INTERPLANETARY DUST
The space between the planets is a vacuum by terrestrial standards, but it is not devoid of some interplanetary gas and small solid particles called interplanetary dust. The particulate matter consists of particles blown out from the sun's atmosphere by the solar wind, micrometeoric debris scattered by comets, anc~ perhaps less plentiful granular powder strewn about by asteroid and meteoroid collisions.
We have learned about interplanetary dust from several sources. One is the zodiacal light, which' is most easily observed in our Northern Hemisphere in spring after sundown in the west and in fall before dawn in the east. It appears as a faint pyramidal band of light tapering upward from the horizon along the line of the ecliptic. The spectrum of zodiacal light is a faint replica of the solar spectrum; it is produced by small particles lying in the plane of the planets' orbits that scatter solar photons in our direction.
Most direct evidence of interplanetary dust comes to us from spacecraft experiments. Electronic sensors on the skin of the spacecraft are arranged to count small dust particles as they strike the su rface. From the numbers of impacts it is estimated that the average spacing between interplanetary dust particles is many meters. The total mass of dust particles is estimated to be about 1020 grams, or about a hundred-millionth of the mass of the earth.
