Abstract
We combine observations from the DIRBE and FIRAS instruments on the COBE satellite to derive an annually averaged spectrum of the zodiacal cloud in the 10-1000 μm wavelength region. The spectrum exhibits a break at ~150 μm that indicates a sharp break in the dust size distribution at a radius of about 30 μm. The spectrum can be fitted with a single blackbody with a λ-2 emissivity law beyond 150 μm and a temperature of 240 K. We also used a more realistic characterization of the cloud to fit the spectrum, including a distribution of dust temperatures representing different dust compositions and distances from the Sun, as well as a realistic representation of the spatial distribution of the dust. We show that amorphous carbon and silicate dust with respective temperatures of 280 and 274 K at 1 AU, and size distributions with a break at grain radii of 14 and 32 μm, can provide a good fit to the average zodiacal dust spectrum. The total mass of the zodiacal cloud is 2-11 Eg (Eg = 1018 g), depending on the grain composition. The lifetime of the cloud, against particle loss by Poynting-Robertson drag and the effects of solar wind, is about 105 yr. The required replenishment rate is ~1014 g yr-1. If this is provided by the asteroid belt alone, the asteroids lifetime would be ~3 × 1010 yr. But comets and Kuiper belt objects may also contribute to the zodiacal cloud.
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