[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

[Guest_Richard Trigaux_*]

To be clear, it seems that we have two kinds of dunes, with two materials:

-A material which is a mixture of blueberries and fine basaltic dust. This material forms a constant layer on the ground at Meridiani, and is more or less undulated on places.

-A material with only fine basaltic dust, forming all the dunes visible at Gussev (Spirit trenched in one at the rim of Bonneville) and sometimes at Meridiany (somme examples in Endurance, some hollow fillings near Eagle. Perhaps the dunes in the bottom of Endurance were of this family.


We understand well how fine sand dunes could evolve (although there are no such dunes on Earth) but not how the mixture dunes formed and evolved. Also the flat layer of mixture is involved in the erosion process which gives Meridiani its very flat look. This erosion process was very visible around Endurance, where random shaped sulphate rocks of the ejecta where like cut horizontally, to the point of forming something looking like an artificial pavement.


On Earth we observe that the grain size is relatively constant, whatever the region the dunes formed, wind speed etc. So, the dune grains size would be a function of atmospheric pressure rather than of wind speed, and whatever the process (flying grains, saltation or rolling grains). In this case, no astonishing to find only these fine dust dunes on Mars, from the lower air pressure.

The problem is the bimodal repartition of the grains (fine dust + blueberries). If there was wind conditions able to move blueberries, they would make dunes with only that grain size, and blow away the fine dust. (When there is wind in Sahara, only large grains are retained into the dunes, the dust is thrown away so far than Europe where we find yellow dust on our car windshields).

Could make a synthesis of this?
It is clear that there is an erosion process flattening the sulphate rock. This erosion makes a layer of free blueberries. Fine basaltic dust would travel all around Mars, forming ordinary dunes like at Gussev. But in Meridiani, this fine dust would be entangled in some way with the blueberries, forming harder dunes which would not really travel.

And the problem with Oppy's sand trap, would be that she went on a ground where this process was still acting, but with a lesser blueberries content.