Press release

Well we would like to see some proof of those clays in a ...delta.

defenetly?

Compare that news release with the July 25 story by Science reporter Richard A. Kerr here.
His title "Water everywhere on early Mars but only for a geologic moment" expresses the contradiction between a planet that has apparently almost always been dry and cold with one containing abundant geomorphic and mineralogic evidence for the presence of liquid water (at least in the distant Noachian). His subtitles "Dry, with wet moments" and "Paradoxical puddles" express the flavor. About Meridiani cross-beds, he gamely states "Only water flowing on the surface - not groundwater - could have formed ripples with their particular shapes" (perhaps wisely not mentioning alternative interpretations).

He ends up concluding that brief episodes of impact vaporization followed by liquid condensation could explain these paradoxical observations, citing work by Teresa Segura et al. from 2002 and in press in JGR. Compare her 2008 LPSC abstract here.
Following a major impact, a few years or even centuries of hot acid rains on finely divided basaltic regolith could presumably form significant amounts of clays, without this being more than a minor excursion relative to the overall climate (cold and dry for "maybe 999 years out of 1000" is how Kerr paraphrases fluvial specialist Bob Craddock of the National Air and Space Museum).

Where he doesn't go far enough is in not recognizing that major impacts could produce major sedimentation episodes (not just climate excursions), and that even minor impacts must produce (or at least excavate and redeposit) sediments. Of course, I might just be prejudiced.

-- HDP Don

Forgive my ignorance of the literature here, but is this where the D to H ratio would come in handy?

Specifically, the rate of change of the D to H ratio over time.

If there were a long cold dry periods then most of the water would be locked up in ice. No water vapor cycling, no D to H ratio change. So I would think the D to H ratio should change very, very slowly. (stepwise, if you could measure it over a fine enough time interval)

During the time that water was cycling, presumably some was being split in the upper atmosphere. So if there were an active and long-lived water cycle on Mars, the D to H ratio should be increasing constantly during that time.

Is this something that could be detected from orbit or based on a decent sample size (spanning a wide enough period of time) of hydrated minerals [meteorites, data from landers etc.]? (Can deutero-oxyated minerals be detected by IR from orbit? Are their frequencies shifted enough?)

Are there any definitive experiments that could discriminate between "1 wet year/999 dry ones" or the "marshy Mars" scenarios?

-Mike

Mike - Not my field, and so I'll provide no references, but my understanding is that the very high present D/H on Mars is commonly interpreted to indicate the loss of a huge proportion of the original H2O. OTOH, that water would be the H2O in communication with the atmosphere (hydrated salts, ground ice, polar ice caps, etc.) and not necessarily any permanently frozen very deep cryosphere or underlying hydrosphere (if present). A record of D/H change over time might be preserved in ancient clays, but AFAIK you'd need an in situ mass spectrometer type instrument or Mars Sample Return mission to measure them. Also, reduced iron-rich clays (such as saponite, a common alteration product of basalt) can easily lose H over time, forming oxy-clays (I've published papers on such clays). Oxidation via hydrogen loss would presumably alter their original D/H ratio. A lot might happen to a clay mineral in nearly 4 b.y.

I won't even attempt your intriguing final question about experiments to differentiate between enduring vs. temporary "warm, wet" climates. Figure it out yourself and get world famous. I will note that virtually all evidence for "warm, wet" seems to date from the hypothesized Late Heavy Bombardment (LHB) period. Merely a coincidence?

-- HDP Don

I suppose one way to differentiate between "warm and wet" and "cold and dry" would be a detailed look at some of the delta deposits. Layers of dust between deposited fuvial sediments would point towards extended cold and dry periods. (I'm assuming dust deposition would be the same or increase during cold dry periods).

Eberswalde, anyone?

Or maybe not.

Interesting idea, but recall that that sediments can be eroded as well as deposited. My guess would be that briskly flowing water would partly or completely erode any loose dust or sand layers, unless those layers were already thoroughly cemented (on Mars, probably by salts). Also keep in mind that far and away the biggest source of salty dust and sand would probably be the impact event itself - also the source of the water vapor. Co-deposition of salty sediment and condensing acid steam in a rapidly outflowing radial density current (ground-hugging impact surge cloud) is exactly what we have already hypothesized might have deposited the surface sediments observed by the rovers. This would occur relatively late in the bombardment process, when impacts were too small and infrequent to be producing hot acid downpours, and were mainly reworking earlier sedimentary material (including older impact debris). Earlier, deeper (i.e., buried) fluvial sediments probably would partly represent reworked and altered (to clays) impact debris too, especially if the debris (dust, sand, and rock fragments) and condensing acid steam (falling as downpours) were produced by the same major impacts.

In short, extended cold and dry periods (e.g., the present era) certainly experience wind erosion, transport, and deposition of dust and sand, but how would the resulting loose deposits be preserved during fluvial episodes?

-HDP Don

I was assuming that the same cementing that causes dune fossilization is occurring on Mars. Any fluvial deposition might be gentle enough to plop sediments on top of the cemented dunes rather than wash and erode the dunes away.

A terrestrial example would be the stream-deposited Kayenta Formation sandstones and mudstones lying on top of the wind-deposited cross-bedded Wingate Sandstone, both of the Early Jurassic in SE Utah. 'Course this is a terrestrial example, and the time between deposition periods was millions of years. [Reference here.]

-Mike

Thanks for stating your assumptions. In the first place, fluvial erosion (not deposition), especially that resulting from torrential downpours, is not particularly gentle. Second, dune "fossilization" (actually cementation or lithification) on Earth is caused by soaking the quartz sand for millions of years in liquid water during deep burial. Before lithification, the units were preserved by slow plate tectonic subsidence and/or sea level rise, allowing younger units to be deposited on top of older units, with minimal erosion in between (i.e., the eolian dunes were preserved from erosion because they were already safely buried under water when the river washed new sediment in on top, not because they were already lithified). Most preserved sediments were originally deposited just below (in the case of, e.g., reef limestone) or just above (in the case of, e.g., fluvial units and dune sands) sea level, on subsiding continental shelves (unknown on Mars). A record of such subsidence spanning nearly 250 million years (the Paleozoic Era) is preserved in, e.g., Arizona's Grand Canyon. A somewhat shorter interval is represented in the Mesozoic (dinosaur) Era rocks you mentioned. Later plate tectonic uplift and fluvial erosion has exposed these once deeply buried rocks at the Earth's surface.

Somewhat analogous multiple water soakings of what were assumed to be dune deposits have been proposed for one small region of Mars, Meridiani, by supposing that this region was a long-lived giant desert oasis of sorts, with groundwater (actually, many salt-saturated brine) welling upwards beneath and even flowing across a wind-swept plain, but such a site-specific, remarkably complex scenario seems unlikely for Mars in general. In any case, gradual plate tectonic subsidence/uplift and sea level rise are unknown on Mars (no known plate tectonics or permanent seas or subsiding continental shelves). The biggest basins, such as Hellas and the Northern Plains, are commonly assumed to represent the sites of giant impacts.

Contrast "gentle" geologic processes with the seismic shock of a major meteorite impact that might imediately toss all loose sand and dust into the atmosphere, all around the planet. To get a feeling for such blast processes see, e.g., the beginning of the latest (2008) Indiana Jones film, in which Indy gets an unexpected ride in a refrigerator.

In sum, a most logical proposal, but its assumptions might not be valid for most of Mars, especially for impact episodes. One possible exception: at the bottom of a big old deep crater (e.g., Gusev), you might find some fluviolacustrine (stream and lake) deposits, buried by eolian sands (or lavas), buried by more fluviolacrustine deposits, buried by more eolian deposits, and so on. You might also expect to find layers of ballistic (i.e., boulder) or surge-type impact deposits separating the eolian sand layers (or lavas) from the overlying fluviolacustrine layers, if a major impact generated each fluvial episode (in the sequence blast --> flood --> freeze dry). The major problem for Mars: exposure. The small craters investigated by the two rovers just aren't deep enough.

-- HDP Don

A story on how long the rains lasted on mars popped up last night, no mention of when the wet period occoured but it fits best with the chronology of mars at the end of the late heavy bombardment.

And two more, here and here.

Thanks for those two useful links. The second one discusses short-lived fluvial episodes at about 4.0-3.8 Ga that filled older impact craters with lakes and deposited deltas, shortly before Mars dried up and froze down. Although LHB-related vaporization and precipitation are not related to the fluvial episodes in the press release, what is stated is clearly consistent with this hypothesis.

The first link merits a bit more discussion. Weitz et al. (2008, in press, Geophys. Res. Lett.) discuss light-toned, polygonally-cracked, finely layered sediments that seem to drape high spots next to Valles Marineris. They compare them to the allegedly aeolian-lacustrine sediments of Meridiani, although theirs contain (on the basis of CRISM data) opaline silica as well as sulfates and are cut by "inverted" fluvial channels (now visible as ridges). These beds are alleged to be Hesperian (i.e., relatively young), based on the fact that they overlie lavas that are assumed to be young. On the basis of the inverted channels crossing them, the authors conclude the beds are fluvial in origin even though they admit that there is no topographic reason for them to be where they are (i.e, so high up). The beds seem to be remarkably continuous laterally ("Where exposures allow us to trace individual beds over several kms, there is no apparent change in their thickness") but friable (easily eroded by the wind). Their erosional properties and appearance are contrasted with those the fluvio-lacustrine beds exposed inside (at the bottom of) Valles Marineris. The possibility of a volcanic (pyroclastic) or aeolian origin is considered and rejected but, of course, the possibility of impact surge deposition - impact rain erosion is never mentioned, and the work of Segura et al. (not to mention Knauth et al. for Meridiani) is not cited.

Thanks again.

-- HDP Don

I think that this link also deserves your comment. This does indeed seem to address the period immediately following the LHB. But did the LHB cause the extended liquid spell or did it end it, with the bombardment destroying most of the evidence of a warmer wetter period?

Aussie - Thanks for your request for a comment. I didn't comment on that story because I hadn't read the original article (and still haven't). Besides, the news release didn't sound all that exciting - another computer simulation, based presumably on terrestrial analogs, and a purely semantic argument involving "big floods" vs. "little floods," and "a long time" vs. "a short time."

Personally, I find such semantic arguments uninteresting. In any case, the story appears to be talking about the period of the LHB itself (the so-called Noachian) - not beforehand and not afterwards. Whether episodic fluvial activity and the LHB are intrinsically related or simply occurred at the same time by coincidence is the question of interest (one not addressed by the news release).

At the very least, by the end of the LHB conditions favorable to the occurrence of liquid water (and rain and fluvial networks and surficial clay minerals) on Mars clearly had ended. Presumably, impact erosion of the atmosphere (i.e., blasting most of it into space) was nearly complete, although other, more gradual processes have continued to erode the atmosphere over time.

-- HDP Don

It snows on Mars !!
http://www.nasa.gov/mission_pages/phoenix/...x-20080929.html

Early in the Martian winter..

If this is in the wrong place someone please move it: A new chapter in the debate over a Martian ocean.