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Meridiani Ice theory, New theory for the formation of the Meridiani bedrock
Doc
post Feb 17 2009, 12:48 PM
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Hi all, long time no see.
Yet another theory to explain the origin for the bedrock encountered by Opportunity.
I first heard of this new theory from Universe Today;

http://www.universetoday.com/2009/02/16/ne...t-mars-equator/

An abstract is available at http://www.nature.com/ngeo/journal/vaop/nc...bs/ngeo438.html

The theory is interesting, but I would really be interested to know how it explains the spherules seen by Opportunity not to mention the vugs.
Thoughts any one?


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Juramike
post Feb 17 2009, 02:22 PM
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Interesting!

(Corrected Universe Today link here: http://www.universetoday.com/2009/02/16/ne...t-mars-equator/)


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centsworth_II
post Feb 17 2009, 03:51 PM
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QUOTE (Doc @ Feb 17 2009, 07:48 AM) *
The theory is interesting, but I would really be interested to know how it explains the spherules seen by Opportunity not to mention the vugs.

Right. From the article:
Michalski said the new theory gets around a lot of the sticking points in the older ones.
"It doesn't require a basin to be present; it doesn't require the groundwater," he said.


But the formation of concretions would require groundwater. Or does he not think the spherules are concretions?
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Doc
post Feb 17 2009, 04:06 PM
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Exactly Centsworth, c'mon people start brainstorming!


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sariondil
post Feb 17 2009, 04:36 PM
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Regarding the vugs, the article says they originate

"presumably from recrystallization and dissolution processes that occurred during diagenesis".

The implication seems to be that the sediments only were altered by water some time after their deposition in the Late Noachian or Early Hesperian.
Concerning the spherules it is stated that

"The thermal infrared spectrum of each of these areas [Meridiani Terra] shows an alignment of the haematite c axis, which is rare in terrestrial rocks and is probably caused by spherule growth."

Is this alignment anyhow related to a specific formation mechanism for the spherules?
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centsworth_II
post Feb 17 2009, 04:45 PM
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QUOTE (sariondil @ Feb 17 2009, 11:36 AM) *
Concerning the spherules it is stated that

"The thermal infrared spectrum of each of these areas [Meridiani Terra] shows an alignment of the haematite c axis, which is rare in terrestrial rocks and is probably caused by spherule growth."

This looks to me like a description of how the spherules affect IR spectra of the area taken from orbit and not a description of how they were formed.
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ngunn
post Feb 17 2009, 04:49 PM
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Once you postulate a thick ice cover I suppose you can imagine subglacial hydrology going on concurrently with sublimation from the upper surface. (In order to pond at a given spot subglacial liquids don't require a basin in the topographic sense, just a closed isobar.) Once the ice had gone for good you might be left with the products of both sublimation and briny subglacial wet chemistry interleaved in complicated ways. OK that's my brain sufficiently stormed for now.
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imipak
post Feb 17 2009, 09:13 PM
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ngunn, could you elaborate on what a "closed isobar" means in this context? I'm having some difficulty picturing the topology of a blister of liquid beneath an ice-sheet. I wonder if this model has any implications for the expected local or regional topology of resulting deposits on the surface today - might the outlines of this area be evidenced in remaining terrain? The article mentions Antarctic examples of this phenomena but if I submit to the call of the Google, I'll never get this pile of work done... sad.gif


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Paul Niles
post Feb 17 2009, 10:25 PM
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I'm glad to see a few people here that are interested in this. The idea is that we have two stages of aqueous alteration here.

The first stage caused the formation of sulfates and weathering of primary basaltic material into silica and phyllosilicates. We propose that the silicates weathered in very small pockets within the ice where liquid water films+silicates+acidic aerosols can all react together. The reason we call on this idea is because the chemistry of the Meridiani outcrops shows that all of the major cations are still present in the abundances you would expect from a fresh basalt, but are present in strongly altered mineral phases (sulfates, silica, phyllosilicates). So it is as if the whole outcrop was weathered in a closed system.

The second stage occurs during diagenesis of these materials after deposition. This is very similar to the ideas already proposed by Squyres et al.. Here we propose that the grains, made up of highly hydrated phases from the sublimation residue of this massive ice deposit, are reworked by aeolian processes and deposited in a crossbedded sequence. As they are buried they dehydrate -- and generate enough water to power a very limited diagenesis to form the blueberries. It has to be very limited because of the presence of both hematite and jarosite which indicates that whatever reaction happened, it didn't go to completion.

Paul
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ngunn
post Feb 17 2009, 10:31 PM
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QUOTE (imipak @ Feb 17 2009, 09:13 PM) *
"closed isobar"


Under the ice water flows from higher to lower pressure locations. Elevation is only one of the parameters affecting the pressure, and not necessarily the dominant one. So 'lakes' don't have open surfaces but rather roofs which are in general not flat. They are places where the pressure gradient is inward along the whole circumference - hence closed isobar. Water can flow uphill as well as down, depending on the overburden in different places. I only know what I've read in popular sources but it all sounds horrendously complex - and confusing.

I raised this because I think that on Mars subglacial waters could have gone on doing their work long after surface waters (or even near-surface waters) ceased to be viable. All you need is a large persistent ice layer protected by a thin crust of sublimation residue. Large buried ice deposits outside the polar regions have indeed been discovered in recent years, so some of the ice is still there. It may now be frozen right down to its base (I don't know) but it need not always have been, and there could once have been a lot more of it.

Normally I keep quiet on this, but whan someone proposes massive equatorial ice deposits on Mars I think of Antarctica.
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ngunn
post Feb 17 2009, 10:39 PM
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Hey - Paul Niles, welcome! More please - most of us don't have access to the full paper.
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dburt
post Feb 17 2009, 10:52 PM
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QUOTE (sariondil @ Feb 17 2009, 09:36 AM) *
...Concerning the spherules it is stated that
"The thermal infrared spectrum of each of these areas [Meridiani Terra] shows an alignment of the haematite c axis, which is rare in terrestrial rocks and is probably caused by spherule growth."

Is this alignment anyhow related to a specific formation mechanism for the spherules?

The Nature Geoscience article, which I've seen in its entirety (as a reviewer), allows for spherule formation via one of two mechanisms: First, "low water/rock ratios and low temperatures" that made possible "the forced hydrolysis of jarosite to haematite" without "wholesale re-equilibration of the deposit and complete conversion or jarosite to haematite" or second, "primarily through impact events," a process that "also provides a means for precipitating the haematite spherules without invoking extensive diagenesis" [and also, I might add, explains their high temperature or gray nature and their unusual uniformity in size and shape].

In other words, the article suggests that spherules could have formed either through local low temperature diagenesis or through high temperature impact-related processes. That the hematitic spherules could be impact-related accretionary lapilli of unexpected chemistry has previously been discussed on UMSF in a separate thread (long since closed).

In its summary, the article describes a possible ice-related geochemical formation mechanism for the acidic sediments, and then states that they appear to have been "reworked by eolian or impact processes" and then underwent "limited diagenesis".

This article examines one hypothesis for how to make and preserve acid sulfates on a basaltic (basic) planet: Keep them mostly frozen to prevent the acid from being neutralized by the basalt (a necessity earlier recognized by Roger Burns and by many other scientists, including Paul Knauth and me - and later validated by alkaline soil pH findings of the Phoenix Lander). As we earlier hypothesized, and this article agrees, no early "warm, wet" greenhouse is indicated by or required to form the Meridiani deposits, because no acid seas, lakes, or groundwaters, except mostly frozen ones, are possible on a dominantly basaltic (and, we emphasized, impact-pulverized and therefore highly chemically reactive) planetary surface.

-- HDP Don
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Floyd
post Feb 17 2009, 11:39 PM
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Welcome Paul Niles. It is always great to have authors clarify questions on their publications. UMSF has members with great interest in space exploration--with really varied backgrounds and expertese on any particular topic.


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Paul Niles
post Feb 18 2009, 01:00 PM
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For those of you so inclined, you can read my LPSC abstract on the topic. It is a shorter version of the paper -- although not much shorter, and accessible by all.





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ngunn
post Feb 18 2009, 01:28 PM
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Thanks for that - it's a very interesting read. You mention 'a common formation process which must have acted over a large area of Mars'. Do you have a view on how much of Mars this ice sheet might have covered at its maximum extent?
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