Exploring Mt Sharp - The Dunes - Part 2: Naukluft Plateau, Sites 53-54, Sols 1274-1352, March 7-May 27 2016 Options

[Floyd]

Actually the article does state the following on page 11:

Lastly, mounted to the DPM housing is a series of six normally open reed switch sensors which provide coarse hammer position telemetry. The small reed switches are robust to large dynamic environments and were easily integrated into the existing rover avionics. The switches are activated by a magnet mounted to the hammer assembly. The activation regions of the adjacent switches overlap providing up to 12 position states. Figure 9 shows reed switch performance across the mechanism range of motion. These sensors provide the only direct telemetry of the hammer motion and thus are useful for operational diagnostics. These sensors are not used for feedback control. Additionally, drilling various rock types during the development test program has shown a correlation between max hammer motion and rock strength. If this carries over to the flight Drill, these sensors may also provide interesting science data about the composition of the Martian rock.

This is a great article with humor and sage advise how tricky communication between engineers can be under time pressure.

[Gerald]

Very simply put, "no mud, no mud cracks" ...
BTW, the most likely cement for the friable (weak and easily-eroded) clastic sedimentary rocks of Mars (not Earth!) is some type of salt, which could not weather to clays, but could absorb or give off moisture.

It doesn't look that straighforward to me. If you take a look at e.g. the Cumberland CheMin analysis results:

"This table contains mineral abundances ... for the Cumberland drill sample.... The abundance of clay
minerals was estimated ... at ~19 +/- 9 weight% of the total sample mass. In addition to the crystalline phases, a
broad rise in background ... indicate that ~30 +/- 18 weight% of this sample consists of X-ray amorphous material.

The current unit may be compositionally different, but I would't rule out the presence of clay-rich layers here, without preceding analysis.

[dburt]

... I would't rule out the presence of clay-rich layers here, without preceding analysis.

Yes, water locally altered some of the granular basaltic rocks to primitive expanding clays (smectites) after deposition, but not necessarily in layers. Expanding clays are generally not regarded as a rock cement, because their formation causes the rock to self-seal, abruptly blocking permeability. On Earth they mainly form by weathering. Similarly, descending surficial acid fluids locally altered (via alkali leaching) another rock layer to silica, but again, this likewise is not regarded as a cement. The same silicification process was observed in a rock layer near Home Plate, in Gusev Crater, and was ascribed to alteration by an acid hot spring. Do you have a suggestion for what might be cementing the vast majority of granular fresh basaltic layered rocks in Gale Crater, if not the salts (mainly gypsum and related phases) that universally vein them? I'm afraid I don't.

For Serpens (above): Just because the Stimson unit seems "stronger" (more resistant to drilling) than the Murray, doesn't imply either is particularly strong in any absolute sense, does it? Shouldn't resistance to drilling be a function of the nature of the grains in a rock, not only their cement? Both units seem to have been etched to all to hell by physical weathering, probably mainly caused by wind abrasion and daily temperature extremes. Wouldn't that imply that neither is particularly strongly cemented?

DBurt

[Gerald]

Do you have a suggestion for what might be cementing the vast majority of granular fresh basaltic layered rocks in Gale Crater, if not the salts (mainly gypsum and related phases) that universally vein them?

Yes, besides clays, the sulfates (including jarosite) and halogenides, any precipitating mineral can take this role.
Most common at Gale is probably hematite. But other secondary iron-bearing minerals, like pyrite and pyrrhotite are other good candidates.
Akaganeite might play the role of a soft cement in some cases.
At some locations, quartz varieties, some of them hydrated, may serve as cements, i.e. microcrystalline opal, cristobalite, and tridymite.
There is a sufficient overlap with usual cements occuring on Earth:

The most common cements are quartz, calcite, clay minerals, and hematite, although other minerals like pyrite, gypsum, and barite can also form cements under special geologic conditions.

Whether the rocks at Gale are actually "fresh basaltic", is another question. I'm referring to actually analysed samples.
Tridymite is very strange, but I suppose, that it formed by precipitation at Gale.
Quartz can be a remnant after leaching, or a result of precipitation.

[dburt]

...Most common at Gale is probably hematite. But other secondary iron-bearing minerals, like pyrite and pyrrhotite are other good candidates...
Whether the rocks at Gale are actually "fresh basaltic", is another question. I'm referring to actually analysed samples...

Umm. If hematite were present as a cement, the Gale rocks would be red, not gray, inside. Pyrite and pyrrhotite, for which there's little evidence, would probably make them black, not gray. The links you supply refer exclusively to terrestrial quartz-rich terrestrial sandstones, and therefore seem completely irrelevant with regard to the basaltic sandstones and siltstones in question. These rocks appear to have been largely deposited as fresh basaltic grains (together with some mineral grains such as olivine), although, as mentioned, they have locally been altered by fluids during the three billion years or so after after deposition. Such minor alteration, unless it were pervasive and directly related to burial, probably would not be considered as cementation to a geologist. Certainly phases other than salts (halogenides and sulfates can be considered salts) are possible as cements, but as yet there is little evidence for them.
DBurt

[HSchirmer]

Umm. If hematite were present as a cement, the Gale rocks would be red, not gray, inside.

Well, hematite can actually be red, brown, grey, or something that reminds me of the horta rock-monster from Star Trek.
http://www.minerals.net/mineral/hematite.aspx


Perhaps some new facts would help channel the speculation?

AUTHIGENESIS/DIAGENESIS OF THE MURRAY FORMATION MUDSTONE IN
GALE CRATER, MARS
http://ntrs.nasa.gov/archive/nasa/casi.ntr...20160006677.pdf

Acid Sulfate Alteration on Mars
http://ntrs.nasa.gov/archive/nasa/casi.ntr...20160006674.pdf

[Gerald]

Thanks for the links to the summarizing papers! I was just going to reference the CheMin analysis results in the PDS, where it's easily seen, that hematite is rather abundant at several locations,
here some arbitrarily chosen CheMin analysis (cmb_476051894min08850450000ch00113p1.csv) of this PDS directory:

MINERAL,PERCENT,ERROR
PLAGIOCLASE,55.1,4.5
AUGITE,2.6,0.6
PIGEONITE,12.7,2.6
FE-FORSTERITE,2.0,1.3
MAGNETITE,6.8,1.0
QUARTZ,1.5,0.6
HEMATITE,7.4,1.1
ILMENITE,0.9,0.4
JAROSITE,6.8,0.8
APATITE,4.2,1.2

Hematite occurs frequently, not always. Sometimes magnetite is more abundant. Or we see very SiO₂-rich samples.
Composition varies; therefore I've enumerated several cases of possible cements.
Calcium sulfates, which fill the veins, may well be a result of hydrostatically fracturing liquids, well after significant lithification with other cements. The rock between the veins appears to be rather poor in calcium sulfate(s).

[serpens]

Just because the Stimson unit seems "stronger" (more resistant to drilling) than the Murray, doesn't imply either is particularly strong in any absolute sense, does it? ........Wouldn't that imply that neither is particularly strongly cemented?
DBurt

True the lithification of the sedimentary deposits analysed so far is reasonably weak and the correlation between drilling energy and time for a depth of hole and the hardness of the rock is relative rather than absolute. Particle size, thickness of laminations, cementing agent(s), voids etc will affect the drill and I cannot see any way to differentiate between the various stress modes (compressive, shear etc). However stepping back to the original "levitating lamination", despite the benign environment this has to be pretty well cemented and it seems to me that the abundance of gypsum is pretty much restricted to the Murray formation. I don't think we can completely discount silica cementing can we?

[HSchirmer]

I continue to be intrigued by the strength of some of these laminations.
Despite the low gravity and benign environment the normal stress on the "levitator plate"
would be reasonably significant indicating low porosity and a strong cementing agent.
The compaction necessary would seem to imply a significant overburden during lithification.

I also notice this, the laminations seem quite strong.
Hmm, I wonder, could frost, acids and silica cementing be reinforcing those blades?

The recent Acid Sulphate Alteration paper (above) mentions acidic fog as a way to generate amorphous silica at a rock surface.
That sounds like the "gel weathering" described at Gusev, where it seems that volcanic acid droplets
dissolved silica grains to a gel, the gel filled in a crack, then dried and sort of glued the rock surface back together again.
No, I'm not making it up, they actually imaged a rock fracture being "healed" and filled in.

So, looks like Gale has water, as frost, probably some near surface adsorbtion; if there is acid around, then the exposed surface might become slowly cemented by superficial silica glass. Could be recent, could be ancient.
Really interesting that the thin blades should cool and frost over easier,
thus collecting more water, which would lead to preferential silica cementing.

[dburt]

Thanks for the links to the summarizing papers! ...
... Hematite occurs frequently, not always. Sometimes magnetite is more abundant. Or we see very SiO₂-rich samples.
Composition varies; therefore I've enumerated several cases of possible cements...

Umm again. Hematite cementing sandstone (look up "red beds") is fine-grained and therefore reddish. Specimens of coarsely crystalline platy hematite may be black or even iridescent blue (cf. Martian "blueberries"), but this fact seems irrelevant. The two abstracts linked both refer to local aqueous alteration of previously deposited fresh basaltic rocks, as producing a variety of mineral compositions at certain locations near the surface. As I stated above, such alteration is not normally regarded as sedimentary cementation or lithification owing to burial, whatever produces it (weathering or "acid fog" or descending oxidizing fluids of unknown origin). The abstracts both contain a great deal of speculation, and note an inability in some cases to differentiate between detrital and diagenetic minerals. One abstract mainly refers to Meridiani and Gusev and the other exclusively to the lowermost Murray Fm. The Ming and Morris acid sulfate alteration paper does contain a couple of statements that I agree with, first that "sulfate has played a major role in aqueous processes at all landing sites on Mars" and that "materials encountered by the rovers were derived from basaltic precursors by acid sulfate alteration" but, again, these do not relate to the general question of bulk cementation (lithification) of the layered rocks observed by the rovers.

And yes, certain surfaces could be made locally stronger and more resistant during surficial alteration and leaching (e.g., silicification), possibly leading to rock overhangs, but again, such an alteration is not bulk cementation (rather, it is a special type of weathering) and calling upon it perhaps neglects the weak gravity of Mars.

DBurt

[Gerald]

...Hematite cementing sandstone (look up "red beds") is fine-grained and therefore reddish....

Indeed, it is.
However, Mars is very dry. So, it's not quite as obvious as on Earth. But if you compare hematite-rich with hematite-poor drill tailings directly, you see the difference.

[HSchirmer]

Indeed, it is.
However, Mars is very dry. So, it's not quite as obvious as on Earth. But if you compare hematite-rich with hematite-poor drill tailings directly, you see the difference.

Eh, with rocks, looks can be deceiving. Actually, redbeds and hematite is a good example.
Redbeds are red, they're red because of hematite, but that doesn't always mean they're mostly hematite.
The brightest red comes from water weathered hematite clay, so a little hematite may go a long way.
They can just as often be 99% something else...

Sedimentary Basins: Evolution, Facies, and Sediment Budget §6.3 Red Beds
"To generate the red staining of clays and silts, less than 1% of hematite is sufficient ...
The red color of sandstones is caused by grain coatings containing hematite;
their hematite content maybe even less than that in claystones." (Italics in original)

So, it's really more of a question of what sort of hematite, black, brown, orange or red?

[serpens]

Since Curiosity arrived at the Murray formation (Pahrump Hills) it has become obvious that in this area at least, a reasonably narrow stratigraphic column contains a number of different facies. Pahrump Hills spanned siltstone, mudstone and cross stratified sandstone with varying diagenetic characteristics. The sometimes startlingly different lithologies indicate changing aqueous conditions, although deposition appears to have been primarily in a lacustrian, reasonably neutral pH environment. Generalisations on cementing agent(s) and surficial influences can give rise to somewhat narrow hypotheses. After all the findings at nearby Marias Pass do not slot neatly into the Pahrump Hills scenarios, with up to 90% silica identified, including the finding at the "Buckslin" drill site of a rare (on both Earth and Mars) quartz polymorph, Tridyite.

[Gerald]

So, it's really more of a question of what sort of hematite, black, brown, orange or red?

As a fine grained pigment, hematite is red or reddish (streak color).

Even though hematite has a highly variable appearance, it always produces a reddish streak.

Other colors from yellow to brown would hint towards the presence of e.g. limonite or goethite.
Abundance is correlated with (color) saturation. When comparing colors with abundance of pigments on Mars, it seems, that colors look less saturated on Mars than on Earth for the same pigment abundance. I'm considering this effect as related to the very low humidity on Mars.
Greenish or bluish casts hint towards less oxidized iron minerals, like some clay minerals.

[HSchirmer]

Since Curiosity arrived at the Murray formation (Pahrump Hills) it has become obvious that in this area at least, a reasonably narrow stratigraphic column contains a number of different facies. Pahrump Hills spanned siltstone, mudstone and cross stratified sandstone with varying diagenetic characteristics.

Yes, different grain sizes, different cements, far more variety that you might expect for a lake in a closed basin.
I'm beginning to wonder whether "pile of windblown stuff accumulating in a crater lake" is a sufficient explanation.

Given the amount of silica in some rocks, faults and groundwater seem to play a larger role than initially expected.
There's an interesting suggestion that faulting may play a part in raising the hill, and creating the depressions-
Tectonic Formation of Mount Sharp, Gale Crater, Mars
If there are faults causing the crater floor to sink, then there are interesting possibilities for sediment r-working. Sediment could undergoing multiple rounds of erosion, deposition, exposure and re-erosion and re-deposition, because the location of the "deep water" portion of the lake would change over time.