iampete wrote:
On the right inside wall of the scoop, the appearance reminds me of a slurry of cement that has just barely begun to harden, thus the adherence to the side.
Right side of the scoop on the image or right side of the scoop as seen from the hinge? At the left outerside of the scoop, as seen in the image, some material is adhering. Images as when digging in wet clay pop up with me.
iampete wrote:Further, inside the scoop, behind the frontmost material, it appears as if the soil is adhering/clumping together, and not falling downward to the bottom of the scoop as one might expect.
None of these manifest the behavior I would expect of either dry sand, or dry sandy soil. The adherence effects of sand or soil I'm familiar with here on Earth are due to either moisture and/or organic matter.
When you examine the dust that has been picked up when Phoenix landed, there are only a few grains are sufficiently large to be characterized as sand. These are mainly found at the right side of the image. They are about 100 μm. Nevertheless inside the scoop larger clumps are visible.
In this image, mainly at the left side, the small grains are smaller than 63 μm, below which granular meterial is called 'silt'. As to what i see, there is material in the range of 10 to 20 μm. This material is very angular, compared with the larger grains, which are more rounded. Angularity enhances both the angle of internal friction (φ) and the angle of repose of a material.
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Based on the image above i wrote a very rough particle size analysis script, using image processing techniques. These provide the following results:
- d10 = 12 μm
d30 = 20 μm
d50 = 25 μm
d60 = 38 μm
d90 = 120 μm
p = d90/d10 = 10 (gradation)
U = d60/d10 = 3 (Coefficient of uniformity)
Cc = d30*d30/(d60*d10) = 0.9 (Coefficient of curvature)
d10 is a particle size of which 10% of the volume of the particles is smaller.
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I would not be suprised that when a
triaxial test could be performed on the material, values for the friction angles of 45 degrees would be found. A rough translation: a heap of material with a slope of 45 degrees is on the brink of becoming instable. Ordinary dry sand has a friction angle of 30 to 40 degrees. Very rounded gravel has an angle of 25 to 30 degrees. Crushed or broken rock (very angular!) 40 to 50 degrees, sometimes 55 degrees.
For a dry soil it does not matter whether it is frozen or not. If the soil was frozen with some liquid in between, angles of 90 degrees are possible. Water is a possible candidate for a pore liquid, specially due to the nature of water to adhere at the contact points between grains. Frozen carbon dioxide is rather unlikely, however not to be excluded.
Another effect that may contribute to the steep angle of the material inside the scoop is the effect of the sidewalls of the scoop. These enhance the slope stability, by means of extra friction.