[X] My Assistant

[Gerald]

If we believe that assumption, then we have two plausible scenarios:
- Either there is just a pebble at the tip of the mole, or
- the mole is hitting bedrock.

In both cases, the mole needs to find a way to return to partially inelastic collisions to make any significant progress.
In the case of a pebble, it may either be possible to destroy it or to work around it. The loss of surface material might hint towards a pebble sinking into other regolith and leaving a void.
In the case of bedrock there may exist two options:
- Find a fracture or a soft fracture fill, or
- find a way to reduce elastic bouncing when hitting hard bedrock,
-- either by a larger force of the scoop pushing the mole, the risk of which needs to be assessed thoroughly by the mission engineers, or
-- by fixing the mole to a larger mass, which is looking even more challenging to me, since sand probably won't do it.

By the images, I'm inclined to presume that they are trying to work with a larger force of the scoop, but very cautiously at the same time in order to reduce the risk of damage.

[JRehling]

The distinction between elastic and inelastic collisions is not present in the mole team's description of its intended operation.

• The design: "for loose soil to flow around it, providing friction that keeps the mole from bouncing backwards with recoil"
• The problem: "a lack of friction in the soil"; "The soil around InSight provides much less friction than what we've seen before on Mars."

The expectation is that the collision will be elastic, drive soil out of the mole's path, the friction from soil lateral to the mole to prevent significant recoil, and for the mole to thus fall a bit into the cavity thus created, and experience a net downward motion each time. The problem isn't that the mole is failing to sticking to the soil in front of (below) it.

https://mars.nasa.gov/news/8444/common-ques...e/?site=insight

[Gerald]

Thanks! So, the mole in the soil seems to be considered to behave similar to the way it would behave in some kind of non-Newtonian fluid, quicksand for instance, with low (inelastic) resistance against low velocities during recoil and high (elastic) resistance against high velocity when the hammer is striking.
That way, the mole will counterintuitively move in the wrong direction.
Something for my backhead to think about.

[Steve G]

It's amazing how little we still know about Mars. Years of research and tests are done on equipment, such as the mole, and the soil is not what they expected, so it doesn't work. Or even the unexpected wheel damage on Curiosity. Years of testing means nothing when the planet is holding the cards so close to its chest. But these setbacks only enrich our learning and experiences.

[Phil Stooke]

https://twitter.com/NASAInSight/status/1310978911002484736

More scooping and hole-filling to come!

Phil

[PaulH51]

Blink GIF of processed / cropped IDC images from sols 651 and sol 654. A little movement on the science tether. Wind, settlement in the filled pit, or thermal variations in the cable?

[PaulH51]

Blink GIF of processed / cropped IDC images from sols 654 and sol 656, more movement of the science tether!

[JRehling]

It feels like reading tea leaves here, because we don't see what might have happened between frames, but the tether seems to buckle upwards in response to a tiny pull downwards. The scoop does not move between the frames, so that would seem to indicate that the mole moved a small distance downwards, which would be a positive sign.

However, the dynamics of a flexible object can be hard to interpret and it's not really going to be a good sign until we see the tether pull at least a couple of cm in the right direction.

[Gerald]

It's amazing how little we still know about Mars. Years of research and tests are done on equipment, such as the mole, and the soil is not what they expected, so it doesn't work...

The only fairly plausible idea my backhead came up with in the meanwhile to explain the strange behaviour of a soil poor of pebbles was triboelectricity between very dry grains of different composition within a very well-mixed fine dust phase. I discarded van-der-Waals forces and magnetism as much less plausible primary causes. Triboelectricity should be strongest where hammering forces are strongest and might make dust behaving temporarily like an elastic solid.
I think that it's fairly difficult, but not impossible, to produce such a kind of Mars analog in a lab and to use it in a test bed to develop a hammering strategy. A thorough mixing would probably require the mixing and settling of aerosols of the different mineral species rather than mixing powders directly. That's making the whole experiment less easy.

That said, thanks Paul for keeping us up to date!
I'll have to return to the next planet further out with my active participation.

[Gerald]

I couldn't help to dig a little deeper in literature, and found that, in contrast to my first expectations, mere van-der-Waals forces can already be sufficient to cause temporary jamming (transition into a solid) of fine powders on high stress (Jamming Threshold of Dry Fine Powders, by J. M. Valverde, DOI: 10.1103/PhysRevLett.92.258303, see especially Fig. 1, investigated in the context of toners).
More tricky electrorheological properties of the dust phase in the Martian soil would still remain possible.

[HSchirmer]

Triboelectricity ... might make dust behaving temporarily like an elastic solid.

Great- so it is basically "electric oobleck"?

[Gerald]

I started with an electrostatically stabilized suspension (but a mineral powder replacing the liquid electrolyte), considered a rheopectic fluid model induced by tribolectricity, and got stuck somewhere between a shear-thickening Herschel-Bulkley fluid (related to oobleck, but with a positive yield shear stress) and the Casson model.
It appears that you'll need a Mars analog testbed with a sufficiently large dust phase (the fluid) of a realistic composition to find out what's really going on.

[PaulH51]

Arm / Scoop activity during Sol 659: Simple blinking GIF, using just 2 images from the Instrument Context Camera (ICC)
Appears to show the robotic arm pulling up from the mole and folding back the scoop.
I guess there will be a few more images in the pipeline, but they may be setting up to scoop more material on top of the mole at a future date

[Phil Stooke]

InSight has now been on Mars for a full Martian year.

Phil

[Phil Stooke]

More soil scraping over the pit on sol 673.



Phil