Zodiacal Dust

On a clear, moonless night, far from any city glare, a keen-eyed observer can see in the sky a faintly glowing cone. This is the "zodiacal light," which astronomers believe is sunlight reflected from dust particles revolving around the sun like microscopic planets. In Sky and Telescope, Astronomer Otto Struve of the University of California tells how he thinks the dust got there.

It was not left over from the original material out of which the planets condensed. That has long since disappeared. The smaller particles of it were blown out of the solar system by the pressure of sunlight. The larger ones were swept up by the planets or captured by the sun itself. The existing particles are comparatively "young" (as ages are measured in astronomy); they were formed, thinks Struve, by the "gravel-mill" action of the bodies in the solar system clashing against one another.

There are thousands of asteroids (minor planets) and billions of smaller objects wandering around the sun. When they collide, as they often do, they turn a great deal of rock into flour-fine dust. If the collision takes place, as on earth, under a thick atmosphere, little of the dust escapes. When neither of the colliding objects has an atmosphere, much of the dust splashes out into space, each particle revolving around the sun on an orbit of its own.

The level parts of the moon, for instance, are covered by a three-inch layer of dust tossed out of meteorite craters. Last year a cloud of yellow dust was seen for a few hours on Mars by Tsuneo Saheki. Struve thinks it was probably stirred up by a meteorite striking through the thin Martian atmosphere and shattering Martian rock.

Collisions between two small objects (asteroid size or less) are probably quite frequent, and in such cases there is no atmosphere and little gravity to keep dust particles from wandering off on their own. It is mostly these small encounters in the solar system's gravel mill that keep the zodiacal light shining faintly.

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