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

[edstrick]

Shaka: "Dam' How can something look so prominent from orbit, and so underwhelming close up? "

Uh....
Take a look at the RAW Mars global surveyor image data!
Granted the shot is looking down through the hazy atmosphere, but there has been MASSIVE contrast stretching <usually saturating "ultreya" black> on presented versions of the orbital images.

[Bill Harris]

And also, remember that we are used to seeing Homeplate in images that have been stretched 3x or more. The view that we are looking at is also physically flat, too.

--Bill

[sattrackpro]

The view that we are looking at is also physically flat, too.

Is it really? How about the top layer of PM? It isn't flat, but an apparent duplicate of the same kind of material that makes up HP lops off over the edge - or, is it the same material... then, there is that slab to the SE of PM, it also appears pretty flat... but, it isn't the same kind of material, is it?

I lean away from HP being a deposit blown in or carried in by wind, water or liquid CO2 - seems more likely to me that Richard Trigaux was closer with his suggestion of small volcanic chimney(s) - but rather than being filled by outside forces, HP could have came up the chimney as hot semi-liquid material that didn't overflow... just stopped rising where it sits today. That sort of small vent action could explain the material that lops off over the side of PM too - sort of freezing in place.

Whatever it is - the speculation here has been most interesting and fun! Hehe... though, at times I think I learned more about British ball games than about geological formations.

[Bill Harris]

Umm, I wasn't too clear. Although there is appreciable relief all around, the slopes around the edge of Homeplate are fairly gentle. We have become accustomed to seeing Homeplate in the views of the Inner Basin which frequently have vertical exaggeration. In reality, the scarp on Homeplate is not as prominent as we tend to think, either physically or tonally.

Ed made a good point, too. The area is also optically flat and the contrast has been exaggerated, too. Take Ultreya: from MGS views it looks like an inkblot, but from the ground it is barely a smudge.

--Bill

[Guest_Richard Trigaux_*]

It all looks strange, perhaps the strangest of all.

The strange is that Homeplate seems to have a precise color limit, which don't really coincide with its shape. As if some joker had painted an ordinary mound.

The first idea we get is that Homeplate is a different rock of the surrounding; but the sharp limit is surprising when you see everything else blurred and mixed by billions of years of erosion.

Especially this yellow sand and dust cover we find everywhere should also cover Homeplate evenly, with only outcrops of rock of a different color. So it looks as if Homeplate's material was added recently and roughly spread wih a buldozer.

So I don't know what to say.

[Guest_Richard Trigaux_*]

So I don't know what to say.

Pétanque or cricket?

[edstrick]

I'll stick with my fossilized pizza-crust-of-the-gods idea.

Keep your eyes peeled for fosslized manhole-cover sized pepperoni and meter long anchovies!

[Bob Shaw]

I'll stick with my fossilized pizza-crust-of-the-gods idea.

Keep your eyes peeled for fosslized manhole-cover sized pepperoni and meter long anchovies!

Did somebody mention manhole-covers? Irradiated or subject to chocolatier alteration?

Bob Shaw

[Bill Harris]

Bob, Ed-- shush.

I've been playing with a couple of our Homeplate images to have something close-in to work with while we're moving around HP. Sort of localized route maps.

Attached is an enhancement of the HP-regional image, contrast tweaked and stretched 3x, and a color Vicinity Map clipped from Alan's Homeplate route map, both at more-or-less the same size.

On the HP-regional image, look on the near slope of the scarp, you can see a crescent-shaped failure surface of an incipient slump. And other goodies.

Interesting place, this. Next week is going to be great.

--Bill

[tty]

Here's the unfiltered navcam image of the same area, the above R1 image is the area right below and to the left of the white mound, and shows the same isolated rock at its left side (topmost large rock),  and you can see how white the dust pan above it is in the unfiltered image but the layer in question doesnt show up as white, so i dont think that line of strata is really part of the white part of the homeplate, its probably an older layer below it.

There is a lot of small whitish rock fragments in the loose sand in the foreground. Could they be remnants of the "Home plate layer" that has been eroded away?

tty

[Shaka]

Bob, Ed-- shush. 

...snip....  And other goodies.

Interesting place, this.  Next week is going to be great.

--Bill

You're right, Bill. This is a special moment. After such long anticipation, it's here, and I'm feeling that our waiting will be rewarded.
You rockhounds go and do your work. I'll just watch.

[RNeuhaus]

I will put two interesting observations which are :

The HP is not niveled but it has a slight slope toward west and the west ridge has a small channel where there is the best way for Oppy to climb into. I tought it might be what the mud or some kind of thick water had drained from HP. It does not look very eroded by the liquid but it seems more as a drift of a some kind of mud.

Rodolfo

Click here to see the HP.

[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...

...gigantic surges of water, and even more likely mudflows...

If you look backward to this post, where I explained the likely geological processes which formed the Homeplate context, I coul add a new possible explanation:

Homeplate could be a blob of lava formed during Gussev impact and which fell here.

But I note that surrounding smaller hills also have smaller homeplates on them, as if an unique layer was cut in several hills by erosion.

Which erosion? in the context only wind deflation can do the job.

[Bill Harris]

I look at this and I keep thinking of Whiterock Formation of Pollack crater.
http://www.msss.com/mars_images/moc/dec00_seds/pollack/

A light-toned, layered, widespread unit, present, as far as I recall, from Syrtis to Marineris on the other side of the planet. Initially I tried to relate it to the evaporites at Meridiani, but it seems to be possibly some manner of windblown ashfall.

Aeolian erosion is a major player on Mars, but it surely didn't create the Ma'dim Valles nor it's delta in Gusev. Other processes have been at work...

The principle of Uniformatarianism assumes gentle, consistent processes over geologic time, but on Mars we should consider that catastrophic events punctuate the quiet progression of time.

--Bill

[Guest_Richard Trigaux_*]

The principle of Uniformatarianism assumes gentle, consistent processes over geologic time, but on Mars we should consider that catastrophic events punctuate the quiet progression of time.

--Bill

The "debate" between uniformatarianism and catastrophism was hot one or two centuries ago, when the origin and duration of geologic processes was still largely unknown. For instance, when seeing a thick layer of sediments, was it formed into only one short (and thus caastrophic) event, or with the slow action of weak forces accumulated over large spans of time?

Today that geologic processes are much better knwn, this debate is now irrelevant, as we fairly known that many Earth Processes are fairly slow, when some others are catastrophic. On mars, where geological forces are much weaker, catastrophes play the main role.

In Gussev we can see two types of catastrophes:
-Impacts
-Mud flows.

Eventually we can too see volcanoes, but it seems that all the lava flows we see in Gussev are much older than Gussev itself.