[X] My Assistant

[helvick]

Fascinating stuff.

I'm a complete amateur in terms of the geology side of things so bear with me if I'm asking really stupid questions.

First I have a question for everyone about the MER team's hypothesis.
Can anyone explain where the water comes from and how it stays around? I could find the layering we see geneuinely plausible if this was a wide area fairly calm "shallow" sea with some depth of water acting as a fluid buffer to help create the relatively fine layering. My understanding is that the sea idea has lost favour and the hypothesis is now talking about periodic pools and mostly subsurface water. Does that mean that the hypothesis now describes a predominantly dry "dust"deposition process with water mostly seeping up from the subsurface and occassionally pooling. I'm still at a loss as to where the water actually comes from - where's the other side of the water cycle? The evaporation \ clouds \ rain bit? Or am I missing something?

Moving back to the impact surge hypothesis I have no problem visualising thin, uniform, laminar deposition of layers from impact surges - lots of distant (large) impacts should average out with wide ranging thin deposition at their edges which might be thousands of km from the impact - however I cannot see how accretionary bodies formed a la hailstones can fall out of the same sort of "distant edge of the surge" fluid and I really have a problem with the lack of any obvious small scale deformation of the layers caused by the impact of these things as they land.

To go somewhat further afield and across to Spirit I have been very surprised by what appears to me to be remarkably well sorted "piles" of dusty materials. Specifically the exposures at Tyrone and Gertrude Weise seem hard to explain using any process. How does that sort of sorting happen in an impact surge?

[dburt]

Also, I find it hard to see how the "berries" could form of a different material than the general
population of dust and grains present in the violent surge outflow. I can see them forming slowly
from later, dissolved, minerals as concretions, but how were the spheres differentiated from the
rest of the base surge materials almost instantaneously?

See my previous post. The "dissolved materials" in this case were dissolved in a very hot, unstable, turbulent, multicomponent vapor. As it moved away from the impact site, this gas expanded, cooled, and various constituents condensed (precipitated) out of it, in succession. The system was highly dynamic, and moving along initially at supersonic speeds, so different things were probably happening in different parts of the cloud at the same time, and reactions were getting "smeared out" over great distances (more than 100 km in the case of Meridiani). At the same time, particulate matter was dropping out, more or less in order of decreasing grain size (with extremely poor sorting initially), and getting rolled along the ground, and bouncing, and earlier deposits were getting sheared off, forming cross-beds. Not that different conceptually from a flash flood or an Arizona dust storm or a submarine turbidity current or an air-supported large landslide, except that it was mainly a hot gas, containing lots of dissolved vaporized rock, made dense by the particles within it, and probably moving much faster than any of the conceptual analogs. Accretionary lapilli in volcanic deposits do not do not usually deform the beds around them, because they were deposited along with those beds.

As a gas, despite its turbulence, the surge cloud couldn't support blueberries larger than a given size (about 5 mm at Meridiani - probably unusually large), which is why they are so well size sorted as well as spherical. Actual concretions are supported by a rigid rock matrix as they grow in a leisurely fashion from an aqueous fluid, and so they are unconstrained as to size (their size is controlled solely by when they nucleated and how fast iron in solution diffused towards the growing concretions). They may incorporate the matrix (if it is quartz sand) or replace it (as for chert concretions in limestone). Concretions commonly are rounded, if they grow in a rock of uniform permeability, or highly flattened if they grow in strongly bedded rock, or elongated in the direction of fluid flow, if the fluid from which they grew was moving. As they grow, they commonly clump together in large nodular masses or beds. See previous posts for more on their typical distribution in relation to brine mixing and flow. The Meridiani blueberries appear to exhibit no such special distributions, or appropriate size and shape variations, and their very limited clumping (in rare doublets or extremely rare tiny triplets) can be explained by their inherent stickiness as they grew in the cloud - or by postdepositional salt coatings where they touched once deposited. They are NOT concretions, no way, no how (unless you really, really want them to be, and then we are talking about a belief, not science ).

The short answer to your question would be: because different minerals or fluids condense out of the expanding, cooling vapor cloud at different times, as controlled by physical chemistry. (But you know how I hate short answers. )

--Don

[dburt]

In discussion of the blueberries, I fail to see how they could be accretionary lapilli (or anything similar) from within a surge cloud and also exhibit the dimple/stalk morphology that we see in many/most of them. I can see stalks forming if they are concretions that built up from small voids in the salty rocks, not in lapilli.

And we don't see them *ever* deforming the layers in which they appear, as you would imagine they would if they fell onto newly-formed layers in the salty rocks. They are embedded in a fashion which screams (to my eye) "concretions formed in place" and not "lapilli that fell onto these layers." They are not organized along specific layers, they are scattered like shotgun-shot all throughout the layered rocks. If they were lapilli that were just dropped onto the still-fragmented salt dust that was being deposited by a surge, you would also expect a *lot* of signs of turbulence in the layer deposition "downwind" (or "downsurge") of the blueberries, and we don't. We see them perfectly embedded in layers that are otherwise laid down quite flat. And if we also buy the theory that each millimeter-thick layer was laid down by a separate impact surge event (which I still have a hard time believing, since the layers are so uniform in thickness), and we know that the blueberries are significantly larger in diameter than the layers in which they are embedded, where is the turbulence we should see "downsurge" from blueberries emplaced by the last surge? I would expect fillets along the upsurge side of the berries, and hollows on the downsurge side, even if the surge flow was relatively slow and non-violent. We see absolutely no sign of this.

I wonder a bit, too, about the lack of shales being definitive proof against a watery environment. The Meridiani light-toned unit is very thick -- if there were simply not enough silicates (especially phyllosilicates) to form a significant amount of the depositional surface, we'd be looking at a large substrate which simply doesn't contain the constructional materials necessary to form impermeable floors (i.e., shales) for standing water. In which case, you'd be looking at standing water *only* when the water table exceeded the level of the surface. As the water table receded downward, it would simply flow through a unit of permeable salty rock all the way down to the base of the aquifer, which (in my thinking) would consist of clays or shales formed at the top of the unit that lies below the light-toned unit. Since *none* of that unit is exhumed anywhere that Oppy has visited, we can't judge on the lack of such materials on the top of the present surface.

Just because Mars may once have had liquid water doesn't mean it would necessarily have formed the same features such water might have created on Earth (like pervasive shales), especially if there are compositional differences in the materials that held the water. Conditions on a hypothetical "wet, warm" Mars would have been very different from conditions on a wet, warm (and teeming-with-life) Earth -- we always need to appreciate that the same water conditions on the two planets could result in some significantly different results when it comes to how rocks were created and altered.

-the other Doug

Other Doug - Are you perhaps getting as chatty as me? If so, real sorry for the bad example I'm setting. Your questions seem to consist partly of vague misgivings, but I'll try to respond anyway.

First, don't confuse the spherules themselves with postdepositional overgrowths of salts and post exposure wind reshaping (especially of the softer overgrowths).

The spherules in matrix at Meridiani look just like accretionary lapilli in volcanic surge deposits - they don't make "bomb sag" like depressions because they don't fall ballistically from the sky like hailstones. Instead, they gradually get too big for the turbulent surge cloud, especially as it expands and drops out particulate matter, and work their way lower until they are incorporated into the rapidly growing sediment accumulation. Drop a marble into a flash flood - is it going to drop ballistically straight to the bottom, or get swept along as part of the general mass movement, and then dumped out?

You don't see turbulence down-berry presumably because such a force would just cause the berry just to keep on rolling. I am not a sedimentologist, however.

Each thin layer was presumably caused by a passage of turbulence/erosion - you can lay down meters of thinly laminated surge beds in minutes, if the steam cloud is condensing and turbulent shear continues at the same time. Depending on shear conditions, other deposits might be massive (more of a dump-out). I repeat, however, I am not a sedimentologist - I just work with one, and I'm still learning.

The lack of shales presumably indicates lack of surface, open, standing water in a lake or puddle, not necessarily a lack of liquid water in general (other observations suggest no liquid water in general). I only say this because Mars is infamously dusty. (Dust on a frozen pond might keep on going, giving you minimal shale, however.) Caution - you might make shale-like beds by depositing a layer of dry dust (such as impact loess) and then burying and compressing it. More likely the dust would just erode, though.

The impermeable shale unit (playa lake beds) that you and the MER team presume underlies the salty beds at depth seems to violate a fundamental rule of geology, superposition (that younger rocks are always deposited on top of older rocks). You assume they are the same age, and yet everywhere the alleged shales must be covered by a thick layer of own alleged erosion products. This is conceptually difficult. (If 90% of Meridiani were an exposure of a playa lake, a source region, with gypsum dunes piled up only at one end, as at White Sands, NM, this might make some conceptual sense. How can you so completely and so thickly cover up your own source rock, though? I know you could do it with some highly improbable 2-step special assumptions, such as pile the dunes up at one end, reverse the wind direction, and spread the salty sand out without allowing it to pile up at the other end, but give me a break, that's just too special - that's worse than saying Grandpa had a sex change, as per the earlier discussion of too many dead grandmothers. More like saying Grandpa personally went to Mars with a rake. Also, salt grains (as opposed to quartz sand) in dunes are extremely soft, and cannot migrate far or long without being ground to dust.

If you want to believe that early Mars was an Earthlike warm, wet world, with babbling brooks, clear lakes, gentle breezes, clean air, and gentle refreshing rainstorms to keep the air clean, feel free, if it makes you feel better. The MER team apparently pictures it as a giant smelter complex and toxic waste dump, with sulfuric acid constantly raining down from the skies and dissolving your clothing and ponding in shallow yellow-brown dead pools and lakes. Neither of those is how I currently picture it though - my picture (at least for the period 3.9 to 3.8 billion years ago) is more like an extended nuclear battle taking place between Peru and Bolivia during an ice storm, with salty surge clouds sweeping across the cratered altiplano, dropping shiny little radiactive black spherules along the way.

The phase diagram for water (controlling whether you get ice, water, or steam) is invariant among the planets, and Mars has always been much smaller and farther from the Sun than Earth. Ice, not liquid water or acid, should normally have dominated the surface. Transient episodes of warming owing to impacts (with possibly some input from early magmatism and volcanism) plus dissolved salts in brines seem sufficient to account for the orbital and ground observations to date. Any more warming and there should be many more clay minerals, for one thing (whereas salts you can crystallize by freezing and/or drying). Ditto acid.

But that's just my take. You're welcome to your own. Mine is again based on Occam's Razor - try to keep your special assumptions (a.k.a. "dead grandmothers") to an absolute minimum.

--Don

[dburt]

Whew! Getting a bit winded from replying on that other thread. In looking at today's new MER downloads, I couldn't help notice what appear to be "festoon" or trough-type cross-beds in the middle of the cliff at Cape St. Mary. The easiest ones to see are just below the exact center of the image, with some more subtle ones above and to the right, just below a pale massive bed (they're small, so look carefully - it's wonderful image, with lots of bedding detail):

http://qt.exploratorium.edu/mars/opportuni...MYP2443R2M1.JPG

I wouldn't dare to comment on their possible significance, but can you see them? (They look similar to those imaged in Erebus Crater, on rock faces named Overgaard and Cornville.) What do you think of them? Am I utterly mistaken, as usual?

--Don

[dvandorn]

Yep -- I see what you're talking about. I have a little wonderment as to how much shock alteration might play a part in the interruption of layers within any of these blocks, but I certainly see cross-bedding in many of the rocks in the cliff face. And there are a few examples of the cross-bedding that the MER team has called festoons, yes.

-the other Doug

[marsbug]

Forgive me for asking what may be a dumb question, but can the evidence for a northern ocean be squared with the view of mars as an ice ball? The evidence in question may not actually be of an ocean, but if it is then mars must have been warm and wet for at least part of its history, and if its not what is the simpler explanation?
Edit: I should make it clear that by 'evidence of a northern ocean' I'm referring to the traces of coastline around the northern basin, not any more subtle evidence for an ocean which I'm not aware of.

[john_s]

Looks pretty real, and pretty impressive, to me- can't wait to get a closer look (though I guess it's too high up the cliff for Oppy to get a really close view of that particular example). I don't see much evidence for shock alteration of these rocks myself- I bet all those fine-scale structures are sedimentary in origin.

Cool!
John.

[djellison]

I've merged this thread and the Victoria-Festoons thread. We only need the one thread on Don's alternate Meridiani hypothesis.

Doug

[dvandorn]

Forgive me for asking what may be a dumb question, but can the evidence for a northern ocean be squared with the view of mars as an ice ball? The evidence in question may not actually be of an ocean, but if it is then mars must have been warm and wet for at least part of its history, and if its not what is the simpler explanation?
Edit: I should make it clear that by 'evidence of a northern ocean' I'm referring to the traces of coastline around the northern basin, not any more subtle evidence for an ocean which I'm not aware of.

If Mars had developed salty oceans before the late heavy bombardment nearly 4 billion years ago, that might explain the amount of salt that is distributed all across the planet. Impact cratering is generally not thought to be a great horizontal mixer of material, mixing more vertically than horizontally (at least based on the lessons learned from the Moon), but the very magnitude of the LHB could have distributed salts from the bottoms of obliterated southern hemisphere salty seas all across the planet (especially considering how far-flung the LHB must have thrown salty water from these putative seas into the early Martian atmosphere).

I can also picture a Mars which has quickly lost much of its atmosphere after the magnetic field died, cooling drastically, losing much of its liquid water to evaporation and ice sublimation and exposing tens of thousands of square kilometers of salty seabed; winds may then have eroded these salt flats and distributed the salts across the planet. So you don't necessarily need the LHB to explain the ubiquity of salts on Mars without requiring the entire planet to have been covered with salty seas, but it remains one plausible transport mechanism.

If there was a Great Northern Ocean, I'm thinking it must have post-dated the LHB, since there is no reason to believe that the impact flux would have limited itself to the southern hemisphere. I truly believe that, at the end of the LHB, Mars looked pretty much like the southern hemisphere looks today, but all over. The smoothed terrains we see in the north must have been overlain over a rough lunar-highlands-type of terrain, unless you want to try and explain how such a heavy bombardment could have completely missed one whole hemisphere of the planet... (Just trying to apply Occam's Razor, here.)

Nonetheless, it's important to remember that Mars with a thicker atmosphere and any kind of greenhouse effect would have been considerably warmer than it is today -- if you moved the Earth to Mars' orbit, it would be somewhat cooler but generally habitable (our seas and oceans wouldn't all freeze over, etc.). It's a touch disingenuous to suggest that Mars could never have been warm and wet because of its distance from the Sun and the related lower insolation than that received here on Earth. Mars is cold and dry today primarily because it lacks a magnetic field and thus the solar wind has sputtered a major percentage of its original volatiles right off of it. Had Mars not lost its magnetic field early on, it might still be warm enough and have enough atmosphere to support liquid water on the surface.

-the other Doug

[dburt]

massive inline quote removed. - Doug.

Great reply to the original question. I would add only a few comments. 1) Impact cratering on Mars is different from on the Moon, owing to the martian atmosphere and subsurface volatiles. Unlike on the Earth, a very complete record of early martian cratering has been preserved (including, we suggest, layered fine impact-derived sediments). 2) Subtle details of MOLA topography and, more recently, radar imaging has revealed that your Occam's Razor supposition about the LHB affecting the northern lowlands as much as the southern highlands is correct. The Northern Plains appear to be just as heavily cratered underneath. Therefore, deposits sitting on top must postdate the LHB (pretty obvious from their paucity of craters anyway). 3) If there was a Great Northern Ocean, presumably formed by outflow channel brines, it could well have been frozen over, with the ice rapidly sublimating, and still left shorelines visible from orbit. In any case, no implications for ideas about higher Meridiani or Home Plate.

4) Current atmospheric theory, as I understand it, is that something like 99% of any early atmosphere was lost by "impact escape" resulting from the LHB (although the authors of the encyclopedia article, here:
http://www.atmos.washington.edu/~davidc/pa...evised-PDF2.pdf
do not seem aware of the LHB, and conventionally just assume a gradual geometric decay of bombardment rates between 4.5 and 3.5 billion years ago. A problem with this conventional Mars assumption is that, as I recall, astronomical models suggest a pretty clean sweep of the inner Solar System in only the first 50 million years.) After the LHB, the very little remaining atmosphere has extremely slowly been lost by other processes and this slow loss continues. Therefore, to assume a "warm wet Mars" AFTER the LHB is supposing something for which there's little evidence except the probably faulty models of the MER team at Meridiani (contrary evidence is abundant, including a lack of young clays, and fresh olivine on the surface). We can imagine earliest Mars to be whatever we like (e.g., with the beautiful Venus jungle maidens so much in vogue when I started reading science fiction), inasmuch as any evidence was pretty much wiped out by the LHB. (Personally, I imagine pre-LHB Mars, which probably existed from 4.5 to at least 4.0 by, as considerably wetter, but not necessarily a whole lot warmer, than present-day Mars.) Owing to the LHB, it really doesn't matter. Mars history, like Earth history, pretty much started with a clean slate at about 3.8 billion years ago - think of Mars at that time as "Post-War ruins". And of course, I think that many of the layered sediments of the highlands, including Meridiani, could be remaining wartime debris. (Without the contained spherules, and predominance of low-angle cross-beds, and other bedding features, I might say they could be wind-deposited instead. In either case, they couldn't have been soaked in liquid water.)

--Don

[dburt]

massive inline quote removed. - Doug.

Before these great questions get lost...

I hesitate (but obviously not very long ) to respond about the MER team's hypothesis, because it makes little sense to me either and I may be just a wee bit biased (so please feel free to correct me, as always in my posts here). Meridiani is not much of a topographic basin, and there are no early drainage networks or other signs of liquid water leading towards it or away from it (e.g., no alluvial fans or deltas near its edge, no exit channels) so they had to assume (at least in their second iteration) that acid, salty waters mysteriously rose out of the ground, forming an enclosed playa lake (now mysteriously vanished, although they seem to see no logic problems with having it still somewhere underneath - see my other post) that then evaporated to precipitate abundant salts at the surface, both highly soluble and nearly insoluble, and neutral and acid (jarosite). In evaporation of any real playa lake, the salts would form bathtub rings according to solubility, with the least soluble ones on the outside, but this problem was not explicitly considered, I believe. These salts were then mysteriously mixed with each other and with with some type of amorphous particle (not crystalline clays) to produce granular little "mudballs" which the wind picked up and deposited at Meridiani as cross-bedded dunes (evidence: there's a large, high-angle cross-bed exposed at the base of Burns Cliff, the so-called Lower Unit). The mysterious brine rose again, but this time it arrived perfectly saturated with all the salts involved, and with the correct acidity, so as not to dissolve anything (fascinating, but why didn't anything recrystallize?). While rising, these waters somehow penetrated the impermable bottom of the vanished old playa lake, if it indeed lay beneath the cross-bedded dune deposits. The acid, saturated brine then established a new, higher-level water table, and wind eroded any non-wet granules on top leaving a planar surface (called the Wellington Contact in publications; a general term in sedimentology literature is Stokes surface, named after the geologist who first described such planar surfaces in dune-derived sandstones). Mysteriously, this "Wellington Contact" is actually not planar, and has a big trough or scour cut out of it (I think ignored in publications) and, unlike in an actual Stokes surface, no muds were locally deposited in low, wet areas. Next, wind, in a fashion that's completely mysterious to me, brought in more salty "mudballs" from this vanished playa, which by now should have been deeply buried beneath the earlier dune deposits, only this time the wind deposited exclusively a relatively thick sequence of low-angle cross beds ("sand sheets") called the Middle Unit and bottom of the Upper Unit. Next, the mysterious, chemically convenient brine rose again, and sat until it had discolored the top of the middle beds at what was called the Whatanga Contact. This is claimed to be the capillary fringe of subsurface evaporation just above the water table (no such capillary fringe is evident at the lower Wellington Contact - I guess it's supposed to have eroded off).

Then the ever-mysterious brine, without dissolving or recrystallizing away the earlier capillary fringe, rose to the flat surface and flowed ankle- to waist-deep across it, making current ripples (the record of which is allegedly seen in cross section as "festoon"-type cross-laminations, little troughs). The fact that this mysterious brine, still saturated in all the respective salts so as not to dissolve any, would possibly have had the viscosity of syrup, is ignored. The latest iteration of the model (an abstract at the shortly upcoming Mars meeting in Pasadena) refers to "gravity driven, possibly unchannelized flows resulting from the flooding of inderdune/playa surfaces". Where this alleged flood originated from, where it went, how it flowed across a flat surface, how it joined isolated interdune areas without leaving channels, where the playa was, what happened after it stopped flowing, and so on are unspecified details. It must have immediately sunk mysteriously right back into the ground, else it might have formed shales or mud cracks or something.

Next a new brine arrived, differing in acidity or iron content from the previous ones, but still saturated in all the relevant neutral salts. This somehow mixed uniformly with any previous brines to make disseminated jarosite concretions uniformly throughout the entire section without leaving any other trace of its passage. All "concretions" were mysteriously the same shape (perfect spheres), were strongly size-limited, and never clumped together as nodular masses (as discussed in previous posts).

Then yet another brine arrived, less acid and more dilute, and uniformly mixed throughout the aquifer over an area the size of Oklahoma (this is impossible, BTW), altering the jarosite concretions to hematite (mysteriously, the blue-gray shiny or "specular" high temperature form, not explained), but leaving the jarosite in the groundmass unaffected, so that Opportunity could detect it. This new brine, or perhaps a different one (I've long since lost count, to be frank), dissolved away some of the larger salt crystals, leaving crystal-shaped hollows in the rock, but still not recrystallizing or dissolving any other salts. Then everything ended. Whew!.

I admit I haven't completely reread the jargon-packed original papers in making this off-the cuff summary of the published stratigraphic section (doing so makes me too dizzy), or the latest modifications, so I may have minor details horribly wrong, but perhaps you get the general idea. Possible perhaps, but not terribly plausible. It does make quite a detailed, elegant, and magnificent story, though - and it's all based on just 3 meters or so of sandy, salty, spherule-bearing, cross-bedded section in a single outcrop in Burns Cliff (plus the even thinner exposure in Eagle Crater). Remember that there are 800 meters or so of layered rock lying beneath these surface rocks. What other incredibly complex, magnificent stories lie in wait?


Back to impact surge, the gaseous-particulate turbulent mixture is a fluid, not unlike water or wind, but travelling at many 100's of km/hr velocity (at least initially). Owing to shear and steam condensation, it can deposit great thicknesses of finely laminated sediments in a very short time, as covered in a recent post (after your question). You don't need a separate impact crater for each layer in a surge deposit, any more than you need a separate windstorm for each laminar layer in a dune, or a separate flooding event for each layer in a stream deposit. All that's needed is turbulence and flow. I'll repeat this as many times as I have to. (Although I'll also repeat that I'm not a sedimentologist.) Regarding the lack of tiny impact craters for each tiny spherule, I already covered that in a recent post too. Picture injecting a bunch of BB's and sand directly into the turbulent exhaust of a roaring jet engine. Are the BB's going to fall straight down, or go with the flow?

I'm not sure what you're referring to w.r.t. "piles of dusty materials" in Gusev. From posted images alone, most of the rocks of current intense interest over there (subsurface sulfates, high-silica rock fragments, etc.) seem to be sitting on top of (not to be a part of) the layered surge deposits of Home Plate. What they signify is an open question, perhaps irrelevant to the origin of Home Plate and nearby layers and spherules.

All for now. Thanks much for your questions.

--Don

[Shaka]

Re post #86
Prof Don,
At the risk of inflicting on you a horrible typist's RSI, I will ask you for a little more specific detail regarding the deposits left by impact surges (your paragraph #6). Though I have delved extensively into the impact literature, I am not familiar with the "finely laminated sediments" produced by "shear and steam condensation", and I wonder if you can provide me with some paper references that describe these sediments, preferably with photographs, so that I can compare them with the Burns Formation. Thank you,
Cheers,
Shaka

[dburt]

Re post #86
Prof Don,
At the risk of inflicting on you a horrible typist's RSI, I will ask you for a little more specific detail regarding the deposits left by impact surges (your paragraph #6). Though I have delved extensively into the impact literature, I am not familiar with the "finely laminated sediments" produced by "shear and steam condensation", and I wonder if you can provide me with some paper references that describe these sediments, preferably with photographs, so that I can compare them with the Burns Formation. Thank you,
Cheers,
Shaka

Shaka,

Here's a basic reference on volcanic and nuclear explosion surges, by one of my co-authors, Ken Wohletz:
http://www.ees1.lanl.gov/Wohletz/Pyroclastic%20Surges.pdf
A bit technical, I'm afraid, and not the best quality reproduction, but plenty of classic references and diagrams (see, e.g., the sand wave variations on p. 259). If you locate any good references on terrestrial impact surges, please let me know, because they are virtually never preserved on land, and the ones deposited in the sea are altered and reworked, including the spherules (so we are working by analogy from volcanic deposits, mainly). As stated in previous posts, there are plenty of reasons to suspect that Mars may be the best place to study impact surge deposits in the Solar System, and that the two rovers may have been imaging such deposits all along (including exposures in Victoria).

Ken's publications page, here:
http://www.ees1.lanl.gov/Wohletz/Publications.htm
has links to the above paper and many others, including the Wohletz and Sheridan 1983 paper in Icarus that first proposed that rampart crater deposits resulted from surges. I also attach a pdf of our 2005 Nature paper, which has some photos in addition to more discussion. Let me know if you need anything more.

--Don

Attached File(s)

 Knauthnature04383.pdf ( 529.75K ) Number of downloads: 730

 

[centsworth_II]

Here's a basic reference on volcanic and nuclear explosion surges, by one of my co-authors, Ken Wohletz:
http://www.ees1.lanl.gov/Wohletz/Pyroclastic%20Surges.pdf

I'm struck by the pictures and diagrams that do look similar to what the MERs have seen.
I hadn't fully appreciated that this sort of material existed prior to the MER mission. It
gives the whole base surge theory a kind of predictive quality as opposed to reactive.

The picture on page 292 really struck me. The first I had ever heard of a volcanic "bomb" was
when the one at Home Plate was described. This picture looks like my second exposure to
a "bomb". Strangely though, I don't see specific mention of it in the text.

Here is the picture (top) with it's caption. The Home Plate image is below (not to same scale).

"A typical wet-surge
outcrop exposure described
by Sohn and Chough (1992)
showing irregular and scourfill
deposits and massive
bedded deposits. Photograph
from Sohn and Chough"

[CosmicRocker]

Would someone please tell me where the "Festoons in cliff at Cape St. Mary?" thread went. I wanted to comment, but I can't find it. Was it merged with another discussion?