[X] My Assistant

[dvandorn]

We've all been assuming that the evaporite that once contained the blueberries that now pave the surface throughout most of the plains units was eroded away by winds. And that makes sense, winds have been the primary erosional process on Mars for millions and millions of years.

However -- I'm still mystified as to where all the evaporite dust went, and how winds could erode away more than the very top layer of evaporite, leaving an extremely thin layer of blueberries. That's not what we see -- the plains seem to be mantled in a fairly thick (at least 10 to 20 cm) layer of blueberries and blueberry dust (with an admixture of the Martian dust that gets transported globally by the frequent dust storms).

How can wind erode evaporite that's covered by a thin layer of blueberries?

Maybe it wasn't wind.

These plains appear to have been inundated by water cyclically, according to the evaporite record we saw in Endurance. And the chemical content of the water that evaporated, forming the evaporite layers, changed significantly from one flooding event to the next.

Perhaps the final series of floodings were composed of water of a chemical nature that it eroded the uppermost third-to-half meter of the previously deposited evaporite, and then drained or evaporated at a level below the thus-exposed layer of blueberries? If you think about water dissolving the soft, salty evaporite rock but leaving the blueberry concretions that had formed within it undissolved, you get something that leads to the conditions we see today -- a layer of "loose" blueberries sitting on top of many layers of evaporite.

The only problem with this theory is that it would have been the only time in the long history of flooding and evaporation that a new flooding would have liberated a layer of loose blueberries. We don't see any layers of blueberries/blueberry-derived soils between any of the layers of evaporite. So either 1) a final flooding that was capable of dissolving blueberries out of the top layers of evaporite would have to have featured a *very* different set of chemical properties than all of the previous floodings (or maybe was a glaciation and not a flooding?), or 2) all of the floodings dissolved prior evaporite layers, but the blueberries didn't concrete within the evaporite layers until quite late in the process, just before the final flooding events. Either way, the water-erosion theory requires some major change in conditions between a large number of early flooding events and the final flooding event(s) prior to Mars' Great Freeze and Dry-Out.

I hate theories that require you to plug somewhere into the equation a clause that semantically resembles the phrase "...and then a miracle occurs..." And that's sort of what that last little glitch in my theory feels like... *sigh*...

I dunno, what do y'all think of a glaciation as opposed to a late flooding to explain the blueberry paving? That could have mechanically torn the soft evaporite into dust, leaving the harder, more-resistant blueberries to pave the resulting plains after the glaciers retreated? Which do you think might be more likely?

-the other Doug

[Bill Harris]

That's right, Ed, I keep forgetting about the windblown basalt sand, which is a prime player in this drama. Without it, there would be no abrasive to sandblast the evaporite. The source is likely to the northwest and it eventually moves to the southeast (based on prevaling winds, suggested by the "downwind tails" behind craters). I guess that it's time to hit the MOC image archive and see where the source and sink of the basalt sand is. There appears to be a lot of it around, typically visible as crescent-shaped or barchan dunes.

--Bill