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Posted by: BruceMoomaw Mar 17 2006, 11:51 PM

Burt and Knauth's LPSC abstract on their theory is http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1869.pdf . Burt also has an abstract from his poster on the subject at http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2295.pdf -- including a highly acerbic section on a "young sedimentologist" of his acquaintance who misinterpreted terrestrial basal surges as water-laid tuff. Seems the ingrate didn't even acknowledge in print that Burt had instantly disproved his long-time belief... This debate seems to have the potential to get seriously personally nasty.

While Burt and Knauth object for other reasons to McCollom and Hynek's rival theory that the Meridiani deposits are due to a VOLCANIC -- rather than an impact-caused -- basal surge, they seem to agree with them that one of the biggest objections to the MER team's belief that the deposits were laid down by alternating wet and dry episodes on the surface is that it stretches chemical coincidence. In their own abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2023.pdf ), McCollom & Hynek reiterate their argument at the December AGU meeting -- which touched off quite a spectacular brawl itself, which unfortunately I couldn't hang around for the end of -- that the MER team's theory is Rube Goldbergian.

The MER team thinks that the sulfates in the Meridiani deposits were originally produced ELSEWHERE by exposure of basalt to acidic groundwater, then dried out, then got blown by the winds to mix with the native sands at Meridiani (which were NOT raw basalt, but already consisted of siliciclastic sands which had PREVIOUSLY themselves been basalt exposed to sulfuric acid, but had then had all the sulfate salts which had thus produced locally leached out of them). This mix then got exposed to groundwater AGAIN (this time non-acidic, and/or relatively small in quantity), so that it only modestly redissolved the sulfate salts, which then dried out and recrystallized mixed with the same siliciclastic sands -- the whole mess having been changed in physical texture, but not further chemically altered, by that last exposure to kinder, gentler groundwater.

As McCollom and Hynek point out, it's stretching coincidence in that case to assume that the current Meridiani layers just happen to have precisely the same ratios of different elements that you'd get if they had been made far more simply by just ONE exposure, in-situ, of local basaltic particles (sand or ash) to sulfuric acid. In the MER team's model, you have to mix inblown sulfate powder with in-situ siliciclastic sand in just the right ratio, by pure coincidence, to get that same mixture of elements: "... the bulk composition of the [Meridiani] bedrocks would require that the amounts and relative proportions of cations that were removed from the primary siliciclastic component prior to its incorporation into the rocks must be exactly balanced by those added back in by evaporating solutions, so that the final rock has the composition of Martian basalts. Although not impossible, it seems like a somewhat unlikely coincidence that this would occur and that the balance would be achieved so uniformly in all rocks at Meridiani." (They add that it's hard to think of a source of such massive amounts of pure sulfates elsewhere to get blown into the Meridiani region, since Mars Express hasn't seen any comparably huge deposits of phyllosilicate sand or rock elsewhere that would have been left behind after the sulfate powders had been wind-blown out of that area.)

So McC. and H. instead argue that the Meridiani layers are the result of a Martian version of Mt. Katmai's Valley of 10,000 Smokes, but much bigger and a lot more acidic (which is plausible, given that Mars seems to have far more sulfur in its overall crust than Earth does): "In this scenario, the rocks were initially deposited as a series of volcanic ash flows of basaltic composition. Following deposition, the ash deposits were permeated by SO2- and steam-rich volcanic vapors that altered the ash at elevated temperatures. In this process, SO2 and H2O in the vapors combined to form sulfuric acid, which then reacted with the rocks, similar to the acid-sulfate alteration observed in volcanic environments on Earth. During alteration, the original igneous minerals are replaced by alteration products including phyllosilicates (e.g., nontronite, saponite), amorphous silica, hematite and sulfate salts. Morphological features such as cross and festoon bedding, interpreted by the MER team to result from eolian and fluvial processes, are also observed in base surge deposits in volcanic settings and appear to be consistent with a volcanic scenario. The scale of the deposits appears to be consistent with large volcanic deposits elsewhere on Mars." Burt and Knauth agree, except that they don't think there are signs of any volcanic regions in or near Meridiani big enough to do this.

Now, McC. and H.'s main argument, the chemical one, could seemingly be explained just as well if local basalt sands at Meridiani were simply exposed to highly acidic but cool and liquid local groundwater that gushed into the region from somewhere. And Benton Clark's observation that Martian atmospheric processes (due to the fact that solar UV can reach all the way down to the surface) can generate far more sulfuric acid on that planet's surface than on Earth could explain the existence of such cool acid solutions. After the fight at the AGU, I phoned and E-mailed Hynek on this point, and he actually agreed -- but said that the MER team apparently disagreed with that particular theory on some grounds involving grain texture in the Meridiani rocks, about which he really didn't know the details. Now, another print-only LPSC abstract by the MER team ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1655.pdf ) explains their reason for opposing this version: "Observed compositions require either that basaltic sands were altered in place by abundant and pervasive acidic groundwater, or that alteration occurred primarily in the source region, followed by the formation, transport and deposition of sulfate-rich sand grains, with subsequent diagenetic redistribution of the most highly labile mineralogical components. The roundness of observed grains and, especially, the locally complete obliteration of grains during diagenesis strongly favors models in which alteration precedes sand generation and transport." That is, the MER team thinks that if the layers were produced by the in-situ exposure of basalt sand to sulfuric acid solution, MER-B's Microscopic Imager would be seeing both much sharper-cornered siliciclastic grains in the stuff, and a lot more still-recognizable grains of locally crystallized sulfates that had not been ground up into powder.

I wonder about that, though -- especially since Mikhail Zolotov thinks that sulfuric acid solution reacts very dramatically and efficiently with basalt, perhaps so well that by itself it totally dissolves and remixes the resulting sulfate crystals and also rounds the grains of left-over siliciclastic material from the basalt. Knauth and Burt actually use Zolotov's statement, in their LPSC abstract, to argue against the idea of the Meridiani layers being created by ANY kind of exposure to sulfuric groundwater: "Zolotov [8] has presented a compelling argument that regional acid aquifers on Mars are untenable because acid would be quickly neutralized by reaction with basaltic material." But if the surface production of really large amounts of sulfuric acid on Mars really is as feasible as Benton Clark thinks -- by atmospheric rather than volcanic processes, out of Mars' surface water in all its different physical phases plus the planet's volcanic sulfur dioxide -- it might very well be possible anyway to get the huge supply of local H2SO4 solution needed to radically modify all that huge supply of basaltic sand that was originally at Meridiani. Knauth and Burt also do seriously question how much the grain textures in the Meridiani layers really can tell us about how they were formed.

In short, there may still be FOUR different models for how the Meridiani layers were formed -- and the simplest one, that local basalt sands got soaked in local sulfuric acid-rich groundwater, may turn out to be the correct one after all. I've really needed to talk to Hynek about this again for some time, and I especially do now.

Posted by: Bob Shaw Mar 18 2006, 12:21 AM

Bruce:

Sounds like somebody should be trying some Mars analogue minerals experiments - it'd hardly be rocket science! Or perhaps there's something the mining engineers knew all along...

Bob Shaw

Posted by: Shaka Mar 18 2006, 12:25 AM

Lovely! Nothing like a good old-fashioned scientific bun-fight. Everyone gets thoroughly exerted, but everyone walks out alive - by separate exits.

Posted by: nprev Mar 18 2006, 12:38 AM

I have to ask where the blueberries fit into this debate? Concretions would seem to require a substantial period of groundwater saturation, if not actual surface pools, to form...and Meridiani is full of these things, apparently across different geological units based on the observations from Endurance and other exposed vertical sections. To me, this argues for multiple aqueous events rather than one...

Posted by: paulanderson Mar 18 2006, 03:13 AM

Emily now has has a good overview posted of The Great Meridiani Debate, LPSC-style (starting about 3/4 down the page):

http://www.planetary.org/blog/article/00000497

Sounds like a good time had by all...

Posted by: BruceMoomaw Mar 18 2006, 05:17 AM

Actually, Emily's piece was my takeoff point -- I read her to discover what more had been said on the subject at the actual meeting, beyond what was in the LPSC abstracts.

One more note: Burt and Knauth quote Zolotov as saying that it's unlikely that any lake of surface liquid water on Mars could be acidic enough to convert basalt sands in-situ to create Meridiani's layers. But in the Zolotov paper that they cite ( http://www.lpi.usra.edu/meetings/earlymars2004/pdf/8036.pdf ), Zolotov bases this conclusion entirely on his assumption that any acid waters on Mars' surface could only have been produced by the same processes that produce occasional small acid pools on Earth -- nowhere does he consider Clark's theory that atmospheric processes triggered by solar UV may have created far vaster amounts of sulfuric acid on Mars than on Earth.

Clark's idea would also mesh well with the fact, pointed out by Zolotov, that so far there's no evidence for acidic waters touching the Mars meteorites, which on the contrary contain significant traces of carbonates. The SNC meteorites, after all, were buried at least a moderate distance below the surface of hardened lava flows on Mars before the impacts that launched them to Earth, and so would be untouched by Hesperian Mars' large supply of surface H2SO4 (as indicated by Mars Express' map of extensive sulfates) -- if that H2SO4 was indeed created by atmospheric rather than volcanic processes. Such acid would modify the hell out of Mars' upper surface, but be quickly neutralized by that very process before it could sink deep into the Martian subsurface.

Posted by: MichaelT Mar 18 2006, 08:50 AM

Thanks for the highly interesting overview Bruce! Like nprev I'd also be interested in your opinion on how the hematite spherules fit into the whole story. As far as I remember they required water to be involved in their formation, didn't they? So how could they have formed without it?

Michael

Posted by: odave Mar 18 2006, 01:14 PM

This is science at work! An apt analogy I heard somewhere recently is that one group hangs out its hypothesis like a Piñata, and everybody else gets to whack at it

Posted by: Bill Harris Mar 18 2006, 02:42 PM

I think of this as the Basal Surge BS. There may be some features we see that can be caused by this, but for the most part they are straining at a gnat. Hawking pet theories to the exclusion af all others is much like a religion: my cult is right and the chosen word, all otheres are wrong. This rather reminds me of the 1960's volcanism_vs_impact camps of lunar crater formation.

--Bill

Posted by: tty Mar 18 2006, 05:11 PM

I don't buy the Katmai/Valley of ten thousand smokes analogue for three reasons:

1. The "ash" at Katmai is actually an ignimbrite and there is not a trace of ingnimbrite texture in Meridiani. A "cool" ashfall wouldn't be able to mobilize large quantities of water.

2. The "smokes" (steam) at Katmai was derived from deep peat layers immediately under the ignimbrite. It seems unlikely that there would be enough ice/water close to the surface in Meridiani (remember that as far as can be judged from orbit the sulfate depoosits are a couple of hundred meters deep).

3. Where is the volcano?


tty

Posted by: Richard Trigaux Mar 18 2006, 05:31 PM

This idea of meridiani being ash deposits further altered with highly acid water is consistent with my own theory as what some huge water surges on mars could have a volcanic origin, mainly enormous eruptions of steam, carbon dioxid, sulphur dioxid, hydrogen sulphide, and ash. Such an eruption result from the extreme maturation of a magma chamber, and it is extremely violent (like the Pinatubo, but much larger and released in less than one hour). Such eruptions would end in huge rains, soaking all the souther highlands and producing the numerous river traces seen there. From the ash, it would rather be mud flows, which are much more efficient than pure water to carve valleys.

Posted by: Steve Mar 18 2006, 07:02 PM

I wonder.... What would be the most likely meteorological effect of the release of large quantities of water vapor in the low pressure and temperature of the Martian atmosphere? In no particular order, I can think of:

1. Rapid dispersion with little local effect...
2. Local rainfall or snowfall...
3. Local condensation as heavy dew or frost...

A naïve look at that kind of release suggests further immediate local cooling as the vapors expand (think of a CO2 fire extinguisher) favoring items 2 or 3. To get very far beyond this kind of "hand waving" explanation calls for some calculations and/or experiments.

Posted by: Bob Shaw Mar 18 2006, 07:09 PM

Wind?

Bob Shaw

Posted by: Richard Trigaux Mar 19 2006, 07:44 AM

On Earth, such eruptions produce large clouds which can sometimes evolve in ordinary rain or storm clouds. Of course it is mostly mud rain, but water is often the most prominent component.
On Earth too, these steam eruptions are contained by the surrounding atmosphere, and the clouds remain local. In clear, the lava-water mixture expands from a compressend state (hundred or thousands of atmospheres) to a 1 atmosphere pressure. (In the way this liquid mixture boils and is separated into hot steam and solid dust). The net result is a cloud, at one atmosphere, made of dry steam** and dust, at temps between 100 and 500°C or more. Still on Earth, this cloud rises swiftly or falls violently on the ground, depending on its density (its dust ratio). It is not adiabatic* as it quickly swallows the surrounding air, and thus its temperature lowers until the steam condenses. At that stage, all the cloud material falls on the ground, under the form of a water or mud rain. But parts which already reached a high altitude can still expand on hundreds of kilometres large, and give huge clouds of dry steam and dust. For instance, most of the Pinatubo clouds fell of the groud, producing very violent basal surges. But part of them spread at high altitude untill they filled all the stratosphere of the Earth with a fine white dust haze.


And on Mars? There are two differences:
-On Mars the eruptions were probably several orders of magnitude larger, to produce such huge caldeiras of tens of kilometres large. And the larger such eruptions are, the shorter they last (Pinatubo: one month. Taupo, New Zealand: estimated two hours). Probably the caldeiras were produced in some minutes only, releasing in this short time the volume of water of a small sea!!

-On Mars the usual atmospheric pressure is hundred time less, so that the clouds will not be contained, and they will expend radially rather than in altitude. Also the eruptions plumes had to evolve in a more adiabatic* way (no air to swallow) untill a much lower temperature, allowing for condensation. Condensation of steam into rain produces energy which prevents further condensation, so that the condensation could occur only gradually.

So the result, I think, may rather ressembles the high altitude cloud of the Pinatubo, but swiftly rampant on the ground: a fast spreading temporary steam atmosphere. There is an upper layer of dry steam** and dust, which weight allows for pressure building at its bottom. So there is under a lower layer of wet steam** where condensation takes place swiftly, producing clouds and a heavy rainfall of water or mud.

So, starting from the point zero of the eruption, we have a temporary atmosphere of steam, which may allow for a very unusual atmospheric pressure on Mars. This temporary atmosphere carries huge clouds and heavy rain falls. It extends radially all around the eruption groud zero, at a very high speed (extreme winds). Then, with condensation, the total amount of steam decreases quickly, untill only the upper dry steam layer remains, after say some hours. By the way this terrible atmospheric phenomenon had spread on half of the planet or more, producing a rainfall or mudfall of a magnitude unknown on Earth, able to carve in some hours the valleys we find everywhere in the southern highlands, like the M'aadim Vallis at Gussev.

After, remains only the upper layer, an atmosphere of dry steam, which will much slowlier condensate by night, re-evaporate at day, untill it is all trapped as ice on the poles. This condition may last some days or weeks, during which unusual phenomenon can take place: liquid water on the ground, night snow fall, which melt at day, producing the small gullies on sunny slopes, etc. But most of this liquid water may be quickly absorbed into the ground and freeze under the gullie's sediment aprons.


* adiabatic: which evolves without exchanging heat with anything else. Gasses in a engine piston evolve adiabatically, except when the fuel is on fire and produces heat.

** dry steam, which is at a temperature higher than condensation point. Dry steam is transparent, to the countrary of wet steam, at condensation temperature, which is cloudy and produces damp, rain or dew.

Posted by: Steve Mar 19 2006, 01:19 PM

Richard:

Thanks for the nice qualitative description of your model -- surely more detailed than my crude "fire extinguisher." I guess I still see a few questions.

The most crucial, perhaps, involves the thermal energy involved to convert sufficent quantites of water to steam that will produce "rainfall and mudfall of a magnitude unknown on earth" over "half of the planet or more." Since we're probably discussing rare catastrophic phenomena, we can allow for very large numbers, but a rough estimate of the amount of energy involved (and its source) would be helpful.

Almost as important is to estimate the amount of water involved and postulate likely local sources for such large quantities of water.

Posted by: Richard Trigaux Mar 19 2006, 06:44 PM

The energy source is simple: volcanic eruption. On Mars, the overal volcanic energy is much lower than on Earth (perhaps 5% if we assume the same uranium percentage, and no phase transition in the core). But, for whatever reason, the volcanoes are much less numerous (about 16 great volcanoes, against thousand on Earth) and eruptions are much more spaced in time (by periods of tens of millions years). So when an eruption occurs, it is understandable that it is much larger in size and energy, as testify the hugeness of calderas in those volcanoes. This is for the energy source.

And where the water comes from? Understand that before the eruption, this water is mixed with the magma (with lava) a thing which is made possible thanks to the high pressure underground. So it is understandable that when such a mixture arrives in free air, it boils very violently. On Earth, in eruptions of this type, like the Pinatubo, the mixtures boils in the volcano chimney, gets foamy, and with the loss of pressure it cools, and the foam turns to a mixture of steam and dust, which erupts to the surface at supersonic speed through a trumpet-shaped pipe of sometimes hundred of metres in diametre.

And why water would be mixed with lava? On Earth, this happens in subduction zones, when submarine alluvions are dragged inside the crust and mantle. As they are soaked with water, this water acts as a melting agent (the lava-water mixture melts more easily than the surrounding rocks). Being lighter than the surrounding rocks, the mixture rises to the surface into an explosive volcano.

But how water would mix with lava on Mars? Not by subduction (there is none). Two only possible explanations:
-the water is already into the mantle. And it concentrates into magma chambers, as they naturally rise up and maturate.
-the water comes from a huge and deep watertable. (In this case the eruption process is different: it is a phreatomagmatic eruption, where the heat of the lava evaporates vast quantities of water and creates huge explosions). Such watertables seem also involved into huge water surges like in Valles Marineris and around. What drives all this is still largely unknown, some invoke a mixture of water and carbon dioxyd which would boil suddenly and trigger huge floods.

Posted by: CosmicRocker Mar 20 2006, 06:27 AM

The blueberries are definitely an important part of this debate. From what we know of them, they do nicely fit with theories involving a concretionary origin, but I think some may be suggesting that such spherical objects might also be volcanic lapilli or impact melt spherules. I suppose you might also form them as concretions in a pile of acidic, steam-saturated basal surge deposits.

Regarding the general discussion in here, it's been fascinating reading all of the different hypotheses regarding the Meridiani rocks. I mean, really fascinating. It seems to me that in the absence of many of the key observations that one would need to come to some reasoned conclusions, individual imaginations allow for quite a number of different scenarios. Bruce started this off with a summary of the inital four, and several variations have been added. I think I could add a couple more based on things that are not certain about this period of time in Martian geologic history.

It seems to me that as scientists, we need to focus on the few observations that we have precisely framed in their observed physical context, and then look for the simplest models that would explain them. That's not a simple task, considering the important advancements that have recently been achieved in remote observation technology. I thought I was going to come to a conclusion here, but the more I think about it, I think I'll step back and wait for the major contendors to duke it out in the boxing ring. I still like the water hypothesis, though.

Posted by: paulanderson Mar 20 2006, 07:17 AM

One thing I've read is that the double and triplet berries seen early on (where they are "fused together" as such) are evidence of water origins, since that process requires liquid water, and couldn't happen with airfall deposits, impacts, etc.. Plus the high hematite content, of course. The MER team had ruled out lapilli or other types of spherules, in their opinion after long analysis. Can anyone comment further on this?

Posted by: Shaka Mar 20 2006, 07:22 AM

Yeah, Tom. Some are saying the BBs ain't legit. I dunno nothin' about no "volcanic lapilli"; I know somethin' about impact microtektites. I know they are beautiful little balls when they first land (usually smaller than a millimeter), but, the more diagenesis acts on them, the worse they look. They start out as glass beads and end up as clay blobs, and they don't grow over time. How you can start with beautiful beads, form rock around each one, and end up with bigger, even more beautiful beads is way over my head. Unless, they started in that rock, and grew while the passing liquids continued to feed them.

You got it on the water front, Man! 'At's gotta be where it's at! We can't take a dive on'is hypothesis, Tom! It coulda been a contenda'! It coulda had class!

StellaaaAAAAA!!

Posted by: Shaka Mar 20 2006, 07:56 AM

The only thing I know about volcanic airfall deposits is that they are supposed to have as many or more non-spherical products (up to Pele's Hair) as spheres. As to impact microtektites, they can collide while in a molten state to form doublets, but triplets are much more improbable. But, again, other shapes, like dumbbells, teardrops etc, which are produced as the molten droplets spin and separate in the atmosphere, should also be common. I do not recall seeing a dumbbell or teardrop BB in Meridiani. When they have cooled and solidified, microtektites can continue to collide at high speed, causing impact microcraters in their glass surface, again something I have not seen in Meridiani.

Posted by: Bill Harris Mar 20 2006, 01:27 PM

Tom, my first thought upon seeing the Blueberries was "lapilli". But knowing their composition and examining their occurance in place the only thing they can be is concretions. To think anything else is quixotic.

--Bill

Posted by: CosmicRocker Mar 20 2006, 04:04 PM

Don't get me wrong, people. I am not a proponent of a lapilli or impact spherule origin for the berries. I am pretty thoroughly convinced that they are diagenetic concretions. I only noticed that nprev and MichaelT asked about the significance of the blueberries in the context of the basal surge discussion and I was trying to resopond to their question. At the LPSC Grottzinger very effectively countered the other origins with an analysis of the berry distribution. I have been meaning to post more of my notes from that. I'll try to do that this evening.

Posted by: dvandorn Mar 20 2006, 05:06 PM

Nope -- more like rock science...



-the other Doug

Posted by: BruceMoomaw Mar 20 2006, 05:58 PM

There's been at least one LPSC abstract in the last couple of years listing reasons why the Blueberries are concretions rather than lapilli of any sort. Once I can find the damn thing in my records, I'll describe them.

Posted by: AlexBlackwell Mar 20 2006, 06:13 PM

LPSC abstracts are certainly good starting points for discussion and it's true that a lot of conference ideas eventually get published. Having said that, however, one should note that LPSC abstracts, even though they are more comprehensive than the typical conference presentation, are, like virtually every other conference abstract, not peer-reviewed. Just because something was presented at LPSC does not confer upon it some magical status. In fact, in my opinion, a lot of junk gets put into LPSC abstracts, and other conferences, too.

I only note this because you seem to rely a great deal on abstracts and very little on publications in peer-reviewed journals.

Posted by: Steve Mar 20 2006, 08:45 PM

Thanks Richard, I'll take on the problem. Rather than give an exact reply to my own question, here's a quick and dirty calculation which confirms that the energy involved is on the order of the largest recorded terrestrial volcanoes.

If we were to cover half the surface of Mars to a depth of 30 cm (yes I know its a foot but it's also an amount I found scattered in the meteorological literature for torrential rainstorms on Earth) we would get 2.7x10^13 cubic meters or 2.7x10^16 kg of water.

The volcanologists estimate the magnitude of earthquakes using the formula m = log(Mass)-7, where mass measures the ash deposit (as best I can tell from the limited sources at hand). I only have indirect access to the defining article Pyle, David M. "Mass and Energy Budgets of Explosive Volcanic Eruptions". Geophys Res Lett (1995) 5:563–566 as it is cited in Ben G. Mason, David M. Pyle, and Clive Oppenheimer. "The Size and Frequency of the Largest Explosive Eruptions on Earth". Bull Volcanol (2004) 66:735–748). Incidentally, this scale is defined to be consistent with the more widely known Volcano Explosivity Index, which emphasizes volume rather than mass of deposit.

This formula gives a magnitude for the postulated martian volcano of 9.3, ignoring the contribution due to solids. FWIW, cranking in the heat of fusion and vaporisation it would require 5.6x10^22 Joules to convert that ice to vapor (or if you prefer, it's a 1.3x10^7 megaton explosion).

Mason, Pyle, and Oppenheimer assign the largest known terrestial volcano, The Fish Canyon Tuff deposit (La Garita Caldera, Colorado, USA; 27.8 million years ago) a magnitude of 9.1 to 9.2 on Pyle's scale.

Despite all the approximations, the Martian volcano needed to produce the postulated flooding is close to the largest terrestrial volcano. Thus from the question of energies alone this model does not seem as totally unreasonable as I had first suspected.

Correction: For "magnitude of earthquakes" read "magnitude of eruptions."

Posted by: BruceMoomaw Mar 20 2006, 08:46 PM

Alex: "LPSC abstracts are certainly good starting points for discussion and it's true that a lot of conference ideas eventually get published. Having said that, however, one should note that LPSC abstracts, even though they are more comprehensive than the typical conference presentation, are, like virtually every other conference abstract, not peer-reviewed. Just because something was presented at LPSC does not confer upon it some magical status. In fact, in my opinion, a lot of junk gets put into LPSC abstracts, and other conferences, too.

"I only note this because you seem to rely a great deal on abstracts and very little on publications in peer-reviewed journals."

True, and the reason for that is simply that I can't get to most journal articles via Internet. I have to go running all the way down to the nearest university libraries to see them -- when they finally come in, that is, and when the students haven't made off with them -- and that, in my case, involves drives of about 30 miles (to CSU-Sacramento), or 50 miles (to UC-Davis), not to mention their extortionate parking fees.

Posted by: helvick Mar 20 2006, 09:16 PM

Interesting to http://www.lpl.arizona.edu/tekton/crater_c.html - A 14.5km diameter porous rock asteroid hitting mars at 45 deg. gives a yield of 1.29x10^7 megatons and the final crater is ~129km diameter.

There are lots of potential candidates for such impacts in mcaplinger's msss page on http://www.msss.com/http/ps/age2.html which has a nice map of the distribution of craters of 100km diameter and above.

On average I'd be incliined to think that impacts would be a more likely source for such events although clearly there is plenty of evidence for vulcanism.

Posted by: BruceMoomaw Mar 20 2006, 11:51 PM

Big news on this subject, in the form of an E-mail to me last night from Steve Squyres -- who says flatly that not only I, but also McCollom and Hynek, have been totally misinterpreting what the MER team thinks really happened at Meridiani, and that their real views are almost totally identical to my "fourth, simplest" model of how the layers were formed -- namely, that they were formed, on the spot, by exposure of basalt sand or ash to liquid groundwater with a large amount of sulfuric acid mixed in:

"... I would like to correct some points. Our observations at Meridiani reveal nothing about where the original interaction of acidic groundwater with basalt took place. The most likely scenario is probably that it happened right there at Meridiani Planum. Interaction of acid groundwater with basalt will produce both altered siliciclastic materials and -- when the groundwater evaporates -- sulfates rich in the very same cations (mostly magnesium, calcium, and iron) that were removed from the original basalts. This mix of siliciclastics and sulfate salts has then been 'reworked' locally by the wind. 'Reworked' does not mean that materials were carried in from some far-away location. It can simply mean that the wind took material that was already there, blew it around (as sand-sized grains), and piled it up into dunes. And in some locations, where we see the ripples, it was also reworked by small amounts of surface water.

"So, the scenario we have inferred based on our data is much simpler than the one you describe. Acid groundwater interacts with basalt, creating a mix of altered siliciclastics and, when the water evaporates away, sulfate salts. This siliciclastic/sulfate mix gets stirred around, as sand-sized grains, by the wind and occasionally by surface water. While it's certainly possible to invent more complex versions of this story, such complexity is not required by the data.

"PS: I should also mention that there is compelling evidence for subsequent influxes of groundwater after these materials were put in place, changing the chemistry and texture of the rocks deep in the section, and creating the blueberries. But that all happened after the original emplacement of the rock."

If so, this completely demolishes McCollom and Hynek's main argument -- that the MER team's view chemically requires a very lucky, pure-chance mixture of just the right amount of sulfate salts from elsewhere with on-the-spot phyllosilicate sand to precisely simulate what would have resulted from just mixing on-the-spot basalt with sulfuric acid. And when that argument is removed, it pretty much blows the bejeezus out of both McC. and H.'s volcanic basal-surge theory, and Burt and Knauth's impact basal-surge theory. The only other real arguments they have are:

(1) That (to quote Zolotov) there wasn't any adequately large source of such acid-saturated surface water on Mars. But, as I said earlier, Zolotov never even considers Benton Clark's view that large amounts of sulfuric acid were formed on the surface of Mars by its unique atmospheric processes -- far more than could ever have been spat onto its surface from underground by its volcanoes. And so the problem of an inadequate supply of sulfuric acid solution actually seems to strike much harder at the two basal-surge theories than it does at the MER team's theory.

(2) Quoting Knauth and Burt (abstract #1869): "The identification of the well rounded primary grains 0.1 to 1.0 mm is problematical. Sulfates have perfect cleavage and should be more angular if truly detrital. The 'grains' more strongly resemble diagenetic growths or wind-abraded efflorescences. [Also, a] process that would uniformly mix basaltic grains <100 microns into sulfate grains is difficult to envision. Even if the basalt mud were a component of the grains, density differences make it unlikely that its distribution would be so completely uniform in far-migrating wind ripples. In any case, acid waters would completely alter such tiny basalt particles into clay minerals."

But the MER team itself says (abstract #1655) that those rounded primary grains are rounded precisely because the original, coarser basalt sand or ash was first exposed to large amounts of acidic groundwater, and THEN dried out and blown around locally by winds, which both ground up the sulfate crystals into powder and rounded the little phyllosilicate dreg particles left behind by the acid -- and that this mix was later exposed to liquid water (not necessarily acidic) again, which redissolved the powdered sulfates and fused them into a solid matrix as well as creating the Blueberries: "Observed compositions require either that basaltic sands were altered in place by abundant and pervasive acidic groundwater or that alteration occurred primarily in the source region, followed by the formation, transport and deposition of sulfate-rich sand grains, with subsequent diagenetic redistribution of the most highly labile mineralogical components. The roundness of observed grains and, especially, the locally complete obliteration of grains during diagenesis strongly favors models in which alteration precedes sand generation and transport." The mixture MER-B now sees at Meridiani, contrary to Burt and Knauth, IS a mixture of sulfates with particles of "clay minerals", not with particles of "basalt mud".

Hokay. So -- now that my misunderstanding of what the MER team is actually saying is cleared up -- it looks to me as though they have very definitely got the better side in this fight. It's the two basal-surge theories that require relatively unlikely conincidences to work -- namely, how either type of basal surge could have gotten hold of enough sulfuric acid to chemically modify the original Martian surface rocks so thoroughly. Since I just love sprinking snuff on my hair and then sticking my head into a lion's mouth, I'll contact Hynek (and probably Burt or Knauth) directly on this to see what their replies are.

Posted by: AlexBlackwell Mar 21 2006, 12:18 AM

In other words, you've covered yourself by picking every plausible model; therefore, you can claim that you were, ultimately, correct? I like that stock broker-like approach ("The market may go up, or down, or stay the same").

Posted by: BruceMoomaw Mar 21 2006, 12:29 AM

Actually, the MER team's model -- or the version of it which says that the second exposure of the Meridiani material to water was to non-acid groundwater -- also explains the creation of the Blueberries much better than the two basal surge theories, since most models of Blueberry creation call for dry ferric iron sulfates (produced by a previous exposure of minerals to acid water) to be mixed with nonacidic water that makes the iron precipitate out again as concretions of hematite (or goethite, which can later dry up over the eons of Dry Mars' history to turn into hematite). See Ormo's LPSC abstract (#1356), as well as the various reports of what's going on at the strange environment of Rio Tinto in Spain. This would also seem to explain why locations with Blueberries seem to be a lot rarer on Mars (judging from Mars Express' and MGS' maps) than locations with just plain sulfates -- simple exposure to acid water is enough to produce sulfates, whereas a locale has to be exposed to acid AND THEN nonacid water to produce precipitated hematite.

Alex, this is getting tiresome. What Squyres said was just what I said he said: he showed me that I had seriously misinterpreted what the MER team was saying, and that they really support a view virtually identical to my "fourth" listed theory. My only consolation is that he says that the basal-surge advocates have also been totally misinterpreting what the MER team really said.

Posted by: AlexBlackwell Mar 21 2006, 12:39 AM

What's "getting tiresome," Bruce, is your penchant for strawman arguments. I never questioned "[w]hat Squyres said," so stop insinuating that I did.

I was only pointing out that, by your own admission, you laid out four models from other workers. And despite your "misinterpret[ion of] what the MER team thinks really happened at Meridiani," your "'fourth' listed theory" actually does agree with them; in fact, it was, in your own words, "virtually identical."

In other words, you were ultimately right (once again).

Posted by: BruceMoomaw Mar 21 2006, 02:52 AM

What you implied was that I had left myself an out for saying that either of two theories (or maybe four, since I don't fully understand your argument) was right. What I actually did was say (falsely) that the MER Team itself believed one theory, but that I wondered whether another one might match the facts better instead. What I actually found out was that I had been totally wrong in saying that they agreed with the first theory; they actually did agree with the second one. This is hardly the same thing as saying that I deliberately left myself an opening for saying that I was "right" in any case -- what it shows is that I was right on one thing and totally wrong on another. Please don't accuse me of being more egotistical than I actually am.

For whatever it's worth, I hadn't even contacted Squyres yet to ask him about this (although I probably would have tried to do so at some point in the next few days to straighten up my understanding of the whole situation). Instead, somebody else who reads this site (I don't know who) contacted him and quoted me to him, whereupon he got directly back to me.

Posted by: CosmicRocker Mar 22 2006, 05:43 AM

I promised to post some notes regarding Professor Grotzinger's comments about the blueberry spatial distribution. He noted that the berries were dispersed, not organized along bedding planes or cross-strata, as would be expected in the basal surge model. They did a Monte Carlo simulation of the berry distribution, and it was exactly what one would expect for concretions formed by molecular diffusion processes.

I might also note that he addressed the trough crossbedding and said, as others on the team have previously said, that the scale of the bedding or lamination is important, and the scales seen in Meridiani are a better fit with an aqueous origin than a surge origin. He pointed out that the gravitational constant was expected to have an affect on the scaling, but in their determination it was not a large afffect between Earth and Mars. I hope I captured that accurately, but I think that is what I heard him say.