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

[Guest_Richard Trigaux_*]

We have a model of Husband hills right on Earth, at Ries, Germany.

When we look at recent Moon craters like Tycho, we find that the bottom of them is filled with cowpat looking mounds, while the ejecta blanket shows lava lakes and chorded lava flows, perhaps less than some metres thick, closely matching the underneath ejecta form.

On Earth the Ries crater formed on a granitic substrate covered by about a hundred metres of limestone and shale layers. It is perhaps more recent than Tycho (38-30 million years) but more eroded. There is however still enough to see here, for tourists gazing from the central peak and think they are on the Moon. We find again a thin layer of lava (less than 10m) all around over the ejecta blanket. Analyzing this lava shows that it is the molten granite, and it fell all around the ejecta, and apparently AFTER it (some seconds I imagine). This lava also formed nice light-green glassy tektites 1000Kms away, known as moldavites. And into the crater, the cowpat mounds turn to be large blocks of sedimentary layers, lifted up, eventually overturned, but which fell back in place, shattered and torn, (jumbled sayd Edstrick) but with their overal sedimentary structure still recognizable (sometimes inverted).

So how this helps us to understand what are the Husband Hills?

It seems very likely that the Husband Hills are one of the "cowpats" which formed when the Gussev crater debris fell back on the ground. Of course other explanation are possible, but this is the most likely I think.

So the stratigraphy in Husband Hills formed BEFORE the Gussev crater, and it can be eventually very ancient, from the time Mars still had a plate tectonics, even back from the time it was still hot from formation, with many volcanoes emitting gasses and lavas (Husband Hills material are very likely all of volcanic origin). All these very ancient eruptions formed the Mars's southern highlands.

What happened to the Husband Hills after Gussev formation? Volcanoes seem unlikely at this epoch. Impacts are much more likely, there obviously was many, and perhaps 100% of the Gussev Hills surface was ablated by impacts. But the overal layered structure is still visible, and small impacts of this magnitude don't form lava. (So the lavas visible today are not from impacts more recent than Gussev).

But the main event was certainly the formation of the gigantic network of Ma'adim vallis. As far I can guess, there was short events (perhaps only one) but gigantic surges of water, and even more likely mudflows, and we known that such large mudflow can flow very far and form large horizontal surfaces. There is an example in the USA, near the Shasta Mount. This Earthy mudflow shows a region of small ridges perpendicular to the flow, and further a very large and flat alluvial fan, not much smaller than the Gussev plain. It is exactly what we see in Gussev! First parallel ridges barring the exit of ma'adim Vallis, and further a large flat land. We can even see the border of the alluvial fan skirting around Husband Hills and all the other hills in the vicinity! Spirit traversed this little downward slope just before entering the Husband Hills terrain. The land where Spirit landed, with the Bonneville crater and all, was the alluvial fan itself, and it showed to be just a muddle of blocks and sand, exactly as we can see in Earth's mud flows.



Does this tell us what can be Homeplate, the other smaller similar structures, and the darker mounds all around? I am afraid no. But it constrains the various hypothesis explained here by former posts in this thread. I shall say that Homeplate can be:

-A remant of a distinctive "sedimentary" layer existing before Gussev (I say sedimentary as it is layered, but it is more likely various layers of lava flows, pumice, tuft layers, etc).

-A larger block which fell here

-A remant of a VERTICAL volcano chimney (eventualy containing more unusual lavas, in the komatiites or trachytes families, see carbonatite family. Such volcanoes could have formed later than all the horizontal basalt flows we see everywhere, at a further cooling stage).

-Eventually this zone could be the true floor of Gussev, only the Hills being a "cowpat". When Gussev formed, it may have cut horizontally a vertical lava chimney, and eventually a whole system of volcano chimneys, with lavas of different composition, dykes, circular fractures, etc.

The problem with the idea of trachytic volcanoes is that they would be visible on all the southern highlands. But all the southern highlands were so heavily craterized that there is no hope to find traces of the original surface.

If Homeplate is formed of komatiites, there is some chances we find semi-precious stones like jadeite, metal ores, or even diamonds!!!! (But diamond form under continents, not likely on Mars).


I shall add here other phantasms, if I find more. What is there to earn for the one who best guessed what is Homeplate?