When Japan's Hayabusa spacecraft photographed the asteroid Itokawa in 2005, scientists were puzzled by what they saw: The total volume of boulders strewn about its surface were greater than the volume of the craters they were supposed to have come from. Now physicists have an answer... by studying the movement of nuts.
It's called the "Brazil nut effect," named after the anecdotal observation that whenever you vertically shake a jar full of nuts, the largest ones always end up on top. Esther Inglis-Arkell has previously written about this complex phenomenon, but the abridged version is that when there are only a few large particles — and all are of equal density — shaking creates spaces beneath the large particles that small ones can fall into. This has the effect of making the large particles rise.
As the Physics arXiv Blog reports, a new study published by a research team led by Soko Matsumura at the University of Dundee speculates that collisions between asteroids create vibrations that cause large boulders to rise to the surface, like Brazil nuts.
But can that same dynamic occur under low-gravity conditions. To find out, the scientists conducted simulations by filling a contained with 1,800 small particles, plus one large one, three-times bigger, at the bottom:
They simulated the gravity of Earth, the Moon and several asteroids including Eros and Itokawa. And they show that if they reduce the strength of the vibrations to match the level of gravity, then the Brazil nut effect occurs in all cases, provided that the oscillation speeds are above some critical threshold that turns the granular material from solid-like to fluid-like.
The key question, of course, is whether the oscillations in these experiments are similar in strength and frequency to those that occur when rocks collide with asteroids, such as Eros and Itokawa. Matsumura and co. say that the simulations show this is exactly the case. "In both cases, we expect that the critical Brazil nut effect oscillation speeds are comparable to the seismic speeds that can create craters," they say.
And they calculate that large boulders should rise to the surface of Itokawa or Eros in a few hours or about a day. "This implies that unless the seismic shaking lasts for more than a couple of hours, one impact might not be enough for a large block to rise to the surface, and that multiple impacts would be necessary to change the surface signiﬁcantly," they add.
It's an especially appropriate solution to the mystery, since the Itokawa asteroid itself bears an uncanny resemblance to a giant peanut.