Thursday, August 27, 2015

A little more Mars....

There's an interesting website here, discussing the sedimentary rocks of Mars.

Here's an interesting couple of paragraphs from it:-


The first and arguably most important difference between Mars' sedimentary rocks as seen by Curiosity and sedimentary rocks investigated by geologists on Earth is their age. The rocks Curiosity is studying are more than 3.5 billion years old, possibly quite a bit older. When Earth was that old, it was a different place. It's very likely that Earth's rocks were predominantly basaltic then, too. Sedimentary rocks are made out of other source rocks. If all you have is basaltic rocks to work with, your sedimentary rocks are going to look more like basaltic rocks. There was an entertaining presentation by Ezat Heydari that showed that you can pretty much explain the entire diversity of rock compositions seen by Curiosity as a simple mix of two endmembers: chunks of broken-up basalt, and a finer-grained material that looks like the ash you'd get from explosive eruptions of very iron- and magnesium-rich komatiites. Nowadays, Earth makes its sedimentary rocks from all kinds of source rocks, most often from much more silicon- and aluminum-rich rocks that have experienced processing through Earth's active rock cycle multiple times. Mars hasn't had that luxury. In a sense, Mars is giving us a chance to look at what sedimentary rocks on Earth might once have looked like, 3.5 or more billion years ago.
But there's still the pesky problem of having rocks made of very fine-grained sediments that, ought not to have a basaltic composition, because the very water that is required to transport, deposit, and lithify sediments into rocks should've also attacked the basaltic minerals and turned them into something else -- a fact that Allan Treiman emphasized in his presentation on Curiosity CheMin results. That's telling us something important, too. For one thing, Allan said, it suggests that the way that Mars turned basaltic lava rock into sediments was not through the action of water -- that Mars broke basalt into basaltic grains through mechanical rather than chemical weathering. And then whatever water-related experience moved those sediments from their point of origin and deposited them into layers in Gale must have happened quite fast, with the water going away very quickly.
"...a simple mix of two end-members: chunks of broken-up basalt, and a finer-grained material that looks like the ash you'd get from explosive eruptions of very iron-and-magnesium-rich komatiites." - Core material, perhaps? As might result from a collision with something that penetrated the molten core?

And in the second paragraph, about how these were mixed:- "that Mars broke basalt into basaltic grains through mechanical rather than chemical weathering." As in a violently explosive impact, perhaps?

Moving away from rocks for a moment, Mars' atmosphere is mostly carbon dioxide with lesser amounts of carbon monoxide. Both of those might easily be the result of combustion, perhaps on a massive scale.

The picture I'm getting is not one of a peacefully-evolving planet, but rather one with a violent but distant past. One in which oceans that lasted long enough to form all those sedimentary rocks seemed to suddenly disappear. That raises more questions than answers.
 

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