust storms.

- The area around Albert and its neighbors consists of slightly bumpier terrain than that to its north, caused by the remnant ridges of crater rims and ejecta features sticking up over the final evaporite deposition layers. Wind erosion becomes preferential around these ridges, and the preferential deposition of the darker regolith forms the "etched" look.

- After most of these surfaces had become mature and resembled what we see today, Victoria was formed. Additional wind erosion has smoothed over most of Victoria's ejecta blanket, but since the target rock had lost most to all of its volatiles content, Victoria did not leave as noticeable of a splash-pattern ejecta blanket. It simply punched through the existing layers of sandstone and evaporites. The only deposition in and around Victoria since its creation has been aeolian.

This all suggests that we'll find some significant evaporite/sandstone ridges within the etched terrain, excavated by the Albert impact event and which predate the evaporite fill layers around them. It seems to me that, since Albert and the rest of the craters in this cluster are larger than any of the other impact features Oppy has visited, the rocks excavated up to its rim would be from the deepest layers of the sandstone/evaporite beds. Even though the craters have been heavily eroded, I'd think that, of the range of rocks available to you on the surface, you'd still find the oldest rocks from the deepest layers along the crater rim and ejecta blanket ridge lines in this old, degraded crater cluster.

We could also possibly find, in wind-sheltered strips along these ridges, a surface that still closely resembles the original evaporite surface that once covered the entire area -- the *original* dried seafloor. And we'll also find more robust dune development because of the wind-relief effects from the slightly raised rocky ridges.

What do y'all think?

-the other Doug