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Regarding the "Basal Surge" hypothesis, What really did create the Meridiani layered deposits?
Guest_BruceMoomaw_*
post Mar 17 2006, 11:51 PM
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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.
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Bob Shaw
post Mar 18 2006, 12:21 AM
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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


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Shaka
post Mar 18 2006, 12:25 AM
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QUOTE (BruceMoomaw @ Mar 17 2006, 01:51 PM) *
-snip-

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.

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


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nprev
post Mar 18 2006, 12:38 AM
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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... huh.gif


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Guest_paulanderson_*
post Mar 18 2006, 03:13 AM
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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... wink.gif
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Guest_BruceMoomaw_*
post Mar 18 2006, 05:17 AM
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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.
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MichaelT
post Mar 18 2006, 08:50 AM
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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
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odave
post Mar 18 2006, 01:14 PM
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This is science at work! An apt analogy I heard somewhere recently is that one group hangs out its hypothesis like a Piņata, and everybody else gets to whack at it smile.gif


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Bill Harris
post Mar 18 2006, 02:42 PM
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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


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tty
post Mar 18 2006, 05:11 PM
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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
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Guest_Richard Trigaux_*
post Mar 18 2006, 05:31 PM
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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.
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SteveM
post Mar 18 2006, 07:02 PM
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QUOTE (Richard Trigaux @ Mar 18 2006, 12:31 PM) *
... 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.

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 naīve 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.
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Bob Shaw
post Mar 18 2006, 07:09 PM
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QUOTE (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 naīve 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.


Wind?

Bob Shaw


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Guest_Richard Trigaux_*
post Mar 19 2006, 07:44 AM
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QUOTE (Steve @ Mar 18 2006, 08:02 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 naīve 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.




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.
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SteveM
post Mar 19 2006, 01:19 PM
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QUOTE (Richard Trigaux @ Mar 19 2006, 02:44 AM) *
...
-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....
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.
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