[X] My Assistant

[dburt]

...you're killing me, Professor; none of my profs ever had a sense of humor like this...

One quick question re the 'ancient salts' hypothesis: What about the atmospheric effects of comparatively recent vulcanism such as the Tharsis Uplift? (IIRC, that's thought to have happened 100-200 MY BPE). It seems reasonable to assume that most of the outgassing was CO2 and water accompanied by a rise in atmospheric pressure (how much & for how long, no idea...though the Big Four are, like, big, so thinking that this had to go on for some time).

Anyhow, what I'm wondering is how apparently water-soluable salts could have survived near the surface during this epoch; certainly atmospheric water vapor should have penetrated the surface below the first few inches of the soil. This makes me think that these deposits were produced later in Martian history after vulcanism subsided and the atmosphere devolved into its present state.

Owing to plate tectonics, Earth probably has at least as much volcanism today as Mars ever did, plus coal-burning power plants and smelters are continously tossing a sulfuric acid precusor (SO2) into the air. This has minor effects (such as slight climate cooling - which we badly need in Phoenix) but the ground hardly is littered with jarosite, gypsum or other relatively insoluble sulfates. Consider early degassing of an entire molten planet, compared with the little bit of steam and CO2 and SO2 that comes out of a lava flow or even volcanic steam explosion - a drop in the bucket, literally. As I understand it (not my field), the atmospheric pressure on Mars is close to "buffered" by CO2 ice at the poles and in fact Mars has been continuously losing atmosphere very slowly since the end of the late heavy bombardment. Evidence for climate change is mainly blamed on obliquity and orbital variations (how much Mars tilts on its axis and how non-circular its orbit is). Some people try very hard to make Mars temporarily warm and wet with volcanism, which may be possible, given how little atmosphere there is to begin with, but I imagine that this effect could be dwarfed ty the effect of a large impact (especially during the boom boom boom of the late heavy bombardment). Impact was adequate both to warm Mars up (very temporarily) and deposit thick layers of salty sediments. As should be obvious from my abbreviated history, I'm with the very wet, but very cold and salty school of early Martian geology (at least until more information becomes available). Show me the palm trees, I say, before I bring my swimming suit.

The fact that water soluble salts have survived near the surface is just what makes me think that it hasn't rained on those parts of Mars in 3.8 billion years - or at least, not much. (Snow or frost is okay for survival of sulfates - see below.) In Arizona, after a small rainstorm (something else we badly need), the sulfates on mine dumps sink into the ground owing to dissolution and then, over the next several weeks, they reappear as colorful crusts referred to efflorescences (because they can look like flowers blooming). I think that's what we're seeing in the cauliflower-like crusts of salts being imaged just below the surface at and near Home Plate. The low pressure makes the salts effloresce just beneath, rather than above, the surface. The driving force is simple capillarity and evaporation of moisture - dip a paper towel into salty water, the water rises up (wicks up) by capillarity, and then evaporates, leaving salts behind - far above the surface of the water. And many salts generate their own brine from atmospheric moisture, so that capillarity can make deep salts wick upwards towards the surface, without rain. This effect was much discussed with regard to formation of "sulfate duricrust" during Viking days, but seems to have been largely neglected in recent discussions.

My general attitude (Occam's razor) is I don't want to invoke any unusual effect or phenomenon, especially anything I can't see direct evidence of, until I am forced to by the data. That's how I feel about "warm, wet" early Mars - it seems to me that impact alone, for which there's abundant evidence at every scale, may suffice for a lot of what people want to blame volcanism, or greenhouse gases, or whatever on. Occam was apparently a medieval theologian (and logician) who got upset when people wanted to blame every event or phenomenon on mysterious, invisible angels. I feel the same way about invisible geological features (missing playas and missing volcanoes) on Mars.

BTW, if you want surface water under present martian surface conditions, just dissolve lots of salts in it (mainly chlorides, not sulfates - the sulfates litter the surface, and the chlorides don't, because the sulfates have very little abilty to depress the freezing point of ice). If frost lands on mixed salts, the chlorides are leached, and the sulfates are not. Knauth and I published papers on that in 2002 and 2003, as mentioned in a previous post, I think, and blamed the crystal cavities at Meridiani on the same phenomenon in our 2005 Nature paper.

Playing the professor again, I'm afraid. All out of jokes, though.

--Don

[dburt]

So when is some geological society going to host a conference where proponents
of these theories can face off. It would be good to get more geologists with varied
expertise and experience weighing in.

Ever hear of the "lunatic fringe"? That's us, in most people's eyes.

[dburt]

You are wrong.

--Bill

Quite probably, but how? (Despite being a professor, I am teachable, I hope.)

[dvandorn]

...missing volcanoes ...on Mars.

Ummm... if the Tharsis shield volcanoes and dozens of other classic calderas readily observed on Mars aren't volcanoes, and if the very clear and obvious lava flows (which read as basalt from both surface and orbital spectral analysis) aren't volcanic, then what are they?

I hate to say this, but if your theories are based even a little tiny bit on this "observation" that Mars is "missing volcanoes," then you do seem to be ignoring empirical data (labeling it "theoretical") that doesn't fit with your own theories, and that's when the scientific method fails.

I'm sorry -- I simply *must* have misinterpreted what you mean, here, since that statement is so obviously false.

-the other Doug

[dburt]

Why assume that the salts did not form at Meridiani billions of years ago.
Is it only because they had to form somewhere else in order to be moved
to Meridiani by base surge?

Good question. If I understand it, your argument is they had to form somewhere, so why not at Meridiani, where we find them? (Great example of Occam's razor at work, by the way.) I don't want to write another long post, because I'm about to go home, but maybe I can practice with a short one. The basic arguments are several: 1) We see no evidence of a geological environment that would permit salts to grow at the surface - no shales, meaning no standing water for an evaporite basin or even puddle - not even in deep material excavated from Victoria. The playa lake, if any, has "vanished". 2) The Meridiani salts appear to be an intimate mixture of highly soluble and highly insoluble salts (a mechanical mixture, in other words) plus poorly characterized fine debris (not crystalline clays). Salts crystallize out of an evaporite basin in inverse order of solubility - least soluble first, in a "bathtub ring" around the outside fringe (usually gypsum), and then more and more soluble salts in zones towards the center. Meridiani is nothing like this. The MER team agrees with us that the salts had to come from somewhere else - they invoke wind transport and mixing (reasonable - plenty of wind on Mars), we invoke impact transport and mixing (plenty of evidence of impacts, of all ages, but especially for early Mars). 3) The salts are very fine grained and the rocks appear uniformly porous and permeable. Growth from water, or soaking in water, would have caused permeability decrease and crystal size increase. This argues strongly against the MER team interpretation that the salty rocks were soaked in brine many times after they were deposited, and that brine mixing in a huge, uniformly permeable volume produced concretions. (The US Government permanently stores military radwaste in evaporitic salt beds, because they are so impermeable.) There are other more subtle geochemical arguments, but those 3 are the easiest to understand, I think. Let me know if you need clarification.

Well, out to play in the real desert (i.e., go home) . Soon the daily summer afternoon dust storms (haboobs) will begin in Phoenix (sort of a very dilute, relatively slow-moving, cool, dry analog for a ground-hugging surge cloud - but something that a martian surge cloud could easily decay into with distance, perhaps). Thank y'all very for your comments (even you, Bill Harris - I really admire your succinctness ).

--Don

[centsworth_II]

Let me know if you need clarification.

Ok, everyone agrees that the sulfates were moved to their present location from
elsewhere. I'm wondering about the very fine layering seen at Meridiani. I have
a hard time imagining such fine layers being formed by such a violent, sudden
activity like base surge. Do you have any feeling for what depth of layering
would be attributable to a single base surge event?

Apart from the fine scale festoons that the MER team attributes to water, there
are the massive chunks that really do look like fossil dunes (or large wind ripples).
Do you also attribute those to base surge?

Even assuming that the layers and berries could be the result of base surge, I
have a hard time understanding the formation of the large crystals whose
dissolution formed the vugs. You've said:

"No more moisture is required than would be present in the original surge cloud
(mainly condensing steam) or could later be removed from the atmosphere by
water-attracting (hygroscopic or deliquescent) salts."

And:

"Only minor water, such as liquid condensed from the steamy surge cloud or water
attracted from the air by the salts itself, is needed to leave the Meridiani crystal cavities"

Is the formation of such large in-ground crystals by trace water (on Earth) a geological
observed fact? Or is it a "fringe" hypothesis? I have no real geological background.

[dvandorn]

Yes -- I brought up the finely layered nature of the rocks at Meridiani before, and that question was never addressed by the Professor. I can see these rocks being built up by aeolian deposition of millimeter-deep layers over the course of tens of thousands of years, laid down on wet ground (and possibly onto shallow standing water), far more than I can see tens of thousands of base surges, each laying down a very thin, very flat layer of rock of consistent composition to the last base surge, each laying down a very flat, very thin layer with almost no turbulence developing along the surge/ground contact.

Such surges would, I would think, have enough energy in them that we would see scouring and channeling -- the very same types of landforms whose lack that the Professor cites as a disproof of standing water or a playa environment. If these layers were laid down by an energetic base surge, how can they be so overwhelmingly flat, with very little sign of any turbulence? (Remember, the cross-bedding we've seen is the exception, not the rule, in these rocks.) Are you postulating that there were *no* surface features that would have caused turbulence in the surge/ground contact? (We may not be able to watch and observe a surge in detail, but we have a ton of similar surge-emplaced landforms that we have observed in great detail on the Moon, and even with a lack of atmosphere, we see evidence of a fantastic amount of turbulence in the debris flows that generated terrain on Luna.)

I also have a difficult time understanding how these deposits could have been laid down at the end of the Late Heavy Bombardment. There is visual evidence supporting the theory that the rough, cratered terrain generated by the LHB actually underlies the Meridiani deposits. You'd have to assume, based on the range of crater-like landforms, the relative lack of large craters, and the relative flatness of the terrain, that the nearly kilometer of Meridiani deposits were laid *after* the LHB had finished. In other words, if the Meridiani unit was generated by tens of thousands of impacts at the end of the LHB, why would the unit not have been broken up by these impacts as quickly as the base surges laid it down? What makes Meridiani so special that it could be laid down by impacts all around it but not suffer any impacts in the area itself, thus leaving this layered unit (which would have required millions of years of base surges to lay down) mostly intact?

Occam's razor suggests that we're seeing an entire population of dead grandmothers here...

-the other Doug

[dburt]

Ummm... if the Tharsis shield volcanoes and dozens of other classic calderas readily observed on Mars aren't volcanoes, and if the very clear and obvious lava flows (which read as basalt from both surface and orbital spectral analysis) aren't volcanic, then what are they?

I hate to say this, but if your theories are based even a little tiny bit on this "observation" that Mars is "missing volcanoes," then you do seem to be ignoring empirical data (labeling it "theoretical") that doesn't fit with your own theories, and that's when the scientific method fails.

I'm sorry -- I simply *must* have misinterpreted what you mean, here, since that statement is so obviously false.

-the other Doug

You got me. Sorry, I guess I should have explained myself much better. I didn't mean the giant Tharsis and similar basaltic volcanoes, which are nowhere near Meridiani, and are non-explosive (Hawaiian style flowing lava only, in general). I meant the small "invisible" exploding volcano presumed by the MER team to have produced the surge deposits at and near Home Plate (that type of steam explosion generally leaves a highly visible crater or construct - consider Diamond Head in Honolulu, which is of this type - and the deposits are usually very local, extending only few kilometers). I also meant the likewise "invisible" but necessarily very much larger Yellowstone Park-style "supervolcano", that McCollom and Hynek (2005, in the same Nature issue as our impact paper) hypothesized for producing Meridiani. Perhaps they saw the recent Discovery Channel film "Supervolcano." It has an excellent depiction of a giant volcanic surge cloud from Yellowstone killing several geologists fleeing in a truck, wheras their boss, in a helicopter high overhead, survives because the flow was hugging the ground. A similar volcanic surge cloud is depicted towards the end of the film "Dante's Peak". No such giant exploding supervolcanoes have yet been documented on Mars, to my knowledge, although various people have called upon them to produce layered deposits. Impacts, for which there is abundant evidence, seem more logical to me.

You mean you can't read my mind?

--Don

[dvandorn]

That makes sense, Don -- though IIRC, isn't there a large, heavily eroded caldera structure in the middle of Meridiani Terra, which (in global terms) lies adjacent to Meridiani Planum? Could be a source of volcaniclastic materials in the Meridiani Planum area, could it not?

I'm still interested in how you explain the finely layered nature of the rocks seen at Oppy's landing site (and, by inference, the entire 800m thick Meridiani unit). And I'd love to see computer simulations of the turbulence at the surge/ground contact...

-the other Doug

[dburt]

I'm wondering about the very fine layering seen at Meridiani. I have
a hard time imagining such fine layers being formed by such a violent, sudden
activity like base surge. Do you have any feeling for what depth of layering
would be attributable to a single base surge event?

Apart from the fine scale festoons that the MER team attributes to water, there
are the massive chunks that really do look like fossil dunes (or large wind ripples).
Do you also attribute those to base surge?

Even assuming that the layers and berries could be the result of base surge, I
have a hard time understanding the formation of the large crystals whose
dissolution formed the vugs. You've said:

"No more moisture is required than would be present in the original surge cloud
(mainly condensing steam) or could later be removed from the atmosphere by
water-attracting (hygroscopic or deliquescent) salts."

And:

"Only minor water, such as liquid condensed from the steamy surge cloud or water
attracted from the air by the salts itself, is needed to leave the Meridiani crystal cavities"

Is the formation of such large in-ground crystals by trace water (on Earth) a geological
observed fact? Or is it a "fringe" hypothesis? I have no real geological background.

Excellent questions. I'll start with the fine layering. That fine layering and cross-bedding was what most astonished me the first time I was exposed to volcanic surge deposits in Utah, while studying volcanic rocks as possible sources for uranium (under a government-university contract) in the late 1970's. The deposits had been misidentified as "water laid tuff" for about 15 years by a few Geological Survey employees, who couldn't believe it either (they ignored the lava flows on top, which might have suggested volcanism). Of course, they were mineral deposit types and sedimentologists who had never studied volcanoes before, whereas I cheated by having a leading expert on explosive volcanism (former ASU Prof. Mike Sheridan) with me at the time, which unfairly helped the diagnosis. Co-author, Ken Wohletz, a doctoral student at the time, was along and did his ASU Ph.D. thesis on surge deposits, before he was hired at Los Alamos. (BTW, a 1983 paper by Wohletz and Sheridan first suggested that Mars rampart crater ejecta should be examples of impact surge deposits.)

While I'm recounting history, in the mid-1950's Gene Shoemaker (who later became world famous for his studies of impact cratering) started out studying volcanic surge surge deposits in New Mexico and Arizona (looking for uranium too) before returning to school to complete a Ph.D. dissertation on Meteor Crater, AZ. He couldn't believe the giant sandy cross beds (dune forms) he saw were caused by explosive volcanism either, and initially attributed them to wind action. (Even most volcanologists were making similar mistakes in that era.) Since then studies by numerous volcanologists (clued in by nuclear bomb tests, where surges and their cross-bedded deposits were first described) have documented that thin bedding, low-angle cross-bedding, high angle cross-bedding, dune forms, and ripple forms can occur in surge deposits, all features that excellently mimic wind or water deposition. Both air and water are fluids that carry solids. Evidently, a hot, turbulent suspension of divided solids in a rapidly-moving gas cloud behaves as a fluid too. The stickiness that condensing steam gives to particles apparently can help them be deposited more rapidly than normal sediments (truly wet surge deposits can be plastered onto the side of trees). Some terrestrial surge deposits appear so ambiguous that non-volcanologists are still arguing with volcanologists over volcanic vs. wind-caused deposition - and those are the deposits that occur next to a crater (information from Wohletz).

The extremely poor sorting (mixture of large and small pieces), together with the cross-bedding and fine layering, is fairly diagnostic of surge deposits close to the explosion site, but sorting increases with distance, so that a mainly sandy deposit can be hard to diagnose. Bomb sags, caused by ballistic ejecta of rocks by the explosion, have been used to diagnose volcanic surges (e.g., at Home Plate), but ballistic ejecta are also typical of impacts. Disseminated accretionary lapilli resembling the Meridiani "blueberries" (except in composition) are common in some volcanic surge deposits, and would never be expected in a wind- or water-deposited sediment. (Post depositional concretions tend to be much more irregularly sized, shaped, and distributed than accretionary lapilli.)

The thickness of the deposits depends on the size (magnitude) and distance of the explosion - but several meters to several tens of meters are typical of volcanic deposits, where the explosions involved much less energy release than a decent-sized impact releases. For very small volcanic surge deposits (such as those protected from erosion by overlying basalt flows at Peridot Mesa, AZ) you can hike out and follow the gradual transition from several 10's of meters of coarse, cross-bedded ejecta (containing blocks 1 m or more across) to a thin deposit only a few cm thick, consisting of well-sorted fine sand - still finely layered, with shrinkage cracks resembling those at Meridiani.

As for large, in-ground crystals: I invite you to go to an Arizona rock shop and buy a "desert rose". These are fairly large crystals (up to many cm across) that form in damp desert soil from the little moisture available. They are made of gypsum, hydrated calcium sulfate, a salt mineral reported from Meridiani (the least soluble one). Or take some wall-board, which is made of fine gypsum, moisten it in a jar for a few weeks, and watch it "rot" as it recrystallizes (as wall-board in a home will do if made damp for too long). I haven't carried out this experiment: let me know the results if you try it.

We hypothesized that the Meridiani crystal cavities were caused by some type chloride salt crystals that grew right after surge deposition, when things were still warm and very damp - early diagenesis, a sedimentologist might call it. Leaching could have occurred later, by frost.

That's all for now - got to go to a meeting. Hope this answers your questions.

--Don

[Shaka]

Excellent questions. I'll start with the fine layering.

---snip---
That's all for now - got to go to a meeting. Hope this answers your questions.

--Don

Prof Don,
First let me thank you for the time you have taken to explain your hypothesis and clarify questions from our members. Let me also compliment you on the lucid arguments you have prepared. I find it hard to imagine that many of your students would need to participate in the exam-week "Slaughter of the Innocents", given the clarity of your explanations.

That said, I must suggest that your answer above has tended to skirt the issue of the laminated structure of the Burns Formation as it relates to impacts. Your interesting anecdotes do an excellent job of showing how explosive volcanism on Earth could produce laminated, even cross-bedded, sediments of a similar appearance, and I, for one, would agree entirely with such an alternate cause if there were a range of stratovolcanos evident on the margins of the Meridiani Planitia. A series of wind-blown ash clouds from cyclic eruptions or periodic pyroclastic flows might produce something like a Burns Formation, given a long enough volcanic history, but, as you indicate, this kind of vulcanism is not prominent on Mars. Of course, the accumulated ash would grow enormously thicker and more pronounced than the Burns layers as one approached the source volcanos. I think the MOC imagery would have revealed some trace of this by now.

But that is quite apart from the question as to whether bolide impacts could produce an accumulation of ejecta resembling the Burns Formation. Many of us have a real problem with that. Impacts do form layered ejecta, but generally not more than two or three layers per impact, and these layers are usually quite distinctive lithologically. I think there are about three detectable in the mid North American continent from the Chicxulub impact. The lowermost from the high velocity jets of ballistic ejecta, the middle from the fallback surge of the collapsing ejecta column, and the third from the slow rain of fine distal fallout re-entering the atmosphere globally.

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, as can be seen in the Caribbean environs of Chicxulub. It is hard to imagine that the coarser proximal ejecta would not appear anywhere in the series, excepting the Victoria ejecta at the surface.

In many ways this argument is analogous to that which you make for blueberries. How can there be so many, so uniform and limited in size range? Actually I have less trouble believing that a broad, uniform plain of well-mixed thin sand layers, as Meridiani may once have been, could, when exposed to brief episodes of uniform wetting with very limited supplies of slow-moving groundwater, produce a crop of very uniform concretions with very limited growth, before the Big Freeze-Dry stopped everything but the wind.

Furthermore, the berry distribution through the evaporite is even, as might be expected from a growth process strictly confined by a limited supply of solute, rather than random or aggregated as might be expected for accretionary lapilli being tossed and hurled by the violent currents of an impact surge.

I am sure that the MER team model for Meridiani leaves some questions unanswered, but I still think it leaves fewer than the impact or volcanic alternatives. I don't have the chemistry expertise to deal with those issues, but as I drive around the Meridiani Planitia, courtesy of my good friend Opportunity, I get the sense that this has been one of the quieter, more-peaceful corners of Mars for much of its history. It's been a nice place to sit and ponder.
Cheers,
Shaka

[dburt]

I can see these rocks being built up by aeolian deposition of millimeter-deep layers over the course of tens of thousands of years, laid down on wet ground (and possibly onto shallow standing water), far more than I can see tens of thousands of base surges, each laying down a very thin, very flat layer of rock of consistent composition to the last base surge, each laying down a very flat, very thin layer with almost no turbulence developing along the surge/ground contact.

Such surges would, I would think, have enough energy in them that we would see scouring and channeling -- the very same types of landforms whose lack that the Professor cites as a disproof of standing water or a playa environment. If these layers were laid down by an energetic base surge, how can they be so overwhelmingly flat, with very little sign of any turbulence? (Remember, the cross-bedding we've seen is the exception, not the rule, in these rocks.) Are you postulating that there were *no* surface features that would have caused turbulence in the surge/ground contact? (We may not be able to watch and observe a surge in detail, but we have a ton of similar surge-emplaced landforms that we have observed in great detail on the Moon, and even with a lack of atmosphere, we see evidence of a fantastic amount of turbulence in the debris flows that generated terrain on Luna.)

I also have a difficult time understanding how these deposits could have been laid down at the end of the Late Heavy Bombardment. There is visual evidence supporting the theory that the rough, cratered terrain generated by the LHB actually underlies the Meridiani deposits. You'd have to assume, based on the range of crater-like landforms, the relative lack of large craters, and the relative flatness of the terrain, that the nearly kilometer of Meridiani deposits were laid *after* the LHB had finished. In other words, if the Meridiani unit was generated by tens of thousands of impacts at the end of the LHB, why would the unit not have been broken up by these impacts as quickly as the base surges laid it down? What makes Meridiani so special that it could be laid down by impacts all around it but not suffer any impacts in the area itself, thus leaving this layered unit (which would have required millions of years of base surges to lay down) mostly intact?

Occam's razor suggests that we're seeing an entire population of dead grandmothers here...

-the other Doug

You raise a lot of excellent points, other Doug, which clearly indicate careful thought. I'll try to take them in order. The fine layering I already addressed in a previous post - it is highly typical of surges and for me was their most surprising feature. Deposits laid down solely on wet ground owing to stickiness probably would not be cross-bedded (as every layer seen to date in Meridiani is). Sedimentts deposited in standing (as opposed to flowing) water would never be cross-bedded and would be likely to be fine mud instead of sand unless they were deposited right at the shore or during a sandstorm. The lack of mud beds (shales) and universality of cross-bedding provide our main basis for stating that Meridiani sediments contain no record of standing water (i.e., no playa is possible, at least not to the depth of exposure/excavation in Victoria or the other craters visited by Oppy).

To make cross-bedding, you need erosion (scouring) followed by deposition. For the wind, this is how sand dunes migrate - they are eroded on one side and the sand is then redeposited on the other side, giving you the giant, high-angle cross beds typical of old dune fields. Dunes (and, for wet and sticky enough particles, antidunes, where the dune grows upwind) are also common in surges. Flowing water making current ripples (so-called "festoons") works the same way, but on a smaller scale. Similar-appearing features can also form in surges, apparently, although they appear to be comparatively rare. Most cross bedding in surge deposits is at very low angles, because the particles are generally moving very fast ("whoosh!"). Low angle cross bedding dominates the Meridiani sediments, consistent with surge deposition, but probably not wind.

As for scouring, inasmuch as every cross bed is a record of scouring, I presume you really meant just to ask about channels. Local channels are highly typical of many areas of surge deposition. They usually are attributed to a swirling vortex (part of the turbulence) moving radially outward from the explosion. Linear, radial channels in volcanic surge deposits (such as those resulting from the 1980 eruption of Mt. St. Helens, WA) have been documented by, e.g, Grant Heiken and Sue Kieffer. By our hypothesis of impact surge deposition, the linear grooves visibly radiating outwards from a great many Mars impact craters (as imaged by both MOC and Themis) would be evidence of just such vortices in an impact surge cloud. Burns Cliff itself appears to contain an example of such a scour - it occurs just beneath what the MER team called the "Wellington Contact" and is near the center of, e.g., this raw image from Sol 287:

http://marsrovers.jpl.nasa.gov/gallery/all...MIP2270L5M1.JPG

Unfortunately Oppy was unable to examine this scour up close, but I saw similar appearing scours while examining various surge deposits in Oregon last summer (described by Grant Heiken and visited by Apollo astronauts, in case some lunar craters turned out to be volcanic). The MER team interpretation is that the "Wellington Contact" is a former water table of regional extent (although it has only been imaged only in Burns Cliff), and the dune-deposited sand was wind-eroded down to this planar water table. How the wind could erode such a localized channel BELOW the water table was never addressed, to my knowledge. (The eroded channel is unlikely to be a stream channel, because it contains no coarse material at the bottom, plus it would be conceptually difficult for water OR wind to erode a stream channel below the water table.) Since they made this suggestion, I have examined several former water tables in exposures of the Navajo and Page Sandstones of Arizona, and I have never seen such a channel feature - the contacts are planar, commonly with shale along them where there was local standing water. That's also where lots of hematitic concretions tend to occur, all clumped together in planar masses. Our impact interpretation would be that the "Wellington Contact" is just a large cross-bed, perhaps marking the contact between two successive surge clouds, with the localized scour marking a turbulent vortex in the younger surge cloud. If so, finding such channels on opposite sides of a given crater exposure might indicate a direction radially towards or away from the parent impact crater (something to look for if Oppy successfully enters Victoria).

Your age question I would answer somewhat speculatively by saying that there has been considerable wind erosion of fines in the past 3.8 billion years. Surge clouds usually are energetic enough to ride up over highlands and settle in lowlands (as notably happened in mountainous areas near Mt. St. Helens in 1980, as today evidenced by the pattern of downed trees). The deposits in highlands tend to be thin and to consist mainly of fines, whereas the lowlands deposits are thick and coarse (more resistant to erosion). In other words, the fluid-like surge clouds tend to "pond" in relatively low areas, despite their energy and high velocity, because they are density currents. The Meridiani lowlands were, in addition, selectively protected from later erosion by their "desert pavement" of dense, unusually large (1-5 mm) hematitic impact-related spherules, which may have been lacking or abraded away in the nearby highlands (pure speculation). BTW, areas of layered deposits in the nearby cratered highlands are reported by Edgett (2005), in his thoroughly-documented article in the first issue of the on-line Mars Journal. As mentioned in a previous post, Edgett also documents that cratering was concurrent with deposition of the main Meridiani sequence (and that some of the lower layers appear to be channeled). With regard to flatness - the wind has done a pretty good job of keeping the terrain flat by erosion (clearly visible at Victoria - look at what happened to the coarse fragments of ejecta); Edgett earlier predicted this from orbital imaging. Also, as mentioned above w.r.t. erosion, although surge clouds can override obstacles (unlike liquids like water or lava), they also tend to fill in lowlands ("pond") and, along with later wind erosion/deposition, could have covered up older or coeval craters.

With regard to comparisons with the Moon: The extremely important difference between Mars and the Moon is that Mars had an atmosphere and lots of suburface brine or ice at the time of late heavy bombardment (and still does today, to a lesser amount). In the dry vacuum of the Moon, impact implies predominantly ballistic processes (like fragments shot from a cannon) no matter how small the particle. The only vapor formed was vaporized impactor and vaporized silicate rock, and these vapors apparently condensed almost instantly into tiny glass spherules. It has been suggested that particle-to particle interactions in a vacuum could have produced a gas-like surge cloud on the Moon, but, to my knowledge, no such finely layered deposit was ever spotted by the Apollo astronauts (who were specifically trained to look for them). Constant "gardening" by micro impacts may have degraded any exposures, however (whereas Mars has enough of an atmosphere to largely protect it from most such "impact gardening" - at the outcrop scale, at least.) On Mars the impact vapors must have contained lots of steam (and vaporized salts), allowing turbulent surge clouds to form that in many ways resembled those formed by smaller explosive volcanic eruptions, where the dominant gas involved is also steam. As each turbulent, scouring, particle-rich surge cloud flowed radially outwards along the ground, it mixed with the atmosphere and decompressed and cooled. Steam (and probably vaporized salts) then condensed onto particle surfaces, making them sticky, and causing them to stick together and agglomerate or acrete into spherules called "accretionary lapilli" (a possible origin for the Meridiani "blueberries" - like a tiny rolled snowball made of sticky particulate rock). In this regard, the so-called rampart craters appear to be unique to Mars, and have always been attributed to either the subsurface volatiles ice or brine or the atmosphere (people still argue - Mars scientists will argue about anything ).

That's rather a long post, but then you asked rather a lot of questions (all great ones, BTW). I agree with you that there are far too many "dead grandmothers" (a.k.a. implausibilities) still associated with the various Meridiani hypotheses (not unusual for Mars). We arrived at the impact hypothesis by the process of elimination - impact seemed to offer fewer implausibilities than either wind/water or volcanism. For example, if Mars wind today can only erode a few meters per billion years, how could it have deposited nearly a kilometer of sediments at Meridiani? And if all those sediments are sulfate-rich, you might have to evaporate a playa lake deeper than Olympus Mons is tall (sulfates aren't all that soluble). I'll stick with the impact hypothesis for now - impact seems like the only process able to do all the work required (until a better idea comes along). Also, as mentioned in an earlier post, we cannot exclude water, wind, or volcanism in any combination as having contributed to the unstudied deeper layers at Meridiani or anywhere else, although we suggest that they might not much be needed.

Impact surge isn't the only plausible depositional process associated with impacts (ignoring the obvious ballistic ejecta, think about all the dust produced! and all the steam! and all the gases other than steam! and all the heat! and all the melt!) - it's just the only one that seems to explain the sandy, salty, cross-bedded, spherule-bearing beds imaged by the first two rovers at both Meridiani and Home Plate, on opposite sides of the planet. It also predicts that similar salty, cross-bedded, spherule-bearing beds could be found at the surface by later rovers (HiRISE imaging, as did earlier MOC imaging, suggests such layers could be common). The most elegant and marvelously detailed Meridiani hypothesis (vanished, highly acid playa lake/wind/soaking acid water/more wind/more soaking acid water/deep flowing - but never standing - concentrated acid brine/magically-diluting-and-mixing water), even if you ignore all of its special assumptions and internal inconsistencies (such as why cold water crystallized the high temperature, blue-gray form of hematite, and why that hematite should be enriched in Ni, and why the soluble salts never recrystallized, and many more I haven't mentioned), at best predicts and explains nothing at all about anywhere else, not even about the almost identical appearing beds at Home Plate.

So when is someone going to give me a hard time about Home Plate? (Although I'm perfectly willing to try to revive more "dead grandmothers" at Meridiani, if I can... )

--Dr. Don

[ElkGroveDan]

For example, if Mars wind today can only erode a few meters per billion years, how could it have deposited nearly a kilometer of sediments at Meridiani?

I think it's more than a little disingenuous of you to assume constant rates of erosion over 3.8 billion years, in of all places, Mars. That Mars has experienced a history of varied atmospheric densities isn't even in dispute. Indeed I can't image your "surge cloud theory" working under present 6 millibar conditions unless you admit that atmospheric densities were higher in the past. And of course in that case we are looking at higher rates of erosion and deposition.

(I also believe that the .38g on Mars will contribute to a much stranger movement of particle masses than has occurred in the Cascades, but I'm not prepared to wade into that debate right now.)

Don, you have some interesting ideas and your theories are certainly contributing to the discussions, but a lot of this minuscule "evidence" you cite is a bit tortured. Sort of like finding a candy wrapper and declaring "children were here!" I think you've killed off more than a few grandmothers yourself on this thread.

[David]

What does this "kilometer of sediments" refer to? Is that supposed to be the depth of the stratigraphy? How could we know it actually goes that deep?

Sorry for the ignorant question, I just feel I'm missing something here...

[dburt]

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...your answer above has tended to skirt the issue of the laminated structure of the Burns Formation as it relates to impacts.
[/quote]

See my discussion of surge deposit textures that was posted after you wrote this. Fine laminations and low angle cross-bedding, presumably caused by shear, characterize many surge deposits, despite their rapid deposition. Early in Mars history, there seems no limit to how many distant impacts could have contributed to the layering, although one big one might have been enough. Unlike with volcanism, impacts can occur anywhere, anywhen, into any available target, from a variety of possible impactor types, and be of any size (up to destroying the planet). A single volcanic surge eruption can result in many meters of section, containing many dozens of layers, varying in character between dune-like and relatively flat-bedded, with some containing disseminated accretionary lapilli and some not. Kilbourne Hole, New Mexico is a famous example of such a deposit, with slopes that resemble Burns Cliff, and dune forms that Gene Shoemaker initially ascribed to wind.

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...explosive volcanism on Earth could produce laminated, even cross-bedded, sediments of a similar appearance, and I, for one, would agree entirely with such an alternate cause if there were a range of stratovolcanos evident on the margins of the Meridiani Planitia.
[/quote]

I am reduced to citing volcanic analogs because, to be frank, no one has seen a surge formed by an impact (except presumably the dinosaurs and pterosaurs, who didn't live to tell about it), on this planet or any other. Impact deposits on this planet (except the coarse fraction - commonly called suevite if it is glassy) are weathered and eroded virtually immediatedly, unless they settle on the ocean, in which case they are water reworked, resorted, and highly hydrated (the fragmentary record of Chicxulub, the dinosaur-terminating impact, is largely written in gummy clay pseudomorphs). Owing to its dry, cold, near-vacuum conditions for most or all the past 3.8 billion years, Mars may turn out to be the best place in the Solar System to preseve a record of impact processes on a planet with subsurface volatiles and an atmosphere. Volcanoes, on the other hand, on Earth explode almost every week or month somewhere, so surge processes can be observed, and deposits can be fresh and unaltered, plus older deposits are commonly preserved under a capping lava flow, scoria deposit, or ignimbrite (welded ash flow tuff). Stratovolcanoes, as you probably know from your use of the word, would not be expected anywhere on Mars, owing to the lack of plate tectonics (they generally form only above subduction zones), nor would Yellowstone Park caldera-type supervolcanoes, because there is no hydrous granitic crust for a mantle plume to melt. In any case, even if Meridiani were a record of such an eruption, we'd might never know it, because the surge beds would probably be buried beneath a much thicker layer of erosion-resistant ignimbrite (welded tuff).

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...But that is quite apart from the question as to whether bolide impacts could produce an accumulation of ejecta resembling the Burns Formation. Many of us have a real problem with that. Impacts do form layered ejecta, but generally not more than two or three layers per impact, and these layers are usually quite distinctive lithologically. I think there are about three detectable in the mid North American continent from the Chicxulub impact. The lowermost from the high velocity jets of ballistic ejecta, the middle from the fallback surge of the collapsing ejecta column, and the third from the slow rain of fine distal fallout re-entering the atmosphere globally.
[/quote]

Your guess is as good as mine - because, like all scientific hypotheses, ours an informed guess with considerable evidence to support it and apparently nothing impossible (like a little boy claiming his grandmother was an ant or an elephant instead of merely dead on a test day) against it. Chicxulub, for which a highly imperfect record is recorded in a few spots fairly close by, impacted into the sea, with a rock target of layered carbonate rocks and anhydrite, on a planet with a strong gravity field and a relatively dense atmosphere, so it may not be representative of Mars processes.

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...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, as can be seen in the Caribbean environs of Chicxulub. It is hard to imagine that the coarser proximal ejecta would not appear anywhere in the series, excepting the Victoria ejecta at the surface.
[/quote]

Congratulations! You have put your finger right on the weakest aspect of the impact surge argument. This group is really as sharp as I'd hoped it would be! 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. Finally, in its rush to get to Victoria (and not get stuck again), Oppy by-passed several areas of coarse broken rock imaged at a distance by the Pancam. These appeared to be lag deposits, and could imply wind erosion of a coarse layer stratigraphically just above the layers now exposed. 3) Sand grains carried by the wind whipping across the plain of Meridiani would eventually plane off and erode any coarse ejecta, unless it were buried - look at what has happened to the coarse ejecta blanket of Victoria (I admit that, given the slow rate of erosion on Mars, this is kind of like my arguing to teacher that my third dead grandma had an unusual sexual preference for the time or my grandpa had a sex change operation). 4) Perhaps my best answer is to simply cite William K. Hartmann, in his marvelous 2003 book "A Traveler's Guide to Mars", as noting that impact into sand or soft sandy sediments is going to mainly scatter more sand. (I don't have the book here at home, but I believe he used the phrase "produce a kablooey of sand and dust" which is not a bad description of a turbulent surge cloud.) In other words, by the end of the late heavy bombardment, much of Mars may have been so beat up that many impacts were "beating a dead horse" in terms of producing coarse ejecta.

But hey, each kid (or hypothesis in this case) is allowed up to two dead grandmothers, isn't he? This is only the first that I'm aware of. (Note, we can't allow two per author, because that would give the Athena Science team an unfair advantage, although, IMHO, they might have exceeded their allowance even with that unfair method of counting.)

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...In many ways this argument is analogous to that which you make for blueberries. How can there be so many, so uniform and limited in size range? Actually I have less trouble believing that a broad, uniform plain of well-mixed thin sand layers, as Meridiani may once have been, could, when exposed to brief episodes of uniform wetting with very limited supplies of slow-moving groundwater, produce a crop of very uniform concretions with very limited growth, before the Big Freeze-Dry stopped everything but the wind.

...Furthermore, the berry distribution through the evaporite is even, as might be expected from a growth process strictly confined by a limited supply of solute, rather than random or aggregated as might be expected for accretionary lapilli being tossed and hurled by the violent currents of an impact surge.
[/quote]

I obviously can't stop you from believing what you want, but I thought I'd already covered this in previous posts, to a certain extent. I've spent much of my life looking for signs of fluid flow (mainly of hydrothermal fluids) and so far none have been imaged at either landing site, IMHO. (White deposits in surface cracks don't count because, like white desert caliche in cracks, they mainly indicate capillary action of moisture near the surface - an expected finding in the presence of water-attracting, soluble salts.) On the other hand, I can go anywhere in the Navajo (or Page) Sandstones, cited by the MER team and Marjorie Chan as a Mars analog, and see that the distribution of hematitic concretions (all red-brown and never blue-gray, of various sizes and shapes, commonly clumped together in nodular masses) is related to brine mixing and flow - distribution along former water tables, along fractures, both cutting across and along specific bedding planes, sudden lateral terminations, etc.). This distribution bears no resemblance to that at Burns Cliff or elsewhere.

If you put a less dense brine on top of a more dense brine, as the MER team proposes formed hematite from jarosite, it will sit there essentially forever - diffusion is possible over at most a meter or two in a sandy aquifer (water-saturated porous, permeable rock). How thick is Burns Cliff, which appears to be "spheruled" throughout? If you inject a less dense brine into a more dense brine from below, it will simply rise to the top as a plume, and you have the same problem again. How do you uniformly mix such brines across an area the size of the state of Oklahoma? It shouldn't even be possible in one place. (I trust some hydrologists will back me up in this.) Besides which, as mentioned in previous posts, rocks so rich in soluble salts should become impermeable owing to recrystallization within a very short time of being immersed in a saturated brine.

In our 2005 Nature article, Knauth described how presumed layered impact spherules (altered accretionary lapilli, as for Chicxulub, about 5 mm in diameter, as for Meridiani) in Archean (oldest known terrestrial) layered rocks are widely and uniformly distributed across areas of South Africa and Australia comparable to the Meridiani hematite area. Therefore, impact still seems like a far more reasonable process than brine mixing to scatter uniformly tiny hematitic spherules across such a wide area. I would be willing to wager that, no matter where future landers might land, no hematitic spherules or small groups of spherules larger than could be supported in a turbulent surge cloud will be found. (Of course, Oppy may prove me wrong as soon as it enters Victoria, or it may find some shale layers suggesting standing water (or compressed loess, deposits of airborne-dust), but right now I'd be willing to bet against either possibility. Note: some exceptionally big hailstones, such as those in the US national news last week, are large enough to break car windshields. I suspect that the Meridiani "hematite hailstones" (if that's what they really are) may themselves represent an exceptionally coarse deposit, judging from their apparent uniqueness on the martian surface.)

[quote name='Shaka' date='Jun 22 2007, 06:12 PM' post='93253']
...I am sure that the MER team model for Meridiani leaves some questions unanswered, but I still think it leaves fewer than the impact or volcanic alternatives. I don't have the chemistry expertise to deal with those issues, but as I drive around the Meridiani Planitia, courtesy of my good friend Opportunity, I get the sense that this has been one of the quieter, more-peaceful corners of Mars for much of its history. It's been a nice place to sit and ponder.
[/quote]

We weren't there 3.8 billion years ago, and neither was Oppy. Everything since has been pretty calm, perhaps, but no more so than across most of the rest of Mars. (As mentioned above, if it hadn't been a calm spot since then, Oppy wouldn't have landed there. Also, remember what Bill Hartmann said about a kablooey of sand likely obscuring part of the cratering record.)

BTW, please excuse any typos in this and several previous posts. These are proofread by no one, least of all me. I say that because doing these posts is an exercise like taking a very, very long essay exam, and I don't want you taking any points off for my poor proofreading. (Writing essays is something I haven't had to do for 40 years or so, and back then it was all in longhand.) Lots of role reversal going on here, and this group has some potentially great professors!

--Don