Elk Grove Dan - I didn't make up the rates of erosion - they were order of magnitude quotes (from my admittedly increasingly faulty memory) based on numbers published by Matt Golombek et al. for the MER landing sites, with Meridiani, if I recall correctly, being slightly faster than Gusev (it's not in a crater and the rocks are softer). In any case, no more than 10 meters or so ever removed from either rover site in nearly 4 billion years - sort of hard to get used to for we terrestrials. (Note that long term burial in, e.g., drifting sand, followed by recent erosion, could likewise minimize observed erosion.) The atmospheric density probably varied considerably, but it was still a near-vacuum (the difference is between hardly any and not much at all).

Most people who study Mars rampart craters believe that the presence of subsurface volatiles is more important than the density of the atmosphere, because the impact vaporizes its own local atmosphere (mainly steam) to make the turbulent flow called a surge. The greater density of the surge is provided by the particulates. A lower atmospheric pressure might allow the surge to run out further and faster, but also would favor more rapid steam condensation, meaning the surge would "run out of steam" (literally). This is the explanation offered by Wohletz and Sheridan in 1983 for rampart crater deposits as surge deposits (most others favored some sort of mud sploosh at the time).

The weak gravity of Mars presumably likewise enables longer runout distance for Mars surge clouds (other things being equal, which they never are), but such modeling is still in its infancy.

BTW, what if I had discovered that hypothical candy wrapper (Meridiani exposures, in this case) on the day after Halloween? (formed right after 3.8 billion years ago, in this case) And the ground was coverd with tiny footprints? (millions of little and big impact craters, in this case). Would you still think I was jumping to conclusions?

So please specify those "dead grandmothers". Shaka pointed out a pretty good one, I thought, related to an apparent lack of coarse material at Meridiani - can you point out a second or third implausibility (based on logic, like he did - not based on the way we might feel things ought to work on another planet, based on our terrestrial predjudices)? And remember, I should be allowed at least one more.

Or better yet, specify some impossibilities. Then I can hand in my essay exam and get it graded. Thanks, Prof.

--Don

From satellite images of eroded terrain around the Meridiani
region in which hundreds of meters of layering can be seen.
Below are links to a couple of articles.

http://www.space.com/scienceastronomy/0606..._meridiani.html
http://www.psrd.hawaii.edu/Mar03/Meridiani.html

Great, thanks -- but how do we know that these layers are of the same type as the relatively short column we see at Victoria, formed by the same process -- which I gather is what Dr. Burt is claiming?

Are any of these "numerous volcanologists" coming forward to support the credibility of your claims?
You mentioned that the surge theory was considered "lunatic fringe". How can that be if the sorts
of things that Opportunity is seeing are well documented in surge deposits from explosive events on
Earth? Is there even much discussion about this going on in geological circles? Are there any that
privately say you have a point or even agree with you but do not want to "go public"?

I'd love to see an "is Pluto a Planet?"-style public debate go on even though that was about
terminology more than basic science.

Dr. Burt would probably hope to see layers different from those seen in Victoria. Note the recent
exchange with Shaka (see below). Maybe MRO can get a closer look at those layers.

Shaka: "...We can see scores to hundreds of layers in the lower parts of the Victoria capes.
They are remarkably uniform in scale and appearance. Since a rain of meteorites would distribute
more or less randomly over Mars, it is hard to credit that some would not land closer to Meridiani
and produce much thicker (meter-scale) layers...."

dburt: "Congratulations! You have put your finger right on the weakest aspect of the impact
surge argument.... I can answer you in several possible ways, none completely satisfactory. 1) Oppy
has imaged only a small portion of the Meridiani layers, those at the very top, which, being the
youngest, could have formed when impacting had tailed off, and been distant (its lack of coarse
surface material was, after all, what moved it to the top of possible landing site choices - it's
possibly a biased sample, in other words). Coarse ejecta or surge layers may lie below the layers
exposed, or may even be exposed somewhere deep in Victoria. Such a finding (of coarse pieces)
would still be ambiguous, however, because ballistic ejecta could in theory land anywhere on Mars,
at any time, on top of any type of sediment (and dust could settle, but it wouldn't stick around,
unless the surface were sticky). 2) Coarse surface ejecta has been found at each landing site to
date (and at others abandoned from consideration when too many surface boulders were found).
Also, coarse layers of boulders in the midst of fine layers have been imaged by HiRISE in various
spots - as noted by Emily in the post that inspired me to stop lurking here about a week ago.

Speaking for myself, I'm operating on a very intuitive level here, not having
much knowledge and no expertise in geology. The base surge hypothesis is
counter intuitive at Meridiani and so raises the most questions in my mind.
It was so simple to see things the way the MER team described it.

At Home Plate, things seem a lot more complicated, even for the MER team.
In the Columbia Hills, even Steve Squyres has mentioned the possibility that
rock alterations were the result of volcanic steam. I don't know if he would
include impact steam as an alternative. In any case, the formation of the
Home Plate area is hypothesized to be the result of violent processes.

It's less of a stretch from volcanic to impact hypotheses for Home Plate
than it is from water/wind to impact at Meridiani.

You're pretty perceptive. Yes, many agree in private, but for entirely obvious reasons they don't want to stick their necks out. And my use of "lunatic fringe" can be thought of as a triple pun on 1) lunatic, 2) luna (no self-respecting Mars geologist would ever admit that Mars resembles the Moon in as many ways as it is different - might be bad for funding), and 3) Gerald Wasserburg's "lunatic asylum" lab at Caltech, which is where many of the Apollo samples were dated by the rate of decay of their contained uranium and other radioactive elements. There were two huge scientific surprises yielded by the Apollo program (not counting the expected result that impact rather than volcanism caused lunar craters). One was that returned lunar rocks were dry as a bone (unlike terrestrial lavas and other igneous rocks, which are relatively hydrous); this dryness and lack of much of a lunar core was later explained by the giant impact theory of the origin of the Moon (that the Moon was formed by a Mars-sized impactor that hit the early Earth a more-than-glancing blow).

The other was that all the lunar impact melts (a few other rocks were older, whereas surface lavas were younger) all seemed to have formed at about the same time, 3.9 to 3.8 billion years ago. The "lunatics" hypothesized a "lunar cataclysm" or late heavy bombardment, which had almost erased the record of everything earlier. The modelers of the time couldn't deal with such a unique event (they favored a geometric decay scheme, somewhat analogous to radioactive decay, so that half of the impactors would be swept up by the planets in a given time, then half of those remaining, then half of those remaining, and so on until 3.8 billion years ago - such a model couldn't handle a sudden huge increase in impactors very late in the game). Modelers, because they are held by most people to be "smarter" than observationists (who, I guess, are supposed to be robots like beloved old Oppy), commonly rule popular opinion in science, despite observational evidence to that contradicts their ideas (look up the history of determining the age of the Earth from cooling rates, or of what happened to Wegener's definitive field evidence proving "continental drift" in the 1920's for examples of the influence of modelers on scientific progress). More than 35 years late, the modelers have apparently caught up with those early Apollo results (which, because they could not be modeled, were not widely accepted then or later) - see, e.g., this balanced discussion by Jeff Taylor:

http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html

The majority of traditional Mars geologists seem as yet unaware of the probability of a late heavy bombardment in inner planet history and its implications, some of which were noted in passing by Jeff. For example, he noted that the late heavy bombardment neatly explains why the oldest continental fragments known on Earth are about 3.8 billion years old, although he didn't mention that the Earth, with its much stronger gravity field, should have been hit by far more impactors than the Moon. Also not mentioned: Mars, all alone in its immediate region of the Solar System, and with a stronger gravity field than the Moon, probably was hit much worse too. Jeff does mention the possibility that catastrophic impacting could have caused catastrophic rainstorms on early Mars, thereby neatly explaining drainage networks on some of the oldest, most highly cratered areas of Mars. If such rains ever fell, many of the lower layers at Meridiani could represent water-reworked impact debris, stripped from the nearby highlands (that would help with the total thickness problem, which some have objected to in this thread - although the tendency of surge deposits to pond in lowlands works too, at least for me, with wind stripping fines from the nearby highlands). The impact surge hypothesis strictly applies to only the uppermost rocks at Meridiani, those imaged and measured by the rover (plus many similar areas imaged from orbit on Mars, plus, of course, Home Plate). The observed abundance of soluble Mg-sulfates and the lack of stream channels probably rules out much rain at that time (although it doesn't necessarily rule out cold frost or snow, which would tend to leach only chlorides - see my earlier post). The observations also come close, IMHO, to ruling out the magnificently detailed model arrived at by the MER team, as described in earlier posts (hey, at least we agree that it didn't rain and that the odd salt mixture requires transport from elsewhere). BTW, "model" is the operative word - not "discovery," as it was reported by the popular press, journals that should know better, and the freshman textbook I currently use. What was "discovered" was some salty crossbeds containing tiny blue-gray hematitic spherules, and even that discovery represented considerable interpretation of raw spectral and analytical data. [...Darn preachy old professor! Almost as bad as that guy on the Bad Astronomy website, 'cept he's younger...]

If impact on Mars is the proverbial "elephant in the living room" that no one wants to look at or mention (except for dating younger surfaces and determining possible depths to ice), wasn't there another tale involving an elephant and a group of blind men (or, aaah, was it a group of highly specialized distinguished scientists, each afraid of contradicting the other regarding his or her specialty, and golly, I can't remember how that story goes)? Or was it something involving a camel? If I can't remember any animal story (excuse me for mentioning it; in fact, I don't even remember mentioning it), then no reasonable person could imagine I meant any possible relationship to any real people, living or dead. Also, no animals were injured while I was writing this, I'm writing this at home, and the usual disclaimer regarding colleagues and employer. (There's certainly a lot to be said for anonymous posting. )

To summarize, yes, in private (definitely not for attribution) many say that they agree with us.

--Don

Thanks much for taking the bait, albeit reluctantly. If I showed Pancam photos of Home Plate vs. comparably exposed areas of Meridiani to one of my classes, I much doubt they'd be able to tell them apart (currently, Pancam photos of the two sites are biased in favor of the spectacular cliffs of Victoria - infinitely nicer than any Home Plate exposures). Except for a few cm. of poorly sorted gritty material with spherules at the base (extremely typical base surge deposit, BTW - although for obvious reasons that term is not being used), the rest of Home Plate looks very much the same as Meridiani (including Victoria cliffs) in terms of grain sizes and bedding textures - lots of low angle cross beds, but no large dune forms. Unlike Meridiani (but like most landing sites), it's littered with coarse impact debris. It has no particularly obvious unconformity (prominent former erosion surface) relating putative early volcanic to later wind deposition. The evidence for Home Plate beds being volcanic is, as I understand it, 1) their salty and basaltic composition - also likely for any impact debris anywhere on Mars, and 2) a single apparent "bomb sag" supposedly formed because a large rock was tossed out of a nearby invisible volcano (so-called ballistic ejecta). Ballistic ejecta are even more typical of impacts, so this feature is hardly diagnostic and, if there had been a volcano nearby, I might expect to see several more bomb sags in an area of the size already imaged by Spirit (an erupting volcano that's sputtering explosively because of steam explosions is hardly shy about ejecta). Rapid lateral changes in mean grain size are also typical of surge deposits caused by such small steam explosions, so I'd expect to see the gritty layer get much coarser in one direction, and much finer in the other, if the rover were traversing it towards or away from the putative volcano (a possible test of the invisible volcano hypothesis).

The interesting rocks showing sulfates or silica-rich alteration are all apparently sitting on top of the Home Plate layered rocks - they don't seem to be an intrinsic part of the package. To me they could have come from anywhere, as impact ejecta, and represent a possible sample of a volcanic or impact-related hydrothermal system somewhere else. Our hypothesis would relate the acid sulfates, at least, to sulfide oxidation (as originally proposed by Roger Burns for Mars in general). In other words, the top of Home Plate resembles coarse dump material near an open mine, with the blasting having been done by impact rather than by chemical explosives. See, e.g., my 2006 "Mars and mine dumps" Eos article attached (member educational use).



In response to your earlier question about volcanologists who might secretly agree with us, BTW, there's one who wasn't afraid to say so (a volcanology grad student who simply web-posted his conclusion and then returned to his thesis). He arrived at the same hypothesis about the spherules as we did, at about the same time (March, 2004), although he wasn't considering impact processes. Here's what he said:

http://www.geo.mtu.edu/~ajdurant/mars_acclaps.htm

Note that he was NOT talking about the spherules at and near Home Plate, which weren't discovered until much later (the MER team IS calling those accretionary lapilli). He was talking solely about the initial imaging of blueberries in Eagle Crater. Sorry, I hadn't located the link last night.

Think of a Meridiani surface surge as a fast-moving "kablooey blast of steam and dust" (the correct quote from Hartmann, 2003, A Traveler's Guide to Mars, p. 272 - sorry for my faulty memory in the previous post) and you can perhaps understand why coarse material might be lacking there - sand need not indicate a passive or calm environment (e.g., Hawaiian beaches during a storm). Only mapping and imaging over a truly regional extent (far more that these two rovers are capable of) could reveal lateral grain size variations, radial erosional channels caused by a vortex, or other indicators of a specific source location.

Anyway, if Home Plate is from a local volcano ("hydrovolcanic" or "maar-related" in geojargon), Spirit should be able to discover lateral grain size variations and more bomb sags at the scale of the HP outcrop; if distant impact is responsible, no lateral size variations might be expected at such a scale, and ballistic bomb sags should be rare. Hardly definitive, but they're both basically the same process, from a different explosion source in brine-soaked basalt. (Definitive diagnosis of impact would require, e.g., instrumentation able to detect microshattering of minerals or ultra high-pressure phases, such as diamond. Imaging impact melt splash droplets - tektites - might offer a diagnosis too.)

Thanks for politely giving the old Prof. another excuse to be boring. Feel free to be impolite, however. I can't but you can, and I'm still seeking intrinsic flaws in our reasoning.

--Don

I don't read this good forum often enough because basically I am too busy to dive into it. Centsworth brought my attention to this interesting thread. I am not a geologist, rather a chemist....

Because of a lack of direct observational data, I don't think that we know enough about the behavior of fine dusts lofted by meteorite impacts in the atmosphere, and how they settle out globally or regionally. I am not sure that volcanic dust clouds are a good model for metoeric dust clouds, because I think that the particle size distribution might very well be quite different, and this would affect rates of settling.

If the dust cloud spreads, it's density over a region should be rather uniform (except right next to the impact crater, where there are surges). I have wondered if dust clouds from impacts could slowly settle down from the martian atmosphere to give fine layers of approximately even thickness? Could dust devils disturb the top one or two layers enough to give a festoon-like pattern in cross-section? I don't know. I also don't have a good explanation for how the layers get cemented together (unless there is rain or subsurface moisture).

I read through this thread and the "surge" explanation seems unconvincing to me for the creation of so many fine layers.

Nice to get new questions from a new face and fellow scientist, and welcome to the discussion. As partly detailed in posts above, our "impact surge" (initially called "brine splat") hypothesis was offered as an alternative explanation solely for the sandy, salty, cross-bedded (mostly at low angles), spherule-bearing layers imaged by rovers at Meridiani and, considerably later, at Home Plate. It does not deal explicitly with post-impact dust deposits or distributions (which, as you note, are likely to be far more extensive, but much thinner and much less permanent, owing to wind action), or with near-crater coarse ejecta. Neither deposit type has yet been imaged by the two rovers (although we speculate that the crudely-layered, coarse, hilly material in Gusev could be impact coarse ejecta), and orbital observations, even the latest HiRISE images, yield ambiguous results (large cross-beds are visible in some layers, possibly indicating migrating dune or surge deposits, and visible boulder beds are likely coarse ejecta, although they could form in other ways (e.g., landslides near a slope, former glacial moraines, stream channel boulders, and so on). What "so many fine layers" are you referring to - those imaged from orbit or those imaged on the ground by the two rovers? (See earlier posts for consideration of the latter in surge deposits.)

Our hypothesis originated when the MER team announced that Meridiani was a windy, dried up acidic playa lake or some similar evaporitic occurrence, that the hematic spherules were concretions proving that the subsurface had been soaked in liquid water, and that liquid water had later flowed across the flat surface, making current ripples (somewhat obscurely called "festoons"). A later minor modification to this model allowed that the incompatible (mixed soluble with insoluble) salt mixture had been wind-transported and deposited from an unknown playa-like source (somehow presumed to lie under the present exposures), but the complex model was otherwise unchanged. We were from the beginning puzzled by the numerous contradictions inherent in this model, some obvious and some subtle. As obvious examples, why should water flow across a flat surface, why don't the spherules look or behave like actual concretions in their shape, size or distribution, why is the hematite in them the blue-gray high-temperature form (specularite) unknown from sediments or sedimentary concretions, why are the spherules distinctly Ni-enriched compared to surrounding rocks, why are there no crystalline clays or clay deposits (clay rocks called shales are, other than salts, usually the only sediments deposited in playas - and dusty Mars should be covered with shales wherever there was liquid water at the surface), why should the youngest (uppermost) rocks exposed on chilly Mars indicate such a distinctly non Mars-like environment (more like Death Valley, CA in summer), how could nature so uniformly mix different-density brines in the pore spaces of sandy rocks to produce the alleged concretions, and why did all this alleged acid liquid water leave no trace of itself in the form of discolorations along fractures, recrystallization of and permeability reduction in soluble salts, dewatering textures in the rocks, visible flow channels on the surface, dried up mud cracks in puddles, or any of the numerous other water features so typical of terrestrial deposits?

As a chemist, perhaps you can appreciate some of the more subtle geochemical contradictions. For example, in the spherules, why should the Ni correlate with Fe, given that Ni2+ cannot be oxidized in aqueous solution and therefore cannot substitute for Fe3+ in hematite during crystal growth from aqueous solution? In aqueous solution, it should should instead subsitute for (partition into) the similar-sized Mg2+ in competing phases such as the abundant Mg-sulfates or Mg-bearing amorphous clays (if present). Substitution for Mg2+ is always dominant where Ni2+ occurs in natural sedimentary laterite (an oxidized type of soil) deposits on Earth (that's one reason why Ni is an expensive metal - it's expensive to extract from clays). High temperature, non-aqueous Fe2O3 can allow Ni2+ substitution by incorporating defects into its structure.

Another contradiction is that the Meridiani deposits are reported to have very high Br/Cl ratios. This is almost impossible to do in a brine without extensive fractional crystallization of chloride salts such as NaCl. (The larger, very rare Br anion freely substitutes for the very common, smaller Cl anion, but it shows a slight preference to remain in solution as chloride salts crystallize. You have to crystallize an awful lot of salt to build up appreciable Br concentrations in the brine.) If Meridiani were an evaporite deposit enriched in Br, chlorides should be the most abundant salts; instead, they are only a minor constituent compared to sulfates. Where did all those chlorides go?

Another, more obvious chemical problem relates to the fact that, in aqueous solution, acids are readily neutralized by bases producing salts. The Martian ferric sulfate mineral jarosite (on Earth nearly always formed by iron sulfide weathering in a moist, oxidizing envrironment, usually a desert mine dump or oxidized ore outcrop called a gossan - Roger Burns suggested the same mechanism should operate on Mars) was held to have grown in an acid underground reservoir the size of Oklahoma, on a planet utterly dominated by finely divided basic (MgO,CaO, Na2O-rich), olivine-rich lavas (normally, basalt). The acid underground solution and the lava fragments should have neutralized each other immediately, as has occurred in all experiments performed under comparable conditions, and gelatinous clay minerals should have been a by-product. Such a persistently acid solution seemed impossible, on Mars or anywhere else (except an inert, clay-lined basin, which describes some small acidic lakes in Australia, or in a volcanic crater lake, where new acid is being added constantly). Acid mine drainage at Rio Tinto, Spain was the offered Mars analog, but sulfide weathering wasn't included in their hypothesis (although it is included in ours).

We therefore sought an alternative explanation also involving sandstone, spherules, and a salty brine, and immediately ruled out wind because it didn't account for the spherules, and couldn't erode or transport them besides. Surge deposition came to mind (owing to extensive 1970's field experience), inasmuch as it commonly produces bed forms resembling all of those imaged by the rover, and sandy deposits commonly contain steam-condensation spherules called accretionary lapilli. Although most of our experience was with volcanic surge deposits, those seemed unlikely at Meridiani for a variety of reasons (wrong magma type and plate tectonic environments for Mars, wrong scale, nothing volcanic in the vicinity, no overlying welded tuff, etc.), so that left impact surge to consider. Impact provided us a lot more theoretical leeway (any scale, any distance, any target composition, a variety of impactor types, any time up to the present). The closer we looked at the images, and the more data that was returned, the better that hypothesis seemed. It accounted for EVERY feature imaged and reported (and still does, including ones not considered by the MER team) and seemed to solve ALL of the geological and geochemical contradictions inherent in the MER team interpretation.

Of course, as a new hypothesis, it raised a few problems of it's own. The most serious one, astutely noticed by Shaka in this thread, was why were the Meridiani materials all sandy (where were the coarser fragments)? If the responsible impacts were all relatively distant or targeted sandy deposits themselves, that seemed surmountable (also, the total thickness of section exposed to the rovers was less than 10-15 meters, and was not necessarily representative of deeper deposits). Another one, which no one in this thread has yet mentioned, was why the Fe-rich composition of the spherules? On a planet where the lavas are 2 to 3 times as Fe-rich as on Earth, where the impactor could have been an iron meteorite, and where Roger Burns had proposed abundant Fe,Ni-sulfide magmatic sulfide deposits in the subsurface, this didn't seem to be a serious problem. Somewhat surprisingly, we haven't thought of any other serious problems (and that really worries me, which is why I've been asking for input in this thread). Some here have objected that individual impacts might produce deposits that were too thin (whereas you seem to be saying the opposite). With almost an unlimited number of impacts available between 3.9 and 3.8 billion years ago (so-called late heavy bombardment, discussed in an earlier post) and with impact cratering an important process right up to the present, thickness or thinness of the deposits hardly seemed a problem either.

Once the sandy, salty, cross-bedded, spherule-bearing deposits at Home Plate in Gusev were discovered, including even the imprint of a small ballistic impactor ("bomb sag"), we hoped the argument might be over. But no, the MER team decided that HP was a hydrovolcanic surge deposit related to an invisible volcano erupting explosively owing to reaction of molten magma with a subsurface brine (just as Meridiani was allegedly related to an invisible playa lake containing a subsurface brine). Why are most Mars geologists apparently so blind to impact cratering as an important geological process, for deposition of ejecta as well as erosion of craters? I can only blame it on their strongly terrestrial (or possibly lunar) bias. We certainly agree with them about the subsurface brine (the basic goal of NASA's "follow the water" mantra) - it's difficult to account for the salts otherwise.

We may well be wrong in hypothesizing that Meridiani, Home Plate, and many other finely bedded deposits imaged from orbit could be caused by impacts, but it seems simpler than hypothesizing a special, unique sequence of geological events for each area, especially if those events all depend on a geological influence or phenomenon for which there's no direct evidence (e.g., an invisible volcano for Home Plate, or an invisible playa lake for Meridiani). Impact craters can be seen everywhere on Mars, of all sizes and ages. Occam apparently stated "Occam's razor" (sometimes called the K.I.S.S. principle) in response to parishoners blaming everyday events in their lives on the influence of "invisible angels". Have we really advanced very much if we always need to call on the influence of invisible geological agents to make common rocks on Mars?

If you see a volcano, then look for lavas (or other types of volcanic deposits). If you see lava, look for a volcano (on Mars only after you've made sure it's not an impact melt, perhaps). If you see a crater, think about both coarse and fine ejecta, and what might happen to the fines on a planet with an atmosphere and subsurface volatiles. If you see sandstone, think about the many ways there might be to make sand, including both impacts and exploding volcanoes, and go on from there. Be willing to change your mind as new data and new ideas become available - that's the only way you'll make important scientific discoveries, even if it runs counter to human nature.

Sorry post this is, as usual, too long - I meant it as more-or-less of a summary essay, so you don't have to read all the posts that precede it. Also, I think I've said just about everything I have to say in this thread, unless new questions or new objections arise (I'm really hoping for the latter). Thanks.

--Don

Don,

Thanks for taking the time for a detailed reply.

I was thinking that deposition of suspended dust clouds from impacts could give uniform thin layers, as we saw in Meridiani. However, I do not have a good mechanism for glueing these settling dusts down into a layer unless the substrate had a moisture content.

Or perhaps, the impact dusts settled down onto a very shallow lake, and left layers underwater as they settled below? And the festoons are ripples from wind driven flow? Sorry, I don't know enough geology to differentiate if one possibility is more likely than another.

However, I don't see how surges could have produces all the thin fine bedded layers of similar thickness we have observed at Meridani. If it were surges, I would have expected some layers to be very thick, some thinner, and some containing jumbles of debris of various size. We haven't seen this.

On your criticism of the Fe/Ni ratios on the spherules, you do bring up some good points. However, I don't see how a surge mechanism would create spherules of this composition either.

Same goes for the Br/Cl ratios - the high Br levels always struck me as 'odd'. I wondered if Cl salts are more friable and thus more likely to be blown away, or something like that. Preferential erosion of Cl slats would ultimately cause the Br salts to enrich. All that said, I don't understand how a surge mechanism would account for the Br/Cl ratios either.

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?

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.

Just my $.02...

-the other Doug

Silylene,

Perhaps you should read some of the earlier posts - some of this has been discussed. As for dust, I'm not going to worry about as a sedimentary deposit until it is imaged. If there was ever standing water, dust falling into it should have yielded finely laminar shale, likewise not yet imaged (and a strong argument, in my mind, against arguing for standing or flowing water at Meridiani). The "festoons" are allegedly ripples that are utterly unique to water-driven (not wind-driven) flow, according to the claim, although very similar features have been imaged in surge deposits (see above posts), so I am unconvinced, and most of the ones allegedly imaged by Opportunity appear to be topographical artifacts of the downward viewing angle (bedding contours wrapping around small ridges and V-ing up cracks).

The fine laminae and shallow cross-beds typical of Meridiani are also very typical of surge deposits, even quite coarse-grained ones, presumably owing to shear between the very high speed turbulent particulate flow and the substrate. Consistent thickess of laminae may indicate consistent velocity of the passing surge cloud - in any case, it is also typical (as it is of comparable wind and water deposits caused by turbulent flow).

All impact spherules are caused by vapor condensation in a hot turbulent cloud. Specular (blue-gray) hematite typically forms in steamy volcanic fumaroles by condensation and reaction of volatile Fe-chlorides or other volatile Fe species, and this is a very similar environment to that in a steamy surge cloud. The Meridiani difference is that some other sticky condensate must have caused the hematite flakes to preferentially adhere to each other and other particles, and grow as a snowball does, until they got too large, and settled towards the ground, where they were incorporated into the rapidly growing sand deposit, commonly in disseminated form. I don't pretend to understand all the chemistry going on in a dynamic, disequilibrium system like an impact surge cloud. Present were plenty of volatile iron species, at least possibly two sources of Ni species (Fe,Ni impactor or subsurface Fe,Ni sulfides), and abundant volatile salts and steam, and what resulted after condensation and crystallization were "blueberries". I don't know the details. The important point is that their mineralogy (specular hematite), high Ni content, size limitation, perfectly spherical habit, enormous extent, and failure to be distributed along fluid passageways or mixing zones indicates that they cannot be concretions. Therefore they must be condensates, analogous to hailstones. There is no reason to expect their Fe/Ni ratio to match that in meteorites, BTW (contrary to a claim made by the MER team).

As regards Br/Cl, see the above post on salts. By our hypothesis, extreme fractional crystallization of chloride salts, yielding high Br/Cl in residual brines and the last crystallized salts, occurred long before the impact episode, owing either to downward freezing of brines in the regolith (our favored mechanism) or surface evaporation of brine lakes (too cold, by our thinking, although surface freezing followed by sublimation works). These Br-enriched brines or late salts were then available to be incorporated in the impact ejecta, and the brines, at least, could have flowed laterally quite far away from their parent chloride salts. Frost leaching would preferentially remove surface chlorides, leaving surface sulfates, as covered in our 2002 and 2003 publications.

These are very good questions, BTW.

--Don

Always flatter your inquisitor!