might as well guess

I've laid out the various things that I think could explain the spherical pebbles. Now I'm going to try to make some guesses to put it all together into a semi-coherent picture. I'm 99% likely to be completely wrong but guessing is fun and I suspect we'll know (more of) the truth in the morning when the experts come out to brief the press.

First, load the mini-TES mineral map because that's my big picture reference here. Here's what I think (wild ass speculation):

We're dealing with three primary materials here. The first is what's thought to be coarse gray crystalline hematite and it's greatest concentration is shown in the mineral map as the brightest red. The second material is the bedrock outcropping and I'm guessing that is basalt. The third material is the small spheroid pebbles scattered across the crater floor, walls, and embedded in the bedrock outcropping. I'm guessing those are amygdules, maybe calcite, or quartz (isn't this a great match with this?).

So, how does that explain the colors on the mineral map? Well, the red is densest concentration of the iron-bearing mineral an the blue is the least dense. The bedrock appears least red and I I'm guessing that would match up with it being a high-silica, low-iron basalt.

The bounce marks from the airbags in the crater floor are also void of iron-rich material while just next to them, there is moderate levels of the iron-bearing material. I suspect that this means there is a layer of hematite pebbles that partially cover the crater floor and those have been pressed underneath the fine-grained, iron-free sand (probably the eroded remnants of basaltic bedrock).

But why then is the crater floor not bright red like the areas above and below the bedrock and outside of the crater? I suspect that is the result of the crater floor having some of all three materials, the basalt (sand), the calcite or quartz amygdules, and the hematite. The amygdules and the hematite are in large enough pebbles to be pushed underneath the fine-grained basaltic sand.

So, that's my guess.

My second favorite explanation is that the spheroids aren't amygdules at all, but rather they are accretion lapilli and the bedrock is a tuff and not a basalt. (coimpare with these wonderful lapilli sitting on tuff.) If it was a tuff that consisted of several layers of ash, that would help to explain what looks like stratification in that bedrock.

My least favored explanations are oolites and spherulites. I think oolites would be a lot more dense-packed than what we're seeing here and I think that spherulites would have formed in a much more glassy igneous rock than the rough stratified-looking bedrock we see in the Pancam and MI pictures.

I'm hopeful that we'll get some more data or, even better, some good science evaluation tomorrow at the morning JPL press brief. I expect to be as wrong in my predictions as is possible ;-)

What's your guess?

Posted by asa at February 8, 2004 08:13 PM