Mid-infrared spectrum of the zodiacal light observed with ISOPHOT

We present 29 mid-infrared spectra of the zodiacal light distributed over the sky. The observed 5.9-11.7 mum spectral shapes are well represented by blackbody radiation with colour temperatures in the range of 255 less than or equal to T less than or equal to 300 K. The spectra are smooth and featureless. The variation of the temperature can be explained by the geometrical distribution of dust in the inner solar system. This result indicates that although the interplanetary dust particles originate from discrete sources (comets, asteroids) the interplanetary cloud of today seems to be well mixed in terms of grain composition and size distribution.