Blueberries In The Drifts Options

[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_BruceMoomaw_*]

I don't think there's any mystery at all. The assumption almost from the start has been that a sheet of dry basalt sand is being gradually blown from somewhere else across the entire top of the soft layered sulfate rock stratum -- across hundreds of km of it, and off the other side -- ever since the layered rock was re-exposed to the surface a few tens of millions of years ago, and that the sand is gradually grinding the soft sulfate matrix to powder, leaving behind the harder Blueberries embedded in it. Thus we have a flat upper surface to the sulfate rock, covered by a thin apron of mixed basalt sand and loose Blueberries which is only a fraction of a meter deep almost everywhere.

[Guest_Richard Trigaux_*]

I think this thread is about the Microscopic Imager from sol 505

Thus we have a flat upper surface to the sulfate rock, covered by a thin apron of mixed basalt sand and loose Blueberries which is only a fraction of a meter deep almost everywhere.

Yes, but the problem is, if the wind is unable to carry the blueberries, they should form a layer at the bottom of the sand layer, and the dunes should be formed only of thin basalt sand without blueberries. Right on the countrary it seems (if this photo was taken on a dune) that there are blueberries even at the top of the dunes. There is something we do not understand, perhaps the dunes formed at an epoch where there was more atmosphere to carry the berries.


Or the coarse grains we see here are not berries, but "cinder hail", coalesced dust grains, that are observed on Earth into certain volcanic ash falls. The wind may roll them but not make them fly.

[RNeuhaus]

I agree with BruceMoomaw's Theory.

This zone is flat, and has great uniformity of height of sand (not much difference as I see on the Earth) and the shapes of dunes are smooth. Martian's winds aren't strong (160 less density than ones of Earth's) enough to fly Blueberries except to roll it and the wind can only fly to very fine sand and needs millions of years to build many small dunes" I see that the wind direction from that zone is constant and the crest are build up prependicularly to the wind direction. This means that the crest of surface were not formed by the water current. Then I starting to think that here there were a very calm or not water current.

As I recall, that zone has found many plates under many meters below of the surface as ones that was found from the Endurance Crater as an example. So I suppose that under the dunes there is a flat sulfate rock.

Rodolfo

[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

[edstrick]

"Granules", which are 2 to 4 mm rock particles (coarser than sand) cannot normally be wind transported on Earty, any more than on Mars. But they can be transported by creep, where saltating sand grains bombard them and move them a fraction of a millimeter with each impact. They can form desert pavements on Earth, and apparently are readily sorted from coarser and finer particles on Mars. We see granule pavements on many drift surfaces at Gusev as well as Meridiani. It's largely concidence that at meridiani, the blueberries are in this granule size range.

[Burmese]

OK, there are blueberries (or blueberry-sized objects) all through the upper segments of the dunes. If the dunes were 'built-up' by the wind then the blueberries had to be placed there somehow. The alternative is to say that they are at the level they have always been, the softer sulfate rock was scoured away from around them, and that the crest of the dunes represents what was once the top of the solid strata. That option doesn't seem at all feasible. We are left with the idea that something pushed the bueberries uphill at some point in time (or outright picked them up and tossed them around). Was it the wind or some other mechanism? If it waws the wind, then we need to modify our undestanding of what we think it is capable on Mars.

[Bill Harris]

Originally I thought that the blueberries were a lag deposit, a desert pavement eroded out of the evaporite. That still seems likely, but there are so many quirks about the blueberries that I think the truth is stranger than fiction. We'll keep looking and speculating...

Since Oppy was immobile for a period of time, the MI and Pancam should have taken a lot of images of the same area(s) over time to determine precisely what changes took place.

Over in "The Other Mars Forum", Horton (Marvin Lipford) has images of a blueberry moving a couple of inches. I disremember where the message is, but wade thru and look...

--Bill

[glennwsmith]

Dvandorn has come up with a VERY good question about why the blueberries (if that's what they are) seem to be sprinkled evenly across the dunes -- almost like nuts on an ice cream cone -- and without shoaling anywhere. And I've read all the responses with interest . Good stuff!

[Bill Harris]

The Dunes of Mars will prove to be a course of study unto themselves. The Blueberry Dunes of Meridiani are fascinating, but some of the most beautiful dunes are the basalt sand dunes of Syrtis Major, near Nili and Meroe Patera. They are black barchan dunes "marching" across a bare rock plain.

--Bill

[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

[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.