Basaltic Sediments, rethinking Mars - again? Options

[ngunn]

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]

.....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]

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]

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.

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]

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]

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.

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:

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.]