From ring particles to whole planets.
Researchers have informally dubbed it the `Velcro effect;' referring to the clingy commercial material whose entangling fingers close pocket flaps and serve in a host of familiar applications. But as Artie P Hatzes of the University of Texas at Austin and his colleagues use the term in the January ICARUS, it describes the effect they think holds together frost-covered ice particles in the rings of Saturn.
In laboratory tests, Hatzes - together with Frank Bridges, D.N.C. Lin and S. Sachtjen of the University of California, Santa Cruz- studied the influence of tiny frost particles on the ability of solid ice chunks to stick together until they build up into larger bits. The scientists used 2.5-centimeter ice balls, swinging them on a pendulum at different speeds to hit an ice block about the size of a brick. Lin notes that separate frost particles may exist in space either because they somehow failed to coagulate into a larger lump, or as the result of previous collisions that broke up larger chunks.
"Collisionally induced cohesion is one process by which small particles may coagulate and large particles may grow in size," the researchers suggest. If the surfaces of both the ice ball and the ice brick are frost-free, the two objects rarely stick together, they found. Using a chilled test chamber, the group produced frost on the two objects by blowing a stream of water saturated nitrogen gas past both ball and brick.
Even with frost on them, the icy chunks would not stick together at all if they hit at speeds of less than about 0.02 centimeter per second or greater than about 0. 1 cm per second, Hatzes says. Within that range, however, the objects almost always stuck together.
Lin thinks the sticking effect may also have played an important part in the origin of the solar system. The cores of the giant planets probably began when particles - either ice or dust - aggregated by sticking together, he says. This suggests that the sticking effect is significant in lumping together small chunks of other materials, such as rock. Lin adds that gravity provides the only other likely way of combining rocky bits into larger chunks, but he says the mass of the particles initially would be too low to start pulling them together to begin the planetary birthing process.
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|Title Annotation:||what holds together the ice particles in Saturn's rings|
|Date:||Jan 19, 1991|
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