[X] My Assistant

[dvandorn]

OK -- does anyone have a good theory for why we seem to be finding a nice, uniform population of what appear to be blueberries within the upper layers of the drifts? And possibly shot throughout the drift material?

I would think that, whatever they are, they would be far too massive to be blown into the drifts by the Martian winds.

I have my own theory, I just want to hear what y'all think first...

-the other Doug

[Gray]

I think it's important to be sure we correctly identify the larger grains in these pictures. I'm not sure that they are "blueberries". Here'a the image from the berry bowl of long ago. berry bowl


This shows an image of berries and non-berries that was taken at about the same time.

Punaluu.

I think the larger grains that we're looking at on Purgatory are not spherical enough to be called blueberries. The distinction is an important one. If they're blueberries, it would imply that they were weathered from the evaporite layer. If they are not blueberries, then their origin is less certain. In the image (above) of the target called Punaluu, the workers at NASA considered the smaller, angular grains to be wind transported. To me that means that they are not necessarily and erosional lag deposit.

The dunes (drifts) might remain mostly stable for long periods of time with only minor transport of the very fine dust-sized grains. Then during times of massive dust storms, they might experience considerable reshaping when both fines and coarser grains are transported by much more powerful winds.

[dvandorn]

With regards to Bruce, I'm not asking why there are blueberries paving the ground -- we've already discussed the "wash" of basaltic dust.

As far as I can tell, Martian winds *cannot* transport grains the size we see in the top layer of Purgatory (which are similar in size to the blueberries and blueberrie fragments that form the flat plain "paving"). Even during global dust storms, even with the highest straight-line winds we see on Mars, the atmosphere is too thin to pick up and suspend grains of that size. At least, this can't happen according to current theory.

And yet, if you assume that what Oppy has been traveling over are simply wind-blown drifts or dunes (as they appear to be), the grains would have to have been transported by the same process that built the drifts. Occam's Razor, right?

My own theory is that some of these drifts will be found to have a relatively high large-grain content, and some will not. This is because the lighter dunes in the etched terrain are the remnants of big piles of evaporite outcrop, and that the wind erosion has reduced the evaproite to big, wind-blown piles of powder -- but has left a concentration of these harder grains (concretions or whatever) as wind-sculpted piles, left pretty much in place where there used to be large piles of evaporite. So the wind sculpted piles of larger grains in-place and *also* deposited lighter grains of evaporite dust, mixing them as the concretion piles were covered with evaproite dust. These drifts are, therefore, not primarily constructional features, but deflationary features -- they have been sculpted into drift forms, but in fact display more deflation than deposition in their processes.

In other words, it's something like the same process that we see out on the plains, where evaporite is stripped from around the concretions. Except that the current stretch of land was a lot bumpier than the plains, once, due to the piles of ejecta thrown out from the ancient crater cluster of which Terra Nova and Erebus are a part. Outctop was sculpted into drift-like forms as a deflationary process, with evaporite and basaltic dust mixed in through an associated deposition process.

The only other explanation I can think of is that, somehow, we're wrong about the grain size that can be transported by Martian winds, and that these are simply broken-up concretions (or grains made of some different material) that have been blown non-preferentially into the same drift structures made by the much, much smaller evaporite dust and basaltic dust that make up the remainder of the drifts. But if that's the case, why haven't the blueberries paving the plains been swept away? They seem to form a very solid pavement -- if the winds can transport grains of that size, why haven't they been incorporated into the drifts on the plains, or blown away entirely, as the evaporite dust has been?

-the other Doug