Zodiacal emission. I - Dust near the earth's orbit

The infrared emission of interplanetary dust near the earth's orbit is derived from IRAS observations of the gradient of in-ecliptic brightness tangent to the earth's orbit, and the annual variation of the ecliptic polar brightness. Models with five grain constituents and three size distributions are compared with the observations. The observed emission is twice as bright as predicted; this discrepancy is due either to calibration errors or to enhanced radiative efficiency of 'fluffy' particles. Graphite and magnetite particles are ruled out because they are too hot. The size distribution is constrained to be less steep than that derived from lunar microcrater studies, and is consistent with that obtained by earth-orbiting satellites. Nonhomogeneous silicate grains with 3 percent graphite impurity produce the best fit to the spectrum. The model predictions extend from 3 microns to 1 mm, for use in analyzing future infrared background observations.