Down in the Front Page Stories board, Phillip asked what all of us UMSF types think Home Plate might be made of and how it was formed. He actually wants Jim Bell's speculations, but asked for UMSF's speculations, as well.
Since we're getting close to getting there, it's time for any of your uninformed speculations out there to be recorded for all posterity...
I posted the following in that thread, but it really belongs here, so I'm reposting it here and inviting discussion. I figure that a lot of us don't bother to read the boards we don't stay actively involved with, so for all of you, this is new. Otherwise, I apologize for the repetitiion!
Look at the vertically-exaggerated image posted here.
Home Plate seems very obviously, in this stretched image, to be the remnant of an impact crater. There are several impact crater remnants in the inner basin, here. Each seems to have been formed in a surface that was a good many meters higher than the present surface -- those missing several meters have been deflated from this terrain, by some process, leaving the shocked "pedestal" remnants of the deeper cratering forms.
Remember, when you make an impact crater, you don't just affect the surface. The disruption caused by the cratering event goes well under the surface, consisting of impact melt (if the impact is energetic enough) and shocked, brecciated rocks.
The crater remnants we're seeing on the surface look like the brecciated and shocked rocks that were originally created in a bowl-shaped lining beneath this cluster of impact craters. I can see traces of at least five different craters within the inner basin, here. (The ridge of rock Spirit is passing right now is, in fact, a small crater remnant.)
As for Home Plate, it sits within the largest and most well-defined of these crater remnants. Maybe such layers were exhumed in *all* of the craters here, and have since been completely eroded away -- but that doesn't seem right. We have traces of several craters, and in only one of them do we see any trace of this lighter-colored material.
I'd have to think that either the impact target composition was different where the Home Plate impact occurred -- which seems a little unlikely when you consider some of these impacts are only a few tens of meters apart -- or that some other substance was deposited in Home Plate crater that wasn't deposited in the other craters. (Or that has been completely deflated from the other craters, if it ever existed there.)
So, logic *seems* to point towards post-cratering material deposition accounting for the light-rock ring. Personally, I think it could have been water deposition. Home Plate could have been a puddle that was filled and dried thousands of times (maybe with an internal artesian spring) that resulted in aqueous transport and deposition.
Or, it could have just been a good wind trap and it trapped a lot of light-colored dust. Hard to say.
I'm not only interested in the light-rock ring's composition, I'm getting very curious about the erosion process that deflated the original surface. Could aeolian erosion have deflated *that* much surface, even over a few billion years? Do we need to postulate aqueous erosion, or even glacial erosion?
Maybe the specific composition and erosion patterns we see on the light-rock ring will help us puzzle that out.
-the other Doug