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May 09, 2012


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Very, very cool. Also, the geology pun of the week is in this post.

Here are some interesting sand grains in return:

I have a question. While the dynamic angle of repose might not have been determined, why don't we already know the static angle of repose on our moon and whether it is lower than it is on Earth? Let's put it this way: I was surprised that there was confusion as to whether gravity would change the static angle of repose, since we have already visited a lower-gravity body that has various kinds of rocks, dirt, and sand and we brought back samples of these materials and each sample's location was logged, probably at least sometimes with photos of the slope the sample was taken from. Maybe nobody really took notice of slope angle? Were there just not enough data?

It's a case where a non-scientist like me is left wondering why it was still a mystery, and so I ask (undoubtedly dunderheaded) questions while still being impressed by the super-cleverness of the experiment.

Blaize - no such thing as a dunderheaded question in my opinion (and apologies for taking a while to respond).

It's a good question, and raises some points about the profound differences between the history of Mars and that of the Moon. Moon sediments contain relatively small proportions of sand (typically less than 20%) and what there is is mixed up with the predominant dust (plus lumps of rock). This is because there have never been processes on the Moon to sort the sediments into different sizes and separate them - it takes water or wind to do that. All there is on the Moon is the debris that fell back to the surface after an impact, and that debris just lay there (until the next impact). And any pile of such mixed-up sediment really doesn't tell us much about the angle of repose of any particular component, just the mixture whose behaviour is more complex than that of any one constituent. Water may only have flowed on Mars a long time ago, but it did sort things out (as the wind still does for the finer material).

Hope this answers your question!

Thank you, Michael. I realized upon re-reading the article that the question the scientists were asking was more nuanced than just "is the angle of repose changed by gravity" and included "how can we tell whether the angle of repose is caused by gravity or by other forces such as water, wind, (and particle size &c) when we're looking at stuff on Mars." My comment seemed increasingly naive the more I thought about it.

This clarification reiterates and expands the point that I missed when I first read the post, and I really appreciate the extra information.

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