Sticky ice can help planets form quickly

Stars and their planets are typically born in dusty nebulae. However, in just a few million years or less, a newborn star blows away dust from its neighborhood, removing most of the raw material for planet formation. Although gravitationally bound kilometer-sized protoplanets will grow rapidly, getting from dust to protoplanets in a million years has proven elusive using the available van der Waals forces. The outer reaches of our own sun’s early nebula were at temperatures below 120 K, and researchers at the Pacific Northwest National Laboratory have now studied cryogenically formed water ice, which is amorphous and fluffy rather than crystalline and solid. They discovered that it has two other unusual properties that can expedite the agglomeration of fragile, micron-sized dust grains. First, cryogenic ice has a persistent, macroscopic electric dipole that is unmasked when an asymmetry is introduced by, for example, a collision with another ice-coated dust grain. Second, the fluffy amorphous ice is mechanically inelastic. In experiments, the scientists found that a free-falling ceramic bead rebounds from a cryogenic ice film to only about 10% of its original height, compared to about 80% for normal kitchen ice. Taken together, the electrostatic and mechanical properties mean that colliding particles are more likely to stick than bounce and the resulting agglomerates can continue to grow rather than shatter on subsequent impacts. (H. Wang et al., Astrophys. J. 620, 1027, 2005 http://dx.doi.org/10.1086/427072 .)