Columbia Hills Rounded Boulder Populations, Where do they come from? Options

[CosmicRocker]

This has bugged me ever since Spirit arrived at the base of the hills. Most of the boulders we have seen since sol 1 are quite angular and apparently the result of impact fracturing events. Pyramidal forms like Adirondak and Humphrey were extremely common on the Gusev plains, and were also fairly common in the hills. Since Spriit arrived at the hills we have seen several areas where very nicely rounded boulders were common.

There was an area near the base of the West Spur, another one was observed leaving the West Spur, and most recently we passed one on sols 647-648. Each time Spirit has passed these uncommon populations I couldn't help but wonder where the rounded boulders were coming from.

On this most recent passing, several pancams were captured of rounded boulders weathering/eroding from outcrops in the area. I can't help but wonder how these boulders were rounded in the first place.

...an area with rounded boulders...
http://marsrovers.jpl.nasa.gov/gallery/all/spirit_p648.html

...rounded boulders weathering/eroding from an outcrop...
http://marsrovers.jpl.nasa.gov/gallery/all...8P2578L7M1.HTML

...An outcrop from sol 647 also contained previously rounded boulders...
http://marsrovers.jpl.nasa.gov/gallery/all...8P2568R2M1.HTML

[Bill Harris]

Good questions, Tom. I've assumed that the boulders were rounded by the wind either in-place or transported by impact. Different degrees of rounding implied different times of placement or exposure. Some boulders have looked rounded when formed, rather like pillow lava (but knowing that they _cannot_ be that).

This is my simplistic view, but without more data I can't refine it. Do you have any ideas of "why"?

--Bill

[Guest_Richard Trigaux_*]

With my opinion, all the rocks in Gussev plain were coming from lava flows, maybe very old, which naturally fracture when cooling and solidifying. This is very common on Earth and we can even see strange things such as fractures of only some cms long, which occur when the cooling is at a stage of "jelly" solid enough to fracture but still somewhat deform from heat. This too is very common in basaltic lava flows.

The Husband hills arise muh more problems. First scientists do not yet agree if they are
1) central peaks in Gussev crater
2) remmants of more recent impacts
3) volcanoes.
My favourite opinion is that the Husband hills are layered because they were put in place before the Gussev impact, and, at he impact, thrown up as a whole block and falling back in the crater (This looks incredible, but it is however what we observe on Earth in the Ries crater, Germany, where such blocks of limestone/shales sandwiches were thrown up and fell, sometimes upside down, while still keeping a recognizable layered structure.

On the composition of the rocks themselves, after what was published, all are all basalt. Some are plain lava flows, some are tuffs (which implies some volcanic activity before or after the Gussev impact. But large impacts are also expected to form tuffs).

And on Earth tuffs usualy erode in rounded shapes (there is a wellknown example at Montserrat, near Barcelona, Spain). But what is the eroding agent on Mars? certainly not the wind, which gives unsymmetrical shapes. What is required here is an agent which acts evenly on the whole available surface, to give rounded shapes. Maybe there is some erosion from daily thermal shocks, but today nobody knows the answer.

At last some rocks really look like pebbles, and perhaps it is what they really are, but in this case the responsible water flow would be very ancient, older than Gussev.

[CosmicRocker]

What I was trying to point out was that almost all of the boulders we have seen on the plains and on the hills have been very angular. Many of the angular boulders have fairly distinctive pyramidal or triangular shapes which suggest to me that they were created by shock fracturing. But whether it was shock fracturing or not isn't all that important to me.

What seems important is that we see angular boulders almost everywhere, but in a few localized areas we have seen concentrations of rounded boulders. There have been virtually zero rounded boulders observed outside of these localized areas. I've been looking for them ever since sol 1. I don't think they were rounded in-place by the wind, because it's unlikely the wind would selectively round only the boulders in a few areas and none everywhere else. For similar reasons, I don't think spheroidal weathering is a likely culprit, either.

I really don't know the answer, but considering the potential erosion-by-water implications of rounded boulders, it seems we should really stand up and take notice when these rare specimens appear. The thing that was noteworthy in this appearance was the simultaneous appearance of an outcrop containing in-place, rounded boulders. One would have to suspect the rounded boulders are weathering out of that layer. It's texturally similar to the Voltaire/Hausman outcrop, though I think the clasts are larger.

It seems that these things were rounded in a previous existence, and later incorporated into the outcrops from which they are weathering. The interesting questions are, what rounded them in the first place and when did that rounding take place? The only "official" comment I have seen regarding rounded clasts was in Steve Squyres' first August 7th update, where he says:

"Spirit is just wrapping up work in the Voltaire region. We gave Voltaire the full treatment with all of the arm instruments at Bourgeoisie, and then we took a bunch of MI images of a rock right next door named Hausmann. Great stuff... rounded to angular pebbles embedded in a very fine-grained matrix. There's a chance this stuff is what you'd call an "impact breccia": the jumbled up debris thrown out from a high-velocity impact. But we're looking at a few other options as well. After Hausmann we moved on to Assemblee, another rock a few meters away that looked unusual to Mini-TES. We're collecting Moessbauer data on Assemblee as I write this, and we've just gotten down some very interesting APXS data and MI images. The MI images (which should be on the web pretty soon) show more small pebbles embedded in fine-grained matrix, but the APXS data look different from anything we've ever seen before. We'll be very interested in what the Moessbauer has to say about Hausmann."

What might the "few other options" be?