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ngunn
This blockbuster article from Emily seems to justify a new discussion topic:
http://www.planetary.org/blogs/emily-lakda...le-of-gale.html

So many thoughts, so many questions, I don't know where to start. What about Gusev? Were those basalts actually sandstones too?
jmknapp
From the conclusion:

QUOTE
With Curiosity's results, I feel like the "water on Mars" argument is bifurcating; Mars was never wet like Earth, but it is not dry like the Moon. Water is crucially important to Mars' story, and yet Mars is drier than the driest conceivable environment on Earth.


Before the ESA livestream on Siding Spring they reran a webcast titled something like Mars Express: Ten Years at Mars. The speaker made the statement that if all the water in the polar caps was melted, water would cover the surface of Mars to a depth of three meters. I think the figure must refer to average depth, so in reality any Mars oceans would be patchy. It's really not a lot using Earth as a standard--but how much has been lost to space?

The average ocean depth on Earth is close to 3000m, so it's about a 1000:1 difference.
Gerald
Thus far, we've just seen a few data points, with respect of time, and location.
Just think of 500 million years of geologic history on Earth (the latest about 11% ). So many things can happen. We don't even know the distance between Mars and Sun, three or four billion years ago, nor the number of planets / moons / planetesimals in our solar system at that time.
One possibility combining abundant water with low chemical weathering could be low temperatures with occasional melts, be it due to volcanism, impacts, chaotic obliquity, orbital instability, solar activity peaks, whatever.
Or think about the lower gravity with respect to Earth, slowing down e.g. separation of water/ice and rock.

Investigating more layers of Mt. Sharp should reveal more detail about the currently just very crudely known geologic history of Mars, or Mt. Sharp, at least.

With respect to the hard sandstone: What about impact metamorphism?
Julius
Whereas it is true that we require more time and more landing site in situ investigation the likes of gale crater, meridian and gusev crater, I feel that the overall picture of the water story on Mars is somehow limited to volcanic, tectonic, hot spring activity . The next landing site for investigation i believe should be selected based on the presence of chlorides and carbonates rather than clay and sulphates.
elakdawalla
The basalts in Gusev are definitely basalts -- Spirit demonstrated that conclusively. Ken's point is that he's having a hard time telling from orbit which dark, erosion-resistant deposits are seds and which are igneous anymore.
SFJCody
What would sedimentary rocks deposited by liquid CO2 be like? Not that I really think that's the case, just wondering if there's some peculiarly Martian way to get these to form from basaltic source material without significant alteration.
Gerald
My first association would be carbonates or HCO3 salts, if at least some water is present to form H2CO3.
craigmcg
QUOTE (SFJCody @ Oct 24 2014, 11:27 AM) *
What would sedimentary rocks deposited by liquid CO2 be like? Not that I really think that's the case, just wondering if there's some peculiarly Martian way to get these to form from basaltic source material without significant alteration.


to make CO2 liquid you need pretty high pressure, greater than 5 atm.
nprev
Here's my best guess at a brief history of Mars with respect to this subject:

1. The LHB just pounded the hell out of Mars. Specifically, there were not only more impactors (presumably due to the planet's probable proximity to the Asteroid Belt) but also many more smaller ones reached the surface due to the fact that the atmosphere, while still much thicker than today, was nevertheless far thinner than currently thought...maybe 0.25 bar or less, maybe even just above the critical value for liquid water. This resulted in the formation of finer basaltic pieces than is normal on Earth.

2. Episodic flooding due to melting of subsurface ice from various causes (impacts & vulcanism, primarily) caused formation of sedimentary strata. However, due to the low (and dropping) atmospheric density standing bodies of water did not persist for long periods of time at most locales, usually evaporating before basaltic minerals finished transforming into clays.

3. In between flooding events (as in the present), aeolian erosion continued, further grinding down basalt into fine material. Subsequent floods produced fine-grained basaltic sandstones.

4. As the atmosphere continued to thin, small impactors assumed an increasingly influential role in soil mixing and generation of basaltic fines, similar to lunar processes albeit augmented by aeolian erosion.

That's pretty crudely painted, but it seems like a good fit for the observations. Clay-rich sites & sulfate plains like Meridiani are examples of places where standing water persisted longer than normal, presumably as a consequence of geothermal activity. However, even there the evaporation rate was rapid. Most of the water ultimately ended up at the poles except for the buried ice deposits we've seen, which eventually will be melted via impacts over time.

Of course, the pros will come up with something a lot simpler and more plausible, which is why they're the pros. smile.gif
Don1
QUOTE (SFJCody @ Oct 24 2014, 08:27 AM) *
What would sedimentary rocks deposited by liquid CO2 be like?


To get liquid CO2 you need at least 5.2 bar of pressure. For it to be stable enough to be geologically relevant would require over 10 bar.

Among pure compounds the only remotely plausible alternative to water looks to be ammonia. For liquid ammonia you need over 61 millibars, which is about 10 times current Martian pressure. At 300mb ammonia will melt at about -78C and boil at -58C, giving a 20 degree temperature range for the liquid phase.
Don1
Virtually all the knowledge about how rocks weather is for an oxygen atmosphere. Most water on earth has dissolved oxygen in it, and that influences the chemistry. If you put a steel nail in a test tube and add water, the nail will corrode as expected. If you boil the water to drive out the dissolved gases, and add a cap of melted wax to seal it from the atmosphere, then the steel nail doesn't corrode. The point of the experiment is that you require both water and oxygen for corrosion to occur.

Dissolved gases and salts have a big impact on chemistry. I don't know anything about rock weathering reactions, but it wouldn't surprise me if weathering worked differently in atmospheres without oxygen.
ngunn
QUOTE (nprev @ Oct 24 2014, 09:02 PM) *
Here's my best guess at a brief history of Mars with respect to this subject


That's a pretty good story, for me. A few comments and queries I would add:

Comments first. I can't help thinking back to Don Burt's descriptions of rapid sediment emplacement following impacts. Bang, slosh, swish, and a whole sedimentary complex with many varied structures both dry and watery is in place all at once. Later aqueous alteration episodes could be similarly brief. Forming gypsum veins this way is no problem. Only clay takes more time, and as you say that seems to have happened just in certain special places.

But - shouldn't liquid water be stable at some depth in the crust, perhaps below the buried ice deposits already known? Why doesn't alteration of the basalt minerals happen there? Or maybe it does but this material has never been excavated by sufficiently large impacts. Is there a buried wet clay-rich world down there still awaiting exploration?

Queries: We know there's ice in contact with the rocks in many places. Are basalt minerals completely immune from hydration as long as the water remains frozen? A 'Yes' would help here. Basalt chunks on the surface: In the absence of obvious weathering is there any way other than by finding a source outcrop to prove the basalt was formed where we find it rather than being carried in by a sedimentary process? Martian water verus terrestrial water: Could the former be less efficient than the latter at hydrating basalt minerals? Absence of some crucial ingredient peculiar to Earth?? (EDIT Don1 got to dissolved oxygen as I was typing. That's the kind of thing I had in mind.)
nprev
Good questions, and I'll defer to someone with actual knowledge of geology to answer them. My fields or expertise are well known and somewhat disreputable. biggrin.gif
dburt
QUOTE (nprev @ Oct 24 2014, 01:02 PM) *
Here's my best guess at a brief history of Mars with respect to this subject: ...

Nice summary. Agree with you regarding stages 1 and 2. Disagree somewhat beginning at 3.

3. Aeolian erosion yes, but water flood deposition to deposit basaltic sandstone, probably not. Not needed, given your Stage 4 that episodic impact erosion, transport, and deposition has continued at a reduced level up to the present. Erosional scouring, plus transport and deposition can then occur via clastic density currents, the impact explosion equivalent of the pyroclastic density currents that can make explosive volcanism so dangerous. These, like water, are fluids.

Mars rovers can only be landed and operated at relatively safe, flat sites and, lacking significant drilling capability, are only able to observe relatively young bedded rocks at the surface, so it is not surprising that all three rovers have observed relatively young "blast beds" (as I have recently called them) of basaltic composition on the surface, covering whatever older, water-deposited beds might underlie them. Inside large impact craters, such as Gusev and Gale, the sediments have been observed to be to be generally coarser, as topographic lows attract the denser, coarser portion of density-driven flows, as well as other types of mass movements (e.g., debris flows and landslides, a major component of terrestrial alluvial fans).

The atmosphere of Mars, although way too thin to support the continued existence of liquid water at the surface, is more than capable of transporting basaltic sediment in clastic density currents (think desert dust storms), especially if these currents are temporarily beefed up by steam and other gases generated by energetic impacts into brines, ices, hydrous salts, clays, or carbonates.

This simple hypothesis, first presented at the Geological Society of America annual meeting exactly 10 years ago to explain initial Opportunity observations at Meridiani, readily explains the observation of fresh basalt in sediments, the problems of which were so ably summarized by Emily. Incidentally, the dry acid salts observed by the first two rovers provide even better evidence than fresh basalt that liquid water cannot have been present for long, if at all, during and after deposition because liquid acids will chew into and alter everything, especially basalt rocks (owing to their low silica content).

ngunn: Thanks for the mention. Yes liquid water should have been stable at depth, especially near cooling impact craters or volcanoes, or if it contained significant quantities of dissolved salts to suppress the freezing point. Basaltic impact glasses, which should have been exceptionally common on early Mars, are trivially easy to alter to clay minerals, especially smectites, and especially if this alteration occurred by liquid water away from the surface. No need to call on special chemical properties of Martian ice. Note also, as I mentioned here when they were first encountered by Opportunity, that the mysterious spherules called "newberries" probably formed by devitrification of basaltic impact glass, based on their matrix and on the their radiating fibers imaged at high magnification. Other spherules observed at all three sites probably owe their existence directly to impacts.

I've been given to understand that discussion of this particular simple hypothesis on impacts, although it is now fully time- and rover-tested, is not particularly welcome at this site, so newcomers to the idea should probably consult our various publications (mainly by Burt, Knauth, and Wohletz).

Don Burt

djellison
For those who want to read tens of thousands of words by Don on that issue - read this thread http://www.unmannedspaceflight.com/index.php?showtopic=4308
Gerald
QUOTE (ngunn @ Oct 24 2014, 11:17 PM) *
We know there's ice in contact with the rocks in many places. Are basalt minerals completely immune from hydration as long as the water remains frozen? A 'Yes' would help here.

Let me pick out this question. The answer is an "almost".
As a general background chemical reactions tend to slow down by a factor of 2 every 10 K as a rule of thumb.
Another factor of the reaction rate is the concentration, or the partial pressure, in this case of water vapor.
According to this NOAA calculator, vapor pressure is 9.92 hPa at 280 K, but only 0.04 hPa at 220 K. Hence the number of available molecules for chemical reactions decreases rapidly with temperature. Water vapor is of course already much less reactive than liquid water at the same temperature.

A 1990 LPSC paper about basalt weathering in Antarctica and on Mars even neglects possible weathering of basalt by vapor, and only considers temporary wetting.

Ice will lack direct contact with basalt, after a thin weathering crust has formed, as long as there doesn't occur additional physical weathering exposing unweathered basalt.
dburt
QUOTE (djellison @ Oct 24 2014, 04:26 PM) *
For those who want to read tens of thousands of words by Don on that issue - read this thread http://www.unmannedspaceflight.com/index.php?showtopic=4308

Thanks for providing the link. Warning! If that thread seems too long to read that's because it really got too far long to read, so evidently nobody read it and I had to answer the same questions again and again. The moderators and I have NO desire to repeat that experience. For information and updates I therefore recommend instead searching and linking on our relevant publications, especially LPSC abstracts and other abstracts, although they tend to be somewhat abbreviated and technical, or contact me directly.

Don Burt
serpens
In all this discussion we need to differentiate between hypothesis and empirical evidence. For example, despite the faint young sun hypothesis we know there was long lasting liquid water on both Earth and Mars. Empirical measurement trumps hypothesis every time. Reaction between acidic water and mafic rocks commonly yields alkaline ground water and significant quantities of hydrogen so the atmosphere of early mars could well have had a water CO2 sink, a high H2 component and a carbon dioxide/sulphur dioxide/hydrogen sulphide component from volcanic release. This mix would fit empirical measurements at Meridiani including deposition of hematite concretions in a mixing interface in a basalt buffered environment. A high H2 content atmosphere would fit water/atmosphere loss and correlate to the measured deuterium/hydrogen ratio although correlation does not necessarily imply causation. Another point is that while basalt weathering at the earths surface in a wet, oxygen rich environment is indeed swift by geological yardsticks, on the ocean floor it is much, much slower. Even in a highly oxygenated environment like Papakolea beach, olivine resists change. The polar cap water reserve is no indication of the amount of water available to early Mars, nor can we use a Terran based yardstick to measure Martian environmental history.
SFJCody
Some exciting ideas so far! Sorry for the ridiculous CO2 suggestion earlier. Rolled my eyes at my own stupidity when I was reminded of the pressure necessary for a stable body of liquid.
Julius
How long in terms of time scale are we talking here when you refer to chemical weathering of deep ocean rocks takes longer than weathering of surface rocks in an oxygen rich earth atmosphere? Still my impression remains that water contact with basaltic rocks has been brief in most instances probably due to water instability on Mars surface.
dvandorn
QUOTE (serpens @ Oct 25 2014, 05:16 AM) *
...empirical measurement trumps hypothesis every time.


Thank you, serpens. I've been saying this for many years, just in regards the photogeology of the Martian surface vs. what the geologists focusing solely on surface-level measurements keep trying to say.

In essence, even before the chemical cues you mention were confirmed, mapping as early as Mariner 9 clearly showed vast catastrophic flow plains, well-developed river and delta patterns -- many, many macroscopic evidences of liquid water carving the surface at some time in the (as now is known, quite distant) past. And once these extremely convincing pieces of evidence for liquid water became known, it seems like a branch of planetary geology became fixated on proving that these aqueous erosional features just had to have an explanation that didn't require large quantities of liquid water, solely on the basis that since liquid water cannot exist on the surface now, we must assume that it could never have existed there.

But, over and over, we see that in both additional orbital imagery and data from surface assets, the empirical evidence points to the presence of abundant liquid water in ancient times. And the theorists step back, very reluctantly and with great inertia holding them back, with new theories that, well, maybe it was some other fluid carving channels... or maybe there was a little tiny bit of water that only existed on the surface for hours, not thousands or millions of years.... or maybe, just maybe, if you push us really hard, we'll admit that in a very few places some water persisted for longer than a few days... And yet, the empirical evidence keeps pointing to relatively long-lived streams, rivers, pools and lakes. Long enough lived to have created rounded and sorted populations of pebbles, long enough to create conglomerate rocks with clay-like matrices... et cetera, et cetera.

Maybe it is the theorists' jobs to creep very slowly back from their best-understood hypotheses as new empirical data comes in to to challenge them. But at some point, you just have to admit defeat and start to look for a new hypothesis that actually fit the empirical data. Heck, even Harold Urey was able to do so (if not with terribly good graces) when the first lunar samples were returned by Apollo. Urey, a cold-Mooner, had predicted that the Moon and its rocks would be primitive and chondritic, without any volcanic transformation. But when the well-described rocks were initially analyzed, one of Urey's staffers came in to tell him the results, and Urey, in a very irritated voice, cut him off and said "Don't tell me, I know, they're basalts, right?" The staffer confirmed that, and Urey just grunted, seeing his entire set of theories of lunar origin and composition fly completely out the window with the first truly empirical results.

I think we still need that moment to pass through the planetary science community in re Mars, and I'll be interested to see what finally tips them over the edge...

-the other Doug (With my shield, not yet upon it)
serpens
Subduction zones have a way of terminating the ocean floor weathering process however an example of timescale would be basaltic clasts with weathering rinds recovered from the Iberia Abyssal Plain in early Cretaceous sediments (say 135 Ma ago) and reported in the Proceedings of the Ocean Drilling Program. Interestingly enough given Emily's comment re potassium enrichment, these basalt clasts were enriched in K2O, attributed to low temperature alteration.
Gerald
QUOTE (ngunn @ Oct 24 2014, 11:17 PM) *
Martian water verus terrestrial water: Could the former be less efficient than the latter at hydrating basalt minerals? Absence of some crucial ingredient peculiar to Earth??

Three possible approaches:
1. Lower atmospheric pressure on Mars: Due to lower gravity the weight of the atmosphere is lower, even with the same mass per area. This tends to reduce the overall temperature for water being liquid, hence generally slows down chemical reactions with water relative to Earth, since water vapor and ice are less efficient in weathering than liquid water.
2. Less energetic transport processes: Lower gravity on Mars reduces the overall abrasion between grains, since mechanical processes are less energetic on Mars for otherwise the same conditions as on Earth. Abrasion and transport is necessary for fast weathering, otherwise we get a chemical equilibrium preventing from further chemical weathering.
3. Atmospheric hydrogen escape: Olivine is alkaline (e.g. increases pH of water during serpentinization by releasing Mg(OH)2 and Ca(OH)2) and needs acids to weather. Water and additional CO2 or SO2 can act as acid; pure water is amphoteric, hence can act as an acid, too. An acid is by one of its definitions a proton-donor. Due to the lower gravity and the weaker magnetic field on Mars, hydrogen escapes faster from Mars than from Earth, resulting in less hydrogen abundancy. Protons are hydrogen nuclei; less hydrogen should lessen the proton supply, hence result in higher pH. Higher pH means less acids to weather olivine. Olivine is hard and prevents basalt from abrasion.
serpens
QUOTE (Gerald @ Oct 25 2014, 04:32 PM) *
2. Less energetic transport processes: Lower gravity on Mars reduces the overall abrasion between grains, since mechanical processes are less energetic on Mars for otherwise the same conditions as on Earth.


To the contrary. While Mars today is an extremely benign environment, before the atmosphere thinned and when the sedimentary features that Curiosity is investigating were formed it would have been an very energetic environment. Low gravity plus thick atmosphere would have resulted in a pretty impressive sand blasting effect with an energy contribution from the elliptical orbit and axial tilt variations.
Gerald
Changing too many parameters at the same time makes things difficult to understand.
So here two examples with just the gravity as variable:
- A river, the same in all properties besides gravity, is flowing slower in lower gravity (Mars) than in higher gravity (Earth), since the viscosity of the liquid - hence the friction within the liquid and between river bed and liquid - remains the same, but the force downhill is reduced in the lower-gravity environment due to the reduced weight of the same mass. Therefore the kinetic energy released on grain collisions is lower.
- Lifting a grain to a given height difference is propotional to the gravity. The release of energy after free falling from the same height is lower in lower gravity, hence less abrasive.

That way the same energy can result in more impressive structures in a lower-gravity environment.

Ancient Martian weather systems are rather hard to estimate, since we don't know pretty much about composition and density of the atmosphere. Even then, weather models are inherently complicated.
Different climatic and atmospheric conditions may have resulted in very different weathering products. The question has been about water-rich conditions favorable to leave basalt chemically less altered. High-energy aeolian events in the context of basalt weathering become only relevant in the presence of water.
dburt
QUOTE (serpens @ Oct 25 2014, 03:16 AM) *
In all this discussion we need to differentiate between hypothesis and empirical evidence... Empirical measurement trumps hypothesis every time...

Thanks. I might phrase this that in science we need to differentiate between actual observations and various interpretations (or testable hypotheses) based on them. Measurement cannot "trump" hypotheses because scientific hypotheses are based on those very measurements. You seem to be confused about the essential relation between the two. The first yields the second, so the second cannot be "trumped" by the first.

Some people start to do science with simple models (e.g., "warm, wet Mars" or "cold, dry Mars"), and then insist that all of the observations must agree with their models, but such simple models should not be confused with testable, competing scientific hypotheses based on actual measurements or observations.

Sorry for my delay in responding - I was leading a field trip to an alluvial fan complex earlier today.

Don Burt
jmknapp
In terms of fixed, time-honored ideas about Mars, hasn't the notion of early and widespread water and habitability been the prevalent one (notably Lowell) and only very grudgingly amended as ground truth comes in (Mariner, etc.)?
MrNatural
Speaking of empirical evidence, how does MSL's DAN's findings work with these theories? As we know, DAN is detecting water (in hydrated minerals?) in the subsurface, with a patchy distribution, but I am not a geologist/geochemist and am not qualified to interpret these readings. Can anyone here speak to these findings?

http://www.spaceflight101.com/dan-science-reports.html
Don1
I think the most likely scenario is not that Mars was dry, but that the water chemistry was not suitable for weathering basalt. Geological waters are never really pure, but contain a variety of dissolved gases and salts which change the chemistry quite a lot. Articles on basalt state that it weathers by oxidation, but that won't happen in an oxygen free environment.

Some articles on early atmospheres speculate on the presence of ammonia, NH3. Ammonia is extremely soluble in water, and tends to produce alkaline solutions, which may be less likely to weather basalt. It also depresses the freezing point. 25% ammonia in water freezes at -58C.
Gerald
QUOTE (MrNatural @ Oct 26 2014, 04:43 AM) *
Speaking of empirical evidence, how does MSL's DAN's findings work with these theories? As we know, DAN is detecting water (in hydrated minerals?) in the subsurface, with a patchy distribution, but I am not a geologist/geochemist and am not qualified to interpret these readings. Can anyone here speak to these findings?

http://www.spaceflight101.com/dan-science-reports.html

Clay minerals at Yellowknife Bay have been confirmed by CheMin:
QUOTE
The abundance of clay minerals was estimated separately, using the program FULLPAT, at ~22 +/- 11 weight% of the total sample mass.

They can embed water into their crystal structure, and they usually form from basalt in the presence of more or less neutral water (hydrolysis).
Bassanite has also been found. It's one of the most abundant minerals in the veins.
More discussion in e.g. this paper.

But that has been a different unit. The unsolved puzzle is mainly about the dark-toned capping unit.
ngunn
QUOTE (Gerald @ Oct 26 2014, 01:23 PM) *
But that has been a different unit. The unsolved puzzle is mainly about the dark-toned capping unit.


Yes indeed. The implication seems to be that the capping unit has never, over its presumably long history, experienced the same water chemistry as the underlying clay-rich material. That could in the end prove to be as significant as any geological discovery Curiosity will make. Water may have been too brief, or nonexistant, or it may have had a different chemistry that prevented it from attacking the basalt. (For the record I have no horse in this race.)


serpens
QUOTE (dburt @ Oct 26 2014, 12:00 AM) *
.....Measurement cannot "trump" hypotheses because scientific hypotheses are based on those very measurements. You seem to be confused about the essential relation between the two. The first yields the second, so the second cannot be "trumped" by the first.....

Actually no. A hypothesis by definition is a proposed explanation for an observed phenomena, normally based on limited evidence, or indeed a proposition made as a basis for reasoning, without any assumption of its truth. A hypothesis can look most compelling until new empirical measurement or observation, often from sources external to the entity that developed the hypothesis, provides data that was not available for hypothesis development and reveals it to be incorrect. Such new data is a separate event from the hypothesis development process so the logic that the first yields the second does not hold in such cases. However I do agree that trumped was probably a bad word choice although widely understood as a colloquialism. The point that dvandorn made is that human nature being what it is, often the proponent of a hypothesis will raise the status of the hypothesis to that of a belief and continue to argue the case despite the accumulation of compelling, contradictory evidence. We saw that with respect to the provenance of Meridiani and Gale is enigmatic enough to generate a wealth of competing hypotheses.

Emily's article reminds us that Gale, like Meridiani is a basaltic sedimentary environment with a provenance extending back billions of years. Something that we are not exactly over endowed with on Earth. How do we correlate the clay beds exposed at the base of Mount Sharp with the largely unaltered basaltic sediments in apparent lake deposits and deltaic formations? Was there a massive change in the environment that altered the water chemistry? How deep beneath the current floor does the sedimentary bedding extend? There will be some interesting times and interesting hypotheses ahead.
Gerald
QUOTE (ngunn @ Oct 27 2014, 12:21 AM) *
Yes indeed. The implication seems to be that the capping unit has never, over its presumably long history, experienced the same water chemistry as the underlying clay-rich material. That could in the end prove to be as significant as any geological discovery Curiosity will make. Water may have been too brief, or nonexistant, or it may have had a different chemistry that prevented it from attacking the basalt. (For the record I have no horse in this race.)

A try ("hypothesis") to fit things together:

The sandstone of the capping unit is very hard. This requires some diagenesis to form a hard cement. It would barely be explainable by a total absence of water.
Instead a low amount of water, just enough to produce some SiO2, H4SiO4 or other silic acids, but not enough for completely resolving olivine or carry away the silic acids, would fit the observation.
This would also provide one possible explanation of the pores in the rock (sponginess) as a result of the release of H2 during some hydrolysis; voids may have never been filled by minerals, or filling may have weathered out recently.
Basalt and water should have originated from different supplies, otherwise basalt would already have been weathered before the event.
Then a singular event should have mixed the two ingredients.
The hard sandstone should have formed, before water could penetrate the rock once more.

A simple scenario of separated supplies of basalt grains and water could be a basalt dune covered with snow or ice.
A singular mixing event could have been a blast by a nearby impact.
If this happened in a cold and arid/dry climate, the mix of basalt sand and snow would have had enough time to form the hard sandstone by some serpentinization. Serpentinization is exothermic and may have molten the snow to readily react with the basalt.

Appropriate conditions may have been common in the late Hesperian or early Amazonian.

We don't find this type of rock on Earth, since impact events near snow-covered basalt dunes in the absence of liquid water have been uncommon on Earth.
Julius
At what stratigraphic level are the yellow knife bay clay sediments in relation to the rock examined at Kimberly and pahrump hills?
Gerald
Not sure, whether there is a final consensus.
Referring to this suggested cross section, Yellowknife Bay and Kimberly are probably part of the crater floor, and Pahrump Hills belongs to Mt. Sharp.
My current understanding is, that the base layers of Mt. Sharp should be older than the crater floor sediments, the latter with origin from the crater rim.
If this holds, Pahrump should be the oldest of the three, then Yellowknife Bay, and youngest Kimberly.
elakdawalla
There were several talks at GSA trying to address that question, and nobody came to any conclusions about the stratigraphic relationship between Bradbury Rise and Mt Sharp. But YB is the lowest part of the stratigraphic column that Curiosity examined in the Bradbury Rise region.
Julius
And pahrump hills would be part of the Murray formation? Because then if yellow knife sediments are lower than the crater sediments, then the lake bed deposit hypothesis may still be relevant. Has there been any attempt to analyse gale crater floor layers by radar if that's even possible?
Don1
It's been over a month since the Pahrump hills were first drilled, and there has been radio silence from the project since that time, despite opportunities for press conferences at events like GSA. At this point I think it is reasonable to infer that the lab results don't contain any dramatic new results. If the Murray formation material is broadly similar to the Bradbury Rise stuff, then whatever makes up Mt Sharp probably sits on top of the same basaltic sandstone/mudstones that cover the crater floor.
ngunn
QUOTE (Don1 @ Oct 27 2014, 10:00 PM) *
It's been over a month since the Pahrump hills were first drilled, and there has been radio silence from the project since that time


I fear my nightmare has come true: that they've found methane but there's no way to release it without embarassment. rolleyes.gif

More seriously, Mr Natural's question about DAN (post 28) has gone unanswered. I'm curious about that too.
elakdawalla
CheMin and SAM do not produce instant results, unlike, say, APXS. Especially SAM -- they have to do lots of work in the testbed to confirm that the gases that come off are good matches to what they see in the data from Mars. It is very time-consuming. Also remember that it's no longer newsworthy that they got XRD or GCMS results on Mars, so I don't expect to see SAM results in Science unless they do find complex organics. Now the science results will be coming out in papers in journals that have longer lead times, like JGR and Icarus and Geology. This kind of fiddly work needs peer review. Plus, Pahrump being the bottom of the Mt Sharp strat column means it will make more sense to wait to publish until they can talk about how things vary with position in the strat column -- that is, they'll drill more before they publish much. All of which means I don't expect to see any Pahrump results any time soon.
serpens
In the absence of folding it would be anticipated that Mr Steno's law of superposition would hold true, but Gale and Mount Sharp are sufficiently enigmatic that we need to keep an open mind. For example Gerald's suggested cross section link suggests that the Murray formation provides the base of Mount Sharp. But if the cross section was extended to the right there would be an eroded dip just past the hematite ridge, exposing clays. Anderson and Bell suggest that the hematite rich formation and the clays are exposed sections of layers that extend beneath Mount Sharp. Following Mr Steno's wisdom, these layers would have extended over the area being investigated by Curiosity. Given cycles of erosion and deposition I am not sure that we can make any assumptions over whether Pahrump or the clays were deposited first. Pahrump and indeed the exposed Murray formation could be remnants of a deposition sequence that abutted Mount Sharp and occurred after the hematite/clay layers were eroded away.
Gerald
QUOTE (ngunn @ Oct 28 2014, 12:49 AM) *
More seriously, Mr Natural's question about DAN (post 28) has gone unanswered. I'm curious about that too.

First I tried to explain DAN with simple means, but it got a little too lengthy, so I discarded most of that part, and recommend reading this paper, instead, for technical background.
(Very briefly: Think of neutrons as cue billard balls, hydrogen nuclei in the target as slightly attractive billard balls of the same weight, other neuclei as much heavier billard balls. Think about collisions, leading to the cue ball being backscattered to the "detector". The lighter balls will slow down or even capture the cue ball, whereas the heavy balls can backscatter the cue ball. Use energies and times of the backscattered cue balls to infere the composition of the target.)

The referenced DAN Science Reports are first of all a proof of principle for a science instrument, that has never before been testet on another planet.
It's also a first attempt to interprete DAN data according to methods proposed, e.g. in the above paper.
The simplest result which can be obtained from DAN is the overall hydrogen abundance in the target rock.
More challenging is the two-layer model; it tries to find out hydrogen abundance for two layers of rock.

What should generally be expected for a wet rock exposed to dry air? It should dry out from outside, and remain wetter in the inside. That has been the expectation for Mars; deeper rock layers should contain more hydrogen in the form of water than the top-most layer, called the "direct model".
But that's not always the case ("Scientists did not expect to find this many cases favoring the inverse model", Spaceflight101 article).
A plausible reason for the frequent inverse model is water being bound to surface regolith or dust. That's particularly evident for locations with very dry rock below the regolith.
YB rock contains more water, making it less likely, that surface regolith contains more water than the underlying rock.

The other way round: If you are looking for rock with bound water, look for the direct model in DAN data.
stevesliva
QUOTE (dvandorn @ Oct 25 2014, 08:57 AM) *
clearly[/i] showed vast catastrophic flow plains, well-developed river and delta patterns -- many, many macroscopic evidences of liquid water carving the surface at some time in the (as now is known, quite distant) past. And once these extremely convincing pieces of evidence for liquid water became known, it seems like a branch of planetary geology became fixated on proving that these aqueous erosional features just had to have an explanation that didn't require large quantities of liquid water, solely on the basis that since liquid water cannot exist on the surface now...


I, for one, cannot decide if Occam's razor dictates that lotsof water caused the features, or if it dictates that the features were created without lotsof water. wink.gif

That said, slope streaks in contrast-stretched images of dust are just images of clumpier dust, as far as I'm concerned. So there are conclusions not to jump to...
MrNatural
I have to ask the obvious question here. What does the stratigraphy in places such as Jubilee Pass, Panamint Butte, and Upheaval Dome tell us? Are these lacustrian or streambed deposits? There appear to be hundreds of layers visible (and they are probably but a small section of thousands of such layers). If these represent sedimentary cycles, then that implies a lot of water or at least a lot of transient water events.

So much evidence, except the basalt, seems to indicate a very wet environment that I have to wonder if the Noachian/Hesperian water was different than water today on Earth. Can experiments be done with crushed basalt and water that has little free oxygen (or maybe dissolved ammonia or CO2), or is it too slow to observe?
Gerald
Cross-bedding is a strong hint towards either a fluvial or an aeolian depositional environment. Read also Emily's caption to her nice stitches of Cross-bedded rock within Panamint Butte. This kind of sedimentation can advance rather rapidly, and form sediments within hours, of course not lithified.
Mineralogy and cementation (edit: clast sizes and shapes, too) tell something about the presence of water.

Experiments with basalt can be done and have been done. Crushing basalt into a fine powder is a way to accelerate chemical weathering experiments, since powdering increases the surface accessible to the water.
Olivine grains can dissolve rather rapidly - within days - in abundant water in the presence of CO2, if mechanical abrasion removes the rind.
It can take 10s of thousands of years, if conditions are less favorable for olivine weathering.

With powdered basalt it shouldn't be too difficult to compare chemical weathering processes in water or in a wet environment with varying additional compounds.
For slow weathering processes sensitive analysis instruments should help.

At YB the water wasn't much different than today on Earth:
QUOTE
These clay minerals are a product of the reaction of relatively fresh water with igneous minerals, such as olivine, also present in the sediment.


[I didn't provide links to papers about experiments of olivine weathering in the presence of CO2 or other compounds, because most of them imply possible violations of rule 1.2.]
jmknapp
Were the fine layers necessarily formed in a wet environment, or might they have been laid down dry by wind?
Gerald
Fine layers without additional evidence aren't unique for wind or water.
It's more a result of moving dunes or some deltaic deposit, as a result of transport by water or by wind.
But wind isn't able to transport clusts above a certain size in a thin atmosphere.
Grain size distribution tells something about the transport and sorting process. That's one line of additional evidence.
Investigating several lines of evidence will result in an evident or even definitive conclusion, at the end.
serpens
Speaking from the comfortable confines of an armchair it appears that the Yellowknife Bay investigation was somewhat limited in a geographic sense with just two drill holes into the Sheepbed member siltstone/mudstone and none into the nearby sandstone. Every indication was that the Sheepbed material was deposited in a lacustrine environment with the implication that this basaltic material was weathered, eroded, ground down extremely fine, transported some distance, deposited onto a lake bottom and then buried as deposited material built up. The whole process would have almost certainly reflected a low temperature environment so can we posit that the majority of the basalt alteration took place during initial weathering of the source rock followed by erosion of the weathering rind and further alteration during the transportation process which would include a degree of mixing with material from other sources such as ash?

Given the accepted basalt alteration susceptibility sequence of glass > olivine > pyroxene > amphibole > plagioclase > K-feldspar (with some overlap) the incomplete alteration of the basalt could be readily explained. There was limited capacity for oxidisation at the surface and minimal following final deposition. The enrichment of magnetite could possibly flow from palagonitization of basaltic glass in addition to the possible production of magnetite as an olivine alteration product. All conjecture of course.
Gerald
QUOTE (serpens @ Nov 2 2014, 04:19 AM) *
Every indication was that the Sheepbed material was deposited in a lacustrine environment with the implication that this basaltic material was weathered, eroded, ground down extremely fine, transported some distance, deposited onto a lake bottom and then buried as deposited material built up. The whole process would have almost certainly reflected a low temperature environment so can we posit that the majority of the basalt alteration took place during initial weathering of the source rock followed by erosion of the weathering rind and further alteration during the transportation process which would include a degree of mixing with material from other sources such as ash?


From this paper, free access via here:
QUOTE
Despite identifying phyllosilicates in Sheepbed mudstones by XRD ... and inferring them from ChemCam ..., the geochemistry of Yellowknife Bay formation provides scant support for any substantial chemical weathering history affecting the sources or the sediment during transport into the depositional basin.

What makes you rethinking (post-depositional) in-situ alteration?
serpens
Thanks, I hadn't seen that release of Science. My thought process was triggered by trying to reconcile the proposed alteration of olivine (with free Al) to saponite + magnetite + H2 with the low temperature environment that all seem to agree existed at the time of deposition. I stand to be corrected but as far as I know the lower temperature limit for this is around 50 C. Hence my suggestion of palagonitization of basaltic glass as a supplement to olivine alteration, to account for the enhanced manganese. But the paper is pretty clear that depletion of olivine did occur and was isochemical (i.e. it occurred after deposition in a closed hydrologic system).



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