We model the infrared emission from zodiacal dust detected by the IRAS and COBE missions, with the aim of estimating the relative contributions of asteroidal, cometary and interstellar dust to the zodiacal cloud. Our most important result is the detection of an isotropic component of foreground radiation due to interstellar dust. The dust in the inner solar system is known to have a fan-like distribution. If this is assumed to extend to the orbit of Mars, we find that cometary, asteroidal and interstellar dust account for 70%, 22% and 7.5% of the dust in the fan. We find a worse fit if the fan is assumed to extend to the orbit of Jupiter. Our model is broadly consistent with the analysis by Divine (1993) of interplanetary dust detected by Ulysses and other spacecraft. Our estimate of the mass-density of interstellar dust in the inner solar system is consistent with estimates from Ulysses at 1.5 au, but is an order of magnitude higher than Ulysses estimates at r > 4 au. Only 1% of the zodiacal dust arriving at the earth would be interstellar, in our model. Our models can be further tested by ground-based kinematical studies of the zodiacal cloud, which need to extend over a period of years to monitor solar cycle variations in interstellar dust, by dynamical simulations, and by in situ measurements from spacecraft.
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