[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_Edward Schmitz_*]

The dunes are clearly built by the wind. Thus all the grains of the dunes are posible for the wind to move. They don't need to be picked up - they can be rolled. Since it happened, we know it is so. If it does not fit a model, then the model does not work.

We don't know that much about mars. We have a good idea of what is going on, but it is incomplete. Maybe the maximum wind speed is higher than expected. Maybe the density of the particals is less than expected.

Trust what you see. Those are wind blown driffs.

[dvandorn]

Those are exactly the conclusions to which I'm coming, Edward -- that the models we have of how such dunes and drifts are built up are lacking.

Notice that, despite the name I gave to this thread, I don't insist the granule-sized objects are hematite concretions -- the granules *look* a lot like the granules in the concretion paving we saw out on the plains, but in the MI images, the drift material looks almost exactly like the floor material of Eagle Crater. The relative abundance of granules is quite similar to what we saw at Eagle, and the grain size of the dust "matrix" in which the granules are embedded (and upon which they sit) is also similar.

Somehow, I think maybe there is a process of questions we need to have answered about how these drifts are formed and how they evolve, and instead of asking the first question in that process, this is something like question number 23... for example, what is different about this terrain that caused greater drifting than we observed out on the plains? Drifts and dunes develop based on the aerodynamics of the local terrain features, so there must have been something very different about the terrain that underlies these drifts, as compared to the flat terrain to the north.

I'm on record here, in several threads, proposing that the etched terrain was originally the complex ejecta blankets emplaced by a crater cluster, of which Terra Nova and Erebus are near the northen end. (Another large crater, probably the oldest of this cluster, lies north of Erebus and Terra Nova, a series of lighter dunes -- of which Purgatory is a member -- appearing to define the northeast arc of its rim.) I'm going to call this the Southern Cluster, for ease of identification.

We've seen examples of relatively fresh and unaltered ejecta blanket patterns on the Moon, and while conditions on Mars (especially the volatiles content of the target rock) would result in some differences, we can speculate on how the large impacts that created the Southern Cluster affected the surrounding terrain. We can also speculate on how things like the presence of standing water, subsurface water or ice when some or all of these large craters were formed might have affected the resulting terrain.

When you come up with models of the evaporite terrain, after most or all of the craters in the Southern Cluster were formed and after water and ice alteration processes have basically ended, *then* you can start to ask how aeolian processes can be modeled that will result in the terrain as it appears today.

So, for example, if it *is* true that the series of lighter-colored, fish-scale shaped drift forms, where Oppy got stuck, define the northeast rim of an ancient crater, then it would make sense that the formation of these drifts was, at least to an extent, controlled by the underlying rim crest. You could argue that the more jumbled the terrain, the higher the drifts built up, and over millions of years the drifts have been cyclically deflated and reformed, each time allowing the underlying jumble of evaporite and basaltic rock that made up the ejecta blankets of the Southern Cluster craters to be eroded down some more. Each cycle of deflation and deposition would then be enriched by the primary erosion of the original evaporite and ejecta-altered terrain.

But we have a long way to go before we can believably model the original evaporite terrain, much less speculate on the ages of the components of the Southern Cluster and how their ejecta was originally emplaced. Or what its constituents were. And maybe we need to answer that question number one before we can successfully develop a model that explains what we're seeing.

-the other Doug