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Doc
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?
Juramike
Interesting!

(Corrected Universe Today link here: http://www.universetoday.com/2009/02/16/ne...t-mars-equator/)
centsworth_II
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?
Doc
Exactly Centsworth, c'mon people start brainstorming!
sariondil
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?
centsworth_II
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.
ngunn
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.
imipak
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
Paul Niles
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
ngunn
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.
ngunn
Hey - Paul Niles, welcome! More please - most of us don't have access to the full paper.
dburt
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
Floyd
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.
Paul Niles
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.





ngunn
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?
Doc
Wow, thanks for abstract Niles. This is certainly going to be fit nicely in 'bed-time' readings.
Regards.
centsworth_II
QUOTE (ngunn @ Feb 18 2009, 08:28 AM) *
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?

This description from the LPSC abstract seems to indicate that the ice cap was perhaps even larger than today's polar caps, if my understanding is correct that the ice sheet being referred to was the actual polar cap back then.
"... massive dust/ice deposit that was located adjacent to Meridiani Planum. This massive dust/ice deposit... resembled the polar layer deposits that exist today in the martian north and south polar regions. Suspended dust... an important driver for the precipitation of water ice in the present day polar caps... would have been more prevalent on ancient Mars."
Paul Niles
The idea -- as it pertains to the rest of Mars -- is a little vague right now. But, we imagine that other sulfates/layered deposits on Mars formed through a similar mechanism. We are not suggesting that ALL of Mars was covered by a single ice sheet, rather that at different times, during different epochs, ice accumulated in these areas (where we find layered deposits+sulfates) -- and this ice-weathering process took place.

It is striking that almost all sulfate deposits on Mars occur with similar characteristics -- they are in finely layered deposits, intimately mixed with silicates, the deposits drape topography, the layered deposits frequently overtop the rims of the chasm/crater in which they are located, the deposits frequently show evidence for having had a much larger extent and have been easily eroded.

If Meridiani truly formed through this ice-weathering process -- then I would think that it is very likely that other sulfate deposits on Mars formed in a similar fashion given the many similarities that have been noted by a number of other studies. However, this linkage requires a lot more work to strengthen.

Paul
ngunn
Thanks Paul, that's very informative. I can see why the evidence to date doesn't pin down one scenario. But it seems to me that you are postulating either very mobile polar caps (a lot of polar wander) or a rather extensive ice sheet. Both are interesting. Can you post a map showing all the locations involved?
dvandorn
I seem to recall asking, in the extensive and now closed discussions with Herr Doktor Professor about alternate theories on the development of the Meridiani deposits, whether what we're seeing might not be the result of Meridiani at one time being much nearer the pole, i.e., that there may have been glacial alteration in this region.

Nice to see that my vagrant thoughts occasionally pop up in other peoples' serious theories... smile.gif

-the other Doug
Paul Niles
I have no Map nor do I know if a map exists. Definitely something I'd like to see. I'm not sure if someone out there is keeping track of all of the sulfate deposits -- I have seen maps like that - check here. But I think these things are changing on the month to month basis as CRISM and OMEGA make new discoveries.

We currently have no comprehensive theory for ice deposits on Mars or how they got there -- we simply think they were at Meridiani. Now we pose two mechanisms for what may have happened:
1) Obliquity variations: Basically the angle of tilt of Mars' axis of rotation vis a vis the Sun is thought to have varied considerably with time. Sometimes dipping down to as low as 40 degrees from vertical. These obliquity variations could cause ice to be redistributed from the polar regions to the equator. The ice would then move back again when the obliquity became more vertical. This might provide a mechanism for depositing large amounts of ice early in Mars' history on the equator, and then removing it again.

2) Polar wander -- this idea is that the actual map location of the axis of rotation of Mars was in a different place early in the history of Mars (before the growth of Tharsis). As Tharsis grew, it caused the axis of rotation to migrate across the planet until it reached its final location. Everybody is pretty sure this happened, but no one knows how large of an effect it had. It could have only moved a few degrees, while it may have moved all the way from the equator. We speculate based on the antipodal hydrogen deposits, and layered terrain in Arabia, that a paleo-pole was located near Meridiani. But if you ask most Mars scientists who are familiar with this subject you would find extreme skepticism -- for valid reasons. Mostly having to do with age relationships of the terrain in question and the growth of Tharsis.
Shaka
Having at last gained access to and read your paper with Michalski, I can congratulate you on your contribution to the Meridiani provenance debate. I look forward to reading the responses of the MER P.I.s who presently sponsor the "warm, wet playa" alternative hypothesis. Probably some of those responses have already been aired at the LPSC, but those of us unfortunates unable to attend must languish in ignorance of the state of play. It would be tremendously exciting for UMSF members if that discussion could visit this forum, but those involved may prefer to rely on the traditional, slower, more thoughtful avenues of debate in the refereed literature. I'm not sure of my own preference in the long term, but I am eager to see the outcome of this collision of hypotheses.

You interpret the regionally-uniform, but locally-improbable chemical ratios of Meridiani sediments as the result of regionally-uniform, but locally-confined reactions in icy microcosms. I am wondering if your model permits quantification of the microcosm system to such an extent that you might be able to predict, at least approximately, the abundances of chemical species found at present. You emphasize, for example, the very limited abundance of water in your reactive system. Could this serve to predict the limited size of the "blueberry" concretions? This issue is one of considerable interest to many of us, including Don Burt, who has listed it among the reasons he favors his "brine splat" impact hypothesis over the greenhouse-playa model. I am less convinced that a series of random meteor impacts might produce the present picture of Meridiani sediments, but I would be strongly persuaded toward your "icy-test tubes" model if you could predict the size-distribution of blueberries. That's probably asking a lot, but, if beyond reason, might you suggest another quantitative observation that would permit a clear choice from the three models? (Or do you feel you have already done that?)

In any case let me add my thanks for your willingness to discuss your ideas in this forum. I hope your experience here will prove congenial enough to persuade other senior authors to emerge from 'lurk-mode' and join in our "cyber-seminar".


Paul Niles
I love the "icy test tubes model" -- can I use that in the future?

The blueberries are certainly the most perplexing and interesting feature of the meridiani sediments and I'm afraid my paper makes no significant inroads towards discovering how they formed. I think the best work done to date is that by D.C. Golden also here at JSC. He has done some experimental work that shows that one can form Hematite spherules with identical characteristics to the blueberries through forced hydrolysis of Fe by supersaturated solutions. Check out a recent LPSC abstract if you don't have access to the article.

Now, D.C.'s work needs temperatures >100 C to form the hematite - and I don't know how you get those conditions over such a wide area of Meridiani (tens of thousands of square kilometers). Another thing about DC's work is that his blueberries are about an order of magnitude smaller than the Meridiani blueberries. He suggests that lower temperatures and longer time periods should act to make larger blueberries. So size distribution may just be a function of the time, temperature, and chemical compostion of the fluids.

Also, a key observation is that the hematite spherules are evenly distributed within the outcrop making it almost certain that they grew during diagenesis after deposition. I know that Dr. Burt disagrees with me on this, but I think the MER team has done a thorough job making this point. So the blueberries didn't form inside the icy test tubes, but rather formed after the sediment had been reworked by eolian processes (maybe some impacts played a role as well).

Our model predicts that the chemical composition of the outcrops should be very similar to the chemical composition of the dust in the atmosphere (with only very minor loss of the most volatile elements). The icy-test tubes are closed systems so there should be no addition or subtraction of elements from what was incorporated in the ice.
Shaka
Please accept "icy test-tubes" with my blessing. (I suppose that will put me in the cheering section for that hypothesis. wink.gif )
Thanks for the DC Golden reference. I should be able to access it in the library later this week. It sounds like a promising line of research if the temperature and concretion size can be varied experimentally. I suppose the three models fall out on the temperature scale with the impact model on the hot end, the icy test-tubes on the cold end, and the greenhouse-playa somewhere in between? I understood that you assumed concretion formation to occur in the "post test-tube" phase of diagenesis.

I meant to ask how "festoon cross-bedding" accords with your hypothesis. Does your low water-mineral ratio scenario provide enough fluid to produce the occasional festoon? I have to say that for a feature so vigorously celebrated by the MER P.I.s, clear examples of festoons have proven to be 'rare birds'. I was hoping the walls of Victoria Crater would provide a few examples. Perhaps the long march to Endeavour will turn some up.

So then, whose court is the ball in now? (This three-way tennis is a complicated game!) Is someone going to bat for warm, wet playas? (Sports metaphors can get complicated too!) cool.gif
The cheering section is all ears.
imipak
QUOTE (Shaka @ Mar 2 2009, 10:47 PM) *
I have to say that for a feature so vigorously celebrated by the MER P.I.s, clear examples of festoons have proven to be 'rare birds'. I was hoping the walls of Victoria Crater would provide a few examples.

I may be mis-remembering, but I thought unofficial identifications had been made? See, e.g,. http://www.unmannedspaceflight.com/index.p...ost&p=82403 ;
http://www.unmannedspaceflight.com/index.p...st&p=106462

I could have sworn festooned cross-bed forms had been spotted in the ramp surface at Duck Bay, and in Cape St Mary / Cape Verde? unsure.gif
Shaka
I've been looking hard for festoons for years now and I don't remember seeing any convincing examples in close-up (i.e. with diagnostic features such as T-junctions) anywhere in Victoria. I'm sure the MER team would have pointed them out, had they recognized any. Of course, at a distance, such as in the many panoramas of the capes, there are numerous areas with variously 'wavy' laminations that resemble festoons in scale. These would be worth examining up-close for diagnostic features, but I can't remember any that were. I'll stand by my assertion, pending photographs that point out the alleged festoons.
Paul Niles
I've never been too convinced by the "festoon" cross bedding. If you go back and watch the press conference they had on this, the independent expert that was brought in (I believe he was from USGS) showed some duneforms that looked alot like dunes on Mars. I think Paul Knauth has also shown some pretty convincing "festoon" cross bedding that was caused by base-surge deposits. If I had the time I'd go back and post that press conference. Anyhow, the icy test tubes theory is in luck because we've got a kilometers of ice just next door. So if we want some liquid water flow, we can invoke a little melting and boom we've got a valley network and some festoon cross bedding to go along with it.

Although you have to keep in mind that as a geochemist I feel that the chemical evidence is much more compelling than the geomorphic evidence.

Paul
dburt
QUOTE
... I could have sworn festooned cross-bed forms had been spotted in the ramp surface at Duck Bay, and in Cape St Mary / Cape Verde? unsure.gif

Yes, and apparent festoon-like features are also exposed towards the lower right of Cape Victory, imaged just before Victoria Crater was left behind last October:
http://qt.exploratorium.edu/mars/opportuni...G3P2365R2M1.JPG
As Shaka notes, it is perhaps safest not to make too much of these cape exposures, inasmuch as they were not officially commented upon, and could not be examined up close. Of course, it might have been safest for all concerned if apparent festoon-like features seen in exposures elsewhere had not been commented upon either, because the claims initially made for them seem inconsistent with later data and interpretations.

Paul N. - There's no evidence of any "valley network" associated with any festoon-like feature seen yet, or for any channels or other obvious liquid water-related features at all, which is one reason why Paul Knauth and I are, like you, unconvinced. The ephemeral valley networks seen elsewhere on Mars seem to be associated with considerably older surfaces than the top surface of Meridiani.

-- HDP Don
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