[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

[tty]

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

Umm.. no, the soil doesn't look in the least like bottom moraine and there is nary a trace of glacial landforms. Of course glaciation on Mars would almost certainly be cold-based, i. e. frozen to the ground, and in such cases there may be very little trace of glaciation, but then it wouldn't have eroded the evaporite either.

I think the most likely explanation is that the "blueberries" are indeed a sort of desert pavement or gibber plain, Admittedly the layer is thicker than any desert pavement I've seen on Earth, but then we don't have any really old deserts here.

tty

[helvick]

I am not a geologist so I may be way off the mark here but what I see in the blueberry pavement is similar to what I see regularly on stony beach areas. In relatively sheltered sections, where severe wave pummelling is rare, the rocks, stones, pebbles and sand tend to settle into layers that are pretty stable over long time periods with the finest particles lowest down and the largest at the upper layer. This is a fairly stable configuration even in the presence of a lot of tidal sand and dust. The physicalland time scales are vastly different but the very low density fluid involved on Mars (C02 at 1% of Earths atmosphere) should account for most of that.

Basically you start off with the evaporite rock with embedded blueberries and as it erodes the stratigraphy tends towards a compacted lower layer that is mostly dust with a surface layer that is predominantly the much harder blueberries. Even a minimal amount of wind driven motion of any blueberries on the surface would tend to reduce any other softer material to dust fairly rapidly (although that might be millions of years) while erosion of the blueberries themselves is probably a couple of orders of magnitude slower so enugh would survive more or less intact to keep the process running. The resulting evaporite sand and dust might blow away but most probably ends up slowly filtering down through the blueberry layer and probably gets protected by the pavement layer.

There are certainly variations on this - the very flat initial plane around Eagle seems to follow the above pattern, the slightly more rugged and sloped area Oppy is now has typical dune drift patterns that seem to be almost entirely very fine sand and dust. That would be compatible with the above theory - the blueberries would tend to roll down slope over time and the dust and sand on the sloped surface would then be more exposed and subject to dune formation.

Even if this isn't a valid explanation Doug's comment about the ground truth is spot on - the dark zones have so far corresponded with blueberry pavement - follow the blueberry road while you can.

[dvandorn]

That brings up a question -- just what is the resolution of the TES on board MGS, anyway? Or is it even still working? My thought is that if we can get high-resolution TES readings of the site from orbit, we can follow the hematite signature (and thereby follow the blueberries) to find safe routes.

Somehow, I doubt the TES resolution is fine enough to use it for that purpose, though.

-the other Doug