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.

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.

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.

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

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.

The latest images of sol 742 are consistent with a whitish layer, irregularly tilted and torn (this is easily explained with my theory of Husband hills being large chunks of terrain lifted upward by the Gussev impact, and which fell after, eventually upside down).

But I am still puzzled by the neat boundary between the white layer and the other terrain, when everywhere in Gussev everything is blurred and mixed by billion years of wind erosion. Looking on this image without knowing it is from mars I would say that the whitish soil was pushed here by a buldozer.

Soon analysis of rocks...

It seems that it is easier to get on Homeplate by the extreme left (Sorry Doug I know there must not be politics on this forum, but sometimes it is justified)

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

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.

Spirit took at five-colors panorama in Sol 743 which musy be very nice. It shows much more details, but it is difficult to interpret because it is difficult to distinguist what is layering and what is only palalel lines due to shape.

oDoug:

This is another example of our Terrestrial prejudices in action. We expect, based on the Earth, to see processes which are akin to our own, and yet other worlds are truly different places! Our own planet is highly processed thanks to the happy combination of plate tectonics and a massive fluid atmosphere (and a fair old gaseous one, too) and we make presumptions on that basis.

Just because things look like landforms we see at home doesn't prove anything - look at the dunes. They are transient features on Earth, but on Mars a single dune may be geological ages old (or not). We make comparisons at our peril!

Bob Shaw

Hey the guies on the other thread "going to Homeplate" are a bit on advance on us, and I was not seeing the latest close view of Homeplate border

It seems that there are at least two systems (each with many individual strata) a upper clear thick one, and a bottom darker thinner coarser one, as I predicted above.

Just waiting the analysis to confirm...

Eventually the bottom sandstone layer I predicted could be cemented by iron oxydes.


Homeplate could be a reduced model of a similar much larger structure (large crater with white layered structures in the bottom, those structure following the terrain shape as if they were painted on it, sorry I don't recall the crater name).
edited later: the place I meant is Pollack crater and white rock. It shows incredible patches of layered white rocks. Those patches are cut by neat channel where the underlying rock appears, the whole thing without this underlying rock being disturbed, as if the white rock was painted on it!!!! Homeplate and similar smaller patches around would be the same formation as in Pollack crater.

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

The process I describe above involves ice, but not glacial erosion.

Huh?? "Therefore, the currently exposed surfaces of the hills and the inter-hill basins are the result of, first, major glacial deflation"

tty

That was me, not Richard. And keep in mind that the glaciation would have occurred *before* the basalt lake filled Gusev, and it doesn't take a great leap in my mind to see glaciers moving downslope along the curving, bowl-shaped floor of the original impact crater.

And while there are no obvious large-scale glaciation features visible *now*, recall that the vast majority of the landforms that would have shown glacial erosion were covered over by the basalt.

-the other Doug

Besides, according to many documents which I have read, none of them have talked about the Mars glaciation in the lower latitudes such as in Gusev Crater. Perhaps, due to the early erratic Mars axis tilt might have induced to create ices of short duration but not as an glaciation.

An extract from the

Amazonian, Hesperian, Noachian Epochs

The geologic history of Mars has been divided into three broad time periods, or Epochs. From oldest to youngest, these are Noachian, Hesperian, and Amazonian Epochs (named after places on Mars). These Epochs are defined by the number of meteorite impact craters on the ground surface; older surfaces show the scars of more impact craters. The actual timing of the Epochs is not known. The Noachian extends back in time to the beginnings of the planet, and ended sometime between 3.8 and 3.5 billion years ago (according to accepted models). Noachian age surfaces are scarred by many large impact craters. Next in time was the Hesperian period, a time of extensive lava plains. The Hesperian Epoch ended sometime between 3.55 and 1.8 billion years ago; the range here reflects different models of the rate of meteorite falls onto Mars. Finally, the Amazonian Epoch extends to the present day. Ground surfaces of Amazonian age have few meteorite impact craters, but otherwise are quite varied. The Amazonian Epoch has seen the formation of the huge volcano Olympus Mons, lava flows elsewhere on Mars, formation of the landslides in Valles Marineris (like these in Gangis Chasma), and formation of the broad plains and sand dunes near Mars' poles.

Rodolfo

Actually there is some evidence of fairly recent low-latitude glaciation:

http://www.sciencemag.org/cgi/content/abstract/311/5759/368

tty

Many thanks TTY. Very interesting article.

We present high-resolution climate simulations performed with a model designed to simulate the present-day Mars water cycle but assuming a 45° obliquity as experienced by Mars a few million years ago. The model predicts ice accumulation in regions where glacier landforms are observed, on the western flanks of the great volcanoes and in the eastern Hellas region. This agreement points to an atmospheric origin for the ice and reveals how precipitation could have formed glaciers on Mars.

So much tilt up to 45 degree (such erratic) and I doubt about what forces has helped to stabilize the axis tilt. Perhaps, the Mars rotation might have helped to straigthen up the axis-tilt to around 25 degree. Might not it?

Rodolfo

That's right. There were several articles about this just recently, evidence for snow-formed glaciers at lower latitudes, I just didn't have a chance to post or blog them yet. I'll look up the links again as I did bookmark them though.

Diverse hypotheses

As Spirit has wheeled ever closer to Home Plate, and as the rover science team have learned more about what is likely and unlikely in this part of Mars, "I think the hypotheses for Home Plate have gotten more diverse."

Currently these include such possibilities as playa, exhumed crater floor filled with typical Columbia Hills layered deposits, a tuff cone or maar, and a hydrothermal vent area, Crumpler noted. "In fact, it is a healthy debate within the team right now."

But based on previous experience with Mars so far, Crumpler told SPACE.com, it’s not wise to place any bets on initial interpretations. The debate is never fully over until Spirit makes use of its science instrument-tipped robot arm, he said.

As for helping pin down what the rover is seeing at Home Plate, there are a bunch of possibilities, Squyres said: Impact deposits, volcanic deposits, maybe wind- or water-lain sediments.

Given Spirit’s navigation camera images now in hand, many of the Mars rover science team sense that the rocks are possibly explosive volcanic deposits.

"But that’s purely conjecture at this point … a working hypothesis," Squyres said. "Everything is on the table until we’ve gotten more data down."

The above is an extracted article from the URL space.com

Hope that very soon, the hypothesis will be cleared. The bet is still going...

Rodolfo

Although we know little about the lower unit of alternating harder and softer layers, we know the the upper section is a cross-laminated sediment. It seems difficult to think of HP as an "exhumed crater floor filled with typical Columbia Hills layered deposits, a tuff cone or maar, (or) a hydrothermal vent area," unless you want to imagine it as one of those things with sediments on top of it. It seems simpler to consider it a remnant of a once more continuous sedimentary section. The other, more exotic things seem to be unneccesary complications.

I can't rule out the possibility that these sediments originated as a crater fill, but I think we need to see other sides of HP, as well as the top, to learn more about that possibility. It sounds to me as if those interviewed for that article knew more than they were willing to admit at the time.

Are those deposits along a fracture in this image?
http://qt.exploratorium.edu/mars/spirit/pa...55P2271R1M1.JPG

I noticed that fracture fill this morning. It looks quite pronounced when viewed with the corresponding L7 as a stereo pair.

Also, notice the rounded pebbles at the top of the slope that do not look ventifact-like.

--Bill

Hmmm... what happened to todays pictures? According to the tracking site some of them are on the ground??

Tom:

Have you ever take into the account the influence of nerby volcano: Apollinarium Patera at about 100-150 km nortwestern from Columbia and Husband Hills. Around the volcan, usually has subsurface water which might be steaming out. The highest activity of volcano has ocurred during the Amazonia epochs, such as the Olympus has erupted since 200 millions years ago.

Rodolfo

Rodolfo: I suppose that volcano could have generated steam in the past which might have travelled some distance from the volcano. All I was suggesting above was that there seems to be no need to hypothesize such a thing for Home Plate, since we now see Home Plate closely and it seems to be largely, if not completely composed of sediments.

Smiles?

In the lower left corner of this image, I see what looks like the type of crossbedding that was so exciting in eagle crater.