Bye Bye Blueberries Options

[Burmese]

According to Steve Squyres' latest journal entry, Opportunity is now seeing virtually -no- blueberries in the surface matrix anymore.

"...On top of that, Opportunity has stumbled onto something really new and different lately... the blueberries seem to be gone!..."

And a theory:

"But one possible guess at this point is that we have moved "up section" in geologic terms -- to rocks that are higher up in this stack of layered sediments -- and that the rocks at this level never experienced the concretion-forming process. It's an interesting hypothesis with interesting implications, and it's also one that could explain a few other odd things we've been thinking about ever since Eagle crater"

Looks like Oppy will stop for some intense IDD work in the next day or so.

[CosmicRocker]

Organic matter certainly could create locally reducing conditions that would mobilize iron ions. But I'm not certain that organic matter would be necessary to create hematite concretions. I don't know what the oxidation state of the Meridiani sediments was at the time they formed, nor when the concretions formed, and that is critical to what happens later. Iron ions are relatively soluble in water when they are in the chemically reduced Fe+2 state, and pretty much insoluble when in the oxidized Fe+3 state.

Oxidation-reduction (redox) reactions are responsible for much of the secondary, iron mineral redistribution on earth. I've seen many examples of hematite concretions that formed around crystals of iron sulfide or other reduced iron minerals. They are often rounded but irregular in shape; but there are examples of very round and regular ones also. In these cases I think the concretions are formed by the diffusion of oxygen into the reduced environment around the sulfides, eventually converting them to iron oxides.

As I understand it, the early atmosphere on earth was chemically reducing, and the very early oceans were loaded with soluble, reduced, iron ions (Fe+2). Even after life and photosynthesis began, it took a long time before oxygen accumulated in the atmosphere, because initially most of the photosynthetic oxygen produced was consumed in the oceans, oxidizing the Fe+2 ions and precipitating vast quantities of Fe+3 as Fe2O3 (hematite), thus forming the amazing banded iron formations of the Precambrian period. I'm sure the actual reaction mechanisms might have been a bit more complicated, perhaps with a goethite or other precursor mineral. The main point is that the young earth had a lot of soluble, reduced iron available. So may have Mars, and that may have resulted in the precipitation of reduced iron species in the sediments, especially if the conditions were arid, or there was a lot of sulfur around.

If the early conditions on Mars were reducing, as they were on the early earth, there may have already been reduced iron minerals in the young Meridani sediments. They could have been oxidized later as the Martian atmosphere slowly became more oxidizing. It seems that such a scenario would eliminate the need for local concentrations of organic matter to reduce insoluble Fe+3 in order to mobilize the iron for diffusion. It seems that we only need a relatively homogenous distribution of small, iron mineral crystals in the sediments under reducing contions as a starting point. Then slowly and uniformly change those conditions to a moderately oxidizing state to create those lovely and uniform concretions known affectionately as blueberries.

Sorry about the rant folks, but Bruce started it. The diverse manifestations of redox geochemistry have fascinated me for much of my life. I worked on iron formations as a grad student, and my first real job was exploring for roll-front uranium deposits. Later, I studied huge redox interfaces that draped oil and gas deposits. It's a fascinating subject, and it gets more fascinating when you throw in the unknown or poorly known conditions during the early history of a planet. This stuff is really fun!