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

One possible issue with this is the lack of glacial striations. These are 'lines' that appear in the rock as the glacier moves. They are usually made by harder rocks that the glaciar has picked up and then deposits and literally uses as a 'knife' and etches the rock. Up in the Pacific Northwest you can see these all over the place, especially on the islands in Puget Sound. I would expect rocks to be 'etched' all over the place from blue berries being ripped across them, or even larger rocks. Of course this could have been eroded away by the wind now. Some more things to consider though are:

1) Rock 'strength' seems to be relatively consistent here, that would mean that the rocks to do this etching (if not being the blueberries) would have to have been transported from considerable distance (not impossible, some rocks in Antartica have been moved hundreds if not thousands of miles before they are deposited)

2) The possible impact of lower gravity on the ability of the glaciar to generate sufficient pressure to actually do anything. This would also lower the ability of the glacier to erode things in the first place. Does anyone know if water would freeze any differently in lower gravity?

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

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.

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

Doug,

I think the TES on MGS is being kept off for the moment, see

MGS Status Reports and drill down a few pages to the Feb 16th report. They turned the TES spectrometer on to estimate how much life was left in it and decided to keep it off for now. TES has a resolution of 3km at the surface so it's no way useful for this sort of navigation I suspect.

The MiniTes could be used but progress would be fairly slow I assume as Oppy would have to set up, take a MiniTES scan of a number of potential routes, send it back and wait for analysis and directions from earth.

THEMIS on Odyssey doesn't look (to me) as if it can identify hematite over the other oxides and salts, it's still operational though AFAIK, resolution is 100m which might be too low. Worth asking all the same.

CRISM on MRO would do the trick (18 m resolution) I think but its science mission isn't going to start until November 2006 so it isn't going to be able to lend a hand unless Oppy lives past Sol 1000 (November 16 2006 if I calculate correctly).

OMEGA on Mars Express is good for 100m surface resolution and can identify hematite if I recall correctly. That's just a bit to low, just like THEMIS but what the heck it'd be worth a shot.

My take has always been that the blueberries make a desert pavement, and they originated in the evaporite unit, which eroded by aeolian processes and was transported away by the wind. Glaciers may be responsible, but we don't see glacial landforms, although the anatolia lineations might be looked at as striations.

I see aeons of wind.

--Bill

The virtually universally accepted idea is that -- ever since the sheet of soft evaporite rock with Blueberries embedded in it was re-exposed just a few tens of millions of years ago, after being buried for billions of years -- a thin sheet of dry basalt sand from elsewhere has been gradually blown completely across its upper surface from one end of the formation to the other, with the basalt sand grains griding the soft evaporite matrix to powder, to leave behind a residue of hard Blueberries mixed with the basalt sand.

This would lead to mechanical self-regulating feedback mechanisms that have probably made the surface apron of basalt sand very thin for almost the entire extent of the huge formation, covering a very flat and even remaining evaporite surface. (After all, if the sand builds up to a higher thickness anywhere, the lower layers won't be moved along as rapidly by the wind -- and so the sand's abrasive action will slack off there until the higher buildup of sand above has been blown away again.) A remarkable place, but entirely understandable in terms of Martian geology.

That's right, Ed, I keep forgetting about the windblown basalt sand, which is a prime player in this drama. Without it, there would be no abrasive to sandblast the evaporite. The source is likely to the northwest and it eventually moves to the southeast (based on prevaling winds, suggested by the "downwind tails" behind craters). I guess that it's time to hit the MOC image archive and see where the source and sink of the basalt sand is. There appears to be a lot of it around, typically visible as crescent-shaped or barchan dunes.

--Bill