MSL at Rocknest, First scoop samples - sols 57-101 Options

[ngunn]

OK, but leaving dark and light out of it there is still the question about grain sizes. How do you form a powdery dune coated with coarse sand?

[serpens]

This armouring effect is much like Spirit's Serpent Dune and seems widespread. I think there is more to this than simple deflation. Possibly a variation on the 'cornflakes syndrome' where the larger particles rise to the surface. Thermal induced vibration?

[ngunn]

I think there is more to this than simple deflation.

I really don't know what to think. If its deflation there must have been a heck of a lot of it and of course bigger bits in the mix to start with (also required by the cornflakes scenario).

[helvick]

Ngunn , assuming (relatively) rare atmospheric conditions that can move heavier grains and smaller grains together separated by long periods where only lighter grains can be moved it seems logical to me that the process would end up with dunes\ripples that have a much higher percentage of fine grain material internally with a much higher relative proportion of coarser material right at the surface layer. This is similar to the type of sorting process that leads to desert pavement here on Earth but on a different scale. On Mars the time-frames could be very long indeed and the difference in atmospheric density ( and lack of moisture ) would result in this operating on much smaller grain sizes, which is how this appears to me.

I'm not interested to know how this does operate here on Earth so I've now got a half dozen sand dune papers open for bedtime reading. I'll post anything interesting I find.

[dvandorn]

Also, remember that the ripples have a cemented layer at top. Perhaps there were multiple cemented layers produced by multiple episodes of cementation, and the uppermost layers are breaking down into the larger-grain fines?

-the other Doug

[ngunn]

I also wondered if the larger grains could be composites made from the fine stuff, but they really don't look like that. They're a different colour for a start. (On reflection I don't think nprev's dust-mantling argument covers that because then they would look different where disturbed by the scoop and I don't think that's the case in the close-ups we have.)

Helvick, your story might work but I'm struggling to believe it and I wish I could think of a better one. The problem is that dunes/ripples are usually understood as constructive landforms, not residual shapes left by removal of material.

[ronald]

This might give some answers - Soil crusts observed and investigated at the MER landing sites (PDF).

[ngunn]

Very good reference ronald. There's enough in that abstract to show it's not a trivial question! Maybe the experts have it fully understood by now but if not surely Curiosity will finish the job.

[atomoid]

It seems like salty condensation deposited atop ancient deflated pavement armours against further deflation, allowing a nascent film the time to evolve into a thicker crust.

[ronald]

It looks as if the second charge is gone through.

Below just a part of the next exciting pan - sol 66 mastcam:



I totally agree with EdTruthan that it would be nice to know how big those rocks are.

[Explorer1]

I don't suppose the Mystery Man has shown up at Gale yet?

[EdTruthan]

On the topic of particle sizes in dune encrustation as compared to its underlying material and the possible causes for it, there's a pretty good chapter in the 2009 publication "Dark Dunes on Mars" by planetary geologist Daniela Tirsch of the German Aerospace Center Institute of Planetary Research about Martian dune induration and imobilization: Chapter 5.7 "The Immobilization of Dunes - Possible Types of Consolidation" that references Spirit's "Serpent" dune (with photos) and offers some hypotheses about the induration processes at work. The entire publication appears to be available too, in a series of PDF's here. Have only had a chance to scan through some of it, but it looks pretty interesting.

[serpens]

The possible inter-granular cementing to form a duricrust over timescales of say 10^6 years doesn’t really address ngunn’s question. It seems that the there is a very thin layer of cemented sand particles surfacing dunes/ripples that are made up of silt. The question is how this well sorted structure formed.
It seems logical that as the atmospheric pressure dropped erosion effectively ceased, existing sand was consolidated in deposition sites, saltation diminished until even silty dunes froze. Overall Mars seems pretty much sand depleted. But the larger sand particles should have ceased saltating before the silty particles. So why have the larger particles ended up on the surface of the dunes?

[ngunn]

Great stuff, thanks for the links.

EDIT: Serpens I've just read your post. I think we can take it that the large grains formed in situ from the finer dune material (plus surface dust) by some sort of coagulation process, a process sketched out in the literature but not really understood in detail yet.

[serpens]

While there is accretion, the armouring grains are still much larger than the interior silt. For instance 'Gallant Knight' has rounded grains 3 times the size of the internal armouring the surface. Purgatory dune showed larger cemented grains armouring the silt. The missing part of the puzzle remains why the large grains ended up on the surface.