[X] My Assistant

[dvandorn]

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

[Bill Harris]

That is a good analogy, Richard. I've learned to be careful about applying terrestrial analogs to Martian conditions, but I think yours is valid.

We approach to Homeplate plateau (or the crest of Mitcheltree Ridge, whatever) and we see scoria, basalt, sand and a sulfate (salt) sprinkle on the rocks. Imagine that...

--Bill

[Guest_Richard Trigaux_*]

That is a good analogy, Richard.  I've learned to be careful about applying terrestrial analogs to Martian conditions, but I think yours is valid.

We approach to Homeplate plateau (or the crest of Mitcheltree Ridge, whatever) and we see scoria, basalt, sand and a sulfate (salt) sprinkle on the rocks.  Imagine that...

--Bill

Thank you for your support, Bill.

I think it is not only an analogy: the same mechanisms are at work. Of course they will not give the same result, depending on atmosphere, gravity, etc... So we must be wary.


I already note that the crest of Mitcheltree Ridge is not layered, so it is not a native ground feature. Rather some process gathered rocks here, and there are many similar features in the surrounding terrains (around other hills in the vicinity, in place which were not covered by the mudflow). So I think it is an important feature which existed in many parts of Gussev crater before mud filling. It is too linear to be impacts, it has too large blocks to be dunes, and water erosion cannot give this. It either don't look like cowpats. My best guess would be that the Gussev impact hit a thick ice layer, among other terrains. When the whole thing fell back, there was large ice blocks, and the spaces between them filled with loose stones and sand. After, this exposed ice melt or sublimed, lefting those curious small ridges and mounds where there was spaces between ice blocks.


Another "phantasm" I will add here is that homeplate would be a piece of hard sedimentary rock. When the highland formed, there was perhaps permanent water or things like that. So it would not be a surprise to find hardened sedimentary rocks, hard ones to bear the shattering and keep large dimentions: sandstone, and even limestone. The later would be a good find.

[RNeuhaus]

My best guess would be that the Gussev impact hit a thick ice layer, among other terrains. When the whole thing fell back, there was large ice blocks, and the spaces between them filled with loose stones and sand. After, this exposed ice melt or sublimed, lefting those curious small ridges and mounds where there was spaces between ice blocks. 
Another "phantasm" I will add here is that homeplate would be a piece of hard sedimentary rock. When the highland formed, there was perhaps permanent water or things like that. So it would not be a surprise to find hardened sedimentary rocks, hard ones to bear the shattering and keep large dimentions: sandstone, and even limestone. The later would be a good find.

I do believe that the Gusev crater was impacted by an icy asteroide as you tought. The surface Gusev is so plain and has lead me to think that crater was filled by mud (mix of water and sand) and the higher parts of Gusev crater has more boulders or stones than the lower parts, it might be due to the mud water erosion due to the gravity and leave naked stones and boulders.

Rodolfo

[tty]

I do believe that the Gusev crater was impacted by an icy asteroide as you tought. The surface Gusev is so plain and has lead me to think that crater was filled by mud (mix of water and sand) and the higher parts of Gusev crater has more boulders or stones than the lower parts, it might be due to the mud water erosion due to the gravity and leave naked stones and boulders.

Rodolfo

It might well have been, but that is completely irrelevant to whether Gusev has been filled with mud or not. Any water in the impactor is instantly turned into (very) superheated steam on impact and most of it is dispersed at hypersonic speed. Any mud in the crater would be due to either (1) melting of permafrost by the impact, (2) groundwater, (3) discharge from the Ma'adim valley or (4) rain.

tty