Building Theories on Sand

Inside a rocket soaring for mere minutes before falling back to Earth, a process reminiscent of the birth of planets occurred. When shaken a little, a fine, sandlike material aboard the rocket lumped together in clusters, just as dust does in the rings surrounding young stars. The experiment, published in this week's Physical Review Letters, may lead to better models of planet formation.

When granular materials such as sand or dust are shaken, they behave like a gas--that is, the particles collide with each other like molecules in air. Scientists have observed that such granular gases sometimes clump together. But that phenomenon has been difficult to study in the lab, because the grains are always pulled down by gravity, limiting the interactions between particles.

To eliminate gravity's tug, scientists from several French laboratories performed similar experiments aboard a rocket that briefly left the atmosphere before plunging back to Earth. They filled three 1-cubic-centimeter cells with different amounts of bronze "spherules" with a diameter of about 0.3 millimeter. An electrical motor made the cells vibrate, while a tiny camera peered in through windows in the cells to observe the spherules' behavior. Once the rocket's engine was shut off, it went into a parabolic trajectory, with its highest point at 150 kilometers above Earth, providing 200 seconds of low gravity. This allowed the researchers to do nine short experiments, in which they varied the frequency and amplitude of the vibrations.

In the two cells with the highest densities, the researchers observed the formation of a stable and immobile cluster of particles; in the cell with the lowest density, this didn't occur. The clusters are caused by collisions, triggered by the shaking, says Stéphan Fauve of the Ecole Normale Supérieure in Paris. After particles collide they slow down, with part of their energy transformed into heat. If the overall density is high enough, there will be spontaneous local fluctuations in density; and as soon as that happens, more particles start colliding locally, slowing them down more and increasing the local density. Eventually, a stable cluster forms.

A similar phenomenon may take place in the dust circling young stars, says astronomer Larry Esposito of the University of Colorado, Boulder, and might explain how such "protoplanetary rings" coalesce into planets. Indeed, Esposito and his team have observed such clumping in rings around stars in the Orion Nebula. "Potentially we can take results of [Fauve's] experiments and use them to prepare models of ring dynamics and the origin of planetary systems," says Esposito.