Basaltic Sediments, rethinking Mars - again? Options

[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]

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:

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).

[wildespace]

And yet Curiosity finds evidence of plentiful water and water-deposited sediment layers:

http://www.ustream.tv/recorded/56255521
http://www.jpl.nasa.gov/news/news.php?feature=4398

"Observations by NASA's Curiosity Rover indicate Mars' Mount Sharp was built by sediments deposited in a large lake bed over tens of millions of years.

This interpretation of Curiosity's finds in Gale Crater suggests ancient Mars maintained a climate that could have produced long-lasting lakes at many locations on the Red Planet."

~~~

A question just popped up in my head - did Mars ever have rain? All those lakes and rivers must have been fed with water in some way.

[nprev]

It may well have had rain at some time(s), but there's no direct evidence for that as of yet.

Hopefully these fascinating lake sediment findings will be followed by many more clues which may be able to answer that question among many, many others.

[centsworth_II]

Phoenix saw snow falling on current day Mars.

[Harder]

My first action after reading these latest MSL results was to look up an old blog entry from Emily (Edwin Kite – about a very cold and dry early Mars; sedimentary rocks fit snowmelt scenario, see http://www.planetary.org/blogs/emily-lakda...ypothesis.html).

Very interesting to say the least that this latest ground-thruth from MSL gives a much warmer impression of early Mars!

[Gerald]

IIRC, in the telecon they said, that they don't have a good climate/atmospheric model yet, which fits the data.

I'd think, assuming the same Martain orbit around the Sun over billions of years may turn out to be too strict.
My impression has been, that this possibility is considered.

The orbit is more excentric than Earth's. This might hint towards either a close encounter with another planet, or towards some orbital resonance.

[Julius]

I followed yesterday's press conference and the evidence provided seems convincing that support a river, delta and lake system as the source for formation of Mount sharp. However, it remains confusing to me following a recent discussion as to why the rocks examined so far although showing some chemical alteration by water, remain largely of basaltic composition which would seem to limit the interaction of water with these rocks.
Another thing is what significance is there in pahrump rocks showing a higher haematite content compared to rocks in yellow knife?

[Gerald]

The largely basaltic composition of the rocks should imply only short exposure to liquid water.
If physical weathering occurred either in cold highlands on the crater rim or during arid periods, and has then been transported within rather short time via alluvial fans and rivers to the lake, where it has been buried fast enough to prevent water from acting on the rock for too long, this could allow for the preservation of basaltic sediments.
Since there probably hasn't been relevant plate tectonics on Mars, repeated exposure to surface water is less likely than on Earth.

Regarding hematite: Color of the rock and CheMin results show significantly higher hematite at Pahrump than at YB.
The good, and yet unswerded question is: If it's the same type of deposit (lacustrine in a shallow lake), how can the environment change between more and less oxidizing withing a geologically short period?
Maybe the answer can be found by questioning the assumptions, implicite in the question.

[Julius]

The largely basaltic composition of the rocks should imply only short exposure to liquid water.
If physical weathering occurred either in cold highlands on the crater rim or during arid periods, and has then been transported within rather short time via alluvial fans and rivers to the lake, where it has been buried fast enough to prevent water from acting on the rock for too long, this could allow for the preservation of basaltic sediments.
Since there probably hasn't been relevant plate tectonics on Mars, repeated exposure to surface water is less likely than on Earth.

Regarding hematite: Color of the rock and CheMin results show significantly higher hematite at Pahrump than at YB.
The good, and yet unswerded question is: If it's the same type of deposit (lacustrine in a shallow lake), how can the environment change between more and less oxidizing withing a geologically short period?
Maybe the answer can be found by questioning the assumptions, implicite in the question.

Correct if i'm wrong but I believe that pahrump hill rocks are at a higher elevation than yellowknife bay?

[Gerald]

True. Same age is one of the implicite assumptions.
With the new working hypothesis, higher and further to the south means likely younger.
But how does this explain the higher oxidation?

[serpens]

Nice to see the illustrations showing a lake in Gale without Mount Sharp. Personally I feel that a dab of artistic licence to show a higher, less eroded crater rim and ejecta field would have provided a better feel for what it would have looked like. Establishing the time line for the various fluvial/lacustrine artifacts found by Curiosity will be a real challenge. The lower levels of Mount Sharp are a pretty strong indication that at some stage the original lake would have extended over the area of Curiosity's traverse, or alternatively, as the crater filled a bowl shape was retained, confining the lake to the centre of the crater over many millions of years. Mass wasting from an initially high and rugged rim would fit that scenario. The telecon central lake hypothesis for the creation of the lower levels of Mount Sharp, followed by excavation and deposition as proposed by Kite to form the Aeolian upper layers seems a pretty seamless fit although the lithification and evidence of significant water flow on the upper levels provides a good argument that this mountain building occurred when Mars was still experiencing wet cycles.

I am still not entirely comfortable with the early classification of Pahrump Hills as part of the basal layer of mount Sharp, implying it was formed in the original lake. The apparent friability and partly modified basaltic composition would argue against that. Given the position in relation to Mount Sharp a late stage onlap or downlap would seem more logical . The same logic would apply to the Murray Formation.