Reposing differently on Mars

Sand, Mars, and a challenge to conventional wisdom. What better ingredients for a post could I wish for?

The two images above show sedimentary fan deposits, the great  - yes, fan-shaped – aprons of detritus that accumulate at the foot of a gully, driven by gravity, often, on earth at least, assisted by periodic torrents of water. Water is an interesting factor: on Earth these things are often called “alluvial fans” because the occasional flood clearly plays a role.

These images are an opportunity for a quiz, since one is from Earth and the other is from Mars – which is which? Sorry, but I’ll maintain the suspense for a while. Are the fans on Mars alluvial – is water involved? The debate about the role of water in Martian surface processes continues, and it’s fascinating and the evidence for,against, ambiguous, is diverse. One of the lines of reasoning involves the slopes of fan deposits – they can be characterised by lower angles than expected and this is cited as a result of the lubricating effect of water. But this makes one critical assumption: that the slope of a deposit of granular materials (the angle of repose) is not affected by the low gravity of Mars – the angle of repose is independent of gravity. But is this assumption correct? Oddly enough, this question had not been seriously addressed until Maarten Kleinhans of Utrecht University together with Sebastiaan de Vet of the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the UvA and colleagues from Delft University of Technology decided to do so. How? Well, as the University press release describes, they took a plane:

The research team used a parabolic flight campaign to mimic Mars’s lower gravity and test the effect on angles of repose of different materials. As the plane followed its roller coaster style path, slowly rotating cylinders containing different materials experienced one tenth of Earth's gravity, Martian gravity and the Earth's normal pull.

And here, from their paper in the Journal of Geophysical Research, is the gravity-reducing plane.

 
But, before we look at what they found, let’s back up for a minute and remind ourselves about angle of repose. In its simplest sense it’s the slope that sand, or any granular material, naturally settles at after it has been poured. It varies substantially with the nature of the material – grain size and shape and so on. The paper included this helpful illustration of “A pile of rounded gravel at the angle of repose, built by Gijsbert F. Kleinhans.”

Because it’s important in many contexts, I have written several posts on the angle of repose, for example in kitchen physics, and “From Bogart to Bugs,” And it’s one of many principles of physics that can be observed simply by watching the flow of sand in this blog’s icon, the sandglass. But the sandglass, simple though it may appear, illustrates the complexity of many things, among them the angle of repose. For there is not one simple angle of repose for a given material – there is a static one and a dynamic one. Essentially, the static angle of repose is demonstrated when a pile of grains is at rest, but, once they start avalanching down a slope, the constant angle of the avalanche is different, and that’s the dynamic angle of repose, always lower than the static.

There are different ways of studying all this (should you be inclined – and many people are), but a classic is the rotating cylinder. This is basically like watching clothes in the drier – a transparent cylinder is partially filled with granular material and rotated – the sand, or whatever stuff you chose, cascades down-slope constantly at it’s dynamic angle of repose. When it comes to rest, it does so at its static angle. This is exactly the equipment that the Dutch researchers took up with them in their gravity-reducing plane: nine cylinders half-filled with sand, gravel, glass beads, some in air, some in water, their behaviours captured by HD video cameras:

All kinds of clever corrections for the noise and accelerations of the aircraft itself were applied. Each stomach-churning parabola allowed for around 20 seconds of experiment at gravities of down to one tenth the normal earthly value – the gravity on Mars is just over a third of Earth’s.

As the authors write in their paper, “Our main result is surprising.” They found that, with decreasing gravity, the static angle typically increases by around 5 degrees, whereas the dynamic angle decreases by about 10 degrees. And this is true of all the materials and regardless of whether they were in air or water – the angle of repose is not independent of gravity. The difference between the two angles is critical to the natural behaviours of granular materials, and in these experiments it increased by roughly an order of magnitude. Why is this important? If the static angle is higher, then a slope can build more steeply, but, once stability is lost, then avalanches cascade down the slope at the dynamic angle – if this is low, then the avalanches keep going and involve more material.

So, bigger avalanches, lower slopes on Mars are possible without the lubricating effect of water. And, as the authors point out, this gravity dependency of the angle of repose has implications for a wide variety of phenomena on Mars and elsewhere.

Ah yes, the answer to the quiz: it was great fun juxtaposing these images (well, to me at least) because of their remarkable similarities. The image on the left is of gullies and fans in a severely eroded 100 kilometer impact crater in the southern polar region of Mars (courtesy NASA/JPL/University of Arizona). On the right is a Digital Elevation Model (DEM) image of gullies and fans on Svalbard (a.k.a. Spitsbergen) from the Europlanet HRSC-AX flight campaign. The laws of physics are compellingly universal.

 

[Many thanks to Brian Romans of Clastic Detritus for catching this paper for me. Paper citation: Kleinhans, M. G., H. Markies, S. J. de Vet, A. C. in 't Veld, and F. N. Postema (2011), Static and dynamic angles of repose in loose granular materials under reduced gravity, J. Geophys. Res., 116, E11004, doi:10.1029/2011JE003865.]

Comments

[F]

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

Here are some interesting sand grains in return:
http://emilyd47.blogspot.com/2012/01/sand-magnified.html

[Blaize]

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.

[Sandglass]

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!

[Blaize]

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.