But the layers are not dusty, but sandy, as I explained in my reply.

--HDP Don

MarsIsImportant - Regarding volcanic heat, one of the reasons the MER team rejected volcanism was that there were no signs whatever of any volcanic phenomena anywhere in the vicinity. No volcanism = no volcanic heat for melting. Also, unlike on Earth, volcanism on Mars does not ever appeared to have moved around, owing to the lack of plate tectonics. The main reason why the volcanoes are so big is that volcanism has always stayed in the same place.

Regarding crystalline clays - they seem to occur only in the very oldest, most heavily cratered areas of Mars, and are rare there. They probably have nothing to do with Meridiani itself, which appears to be much younger. They could reflect water melting or condensation that occurred at the height of the late heavy bombardment, or volcanic heating, or a different climate back then. No one knows.

I'm not trying to convince you of anything about Meridiani - just trying to open your eyes to all the possibilities, so that you can make up your own mind, based on the evidence as you understand it. And the berries to me are still pretty round near Victoria, although some of them may have been eroded a bit by the wind (everything else was, that's for sure). They also could have been affected by the Victoria-forming impact itself, during the excavation process. Thanks for the discussion.

--HDP Don

Thanks for answering that far better than I did.

--HDP Don

As some of you know, I don't have nearly the time to devote to thoughtful posts as I used to...but be that as it may, I see in my review of the recent flurry of posts that my name has been called upon for some sedimentological input:

- single grain (or a few grain) layer lamination: the most common occurrences of these in the terrestrial stratigraphic record are due to three processes: 1) Stokes settling, 2) upper stage/critical flow regime streaming, and 3) ballistic/translatent ripple migration. 1) and 2) are subaqueous processes and 3) is aeolian. Stokes settling is simple gravitational fall-out of material suspended in a fluid, with particles sorted by their velocity (generally grain size and density, although drag and grain Reynolds number also are important with non-spherical grains). The results are things like varves, laminated lake clays, and deep-sea oozes, with some amount of grading apparent. Upper stage/critical flow regime streaming occurs when the shear stress at the bed is such that bedforms such as ripples and dunes can not form, but is not exceeding a point at which antidunes or chutes and pools form. A plane, flat "traction carpet" of grains forms, and as velocity wanes, a plane, flat lamination forms before dunes and ripples. There may be low relief (1-2 grains thick) stripes that form down-current due to rollers and stretched-out vortices near the boundary layer. These are expressed as parting lineation in indurated sandstones. These features are common in river, tide, and beach (swashface/foreshore) sandstones, the Tb part of Bouma successions (turbidites), and pyroclastic flows and base-surge deposits. The migration of ballistic or translatent ripples in aeolian regimes produces pinstripe lamination, with alternating finer and coarser grained layers only a grain or two thick. Foreset laminae are generally not present in these types of ripples, and cross lamination is rare in most aeolian deposits. Cross-bedding, on the other hand, is almost always present.

- formation of martian sandstones: siliciclastic sand, of course, forms from breaking up big rocks into smaller ones. Because of the upper size limit of sand (2 mm), most grains in sandstones are either mono-mineralic or are made up of 2 or 3 crystals of different minerals still stuck together by other means (igneous or metamorphic crystallization or sedimentary cementation). Chemical and physical weathering processes on earth can be quite efficient in breaking rocks down to sand and silt size, and the abundance of sandstones and siltstones is related to the distribution of crystal/grain sizes in the parent material. Induration can result from geochemical processes related to the original constituents (pressure solution, dissolution/reprecipitiation, etc.) or post-depositional additions (cement, neomorphic clays, etc.). I guess for me, the larger questions of sandstone formation on Mars are the mechanisms under which sorting to sand-sized material (to a deposit that could be preserved as a sandstone) takes place, and how deep burial (sedimentary basin formation) can occur without an obvious plate tectonic engine driving crustal deformation.

- scour surfaces in aeolian deposits (or their absence): scour surfaces are ubiquitous in aeolian deposits. Stokes surfaces, or supersurfaces, form by regional deflation to a water table. Brookfield surfaces form a hierarchy of bounding migration surfaces of dunes, draas, and larger bedform families. Changes in wind speed, direction, or sand supply can result in regional deflation, and local blow-outs form down-wind of and between dunes at zones of air flow reattachment or impingement.

Don Burt's catastrophist/non-uniformitarian ideas have also made me start to think again about the dynamic nature of Mars climate and paleoclimate. I keep having this nagging suspicion that we have yet to completely comprehend the full impact of Mars precession and obliquity cycles and how they control global volatile distribution (and state). This, plus our generally impact-poor view of surficial processes, makes us ill-prepared to, as Bill Shakespeare once said "figure the nature of the times deceased" on Mars.

The sand on Mars is like fine dust on Earth. Looking with the Micro-Imager, the dust looks sandy. The really fine dust in the high atmosphere is not what I was necessarily referring to. You can see the tracks of the rover dramatically change in appearance during the Dust storm because the wind speed dramatically increases. This allows dust or sand as you say to move and be deposited elsewhere. But really, I should not have to explain this to you. Sometimes I can be the Master of the obvious.:-)

http://qt.exploratorium.edu/mars/opportuni...22P2953M2M1.JPG

http://qt.exploratorium.edu/mars/opportuni...02P2956M2M1.JPG

You are correct. It looks sandy to me too. It looks like its made out of the same stuff. Sorry about that. I sometimes get confused when talking about the dust or rather sand on Mars.

I agree that the hard limit in size at 5-6 mm is a key observation. Also, it has been observed and discussed on another forum that the size distribution of the berries in or near the crater outcrops, or the berries which are protected from the environment in the craters, exhibit a tail towards small diameters. No Earth analogy of this feature has been proposed. This property is not clearly seen for the berries of the plains, which are generally of smaller diameters, and may have suffered weathering.
Although the implications of this asymmetry is not clear to many of us, I do think that it is not incompatible with a condensation scenario in an impact surge cloud.
Distinctive morphological details have also been discussed : dimps, seams etc.. To me an account of these features in a surge hypothesis seems much more problematic.

denis

Welcome, Denis! Agreed, and esp. since the anomalous small-scale features you cite seem to be much more prevalent on berries that are embedded in sedimentary strata or under the soil (recall those seen in the trenches dug by Oppy early in the mission). If they are condensates rather than accretions, then I am at a loss to explain their symmetry, unless they somehow formed VERY rapidly while airborne or suspended in solution; haven't seen any 'pristine' berries with a flat side, for example.

"Other Don - That's all well and good, but many things are theoretically possible (including little green men, perhaps). Just show me the evidence, please. More to the point, are there any features at Meridiani (or Gusev, or any of those many other salty layered deposits all around the Mars highlands) that CAN'T be accounted for by the impact surge hypothesis?"


HDP Burt

I agree, green men are possible right up there with impact surge- the structure of your debate on impact surge seems to include any and all features can be linked to a surge. If there was a kitchen sink observed in an outcrop I'm sure there is an explanation for it with impact surge. Do salts in a nonequallibrium condition at endurance point directly to impact surge? Of course not. Do hematite spheres? No. What is the evidence for impact surge, just show us the evidence, please! Difficulties or simply holes in the MER theory are not allowed.

"the other Don"

tdemko, Thanks for your reply 154. It is good to get confirmation that single-particle layers do occur in turbidites and base-surge deposits. You seem to have included base-surge among sub-aqueous processes. Do you think of base surge as a sub-aqueous process? I have read some descriptions that make much of the presence of liquid water in creating the surge layers and other sources that do not mention liquid water at all. I resumed my net search on this topic in the last few days and found a 30 year old Wolletz and Sheridan paper that includes a few lines that spell out how the planar beds were thought to form in base-surge:

“Still further from the vent the cloud deflates to the point where inertial flow dominates and void space is generally less than 60%. Here grain collisions set up shear plains along which grains of different sizes seek their respective zones of lowest shear energy and inversely graded planar beds result.”

That reference to shearing layers has been very hard for me to find and confirms the explanation that I have been using. Unlike many layer-forming processes this one involves the simultaneous presence of a stack of unfinished layers all in motion together and interacting. Herr Doctor Burt seems a little bored with these details, but I think that they may be important in changing minds, especially in explaining impact-surge to non-scientists. Surge isn’t a magic cloud that makes layers but it does make layers in a way that is very unfamiliar to most people, that they can't hope to visualize without some help.

Here is the address of that paper. I hope it works. If not, sorry about my bad net skills on a new site and new browser.

http://articles.adsabs.harvard.edu/cgi-bin...p;filetype=.pdf

This statement, if it is a currently accepted postulate for how planar beds are formed during basal surges, is quite well-suited for testing in re the Meridiani deposits.

It predicts that, for each surge event, you should see inversely graded planar beds. In other words, the largest grains should make up the first beds deposited, followed by progressively smaller grains until the uppermost layers are made up of the finest grains. (At least, that's my reading of what "inversely graded" means.)

Has Oppy found such an inverse grading relationship between the beds in the units it has been able to study? Since the theory above uses as a basic theorem that the layering is entirely due to different grain sizes sorting out, there ought to be an observable change in grain sizes from layer to layer. Even if this delta is so small as to be difficult to measure from one layer to another, it ought to have been enough over the depth of the unit observed within Endurance to be able to be quantifiably observed.

So -- here's a direct test of your theory, Herr Doktor. It's even a test on which you can do a little non-rigorous work using the raw JPG images from back during the Endurance campaign (or more rigorous work using the images already released to the PDS). I have to say, I truly think that if anything of this sort had been observed at the time, we would have heard about it... but I'm prepared to be proven wrong on that if you want to try and pursue this proof.

-the other Doug

"Normal" grading is coarsest grain size at the base, finest at the top. Inverse grading is finest grain size at the base, coarsest at the top. Normal grading approximates what you might see from simple gravitational (Stokes) settling.

Aeolian pinstripe lamina are inversely graded due to winnowing of wind ripple crests...fines are trapped in the troughs and the coarser crests bury this material during migration. Debris and turbidity flows that accelerate rather than decelerate during deposition would also be inversely graded.

I threw in the comment about lamination in base-surge deposits as an afterthought. I think that pyroclastic and base-surge flows act differently from both subaqueous and subaerial sediment transport processes. The evolution of hot gases in the flow during transport and deposition make characterizing the hydro/aerodynamics complex and unpredictable, to say the least.

Nprev (and Denis) - Been taking most of the weekend off from Mars, but have a few minutes now. Just a couple of comments: 1) There are no really comparable deposits on Earth (only volcanic surges, which are real wimps by comparison, and form under vastly different conditions. 2) I suspect that some of the features you describe formed as post-depositional coatings, affected by local bedding - I would only pay attention to the appearance of spherules after any salty coatings have been brushed off by the Rover. Even then, a symmetry (of say a belted waist) is hardly inconsistent with accretion in a cloud. 3) Unlike some people whom I won't name, we don't pretend to have all the answers.

--HDP Don

I disagree with your general philosophy - there were so many inconsistencies and internal contradictions in the MER team theory that we felt compelled to come up an alternative - and this is the best we could do, given our collective backgrounds. Perhaps it's wrong - we almost certainly have some details wrong. I'm open to any and all suggestions for improvement. As for the evidence, it is all around - Mars is drowning in impact craters, of all ages and sizes, hitting any imaginable target composition. It's also drowning in layered sulfate-rich sediments. Both rovers have found almost identical-appearing features in such sediments (including spherules), currently ascribed to completely different causes. If you wish to ignore the hydrosphere (mainly a cryosphere, near the surface) and atmosphere of Mars, and think that impacts on Mars are identical in their affects to those on the Moon (ballistic ejecta only) feel free. Rampart craters, unique to Mars, suggest you would be wrong. If you think that Mars cratering never produced fines, other than a bit of dust and sand on the surface, and that these never were deposited into rocks, other than those deposited by a secondary agent (water or wind or volcanic reworking- never impact surge) feel free. Occam's razor (the simplest explanation consistent with ALL the data, in Einstein's formulation) favors only impact surge, in my admittedly biased opinion. If you think it favors the MER team explanation, or if you reject it as a general philosophy, please explain why.

That's all for now - gotto go eat.

--HDP Don

PS (just ate): Surge is perfectly capable of eroding, transporting, and depositing a kitchen sink (which is more than I can say for the wind); putting it on Mars in the first place might present be a slight problem, though. Outflow channels - no problem moving and depositing it either.

Other Doug - I'm glad Tim Demko has joined this discussion - I hope he feels free to contradict what I say. My impression from when I first learned about inverse grading in volcanic surge deposits nearly 30 years ago was that it was observable mainly in near-vent, coarse-grained deposits consisting of blocks and lapilli of many cm to many mm. In far-out deposits consisting primarily of well-sorted, sand-sized grains you would not expect to see it. No inverse grading has been observed or described from the sandy surge deposits at Home Plate, for example, and yet the MER team is calling them volcanic surge deposits (owing to the presence of a single possible "bomb sag" - there is no volcano apparent, no tear-drop-shaped lava "bombs" have been seen on the surface, and the basaltic bulk composition - and occasional ballistic ejecta sags - are to be expected for practically any impact surge deposit on Mars). So are you suggesting that they are mistaken too? If not, please explain your logic. Thanks for the good ideas, BTW.

--HDP Don

How do you explain the near pure silica deposits found by Spirit? How would that fit in with impact surge?