[X] My Assistant

[JonClarke]

I completely missed it at the time but there is a very interesting article ("Mars and Mine Dumps") by Donald Burt, Kenneth H. Wohletz, and L. Paul Knauth in the December 15, 2006, issue of Eos. In the article, the authors, as an alternative to the Squyres et al. model(s), propose that "the sulfate-rich sediments at Meridiani can be explained in terms of impact surge deposition," which the three of them put forth in a paper in Nature by Knauth et al. [2005].

In the Eos article, Burt et al. note that a feature of their "impact surge deposition" model is "an implied analogy between ancient impact craters on Mars and open-pit sulfide mines on Earth." Interesting, to say the least.

I recommend for those with access to read the entire article, but the last three paragraphs of the Eos article provide a nice summary

Volcanic base surge deposits look nothing like the sediments at Meridiani. No do impact base surge deposits. Nor do mine dumps.

Knauth et al are invoking special pleading whilst desperately searching for an alternative to the bleeding obvious.

Jon

[don]

Jon - I'm somewhat familiar with the Jurassic of So. Utah and I agree there are some features of the Summerville that provide good analogy to the upper Burns fm as examined at Endurance. Take it a step further to the underlying Navajo SS (eolian on a grand scale), throw in a healthy unconformity and there may be further correlation with the Burns section at Endurance. If press releases are any clue to what is going on inside the MER team, we'll be hearing more about an expansive dune field at meridiani, which the Navajo SS clearly represents as a terrestrial analogy.

Alex, I attended the Burt talk at GSA earlier this year and found the correlation to sulfide mine dumps quite interesting. While it seems that Burt and Knauth at times are throwing darts and hoping one occasionally sticks Burt has been preaching the weathered sulfide idea since day one (while giving Burns his due). Burt and Knauth are keeping the MER team honest, as you will notice in publications.

[tty]

I tend to side with Jon. I've seen surge deposits too, and they are very different from Meridiani.

tty

[Guest_AlexBlackwell_*]

For the record, I think that Squyres et al. have made the more compelling case with their model, and I haven't seen any alternative models that come close to dislodging it.

I will concede, however, that Burt, Knauth, and Wohletz raise a cogent point about the apparent lack of clays at Meridiani, which, as they point out, plausibly should have been produced by a quick chemical reaction between the putative acidic waters and the ubiquitous basaltic substrates, thereby neutralizing the solution. And they find the terrestrial Rio Tinto River analogy, which has been invoked by several workers as an analog to Meridiani, to be inapposite "because the sediment from this river has voluminous clay minerals and the acid level is enhanced by over 3,000 years of human mining activity on the Earth's largest-known volcanogenic sulphide deposit. No such clay deposits, upland massive sulphide source, upland drainage channels, or deltas extending into the putative lake have been observed near Meridiani Planum." [Knauth et al., 2005].

That said, we don't have a clear enough idea of the deep stratigraphic record at Meridiani to conclusively rule out clays there, and phyllosilicates have been spotted elsewhere on Mars (e.g., Nili Fossae).

[don]

Roger Burns suggested in 87 that the production of sulfates on mars occur through oxidation of sulfides even going as far to describe the formation of gossans. We know that sulfides are present on mars from martian meteorites. Burns never considered Mars impact cratering or sulfide dispersal by impact surge, but his concept of sulfide weathering stands on its own. Impact surge not likely, but the chemistry of sulfide weathering on mars is interesting.

[JonClarke]

For the record, I think that Squyres et al. have made the more compelling case with their model, and I haven't seen any alternative models that come close to dislodging it.

I will concede, however, that Burt, Knauth, and Wohletz raise a cogent point about the apparent lack of clays at Meridiani, which, as they point out, plausibly should have been produced by a quick chemical reaction between the putative acidic waters and the ubiquitous basaltic substrates, thereby neutralizing the solution. And they find the terrestrial Rio Tinto River analogy, which has been invoked by several workers as an analog to Meridiani, to be inapposite "because the sediment from this river has voluminous clay minerals and the acid level is enhanced by over 3,000 years of human mining activity on the Earth's largest-known volcanogenic sulphide deposit. No such clay deposits, upland massive sulphide source, upland drainage channels, or deltas extending into the putative lake have been observed near Meridiani Planum." [Knauth et al., 2005].

That said, we don't have a clear enough idea of the deep stratigraphic record at Meridiani to conclusively rule out clays there, and phyllosilicates have been spotted elsewhere on Mars (e.g., Nili Fossae).

Certainly way out ideas keep us honest and some turn out to be true.

I think there are at least three possibilities here.

1) The waters were not as acid as has been made out. Jarosite can form at pH of 4, which is not particular acid. remember the hyper acidity models are just that, models. They are not observations. I have a lot of sympathy with this, we don't see the alternation assemblages, apart from some jarosite, that you would expect of strong acid alternation.

2) We don't know the paragentic sequence of the minerals well enough to say whether the jarosite was synchronous with deposition, with diagenesis, or is a weathering overprint, or all of the above. I really think that strong assertions about hydrochemistry of deposition and diagenesis is pointless until we know this simple fact.

3) The products of of acid water-rock interaction at Meridiani may have been amorphous silicates, rather than clay.

Jon

[Guest_AlexBlackwell_*]

1) The waters were not as acid as has been made out. Jarosite can form at pH of 4, which is not particular acid. remember the hyper acidity models are just that, models. They are not observations. I have a lot of sympathy with this, we don't see the alternation assemblages, apart from some jarosite, that you would expect of strong acid alternation.

2) We don't know the paragentic sequence of the minerals well enough to say whether the jarosite was synchronous with deposition, with diagenesis, or is a weathering overprint, or all of the above. I really think that strong assertions about hydrochemistry of deposition and diagenesis is pointless until we know this simple fact.

Also appearing in that same issue of Nature with Knauth et al. is another non-standing-body-of-water-at-Meridiani paper by McCollom and Hynek. When I read the latter paper at the time, I was interested in how McCollom and Hynek would account for jarosite in their model, and I noted the following excerpt from their paper (internal references omitted):

Jarosite, a mineral observed in the Meridiani bedrocks, did not occur in any of our equilibrium models. Because jarosite is unstable in equilibrium with haematite, it apparently persists as a metastable mineral owing to kinetic constraints and may have formed during evaporation or as a weathering product subsequent to other alteration.

In addition, Figure 1 from McCollom and Hynek is a ternary diagram that shows the Meridiani bedrocks falling on a mixing line between martian basalts and the pure sulfur endmember.

Interesting.

[Guest_AlexBlackwell_*]

Check out Emily's latest blog entry: "Mars and Mine Dumps."

[don]

In regard to Fig 1 from McCollom and Hynek, Squyres et al would make the argument that no known martian basalt is a good match for meridiani outcrop (based on the siliciclastic component). The issue is the modest Cr/Ni ratio and high Ni of outcrop while SNCs and basalts from Gusev have an order of magnitude higher Cr/Ni ratios. I believe it was the sample “Barberton” as closest to meridiani outcrop but not an exact match. That said, Squyres et al do acknowledge that McCollom and Hynek can’t be ruled out based on what we know at the time.

[JonClarke]

The idea that jarosite is evidence against liquid water is laughable. It forms in mine dumps because liquid water from rainfall and atmospheric oxygen percolates through the suphide bearing waste. No liquid wter, no jarosite. Sometimes the amount of water is small, but it still has to be present.

Are these guys also arguing for large amounts of sulphides in the Martian bedrock?

Jon

[CosmicRocker]

I must admit that using the mine dumps analogy makes some aspects of the hypothesis more palatable to me. At least I have a better idea of some of the processes they are invoking, since I don't have access to those papers. It still seems like more of a stretch to me than the groundwater/sabkha/dunes concept, but as I said previously, I'm trying to keep an open mind. I'm admittedly not an expert on the geochemistry of mine dumps on Earth, let alone on Mars, but aside from the points already made, I have questions about how and when this apparently massive amount of sulfide was oxidized. During and for some time after the bombardment ("milling process"), conditions were surely highly reducing. It sounds as if they are suggesting that the oxidizing alteration occurred very early.

[don]

If Burt was referring to thin coatings of sulfate salts such as copiapite that can precipitate on waste-rock surfaces as water evaporates (and quickly go back in solution with minimal mositure) I could see his point but coatings of jarosite from Pyrite oxidation are more stable and less soluble than the hydrated iron sulfate salts. Sulfide oxidation will be difficult to demonstrate on mars unless we stumble across recently exposed beds of the material.

[nprev]

Not to sound completely parochial, but I literally grew up on a mine dump that had very little annual precipitation and a deeply buried water table: Butte, Montana, site of the Berkeley Pit copper mine. Spent many, many happy days roaming the dumps with my eyes turned to the ground, looking for interesting rocks & the odd ancient mining tool...

There were two notable features of these old dumps that may be applicable to this discussion:

1. In the presence of any liquid water at all, metallic precipitates readily formed. In the case of Butte, these were usually copper salts; at Meridiani, the direct analog would be blueberries.

2. The dumps were covered with an extremely tough and very thick layer of duricrust (I know; I had ten stitches in one of my knees from an unfortunate fall). This was probably due to a combination of atmospheric water vapor and the annual snow melt. The rather extraordinary 'paved' quality of Merdiani Planum observed by Oppy (and its ability to support the rover's weight) suggests that the mechanical properties may be similar.

No conclusions offered, merely observations!