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Posted by: dvandorn Feb 2 2008, 08:53 PM

Back in the Paolo's Plunge thread, someone recently made a statement to the effect that the idea of an early wet, warm Mars has gone from a proposition to a belief. The statement made it clear that this was a bad thing -- that the concept of an early wet, warm Mars is keeping us from seeing how the planet's histopry has actually played out.

I'm responding to that statement here since I don't want to continue to hijack the other thread. But I feel the following two points must be made:

1) Mars was once warm enough (and had a thick enough atmosphere) to support flowing liquid water on its surface.

2) Mars was once wet enough for that liquid water to form well-developed river drainage systems and for enormous floods to scour thousands of square kilometers of its surface.

Those two statements aren't theoretical. Observed landforms verify them with primary, empirical evidence of river channels and catastrophic flood plains.

Those are statements of fact, not belief. My own feeling is that we must proceed from that point and not continually try and postulate a Martian history which cannot account for these proven facts.

Also, to the comment made several times that the LHB was responsible for the stripping of Mars' atmosphere, I must point out that several reputable studies have shown that the interaction between the solar wind and Mars' upper atmosphere is sufficient to have reduced an atmosphere as dense as Earth's to what we see today over the course of three billion years. And that neither Venus nor Earth seem to have had their atmospheres stripped during the LHB.

Just a few points I felt needed to be made at this juncture.

-the other Doug

Posted by: Aussie Feb 3 2008, 02:00 AM

Good points. I'm not sure that we can state definitely what happened to planetary atmospheres during the LHB but given the smaller size of Mars compared to Earth and Venus the chances of a given large impactor creating a plume exceeding the escape velocity would seem to be much higher. While the LHB would have given the Martian atmosphere a real shakeup, polar deposition and weathering would also have contributed and since Mars lost it's prime magnetic field (possibly as a consequence of the LHB) the minor residual fields would have provided little protection from the effect of the solar wind and the remaining atmosphere would have been steadily degraded despite any recharging from volcanic activity. But the loss of the atmosphere remaining after the LHB would probably have taken some considerable time which fits with the apparent gradual decrease in erosion over time.

Deposition by impact surge requires an atmosphere else all ejecta is ballistically emplaced. So no matter which camp you may follow in the endless saga on the genesis of Meridiani and Home Plate, both require a warmer wetter Mars with an atmosphere significantly thicker than the current vacuum.

Posted by: Juramike Feb 3 2008, 02:38 AM

I had this table handy. Based on estimated erosion rates, it seems that the early [Noachian] erosion rate is comparable to erosion rates in Earth's dry deserts. Later erosion rates (presumably eolian) drop significantly.

A freely available abstract describing evidence for the warm wet early martian climate and determination of erosion rate can be found in:
http://www.lpi.usra.edu/meetings/5thMars99/pdf/6057.pdf

According to the authors "the impact degradation of many valley networks further suggests that they [the valley networks] may have formed at the tail end of heavy bombardment around 3.9 Ga(ref.)."

So it appears that the atmosphere (and water) survived (or was cached during) the initial phases of the late heavy bombardment.

-Mike

Posted by: Gladstoner Feb 3 2008, 08:13 AM

.

Posted by: dvandorn Feb 3 2008, 09:03 AM

Exactly. In point of fact, a good number of the well-developed river drainage systems observed on Mars developed in the crests between crater rims in the heavily battered southern hemisphere.

It rained on Mars after the LHB had pretty much breathed its last.

-the other Doug

Posted by: dburt Mar 3 2008, 11:07 PM

I somehow missed this extemely interesting, short-lived discussion a month ago (because I was in Houston giving a talk on liquid water), but let me comment that most of the Noachian Mars drainage features can and have (by others) been explained as a result of the LHB itself, assuming that Mars was at least icy. They coincided in time - the Noachian basically ended when the LHB did. Large impacts would vaporize sufficient steam and other volatiles to create a very short-lived greenhouse; steam condensation could then create drainage networks accompanying catastropic floods and ice-covered lakes with deltas and even clay minerals (water would convert impact glasses to clay minerals extremely rapidly). Then dry and cold until the next impact. Smaller impacts towards the tail end of the LHB would have somewhat less effect on climate, but should still deposit sediments (cross-bedded surge and fallout deposits), because each impact into ice generates its own vapor. (Example: young rampart craters.) Late, small impacts especially might condense frost or snow rather than liquid. Volcanism would play a secondary, local role by comparison.

Blaming up to 99% of the early atmospheric loss on the LHB (with slow, steady losses afterwards owing to solar effects) is regarded as mainstream among Mars atmospheric modelers (Catling et al.). Early Mars may have been ephemerally warm and wet planetwide, but probably only immediately after a major impact episode. And each major impact brought it that much closer (doomed it to) a dry and cold future (at least on the surface). Therefore all the old craters are still there for us to look at, at least in the highlands. And even old craters on the Moon are degraded, although in somewhat different ways (e.g., no terrain softening from ground ice).

BTW, I agree with Aussie regarding his contrast of Mars with the much larger Earth and Venus, but disagree with his contention that a dense, permanent martian atmosphere must have been present to generate an impact surge, such as at possibly Meridiani or Home Plate. A volatile-rich target region should suffice (i.e., the atmosphere is temporarily generated by the impact; see above), and even the thin present atmosphere of Mars seems sufficient to move dust and sand.

This not to say that a long-lived warm, wet, densely-atmosphered, Earth-like Mars wasn't initially present. It simply isn't required by any data I've seen to date, runs contrary to simple observations like visible surface cratering, planet size, and distance from the sun, and most of the newer scientific data seem to be pointing the other way.

--HDP Don

Posted by: Juramike Mar 4 2008, 07:48 PM

OK...I'll bite.

Scenario A has an early Mars with a long-lived warm and wet climate with really big storms and outflows happening all the time (as part of the natural climate variation).

Scenario B has an early Mars that's pretty much icy-dry and cold, but punctuated by sporadic post-impact wet periods with catastrophic floods during transient post-big impact greenhouse climates.


(And there is the whole spectrum of possibilities varying from Scenario A to B as well as changing through time as well. e.g. from A to B or even B to A)


What observational evidence would allow differentiation of periods of Scenario A or periods of Scenario B?
(BTW, Mars not the only body for which this question seems relevant)

- Mike

Posted by: Patteroast Mar 4 2008, 09:06 PM

Hmm.. is it completely insane to wonder if Scenario B is possible on Ceres?

Posted by: dburt Mar 4 2008, 10:38 PM

Mike - Excellent question. I'll try to answer it, but doubt you'll like my answer. First, if Mars was completely slagged during the LHB (i.e., the surface was remelted), as probably happened to Earth, then the question becomes unanswerable (and probably irrelevant), because virtually all earlier history would be completely erased (except perhaps as zoning in zircons or other difficult-to-melt minerals). Clearly, after the LHB had tailed off, the surface seems to have remained largely dry and cold, or else the LHB cratering record should have been erased by weather.

Second, if the surface was not completely slagged by the LHB, then the earlier Earth-like warm, wet period should have left a record in the form of abundant clay minerals in shales, and possibly carbonate sediments (especially if life was around). But I'm not sure how you'd definitively tell those few sedimentary remnants apart from sediments generated during a temporary LHB greenhouse, without radiometic age dating of interbedded lavas (i.e., without Mars sample return). The apparent restriction of clay minerals to the oldest, most heavily cratered terrain, and abundance of fresh igneous minerals (olivine, pyroxene, and plagioclase) in younger sediments seems to provide independent evidence that the surface remained largely dry and cold afterwards. The abundance of soluble salts and acid salts on the surface also implies cold and dry (liquid water would dissolve or destabilize them). In other words, geomorphology and mineralogy seem to agree about cold and dry afterwards (anomalous Meridiani claims notwithstanding).

So we seem to know that post-LHB Mars was largely dry and cold on the surface (at least at the limits of orbital observation and detection), but the record of anything earlier seems ambiguous (in terms of distinguishing your two scenarios). As our orbital resolution increases, more landers arrive, and Mars sample return becomes a reality, hopefully we'll learn more.

BTW, from what I read, post-Noachian outflow channels seem to involve groundwater (presumably brine) abruptly escaping during collapse of chaotic terrain at their source. I've been discussing long term surface surface climate and weathering, not local subsurface conditions that might have led to catastrophic escape of brine.

Patteroast, regarding Ceres I suspect (without recalling any relevant modeling) that a temporary greenhouse would only work on planets above a certain size (but still too small to hold a dense, permanent atmosphere). Mars qualifies, but Ceres might not.

-- HDP Don

Posted by: Juramike Mar 4 2008, 11:13 PM

Thanks!

Sounds like another good argument for a sample return mission. (And extra care to hopefully finding just the right layering for deciphering the early martian record).

[The other body I was thinking of was Titan. Not sure if the earlier presumably wetter methane abundance was steady (Scenario A) or more post-impact espisodic (Scenario B). Still, chemical analysis of surface materials might be a way to tell, assuming appropriate layering could be found, dissected, and analyzed in a future mission (trapped isotopic gas ratios in ice grains being the timing marker?). Sample return, anyone?]

-Mike

Posted by: dvandorn Mar 5 2008, 03:57 AM

Well... I completely agree with you (especially with that proviso "largely"), as far as Mars' southern hemisphere is concerned. But north of the dichotomy line (which isn't exactly equatorial), very few remnants of the LHB are seen to survive. That land *has* been heavily resurfaced, somehow, since the LHB.

So -- *half* of Mars fits your description above. Half doesn't.

Makes it deucedly difficult to come up with generalized descriptions of the planet's past, doesn't it?

-the other Doug

Posted by: Aussie Mar 5 2008, 09:36 AM

When the LHB petered out Mars would probably have been left with a remnant of its initial atmosphere. But Earth would not have been particularly better off. Earth's atmosphere was recharged from volcanic activity and given the clear indications of significant volcanic activity on Mars, the post LHB build up of a reasonable atmosphere consisting of greenhouse gas would seem an acceptable concept. Mars was just not big enough to hold the atmosphere but while it lasted it would probably have enabled a warmer, dynamic, erosional environment.

The thing I find interesting is that the composition of the major gasses in the remaining Martian atmosphere is similar to that of Venus. Over 95% carbon dioxide and some 3% nitrogen. So Earth is the anomaly (if that expression is allowed with only 3 planets in the goldilocks zone) and it would seem that the key influence for Earth was abundant water and biological processes which over a long period removed carbon dioxide, injecting waste products like oxygen and some nitrogen from denitrification in its place. Following this rather tenuous logic the atmospheric gas percentages in the Martian atmosphere seem to point to a lack of any biological influence in the past.

Posted by: marsbug Mar 5 2008, 12:38 PM

How long would it take earth's atmosphere to change if all life here disappeared tomorrow? And how large does the biosphere have to be to maintain atmosphere in its current state? The compositions of mars and venus atmospheres may well point to no significant biological activity today, but I don't agree that it would rule it out in the deep past. The only other world we know of with a thick nitrogen atmosphere is titan, and the common link there is organic chemistry rather than life (or at least life as we know it).

Posted by: Juramike Mar 5 2008, 02:56 PM

Earth is also the only planet with smoothly running plate tectonics (probably lubed by water).

The exchange with the ocean and precipitation of carbonate salts probably did a great job of sequestering a lot of Earth's CO2 on Earth. Then stuffing precipitated carbonates in a trench due to tectonic action is a really great way to take CO2 out of the game for a nice long while.

I'm not sure of the relative removal rates of CO2 due to geological, oceanic, and biological processes. But I'll wager that all these helped make Earth a really nice place to live. (Most of these processes went away during the putative Snowball Earth phase, leading to the buildup of CO2 which eventually caused the Big Melt)

(I'd speculate that if plate tectonics magically stopped during the Big Melt (or any other time early in Earth's history), our atmosphere would eventually resemble Venus'.)

-Mike

Posted by: tty Mar 5 2008, 03:45 PM

In that case would pre-LHB zircon crystals really have survived intact? And it seems that the Acasta formation in Canada is older than at least the later part of the LHB. If the surface was completely remelted I would think that all the radiologic ages would have been reset.

Posted by: dburt Mar 6 2008, 12:08 AM

tty - From a talk I heard over a year ago (by Stephen J. Mojzsis, Univ. of Colorado), many pre-LHB zircon crystals did indeed survive on Earth, but they tend to show an age anomaly at 3.8-3.9Ga (i.e., age zoning: only the cores give older ages) that suggests crustal remelting during the LHB. Some terrestrial rocks seem to overlap the approximate time of the LHB, as you mention, but 60 years of intense geochronlogical research has yet to find any older on Earth. Also keep in mind that the overlapping rocks could have been dated by their zircons, and I imagine that an average age for the entire zircon tended to be determined in older work (although geochronology is not my field, I freely admit). This could yield spuriously old ages for the rock as a whole (based on the faulty assumption that the zircon crystallized at the same time as the rest of the rock). Sorry, I'm too busy getting ready for LPSC next week to look up the relevant literature.

-- HDP Don

Posted by: dburt Mar 6 2008, 12:46 AM

Good point Doug, but the "resurfacing" of the northern plains need not have been done by a climate greatly different from today's. All the dust raised by impacts has to end up somewhere, and lower elevations seem logical. Ditto for whatever the wind was blowing around (just look at the dust and dunes nearly filling craters like Victoria), plus what was deposited by the groundwater (brine) breakouts of the outflow channels, plus possible volcanic contributions. All fluids and dense suspensions tend to flow downhill, not just liquid water.

Also keep in mind that this cover clearly is very shallow - MOLA maps reveal the ghosts of underlying craters, and recent radar data has revealed that, under its shallow cover, the northern plains are just as heavily cratered as the exposed highlands. Herb Frey has published many papers and abstracts on this topic. So all you seem to be pointing out is the global dichotomy in elevation itself, not anything revealing about climate or erosion or the LHB.

-- HDP Don

Posted by: Juramike Mar 6 2008, 03:09 AM

OK. So my curiosity got the better of me:

From Wikipedia (carbon dioxide):http://en.wikipedia.org/wiki/Carbon_dioxide
and Wikipedia (carbon cycle, with cool-o diagram showing carbon fluxes in GTC):http://en.wikipedia.org/wiki/Carbon_cycle

So, assuming that sequestering and plate tectonics totally magically shut down on Earth. No oceanic exchange, no sediment sequesteration. How long would it take to build up a sizable CO2 atmosphere?

Output of CO2 from Earth volcanoes is: ca. 200E9 kg year-1
(130-230 million tonnes year-1 x 1000 kg per tonne)

[Makes sense, the sediment sequestering flux is about 0.2 GTC year. Up until man got all industrial and stuff, things were pretty much in balance]

Total mass of the atmosphere is 5.14E18 kg. (Wikipedia again)


5.14E18 kg / (200E9 kg year-1) = 25 Myr to generate an extra atmosphere of CO2 (ignoring stuff like m.w. difference between CO2 and N2)

So after 25 million years you would have a pressure of 2 atm and a 50% CO2 atmosphere.

Venus' atmosphere is 90 x that of Earth. So assuming that volcanoes still keep burping out CO2 at the same rate, it would take 2.2 Gyr to build up a Venus-like 90 atm atmosphere here on Earth. [1% N2, 99% CO2]

"Venus, there but for the grace of tectonic CO2 sequesteration, go I."

(Now won't y'all sleep better tonight knowing a global terrestrial ice age would be reasonably short-lived?)

-Mike

Posted by: Aussie Mar 6 2008, 07:49 AM

Pathfinder and the Viking landers didn't exactly find sand seas with particle sizes reflecting those that could be with be mobilised in the current Martian vacuum and to continue the Venus comparison, the impact craters on that planet have resisted weathering despite the presence of a thick atmosphere. (Plate tectonics rule). There would seem to be sufficient degradation of southern highland craters to suggest that significant aeolian and indeed liquid water erosion took place after the LHB ended.

If Mars did have a volcanic recharged greenhouse atmosphere that lasted until volcanic activity shut down, which seems most plausible, then would not this fit both the impact proposition and the Cornell warmer wetter theory. Following the LBH there was a period of unknown duration where an environment fitting the Cornell model existed. The findings from the descent into Victoria do seem to substantiate the Cornell proposition. Once the atmosphere dissipated impact again became the dominant influence, but with a sedate tempo.

Posted by: dburt Mar 6 2008, 07:09 PM

Aussie - Not wishing to revive ancient arguments, but Pathfinder and Viking found a rocky surface made up of what could reasonably be interpreted as young impact ejecta, and no evidence of water other than the ubiquitous salts in the soil. Evidence of wind erosion abounds, but evidence of liquid water erosion after the LHB seems largely resticted to outflow channels (interpreted to be temporary groundwater breakouts). You can hypothesize or model volcanic recharge of a Martian greenhouse atmosphere, but don't confuse this with evidence. Similarly for the descent into Victoria - I've seen no feature that requires more than minimum moisture, despite the models. And Knauth and I noted that concentrated brines could exist under present Martian surface conditions as long ago as 2000 (preceded by G.W. Brass in 1980). No greenhouse required.

-- HDP Don

Posted by: Aussie Mar 7 2008, 08:37 AM

The two statements are not really compatible since the resurfacing must have occurred after the LHB. I think I will sit fair and square in the other Doug's corner on this one.

Posted by: tty Mar 7 2008, 11:09 AM

Actually that mechanism is thought to have ended the very large neoproterozoic ice ages when Earth possibly froze over more or less completely. The large deposits of "cap carbonates" apparently deposited at very high temperatures right on top of the glacial deposits certainly seems to indicate that a lot of CO2 had accumulated in the atmosphere.

However at that time plate tectonics was going at full blast. If plate tectonics somehow stopped as you posited, then volcanism and CO2 emissions would pretty quickly decline (though probably not stop completely) since no new easily-remelted organics-rich seafloor rocks would descend into the mantle.

Posted by: Juramike Mar 7 2008, 01:44 PM

Sorry my post wasn't clear: I was whipping out a hypothetical exercise to see what would happen IF plate tectonics magically stopped on Earth AND volcanoes continued to burp. This was just to show the happy effect that plate tectonics has on preventing CO2 buildup.

I never meant to imply that plate tectonics stopped on Earth at any point in our history. (Although individual plate boundaries might lock up a bit on a fairly long time scale)

But what was cool (!) about this back-of-the-envelope calculation is that the time frame matches up nicely with the estimated time (in Wikipedia) it took to get out of the Snowball Earth scenario.

[From Wikipedia (http://en.wikipedia.org/wiki/Snowball_earth):
"The carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, about 13 percent of the atmosphere.*[12] Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by the weathering of siliceous rocks. Over 4 to 30 million years, enough CO2 and methane, mainly emitted by volcanoes, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of ice-free land and water developed;[13] this would be darker than the ice, and thus absorb more energy from the sun — initiating a 'positive feedback.'"]

-Mike

* actually, I think 13% is one of the lower estimates, other models seem to require a higher amount of CO2.

Posted by: dvandorn Mar 7 2008, 03:45 PM

Just to re-state a bit... in my initial post, I did say that Mars was incontrovertibly once warm and wet *enough* for water to form sinuous, meandering (and sometimes braided) river channels, and to allow for catastrophic floods. That's all I really said. Not that it was that warm and wet for billions of years -- just that it was warm and wet *enough*, for long enough, to allow the features we see to have formed.

I do find it fascinating that, when confronting this fact, those who are belief-level-certain that Mars has always been cold and dry (just as it is today) seem driven to ask themselves how such conditions could be generated on extremely short time scales, such as hours or days, to allow such things to have happened without challenging their dearly-held beliefs. And postulating that impacts had to have both stripped Mars' atmosphere and, *at the same time*, thickened it for brief periods.

I'm not saying that these are impossible scenarios. I'm just saying that my own reaction to them is they "feel" like they push Occam's Razor quite far in order to account for observed features which cannot be created on the Mars they are just certain must always have existed...



For what it's worth, I completely agree that Mars does indeed seem to have been as cold and dry as it is now since the beginning of the lava piling that resulted in the Tharsis Bulge, since those lavas have never been broken down chemically by abundant liquid water. My own opinion is that Mars' magnetic field died as a result of LHB interactions and that its atmosphere, thinned by the LHB to just above a "line" where liquid water was commonly possible, fairly quickly became depleted by solar wind interactions (within a few million years) to where the only further erosion Mars would see from that point on was aeolian- and impact-related. The very last water erosion was probably the catastrophic flooding, set off when frozen reserves of water (and very cold reserves of liquid brine) were mobilized by the heat of the magma rising into the Tharsis region. So, in my own personal perspective, the lavas which define the end of a Mars wet and warm enough for liquid water to be a major erosional factor were actually responsible for the final era of such aqueous erosion.

-the other Doug

Posted by: dburt Mar 7 2008, 08:01 PM

OD - I don't think we disagree about very much, then. From models I've seen, the temporary greenhouse warming following a major impact could last 1000 years or more, "long enough" to allow significant channel erosion following catastrophic downpours (or blizzards), without much chemical weathering. Your stating "hours or days" would be putting words in my mouth. Temporary greenhouse warming would be mainly caused by vaporized steam that would later condense; the atmosphere that was permanently lost "at the same time" was presumably different, mainly CO2. And I've only come around to the "mostly cold and dry" camp very slowly, kicking and screaming as more evidence came in (I initially wanted to study Mars for its clays, which have turned out to be severely time and space restricted - especially surprising for a planet that was supposedly swimming in acids). I don't deny an early volcanic greenhouse effect, I just don't feel that it need be significant, given the overwhelming abruptness and magnitude of the LHB effects (i.e., it was probably a secondary effect, one that declined rapidly with time after the end of the LHB, at the same time that the remaining atmosphere was itself being thinned by solar effects). Enjoyable discussion, although it might save time if we could read each other's minds.

Well, I'm off to LPSC, to blather about this stuff some more, and photograph alligators.

-- HDP Don

Posted by: tty Mar 7 2008, 10:42 PM

I suppose in those models the residual heat of the crater keeps the hydrological cycle going for a while, because the water lofted by the initial impact would certainly rain (or snow) out within a few years at the very most.
Most of the greenhouse warming would have to be due to water, since CO2 by itself is a rather feeble greenhouse gas.
Also I wonder, is a thousand years really enough to create for example the Eberswalde delta? There are deltas created in about that time frame on Earth (e. g. marginal deltas that evolved in Scandinavia during the Younger Dryas) but they are not nearly as complex morphologically.

Posted by: Juramike Mar 8 2008, 05:46 AM

A nice freely available article on the Eberswalde delta:
http://www.beg.utexas.edu/indassoc/dm2/pubs/Wood_Mars.pdf

-Mike

Posted by: dvandorn Mar 8 2008, 05:47 AM

I will say that, on geological timescales, a thousand years and ten thousand years are within the same order of magnitude. And I would imagine that the timing of the loss of magnetic field has a lot to do with how long such a Greenhouse Mars could persist -- without a magnetic field, volatiles (including water, broken down into hydrogen and oxygen) in the upper atmosphere are sputtered off into space far more quickly. The atmospheric water vapor content, which drives the water cycle, would be steadily eroded when the protective magnetic shield collapsed -- but could have reached a comfortable equilibrium while the field was in place.

In any event, my "gut" feeling, here, is that the landscape would probably look rather similar today had Greenhouse Mars lasted for a thousand years or a quarter of a million.

The way in which this whole speculation relates to the question of biogenesis, of course, is whether or not life could develop during the geological instant in which Greenhouse Mars could have existed, and could extremophile descendants have adapted to the conditions that followed? Single-cell organisms seem to have developed on Earth within a very short timeframe after conditions allowed it -- were the "relatively wet and warm" period(s) on Mars long enough to allow for biogenesis?

That's one reason I have a strong desire to find paleobacteria on Mars -- if life could start there under the rather more extreme duress than it found on Earth, well, the implications are profound...

-the other Doug

Posted by: Aussie Mar 8 2008, 06:47 AM

We don't have any clear indicators as to how long the postulated warmer wetter period on Mars lasted. If the Cornell interpretation of Burns Cliff and the Victoria promentaries is correct then it lasted quite a long time. The cratered southern highlands are not really an indication that Mars has always had the environment evident today. Erosion is evident and IMHO weathering of craters would not seem a particularly accurate yardstick by which to assess climatic factors. For example Gosses Bluff is a big (22 km diameter) crater but it is still pretty impressive despite 140 million years of weathering in Earths highly corrosive atmosphere. Wolf Creek is just a tad larger than Victoria and after 300,000 years is in much better shape. The only point I am trying to make is that the greenhouse mars could have lasted considerably longer than 1000 years. Perhaps I am hoping too hard for that to be true and my judgement is affected. But if it lasted a few hundred thousand years or more then the chances of the Other Doug's paleobacteria arising would be much enhanced.

Edit: just noticed this release http://hirise.lpl.arizona.edu/PSP_003077_1530

Posted by: imipak Mar 12 2008, 11:17 PM

On a related note, from an LPSC blog:



http://martianchronicles.wordpress.com/

Posted by: don Mar 13 2008, 01:00 PM

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"

Posted by: dburt Mar 14 2008, 05:33 AM

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

Posted by: Aussie Mar 14 2008, 09:13 AM

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.

Posted by: dburt Mar 15 2008, 02:50 AM

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

Posted by: tdemko Mar 16 2008, 03:49 PM

Nice paper on a possible earth analogue for Meridiani-like early hematite concretions:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4RM7N26-2&_user=10&_coverDate=04%2F15%2F2008&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2ea48c235ade97e30277d9bb667ef890


--
Tim Demko

Posted by: centsworth_II Mar 16 2008, 05:31 PM

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

Posted by: dburt Mar 17 2008, 05:22 AM

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

Posted by: Aussie Mar 17 2008, 10:58 AM

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

Posted by: don Mar 17 2008, 02:58 PM

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.

Posted by: Juramike Mar 17 2008, 03:58 PM

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

http://www.space.com/scienceastronomy/080317-volcanic-mars.html

-Mike

Posted by: dburt Mar 17 2008, 07:52 PM

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

Posted by: OWW Mar 17 2008, 10:16 PM

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!”

Posted by: ngunn Mar 17 2008, 11:24 PM

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.

Posted by: nprev Mar 18 2008, 12:35 AM

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!!!

Posted by: Aussie Mar 18 2008, 02:24 AM

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?

Posted by: dburt Mar 18 2008, 06:14 AM

Aussie - Only the big picture, I might argue. Diffusion is a function of having chemically different pore fluids above and below, not necessarily of what's in the matrix (although that may affect the speed of diffusion). In the presence of liquid water, chemical constituents dissolve and move up and down chemical gradients in the liquid, and they thereby segregate into distinct monolayers, as in the Australian lakes. Basically, if there was brine diffusion, show me the monolayers (including pure salt beds) - not a fine mechanical admixture, approximately uniform over many meters in every vertical exposure. (And I exclude the bright surface coating or efflorescence produced by upwards wicking of moisture and salts - no one is arguing that the rocks weren't once moist).

My basic problem with the brine story is that immersing the mysterious salt-sediment admixture (allegedly produced by wind erosion of granular "mudballs" off the surface of a drying playa) in brine should basically undo the mixing done by the wind and leave you with something close to the playa lake you're supposed to have started with (e.g., separate mud and coarsely crystalline salt layers). No such diffusional segregation process is evident. The inference is that the granular salty sediments, whatever produced them, were not immersed in brine afterwards. The Australian lakes, except for their acidic chemistry (easily explained by the fact that they are at the opposite exteme from Mars in terms of degree of chemical weathering and leaching), are not unusual. They are chemically "well-behaved." Meridiani is not. (In fact, it seems to be extremely naughty.) I hope this explanation helps. And keep in mind that I actually like the idea of brines on Mars!

-- HDP Don

Posted by: Doc Mar 19 2008, 10:25 AM

Well put Burt... I must say you have an almost mystical way of peeling the multilayered Meridiani paradox.
But Im afraid its going to take more than that to make me accept your impact surge hypothesis
(No hard feelings... )
The fact that acidic water cannot exist on Mars is well... but the brine idea has a certain logic in it when it comes to explaining alien places like Mars. But it may make you happy to know that I am looking into the thing afresh...(which is partly the reason why I dont post alot these days ).

Posted by: marsbug Mar 20 2008, 05:34 PM

http://news.bbc.co.uk/1/hi/sci/tech/7294767.stm seems to have been overlooked, and it talks about ice deposits formed during the amazonian, perhaps it's old news that I missed? Anyway if there has been volcanic activity in the geologically recent past, as the article juramike linked suggests, then (IMHO) the chance of there having been habitable niches in the amazonian era take a boost.

Posted by: dburt Mar 20 2008, 09:29 PM

Marsbug - Thanks for the link. No doubt there's lots of ice on Mars. What I found most interesting about that story is that to depth of a kilometer, apparently, they detected no liquid water or brine (or they would surely have announced it) - just ice. This again implies to me that the surface (and shallow interior) of Mars has been freeze-dried for a very long time - possibly since before the end of the Late Heavy Bombardment, despite episodic, highly localized brine releases later. This inference is supported by this http://news.nationalgeographic.com/news/2008/03/080320-mars-salt.html on numerous small, highly localized putative chloride evaporite deposits and their intimate involvement with cratering (3.8 billion year old age in the story) in the ancient highlands terrain. As I've suggested here before, if you want warm, habitable niches today it might be safest to look inside the long-lived giant volcanoes (as indirectly implied by that earlier article on volcanism).

-- HDP Don

Posted by: Doc Mar 21 2008, 09:53 AM

I can agree with you on that statement. Also I can agree that perhaps the sediments that we see rich in clay minerals are not necessarily the best place to look for signs of life. In the end places with past (or current) magmatic interaction could be the new goldmines.

Posted by: SickNick Jun 8 2008, 09:46 AM

What you say may well be true, but it's also misleading.

let us restate the observations:-

2a) The erosion patterns in e.g. Warrego Valles are almost impossible to produce without liquid water and precipitation (which could be snow, with basal melting trickling down snow-covered gullies)
2b) The catastrophic floods in various areas of Mars are most easily explained by Mars being WARM enough to melt subterranean ice in a once-only local event
1) Given the above, Mars must have been at least locally and transiently warm and wet enough for liquid water.

Now let's look at the rest of the context. ESA have published a very useful and compelling chronology of the role and activity of water on Mars, as seen from planetwide distributions of mineralogy:-

1- A widespread and active (but not necessarily permanent) water system producing clays - Back in the Noachian
2- A limited water system with strong acid brines producing sulphates - Back in the early Hesperian or late Noachian
3- An extended and planet-wide aridity lasting to the present day - some 3.5 billion years. Local exceptions occur associated with volcanic heating and consequent floods, but little if any chemical evidence results.

The simplest way to interpret all this is in a scenario where the stable state of Mars is arid and frigid, at almost all times. Back in the Noachian, major impact events caused atmospheric transients (lasting tens of years to thousands of years) during which the atmosphere was kicked into a metastable state with liquid water activity. Essentially, after the Noachian, no impact was big enough to cause a global ocean and rain. After the Hesperian, there was nothing left. Thisd model was originally proposed in concept form by myself but developed more carefully by Teresa Segura.

Essentially, we're looking at the evolution of Mars *as it accretes*. During THIS time, the system is energetic enough to enable liquid water (Recall that on earth, at the same time, the atmosphere was beleived to be superheated steam, or even silicate vapour! Mars is colder - there is "only" liquid water, but it's as stable on Mars as superheated steam is on Earth, imho...)

Posted by: sjbradshaw Jun 9 2008, 10:49 AM

This is a fascinating discussion, and it makes me wonder if anyone here could offer their thoughts on the following question.

If we assume for the sake of argument that infrequent large impacts can produce a temporary warm or wet Martian environment, how long would such conditions last? In particular:

- How dense would the resulting temporary atmosphere credibly be? 0.1 bar, or more?
- What would it comprise - CO2 liberated from frozen deposits, or vaporised rock?
- What would be the process by which the Martian environment reverted to its steady state?

Thanks for any insight you may be able to provide,

Simon

Posted by: marsbug Jun 9 2008, 11:09 AM

My uneducated guess: To produce heating over a long time or large scale an impact would need to trigger vulcanism on a global scale. So I'd think the chixelub impact, where a ten KM body hit mexico leaving a crater 200 odd km across, and maybe triggered vulcanism in the deccan traps on the other side of the earth, as a reasonable hand waving lower limit. As to how long it would last, I'd guess hundred to tens of thousands of years, but not hundred of thousands or millions unless the impact was really huge (just a stab in the dark). I doubt vapourized rock could comprize an atmosphere that would be cool enough for water, so vaporised ices and volcanic gases are the likely candidates. As the vulcanism or residual heat from a really big impact dies down i imagine the surface becomes cooler, gases making up the temporary atmosphere freeze out and atmospheric pressure drops back toward 0.01 bar. And Nick it's good to see you posting here again.

Posted by: SickNick Jun 9 2008, 12:10 PM

Yeah, I took a break. I lost my wife, got over it, got Cancer, got over it, got married again, still can't get over how good it is...

As for Mars, and impacts, you need a planetary-scale impact that will deposit an impact sheet AT LEAST 1 m thick. That will keep the atmosphere hot for 1 year, and the surface warm for 10 years. A 10m impact sheet is worth about 30 years in the atmosphere and 300 years at the surface, and a 100m sheet is 1000/10,000 years

But when you look at the size of impact require to acheive this, the mind boggles. Isidis and Hellas are the sort of thing we need. I think Teresa Segura failed in her effort to show that impacts on mars are important for dynamic/transient atmospheres because she statted too late in Mars time, AFTER the giant basin-forming impacts. If we go back to this time, and to the time of ultra-giant basins like the entire northern plains, then it becomes a no-brainer. Clearly, at this time, Mars had some liquid water. It probably had a lot of steam, and then a lot of rain... it was probably at this time that the water was active on Mars and did most of its doings. Later, Mars was a lot quieter, but we still see the last echoes of that early activity...

Posted by: marsbug Jun 9 2008, 01:24 PM

Congratulations on coming through what sounds like a very rough patch Nick, and congratulations on getting married!
Vulcanism on mars would have been a lot more active at the end of the late heavy bombardment, would that not have kept at least some locales warm between, and for some time after, the giant impacts? I'm not sure when the giant basins were formed. Could a more or less continous stream of smaller impacts contribute to keeping the planet warm? I'm afraid I'm throwing ideas around wildly here as I'm not familiar with how the late heavy bombardment/final planetary accretion was structured in time.

Posted by: sjbradshaw Jun 9 2008, 01:53 PM

Ah, I phrased my original question rather ambiguously. I was wondering about CO2 liberated from carbonate rocks by the impact; I seem to recall a rather extreme terraforming proposal that involved very enthusiastic, er, nuclear engineering to do this.

Posted by: tty Jun 9 2008, 07:18 PM

Sorry, but no. The Deccan eruptions started well before Chicxulub, which is proven by the fact that the iridium layer from the impact is found in one of the intertrappan beds within the Deccan traps. I think something more like the Hellas impact is probably needed.

Posted by: marsbug Jun 9 2008, 08:35 PM

Thanks for the info tty, I was under the impression it was still being debated.

Posted by: dburt Jun 9 2008, 09:35 PM

Hi Nick, I'm also happy to see you posting here again (although I've enjoyed following your occasional Mars-related comments elsewhere). I seem to have ended up agreeing with most of what you've said in the distant past, even about White Mars, except I feel that it's adequate to suppose that the "white" can be some combination of ice and salts (and of course impact-derived steam), rather than some form of carbon dioxide (not that the latter can be excluded). We met in 2001 when Knauth and I presented our low-melting salts idea at an LPI workshop.

Anyway, in response to sjbradshaw, as I understand it, and as you stated above, a major impact will volatilize nearly everything, including silicate rocks (even ephemerally separating, e.g., Si from O), but the silicate fractions will condense relatively rapidly, possibly to glassy spherules, as on the Moon. Carbon dioxide from target rock carbonates was a major vapor component for e.g., Chicxulub on Earth, but it remains to be seen if such target rocks ever occurred on Mars (probably not). The most important impact-derived greenhouse component on Mars is probably simple steam (water vapor) from impacted ice (or brine), together with sulfur dioxide from impacted sulfides and/or sulfate salts (you don't need unusual episodes of volcanism to make it). Condensation of impact-derived sulfur dioxide as ferric acid salts, and/or late oxidation of impact-distributed sulfides, could account for the mine-dump type mineralogy of the surface of Mars, without requiring acid seas, lakes, or groundwaters (which are nearly impossible from a geochemical standpoint).

Nick, I'm not sure that a Hellas-sized impact is actually required, if one accepts the Late Heavy Bombardment (LHB) as having occurred at about 3.9 billion years ago (long after initial accretion at 4.5 billion years). As suggested by marsbug, a concentrated stream of smaller impacts at the height of the LHB might be sufficient to account for the buried clays, drainage networks, and so on. I agree with you that as the LHB was tailing off, and afterwards, impacts were generally too small to yield much of a greenhouse, and, in fact, late impact debris seem to have covered up the clay minerals in most areas of Mars.

-- HDP Don

Posted by: SickNick Jun 11 2008, 12:04 AM

Thanks Don. I also have come agree with most of what you and Knauth have been saying. With time, there is a convergence of concepts, driven by the accumulation of evidence. Most Mars scientists are coming to terms with a long-term dry and cold Mars, but that doesn't stop volatiles having an important role at some times and places. The trick is to understand when, where, and how the volatiles act.

I'm still learning...

Posted by: ngunn Jun 26 2008, 02:20 PM

Another one to chew over:
http://www.sciencedaily.com/releases/2008/06/080625093242.htm

Posted by: dburt Jun 26 2008, 10:26 PM

Thanks. Haven't read more than that news story, but they seem to be comparing the Atacama Desert with Mars, and inferring that preferential chloride leaching from the surface indicates occasional rain or mist, based on the different relative solubilities of chlorides and sulfates. IMHO, possibly the wrong conclusion for the wrong planet - the old problem of using terrestrial processes as analogs for martian processes, rather than depending on basic physics and chemistry. I've been caught there myself e.g., as regards my recent informal discussion with Nick H. over how to make solid ground ice at the Phoenix site (my suggestion involving liquids was okay for solid salts, but probably not for solid ice, because it neglected the relatively high vapor pressure of ice). In other words, my suggestion explained how ice forms on relatively mild Antarctica, but probably not on much colder polar Mars. Nick was kind enough to set me straight, but also was kind enough NOT to rub my nose in the irony of my being victimized by the Rosenthal effect (finding or seeing what you expect), right after having contrasted that with Occam's razor (the simplest interpretation for all of the data).

Similarly, the authors of this study, if that news release is completely accurate, might appear to have neglected that chlorides, in addition to being more soluble, have much greater freezing point depressions than sulfates (more than 50C vs. less than 5C). Therefore, preferential frost leaching of chloride, on which Knauth and I published in 2002 and 2003 (in Icarus and JGR) might seem a more "Mars-friendly" explanation than rain or mist.

A couple of author's quotes from that news release: "Our study suggests that Mars isn't a planet where things have behaved radically different from Earth" and "It seems very logical that a dry, arid planet like Mars with the same bedrock geology as many places on Earth would have some of the same hydrological and geological processes operating that occur in our deserts here on Earth". In this regard, after weeks of 110+ degree (= 43C) days, Phoenix had its first seasonal dust storm last night. Do dust storms make Phoenix, Arizona hydrologically comparable to the Phoenix lander site, Mars? Perhaps not.

-- HDP Don

Posted by: ngunn Jun 26 2008, 10:58 PM

I thought you'd have something to say here about the unreliability of earth analogues.

Posted by: dburt Jun 27 2008, 01:58 AM

In other words, you knew I wouldn't be able to resist.

-- HDP Don

Posted by: marsbug Jun 27 2008, 11:47 AM

The impact in the http://www.unmannedspaceflight.com/index.php?showtopic=5262&pid=119294&st=0&#entry119294would probably meet the requirements