Orbits of inner planets are shown as large circles in this computer-generated snapshot of actual known objects as of July 20, 2002. Green dots represent asteroids in the main belt between Mars and Jupiter. Red dots are asteroids that stray out of the main belt and pose a small but known possible risk of hitting Earth.
Credit: MPC, CBAT, Harvard CfA, IAU
Topics: Asteroids, Astrophysics, Geophysics, Metamaterials, Quasicrystals
Naturally formed quasicrystals—crystal-like solids with supposedly impossible symmetries—are among the rarest structures on Earth. Only two have ever been found.
A team led by Paul Asimow (MS '93, PhD '97), professor of geology and geochemistry at Caltech, may have uncovered one of the reasons for that scarcity, demonstrating in laboratory experiments that quasicrystals could arise from collisions between rocky bodies in the asteroid belt with unusual chemical compositions.
A paper on their findings was published on June 13 in the advance online edition of the Proceedings of the National Academy of Sciences.
At an atomic level, crystals are both ordered and periodic, meaning that they have a defined geometric structure, with that structure repeating itself over and over. To grow such a repeating structure without the original organization breaking down, the crystal can only exhibit one of four types of rotational symmetry: two-fold, three-fold, four-fold, or six-fold.
The number refers to how many times an object will look exactly the same within a full 360-degree rotation about an axis. For example, an object with two-fold symmetry appears the same twice, or every 180 degrees; an object with three-fold symmetry appears the same three times, or every 120 degrees; and an object with four-fold symmetry appears the same four times, or every 90 degrees.
Natural quasicrystals may be the result of collisions between objects in the asteroid belt