QUOTE (don @ Mar 13 2008, 06:00 AM)
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 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