  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.
