Home Plate Speculations, Get it in now, before we know the truth! Options

[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

[dvandorn]

The closer we get, the more this area looks to me like something similar to glaciation may have carved up the Columbia Hills and other similar landforms throughout Gusev.

My thinking here follows this general timeline:

1) Near the end of the LHB, Gusev is formed by impact into an already impact-shattered and brecciated megaregolithic crust.

2) Tectonic stress during the final cooling and "setting" of Mars' crust (the end of any major crustal plate movement) uplifted a string of hills through the center of Gusev. The Columbia Hills are one stretch of a general line of hills that runs off-center through Gusev and extends beyond it, and thus doesn't appear to be related to impact formations. Ergo, tectonic uplift is implied.

3) Before, after or during this tectonic uplift, Gusev filled with water and became a lake. This may have been a relatively long-lived phenomenon, or a short-lived one. Or even a cyclic one. I'm thinking this happened after rather than before the hill uplift, because while some of the rocks of the hills seem to have been fairly strongly aqueously altered, many more seem to have seen very little liquid water in their histories.

4) As Mars cooled further, the liquid water all froze and glaciers formed. Glacial movement deflated much of the original floor of Gusev, including a lot of the lacustrine materials, piling them up in some places and denuding them from others.

5) The volcanic deposition originating from the Tharsis Bulge finally reached the Gusev region, filling over all but the tallest features from the original Gusev floor in a lake of basalt -- the last lake Gusev would ever hold.

6) Therefore, the currently exposed surfaces of the hills and the inter-hill basins are the result of, first, major glacial deflation, and second, long, slow aeolian deflation and deposition, of the Gusev flooring material that was uplifted into the original hills so long ago. It's the last exposed remnant of the pre-lava-flooding Gusev floor. And while that is exactly what the MER Team was hoping for, the long and eventful history of the site jumbles everything and makes it nearly impossible to gain a clean context for most of the exposed rock beds.

One reason it's all so jumbled is that the original Gusev floor was impact melt and breccia, which is jumbled to begin with. That floor then collected layers of volcanoclastic and lacustrine materials (all the while being peppered with impact craters of all sizes), and then was tectonically uplifted, which is a jumbling process in and of itself. Then when the Gusev floor was heavily glaciated, rocks and soil got transported glacially all over the place. Yet more jumble.

I'm beginning to think that it's going to be almost completely impossible to find any geologically non-jumbled sites on any bodies that still exhibit major scars from the LHB...

-the other Doug

[tty]

The closer we get, the more this area looks to me like something similar to glaciation may have carved up the Columbia Hills and other similar landforms throughout Gusev.

My thinking here follows this general timeline:

1)  Near the end of the LHB, Gusev is formed by impact into an already impact-shattered and brecciated megaregolithic crust.

2)  Tectonic stress during the final cooling and "setting" of Mars' crust (the end of any major crustal plate movement) uplifted a string of hills through the center of Gusev.  The Columbia Hills are one stretch of a general line of hills that runs off-center through Gusev and extends beyond it, and thus doesn't appear to be related to impact formations.  Ergo, tectonic uplift is implied.

3)  Before, after or during this tectonic uplift, Gusev filled with water and became a lake.  This may have been a relatively long-lived phenomenon, or a short-lived one.  Or even a cyclic one.  I'm thinking this happened after rather than before the hill uplift, because while some of the rocks of the hills seem to have been fairly strongly aqueously altered, many more seem to have seen very little liquid water in their histories.

4)  As Mars cooled further, the liquid water all froze and glaciers formed.  Glacial movement deflated much of the original floor of Gusev, including a lot of the lacustrine materials, piling them up in some places and denuding them from others.

5)  The volcanic deposition originating from the Tharsis Bulge finally reached the Gusev region, filling over all but the tallest features from the original Gusev floor in a lake of basalt -- the last lake Gusev would ever hold.

6)  Therefore, the currently exposed surfaces of the hills and the inter-hill basins are the result of, first, major glacial deflation, and second, long, slow aeolian deflation and deposition, of the Gusev flooring material that was uplifted into the original hills so long ago.  It's the last exposed remnant of the pre-lava-flooding Gusev floor.  And while that is exactly what the MER Team was hoping for, the long and eventful history of the site jumbles everything and makes it nearly impossible to gain a clean context for most of the exposed rock beds.

One reason it's all so jumbled is that the original Gusev floor was impact melt and breccia, which is jumbled to begin with.  That floor then collected layers of volcanoclastic and lacustrine materials (all the while being peppered with impact craters of all sizes), and then was tectonically uplifted, which is a jumbling process in and of itself.  Then when the Gusev floor was heavily glaciated, rocks and soil got transported glacially all over the place.  Yet more jumble.

I'm beginning to think that it's going to be almost completely impossible to find any geologically non-jumbled sites on any bodies that still exhibit major scars from the LHB...

-the other Doug

I for one can't see anything in Gusev that looks like glacial landforms. Also please note that glacial deflation requires an active wet-based glacier that moves over the substrate. This requires either melting in the marginal zone or gravity-induced movement (i e downslope). A cold-based glacier, that is one that is frozen to the substrate, leaves little evidence of its presence.

tty