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_*]

The latest scenario I suggest, after closest view of sol 742:


Sometimes between the formation of Gussev and the final filling by mudflow, there was a lake (temporary, or permanent). The surface of this lake was about the level of Homeplate, or a little above, so that Husband hills were not covered. Its open surface lasted only some days, and after the surface frozen, and eventually all the water froze to the core in some months or years.

This water was charged with a variety of salts, and these salts were deposited, but very unevenly, from the presence of ice, after one or several of the following processes:

-waves projected water on mounds, where it evaporated very quickly, lefting the salts on priviledged patches. (This is sometimes visible on earth)
-there were faults in the ice cover, allowing strong evaporation of water in some very restricted places.
-There was a continuous ice cover, see the lake froze into its whole depth. But salts were contentred in tiny patches of very salty brines, which can exist at very low temperatures (-50°C for calcium chloride). When ice sublimated, it left the solid salt patches to end drying. Homeplate could be one of these patches. Other were observed all around.

So if this model is true, it predicts that Homeplate and similar smaller patches are just salts. Eventually we may find, from top to bottom:

-basaltic blocks and sands, projected here by more recent impacts
-most soluble salts, such as sodium and calcium chlorides
-less soluble salts, such as jarosite and sulphates
-eventually an iron oxyd layer somewhere in between
-at bottom a sandstone of basaltic sand cemented with sulphates or carbonates (eventually limestone). This layer would be the dark rocky outcrop which seems to encircle Homeplate.
-a "discordance" and under ordinary soil (same as elsewhere around).

If there is a solid layer of salts, it is not astonishing to find windblown salts, as it was found recently.


And why Homeplate would be so precisely bound? The reply is on the other side of Mars: in recent craters like Fram, the jarosite blocks appear with all kind of sizes, as the explosion left them. But in a more ancient crater like Eagle, only large blocks remain. This is very probably because the jarosite is getting "dissolved in air" with he length of time. The process is likely that UV light breaks up water molecules in the crystals, thus those crystals get dry and powdery, exactly as when we prepare plaster in a furnace. This dust is then blown away by the wind. So small blocks disappear first, and bulk layers are the last to stay in place. This would explain why Homeplate is so precisely delineated, when everything else in the vicinity is blurred out by erosion and impacts.

And why we did not found such salt deposits everywhere in the bottom of Gussev where Spirit landed? Because it was filled later by a gigantic mudflow, where water is much less mobile, so the salts present in the mud remained into it. Only little salts were found into sandy crater fillings. By the way the so many small craters (all about the same size) found in the bottom of Gussev would not be impacts, but steam explosions when the mud began to freeze and dry.