Home Plate Speculations, Get it in now, before we know the truth! |
Home Plate Speculations, Get it in now, before we know the truth! |
Jan 25 2006, 04:10 PM
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#1
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Senior Member Group: Members Posts: 3419 Joined: 9-February 04 From: Minneapolis, MN, USA Member No.: 15 |
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 -------------------- “The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Jan 25 2006, 04:53 PM
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Solar System Cartographer Group: Members Posts: 10229 Joined: 5-April 05 From: Canada Member No.: 227 |
Very interesting topic - and a good idea to move it here.
It's not so clear to me that HP is a crater remnant. Admittedly it looks a bit like it in the image Doug linked to. But what about all the other light material on the way, especially just before Comanche? In fact, Comanche seemed to be superimposed on bright stuff, stratigraphically higher. Is HP part of a much wider layer? If so, what is it? Just as a working hypothesis I'll throw out this possibility. The hills are a composite of multiple layers of ejecta, volcanic ash from Apollinaris or elsewhere, and maybe wind-deposited material, stacked up over hundreds of millions of years after Gusev formed. They were sculpted by small impacts (Inner Basin, East Basin etc.). A brief flood - more like a mudflow - from Ma'adim Vallis soaked the area, but never really formed a lake. Later the basalts of the actual landing area flooded part of the Gusev floor. Since then small impacts like Castril and Bonneville have scattered ejecta over everything. Deflation acting over 3 Gy has gradually eaten away at the hills, which were never consolidated and deflate faster than the basalt plains. HP would be part of one such layer. I don't know what it is, but let's say for sake of argument it's a layer of fine volcanic ash deposited over an irregular surface within the stack of ejecta etc comprising the hills. Other parts of the same formation are visible around (under) Comanche. It will be altered by exposure to water. Now it's being exhumed by aeolian action in several places. Maybe HP itself is a place where the layer was deposited in an old depression and has been preserved better than elsewhere. Just a guess! To my mind one of the most interesting things about this will be looking for stratigraphic contacts. Can we see where the HP material sits right on top of the underlying stuff? I have a hunch that the NW corner of it exposes a section at least a meter or two high. That would be my first priority. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PDF: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Jan 25 2006, 06:03 PM
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Member Group: Members Posts: 706 Joined: 3-December 04 From: Boulder, Colorado, USA Member No.: 117 |
QUOTE (Phil Stooke @ Jan 25 2006, 04:53 PM) HP would be part of one such layer. I don't know what it is, but let's say for sake of argument it's a layer of fine volcanic ash deposited over an irregular surface within the stack of ejecta etc comprising the hills. Other parts of the same formation are visible around (under) Comanche. It will be altered by exposure to water. Now it's being exhumed by aeolian action in several places. Maybe HP itself is a place where the layer was deposited in an old depression and has been preserved better than elsewhere. I suspect Phil is exactly right. Homeplate is probably an unusually symmetrical remnant of one of the light-colored layers like the ones we've been traversing on the way down from Husband Hill, maybe preserved because it's at a low point in the undulations that are apparent in the layers throughout the Columbia Hills. I still don't understand all these undulations though- most deposition mechanisms would form fairly horizontal layers initially (draping over existing topography is possible, but usually the deposited stuff just collects in the depressions, and doesn't form a uniformly thick mantle over everything). It's not clear to me how impacts could warp the layers in the broad, coherent way that we see. |
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Jan 25 2006, 07:26 PM
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Member Group: Members Posts: 624 Joined: 10-August 05 Member No.: 460 |
Given the clean lines on the rim, I would go with a snapped off fossilized Sequia - notice the proturding serpentine root structure
Seriously, I vote for a volcanic origin for this very reason: The serpentine hills surrounding homeplate could be volcanic in nature...either that or the grounds keeper left the hose out and running, and it sunk into the mud. Very curious feature. |
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