The zodiacal emission spectrum as determined by COBE and its implications

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 l m wavelength region. The spectrum exhibits a break at similar to150 mum that indicates a sharp break in the dust size distribution at a radius of about 30 mum. The spectrum can be fitted with a single blackbody with a lambda(-2) emissivity law beyond 150 mum 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 mum, can provide a good fit to the average zodiacal dust spectrum. The total mass of the zodiacal cloud is 2-11 Eg (Eg = 10(18) 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 10(5) yr. The required replenishment rate is similar to10(14) g yr(-1). If this is provided by the asteroid belt alone, the asteroids lifetime would be similar to3 x 10(10) yr. But comets and Kuiper belt objects may also contribute to the zodiacal cloud.