In the “follow the water” strategy the discussion of a cold or wet mars and the resulting focus on the presence of free/liquid water is a red herring of sorts. If the current or commonly accepted model holds for multiple episodes of groundwater cycling in the subsurface at meridiani, that alone appears adequate evidence of an incredibly active and long lived hydrologic system (IMO). The variations of diagenetic textural changes observed and interpreted to represent multiple phases of cementation, recrystallization, secondary and tertiary porosity, post depositional dissolution, and mobilization of soluble salts can not be achieved in seemingly simple terms of years or even thousands of years. Consider the dynamic chemical processes - the interaction of the solid phases, the gas phases, and aqueous phases to produce the features examined. The diagenitic variations observed occur because apparently large subsurface water systems were in disequilibrium, and so the question arises, what causes disequilibrium in a seemingly large water source (that by shear volume has tendencies to remain in equilibrium).......and if earth provides any lesson, chemical changes in groundwater systems are often tied to surface processes.

"the other Don"

OD - I've said this before, but if the warm, wet, acid diagenetic history had lasted anywhere near as long as you infer, then Meridiani should consist of a massive layer of mud containing giant salt crystals and concretions sized up to grapefruit (and I exaggerate only a little ). Limited moisture yes, enough to wick upwards and give you a meter or so of bed-crossing sulfate-enriched duricrust at the surface (as originally hypothesized by Viking scientists) and incomplete fracture fills, but not necessarily more. Diagenetic frost leaching of chloride salts (producing crystal cavities) might account for all of it, although the impact surge (if that's what it was) should have been wet when emplaced. Remember that we are dealing with soluble salts there, that will recrystallize in weeks in a jar. Also, when the wind blows across a playa surface, it doesn't mix things randomly, but instead winnows particles or crystals by size, density, and stickiness. This process generally yields pure gypsum crystals in dune fields, in every case on Earth known to me (e.g., White Sands, New Mexico), and presumably on Mars too (e.g., the gypsum dune fields recently reported from the northern plains). Why should Meridiani be so different?

I fully agree with you that the "follow the water" strategy, in its present form, is probably something of a red herring. Given the hostile past and present surface of Mars (whether too cold, too hot, too radiation-rich, too oxidizing, too acid, too salty, or too dry - take your pick), wherever life might have originated, it presumably then persisted and evolved underground, where it was largely protected, yet still had abundant sources of chemical and local heat energy (mainly magmatic, once bombardment had ended). Crater ejecta and walls, especially those located near sources of long-lived magmatism, might therefore be the best place to look - not in sediments deposited at the surface.

---- as an aside ---

My 3 days at LPSC (and 2 days at the earlier Brown-Vernadsky Microsymposium 47, held at LPI) were fun, especially all the talks in various sessions and disciplines (ranging from astronomy to astrobiology) supporting the Late Heavy Bombardment (LHB) hypothesis, and the amazingly detailed and varied OMEGA and CRISM reports of widely-distributed, pervasive clay alteration in the deep basement, presumably dating back to the initial (still very wet and steamy) stages of the LHB. The main disagreement seemed to between those (e.g., Bibring et al.) who inferred formation of clays via surface weathering, and many others (e.g., Mustard et al.) who inferred clay formation by deep hydrothermal circulation in the megaregolith, presumably related to impact heating. The extremely deep (up to 5 km below the surface) occurrence of clays might favor the latter process. Hydrothermal circulation also gives you the large water to rock ratios and warm temperatures needed to form clays quickly (because you can use and focus the same warm water over and over again in a short time). In any case, by the waning stages of the LHB, clay formation seems largely to have ceased, accounting for surficial impact layers rich in fresh olivine and other fresh igneous minerals, together with salts. The clays are best imaged in the walls of canyons and fairly young craters, where they are not obscured by younger cover. Such outcrops apparently are small enough that earlier instruments could not pick them out.

BTW, Bibring argued (in his B-V microsymposium talk) that, if impact-related hydrothermal circulation had formed clays, then clay formation should not have ceased well before impacting did. He therefore favored abrupt loss of atmosphere related to the loss of the martian magnetic field, well before the end of bombardment, and resulting cessation of surface weathering to form clays. After his talk, I pointed out to him that if, in the waning stages of the LHB, the martian atmosphere were already as thin, and the surface already as cold and dry as he proposed, then impact condensates would fall mainly as snow, and the megaregolith would already be freezing down (to the beginnings of today's cryosphere) and drying up (especially near the equator). In that case, impact-related hydrothermal circulation could take place only locally at best, not pervasively enough for clays to be detected from orbit (OMEGA and CRISM instruments). We agreed that the catastrophic loss of atmosphere (owing to cessation of magnetic dynamo and impact erosion), and the resulting change of surface conditions, appears to have been rather abrupt, and to have occurred during the LHB, not afterwards. To me this result presents a possible problem for the current "warm, wet" Meridiani model, inasmuch as Meridiani is usually dated as late Noachian (itself marked by the end of the LHB). That is, Mars as a whole should already have been cold and dry by the time sulfate-rich Meridiani formed, whatever the process. It also supports my argument against looking for biological indicators in post-bombardment surface sediments. What do you think?

My own B-V microsymposium talk mainly dealt with 1) liquid acids always being neutralized by bases (basic igneous rocks), unless frozen or preserved in acid salt crystals, 2) the vastly different freezing point depressions of common chlorides (up to -50 degrees) and sulfates (all less than 5), 3) the resulting preferential downward frost leaching of chlorides accompanied by upward wicking of sulfates that can't be frost leached, and 4) the implication that the transition from clay-rich to sulfate-rich surfaces could be explained simply by atmosphere loss during the LHB, and abrupt temperature decrease, without needing to infer a sudden influx of volcanogenic SO2 that yielded oceans of strongly acid liquid water. I suggested that at least as much acid had been generated during the LHB as by later volcanism, and that the resulting acids had assisted in the formation of clays (as they were neutralized by regolith). Afterwards, Mars simply got too cold and dry for liquid water (other than concentrated chloride brines) to persist, or for frost or snow to leach sulfates. No need to propose life-hostile, extremely improbable (in terms of acid base chemistry), strongly acid liquid ground and surface waters. Several people told me afterwards that they liked my line of reasoning. Are there any parts that you especially disagree with?

-- HDP Don

Dburt,
I am sure that you are right. The ASS (Acid Sulphate Soils) research is wrong and basalt will neutralise everything.

Several people told me afterwards that they liked my line of reasoning You are a well respected doyen and your command of English and rhetoric is impressive. It would indeed be a brave person that tried to disagree with you and to be honest I would have greatly enjoyed having you as a mentor in my early life. But rather than letting the surge concept become an obsession could you not direct your amazing intellect towards reconciling the surge and Cornell theories.

Aussie - Thanks for the compliment, I think, and believe me, I've tried, and so have my co-authors (and we aren't particularly obsessed with impact surge; we just haven't found anything better). My problem is that the spherules just don't look much like concretions and nothing at or near the surface of Meridani looks like it's been exposed to much more than moisture (let alone sulfuric acid groundwater). Although I refuse to buy the festoon argument, I could probably go with wind delivery of the sediments to explain the Victoria cliffs, despite the prevalence of low-angle cross-beds, if they didn't contain all those dense spherules, far too big for the wind to have moved (no problem for impact to move and redistribute). I'd also be happier if the terrestrial analogs (e.g., White Sands, NM) consisted of other than pure gypsum crystals, and if there were a possible (non-buried) playa source near the Oppy site. There might well be clay-rich, water-deposited, playa-type sediments at depths greater than exposed in or excavated by Victoria, but they can't be called on to produce the clearly younger sediments at the surface (except by impact excavation, our mechanism, and even then they would probably contain crystalline clays instead of acid sulfate). So Houston, we seem to have a problem.

Any helpful suggestions? We seem to agree that brines were involved, to expain the salts, erosion and transport, to explain their odd mixture (soluble with insoluble, our initial suggestion), frost leaching, to explain why chloride increases with depth and possibly the crystal cavities (our initial suggestion), wicking of moisture, to explain the bright band at the paleosurface and possibly the fracture fills (our initial suggestion, following Viking investigators), and acid, to explain the jarosite, but we can't seem to agree about the amount and source of acid, the inferred temperature, the dominant process of mixing and sedimentation, a reasonable gray spherule formation mechanism, or just about anything else. Help!

-- HDP Don

Nice paper on a possible earth analogue for Meridiani-like early hematite concretions:
Bowen et al., 2008, Active hematite concretion formation in modern acid saline lake sediments, Lake Brown, Western Australia: Earth and Planetary Science Letters, v. 268, p. 52-63.


--
Tim Demko

Reader-friendly background (with pictures ) here:
http://www.purdue.edu/eas/sed/research.htm

Hi Tim - Thanks much for the link and reference - I'd previously seen that work only in abstracts. Do you really think those irregular red lumps and clumps, ranging in size up to 2 cm, are a good match for the uniformly sized and shaped 4 mm gray spherules at Meridiani? Is their restriction to a single vertical zone a good match for the distribution at Meridiani? Do you think that the Australian acid lakes, in some of the oldest, most weathered and leached terrain on Earth, are a good match for the immature basaltic (rich in basic oxides) regolith of Mars, complete with unweathered igneous minerals such as plagioclase and pyroxene? Have you noticed that when the wind blows across dried playas, it tends to concentrate gypsum, not a random mix of granules and soluble and insoluble phases? Have you noted the abrupt color changes and bleaching, representing fluid flow and mixing, that are characteristic of concretion-forming environments in the Navajo and Page Sandstones, but not Meridiani? Do you think Mars was that wet and warm at the end of the Noachian (3.8 Ga)? And on and on. Just wondering how good the overall match seems to you as a sedimentologist. Thanks.

-- HDP Don

Well if there were acid saline groundwater on Mars in the far past would it not be reasonable to expect to find jarosite, sulfates, silica, phyllosilicates, hematite, vugs as well as some indications of sedimentary structures indicative of water flow. The fact that we find all this is pretty compelling evidence particularly when there are terrestrial diagenic analogues such as the Nippewalla Group. Also Chan did forecast the possibility of hematite concretions and Benison's work substantiates the formation of hematite concretions in an acid saline environment, demonstrating a current ongoing process. While the hematite concretions at Brown Lake may not look like those of meridiani, would we really expect them to given the totally alien environment? Another factor is that the meridiani 'berries' are not spherical but have about 5% elongation in one axis, which would seem indicative of concretion development in a very slow flowing solution. Besides, a Mars which tried to give it a go is somehow comforting.

And the reader friendly background with pictures was a nice touch - thanks C2

DBurt

Groundwater is often mentioned but really only receives lip service – its role is under-valued by surface process myopia................editorial comment.

The shear mass of iron required to generate the quantity of hematite locked up in the meridiani spheres requires a long residence time for groundwater IMO. Obviously iron was present in the aquifer solids and some time was required for it be leached and dissolved. The meridiani equivalent for rapid color changes/bleaching you note associated with concretions in the Page and Navajo SS may have been erased by the subsequent episodes of groundwater cycling (or even frost leaching of sulfates that you have discussed in gully related text). More likely the meridiani “neutralization zone” is not a common migration front, but a groundwater chemistry altering event on a regional scale perhaps volcanism or impact related.

As an aside. grapefruit sized iron concretions (and larger) can be found in the Bisti Wilderness Area of New Mexico, within gypsum dominated sediments.

space.com article summarizing report of episodes of volcanic activity (and presumably water release) on Mars:

http://www.space.com/scienceastronomy/0803...canic-mars.html

-Mike

Aussie - Look at the photos and diagrams in that article some more. Those are typical lake beds, consisting predominantly of mud, with evaporitic salts and a single diffusional mixing horizon with concretions, which little resemble the blueberries. Meridiani is sandy and all cross-bedded, and has dozens of other differences. Acids, brines, and groundwaters (and ice) were involved in its formation, but probably not in the way that has been described. Nevertheless I realize I'm not going to change many minds here, especially of those who find exact Earth parallels "in a totally alien environment" (your own words) somehow "comforting". Thanks for your honesty in that regard.

-- HDP Don

I agree. "comforting" is not very scientific. I think the reluctance to even consider "white mars" or "brine splat" type theories is psychological. We just can't let go of 19th century Mars.
First Lowell's canal builders were not there ....then the plants and mosses were not there...then liquid water was not there. After all these 'setbacks' it is difficult to accept another setback and the only way to keep a remnant of romantic Mars alive is to think it must have been very 'wet'... in the past. Difficult to prove or disprove. Very convenient.

Here are a few relevant quotes from one of my favorite scifi stories, the Gods of Mars, crudely translated from Dutch back into English. Written for the True Believers if all else fails:

“You said Lowell saw what he wanted to see. That's correct, but not the way you meant it.” “Because Lowell wanted to see it, it existed for him! Just like it exists for us – because we want it to exist! We don't have to accept that grey reality of all the small grey men at NASA. They want a Mars with only dust, rocks and dead empty deserts. They like it that way...”
“...But we don't like it! Deep inside, we don't believe in that Mars! We believe in this one – the real one. That's the reason it is here for us! That's the reason it is the way it is – it has been made from our dreams! Who knows what's behind those hills? Fantastic white cities? Four-armed green men? Beautiful princesses? The Twin cities of Helium? Anything is possible!”

Wishful thinking and Mars - you could write a book about it. However I think that most scientists working in the field are genuine enquirers and not coming in with some previous agenda. I know that I just want to understand the truth, whatever it happens to be, and I expect that's the majority view. I think it would be wrong to attribute the current scientific consensus to comfort-seeking motives - even by implication. Having said that I think that consensuses need to be subjected to particularly hard examination because there is a big cost to science if they're wrong.

Comforting or not, it is interesting that the article Mike linked to mentions that there may have been some minor flows as recently as 2 My BCE; if true, then that really increases the possibility of modern, highly localized small-scale geothermal activity similar to what seems to have happened at Home Plate.

Exciting stuff!!!

But the Brown Lake concretions are forming in the single sand layer, so the single diffusion mixing horizon is a function of the matrix and would not seem to be a significant variation from the Meridiani scenario - or am I missing something?