When the original news of Opportunity's watery finds at Merdiani were announced in early 2004 .. we had a region of Mars that to quote NASA was "drenched in water" with long standing pools or lakes on the surface.

However, the most recent science results all refer to "ground water" rather than surface water producing the concretions etc. It also seems that much of the bedrock making up the walls of Endurance Crater... and Victoria Crater are vast "fossilised" dune fields - not very watery at all. Is the story of water at Merdiani drying up?

To some extent, Sunny, that depends on what story you've adopted. When the first convincing clues of a watery past were seen in Eagle Crater, some enthusiasts were talking about a "Meridiani Sea", but that was always the far end of a spectrum of likely water scenarios for the area. Even in Eagle Crater the evidence for free-standing water (i.e. "Festoon Cross Bedding") was restricted in scale compared to the sediments that were more typical of eolian sand dune environments. As Oppy wandered across Meridiani, this ratio (favoring dry surfaces) continued. The festoons were few and far between, and there was no sign of the sediment accumulations characteristic of the bottom of a "sea". So the consensus view swung away from that extreme toward where it stands today. Steve Squyres and colleagues summed this up at the end of their recent paper in Journal of Geophysical Research (vol. 111, E12S12, 2006): "Overview of the Opportunity Mars exploration rover mission to Meridiani Planum: Eagle Crater to Purgatory Ripple". I quote them:

In summary, we interpret the Burns formation to be sedimentary rocks formed in a wind-swept, arid surface environment with a fluctuating water table. Water rose occasionally to the surface, and sulfate-rich sand grains were reworked by the wind to form dunes and sand sheets. The rocks observed by Opportunity to date record a transition from dunes to dune-marginal sand sheets to transient surface water. Multiple introductions of groundwater governed diagenesis, including the formation of the ubiquitous hematite-rich "blueberries."

Nothing we've seen yet at Victoria looks likely to overturn that position. Maybe there does lie such evidence deep down below (hundreds of meters), but we aren't likely to see it on this mission. So we can still believe in scattered rippled ponds between the shifting dunes, but a "Meridiani Sea" continues to elude us.
Pity.

Yes thats how I interpret what I have read also. Sometimes there been pools or small lakes, and on other times a sandy surface with groundwater in the bedrock below.
The era that interests me the most are sadly out of reach here. Lets see about MSL........

A vast number of workers have been skeptical of a "Meridiani Sea" from the outset, even with the MGS TES hematite detection, and this is based on, if nothing else, the topographic configuration of the region. Close observers of the MER science results might note that Squyres et al., while confirming an aqueous origin for the hematite, have been very careful to go only where the data have taken them, e.g., by offering the "shallow lakes" or interplaya dunes models.

I have spent a few years of my life working on salt lakes and there are several things to remember.

One is that even when "dry" most salt lake lake surfaces are actually wet. The surface of the lake is defined by the capillary fringe. Any sediment above the capillary fringe dries out and is rapidly deflated. The water saturated nature of salt lake sediments is why they often enjoy such extenisve diagenesis.

Another is that rippled sediments don't develop in ponds. Wave and wind-driven current ripples require significant fetches and 10's of cm of water depth to develop. In my experience "significant means several km. Furthermore the ripples in lakes with low fetches (less 20 km or so) are very low amplitude, the Burns formation ripples have significant amplitude point to quite large fetches, I suggest (as far as I knwo this has never been quantified).

Aslso, most water laid sediments in salt lakes are flat bedded, not rippled. The water depths are too shallow to support ripples or other bedforms. Rippled units only form in a small proportion of the subaqeous units, those which represent eithere channels or from the episodes when when the surface water is deep enough to support bedforms.

lastly, good evidence for aeolian deposition in the Burns Formation is, IMHO scarce. The only good candiates I have seen have been the steep cross-bedded units at the bottom. Everyting else I have seen is consistent with salt lake deposits.

So the original story at Eagle crater and confirmed by observations at Endurance and stands up quite well, IMHO. Deposition in a groundwater discharge complex with ephemeral saline lakes and marginal dunes with extensive subsurface diagenesis in brine-saturated sediments. If this is not "brenched with water", I don't know what is.

Jon

Jon's comments are interesting, to say the least - can I throw another factor into the equation?

Simply put: ice dams. Although Meridiani's rocky topography may not support basins containing water, when liquid water was (perhaps!) fairly common on Mars it seems reasonable to assume that ice was also. So, catastrophic floods after ice-wall breaches, and 'temporary' lakes are quite possible. The only problem is the, so far as I know, lack of any evidence for ice-related features in the region...

Bob Shaw

Looking at the patterns of stripping of dunes and of layered sediments from older martian surfaces, it's abundantly clear that you can potentially bury a surface with a kilometer of "fines" and then deflate them back down to the topographically nearly unaltered original surface. We have <almost> absolutely no idea of the extent and topography of the Meridiani sediments when all the "interesting" stuff was going on. The topography that retained the ponded water is probably eroded beyond recognition. There will be traces indicating what was there to be deciphered by field geologists, but it's not going to be easy.

True, it's certainly possible that evidence for an enclosed basin topography supporting a regional-scale sea or ocean, as opposed to local-scale ponding, at Meridiani Planum has been "eroded beyond recognition." It's also equally true that Noachian-aged regional-scale basins that have been subjected to extensive burial/mantling/erosion processes can preserve evidence of their margins.

Fascinating and provocative input, Jon. I wish I had the knowledge to weigh your views against those of our illustrious PI's, but that is beyond me. Perhaps some other specialists - e.g. Tim Demko - could contribute here, but meanwhile I can only take at face value what the recent literature contains, and report it. I have already seen the MER team guess wrong (remember the Beacon?), but here we are dealing with the considered and peer-reviewed consensus among a long list of specialists with access to information few others can boast. I would be delighted to hear a debate between them and you on these issues, but, meanwhile, I can just point out the obvious: that no one I have seen has argued the precise parameters of the hypothetical "ponds" or "lakes" or dunes or ripples, or the precise limits of environmental variables in Noachian-Hesperian Mars. We can estimate with some accuracy the force of gravity, but the temperature, atmospheric pressure, humidity, wind velocity, precipitation and ionic composition of ground waters, are all in the "ballpark estimate" realm at best, IMHO. Since we have already seen published entirely "non-wet" interpretations of Meridiani Planum, I would quail at the thought of trying to paint a "definitive" picture of Meridiani history. If Steve and his brethren are not capable of this, I submit no one else is either.

P.S. Speaking of "ballparks", I should take the opportunity to welcome the return of Bob "Baby Mound" Shaw to the fray (and express my sympathy regarding what the Australian cricket team is doing to his mother country). I do so with a measure of hesitance, given the frequency of identity theft these days. Until Bob has launched at least one truly-dreadful pun in these pages, I will wonder whether this is truly he, returned from the wilderness, and not some phishing pretender.

One of my all-time favorite passages from a science paper comes from Craddock and Howard [2002]:

QUOTE

Determining the nature of the early climate on Mars is extremely difficult....However, it would be unwise to force the interpretation of any geologic feature on Mars to fit a given result or scenario. To offer one perspective, climate models basically attempt to predict the weather on an alien world almost four billion years ago; yet, even the daily forecasts for the Earth are frequently inaccurate. As outlined in following sections, the geology of the highlands offers the best record for understanding the climate of early Mars when geomorphic features are examined collectively.

[quote name='Shaka' date='Dec 27 2006, 07:43 PM' post='78756'
Speaking of "ballparks", I should take the opportunity to welcome the return of Bob "Baby Mound" Shaw to the fray (and express my sympathy regarding what the Australian cricket team is doing to his mother country). I do so with a measure of hesitance, given the frequency of identity theft these days. Until Bob has launched at least one truly-dreadful pun in these pages, I will wonder whether this is truly he, returned from the wilderness, and not some phishing pretender.

[/quote]

Shaka:

Have a look in the 'MSL Naming' thread - if not a pun, then there is without question a play on words (or two, one of which is cruder than t'other!).

Yes, I'm pleased to be back, and wish to extend my sincere appreciation to all those who expressed their joy at my return; and as for the rest of you...

Bob Shaw

PS Cricket passes me by even more than football. It's a bit like the eighth planet, I know it's out there, but can barely discern it from my elbow.

I agree with all the comments about the caution in trying to find the limits the basin that constrained the Burns Formation. I would add another one - tectonics. When dealing with flat lying sediments over irregular basement even subtle warping can significantly impact on attempts to outline basin limits.

Three Australian examples I am familiar with are the extent of the Pleistocene Lake Bungunnia that deposited the Blanchetown clay in the Murray Basin. The precise extent of the lake is very difficult to determine due to irregular basement topography, complex original geometry (sub lakes separated by dunes), and subsequent tectonics, erosion, and deposition. Another example is the Miocene deposits of Lake Billa Kalina on the Gawler Craton, clearly lacustrine with magnificent shorelines visible in satllite imagery but, thanks to erosion, we have only a rough idea of its extent, no idea of its inlets and outlets, or of its confining basin. A third example is the Proterozoic Woodline beds, flat flying Proterozoic clastic and chemical sediments in the SE Yilgarn. These are preserved as eroded remnants several 100 metres thick, with no clue as to their original extent.

In terms of textural and depositional analogues to the Burns formation I must say I was very impressed the the Jurassic Summerville Formation in Utah. Flat-flying silts and stones, with rippled and cross-bedded interbeds, abundant gypsum crystals, as well as gypsum veins and nodules. Anyone here seen it? I saw it back in 2003, when I did not realise how good an analogue it might turn out to be.

Cheers

Jon

Hey! I have novel idea. Why don't we send some geologists to Mars? These rovers and orbiters have been amazing, but they are only sending us a part of the story.

Let's!

Jon

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:

QUOTE

By the impact surge hypothesis, the surface of early Mars dried up and froze down while impacting continued. Impacts into partly frozen, salty, locally sulfide-rich basalt, followed by moist weathering, could have produced all of the features seen at Meridiani and Gusev and probably in many layered deposits elsewhere on Mars.

Sulfur-rich Mars sediments then are potentially analogous to weathered sulfur-rich mine waste but with the mining and milling having been performed by impacts. Preservation of such easily eroded, easily dissolved, easily recrystallized fine sediments from their formation billions of years ago until the present implies that any early ‘warm, wet’ period was well over by the time they were laid down near the top of the stratigraphic section, and that near-surface liquid water, other than intergranular films, had little or no role in their deposition or later diagenesis.

Nevertheless, by analogy with terrestrial arid-region mine dumps, these heterogeneous deposits might have provided excellent, energy-rich habitats for any microbial life present on early Mars...

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

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.

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

tty

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).

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.

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

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):

QUOTE

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.

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

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.

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

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.

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.

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!