Experiments show disproportionately large number of big boulders on asteroids likely due to Brazil-nut effect

Jul 17, 2014 by Bob Yirka report
Schematic diagram of a cross-section of the experiment after an infinitely long cylindrical container (with a diameter of 10 cm) is filled up with small particles (up to ∼ 22 cm from the origin), but before shaking begins. The large cyan particle (the intruder) is initially located at the floor. Credit: arXiv:1407.2748 [astro-ph.EP]

(Phys.org) —A team of researchers led by Soko Matsumura of Dundee University in Scotland has found evidence that appears to explain the inordinately large numbers of big boulders found on the surface of asteroids. In their paper uploaded to the preprint server arXiv, the team describes lab experiments they conducted that show the Brazil-nut effect is likely responsible for the seemingly odd phenomenon.

A decade ago, Japan's Hayabusa spacecraft revealed what appeared to be an anomaly, the it was studying, Itokawa, had more large boulders on it than could be accounted for by volcanic activity. Scientists could have just chalked it up to an idiosyncrasy of that particular asteroid, except that five years earlier, researchers studying Eros had found the same thing. In this new effort, the researchers appear to have found a reasonable explanation for what has been observed.

It's all due to the Brazil-nut effect they say, when rocks collide with an asteroid and cause enough oscillation to push smaller rocks down, and larger rocks up. The Brazil-nut effect is what happens when large nuts in a jar work their way to the top due to jostling—the smaller nuts actually work their way down as they fit into smaller spaces below. The team believes they have proved the same thing is possible with the asteroids by conducting experiments in their lab. They put one large sphere in with a group of smaller spheres in a tube and then shook the tube under various simulated gravity states meant to mimic that found on Earth, the moon and several known asteroids. They found that under the right conditions, the large sphere did indeed rise to the surface, even under the weak gravity simulations. Furthermore, other simulations conducted by the team suggest that oscillations that would occur on an asteroid due to space collisions would be sufficient to drive the large rocks to the top (in as little as several hours) offering an explanation of the large numbers seen by space probes.

The of such big boulders on then appears to be the result of many collisions with , each subjecting the asteroid to the Brazil-nut effect.

Explore further: Rock that whizzed by Earth may be grabbed by NASA

More information: The Brazil-nut effect and its application to asteroids, arXiv:1407.2748 [astro-ph.EP] arxiv.org/abs/1407.2748

Abstract
Out of the handful of asteroids that have been imaged, some have distributions of blocks that are not easily explained. In this paper, we investigate the possibility that seismic shaking leads to the size sorting of particles in asteroids. In particular, we focus on the so-called Brazil Nut Effect (BNE) that separates large particles from small ones under vibrations. We study the BNE over a wide range of parameters by using the N-body code PKDGRAV, and find that the effect is largely insensitive to the coefficients of restitution, but sensitive to friction constants and oscillation speeds. Agreeing with the previous results, we find that convection drives the BNE, where the intruder rises to the top of the particle bed. For the wide-cylinder case, we also observe a "whale" effect, where the intruder follows the convective current and does not stay at the surface. We show that the non-dimensional critical conditions for the BNE agree well with previous studies. We also show that the BNE is scalable for low-gravity environments and that the rise speed of an intruder is proportional to the square root of the gravitational acceleration. Finally, we apply the critical conditions to observed asteroids, and find that the critical oscillation speeds are comparable to the seismic oscillation speeds that are expected from non-destructive impacts.

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User comments : 11

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zorro6204
3.8 / 5 (4) Jul 17, 2014
No kidding, I thought the brazil nut effect was that when you eat your way through a can of mixed nuts, all you end up with is nasty tasting brazil nuts. Learn something new every day.
Dr_toad
Jul 17, 2014
This comment has been removed by a moderator.
Doug_Huffman
4.8 / 5 (6) Jul 17, 2014
Any farmer under less than ideal soil conditions knows this phenomena well. I live on the Niagara Escarpment and my gardens grow rocks best. If I pull up a big one - tons sometimes - all its little chillun are hiding under it.

Local traditional wheat fields have been the source for beautiful rock fences and walls and still break plows with new rocks. When our roads were first paved 80 years ago, the adjacent rock fences were the raw material for the rock crusher pavement pounder.
Whydening Gyre
5 / 5 (1) Jul 17, 2014
Canada, Doug? Spent some time in Port Colborne some 35-40 years ago...
baudrunner
not rated yet Jul 17, 2014
Or anywhere around Clinton, Ontario any time. Farmers grow rocks allmost exclusively there.
Whydening Gyre
not rated yet Jul 17, 2014
Or anywhere around Clinton, Ontario any time. Farmers grow rocks allmost exclusively there.

Not much market for them anymore - everyone is building fences from plastic these days....
Doug_Huffman
not rated yet Jul 17, 2014
Canada, Doug? Spent some time in Port Colborne some 35-40 years ago...
LOL An Island off the Door County Peninsula seven miles. N 45°22'58.42" W 86°53'49.80"
DoieaS
3 / 5 (2) Jul 17, 2014
A combined team of researchers from the Technical University of Braunschweig in Germany and Kobe University in Japan has determined that the Brazil nut effect is less pronounced when the gravity is reduced. The gravity at asteroids is very low, which would indicate, the observation of large boulders at their surface has a different origin. IMO such a relative excess could be a consequence of the fact, the small boulders tend to fit the free space between these large ones and the small boulders can be covered in regolite layer. As an example may serve the Hartley-2 comet. This comet looks like being composed of pair of large boulders only. But these two boulders are connected with thick layer of regolite, which may hide many smaller boulders inside the neck of its peanut shape.
Whydening Gyre
not rated yet Jul 18, 2014
Canada, Doug? Spent some time in Port Colborne some 35-40 years ago...
LOL An Island off the Door County Peninsula seven miles. N 45°22'58.42" W 86°53'49.80"


My bad. I figured your "Niagara Escarpment" descriptor was a little closer to Niagara Falls...:-) Port Colborne is in the isthmus tween Lake Ontario and Lake Erie. Bout 15 miles to the falls.
Whydening Gyre
not rated yet Jul 18, 2014
So, anyway - back to the article.
So, what's "shaking the can"...? Gravity? Magnetism? electricity? Momentum? Friction? Can anyone explain the counter-intuitive question of why the denser pieces seem to be replaced by less dense medium?
Just a speculation, but - maybe they're not as dense as the surrounding "medium" (smaller mass pieces) as they aggregate...
Psilly_T
4 / 5 (4) Jul 18, 2014
WHAT if... the visible universe is just a boulder that now floats atop the smaller, more abundant, pieces? I'll stop there! no one get mad =-o
Dr_toad
Jul 18, 2014
This comment has been removed by a moderator.
Whydening Gyre
5 / 5 (1) Jul 18, 2014
That's actually better than turtles all the way down.

Or stacked elephants....

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