The zodiacal light is the dominant source of the mid-infrared sky brightness seen from Earth , and exozodiacal light is the dominant emission from planetary and debris systems around other stars .
We observed the zodiacal light spectrum with the mid-infrared camera ISOCAM over the wavelength range 5–16 \mu m and a wide range of orientations relative to the Sun ( solar elongations 68 ^ { \circ } –113 ^ { \circ } ) and the ecliptic ( plane to pole ) .
The temperature in the ecliptic ranged from 269 K at solar elongation 68 ^ { \circ } to 244 K at 113 ^ { \circ } , and the polar temperature , characteristic of dust 1 AU from the Sun , is 274 K. The observed temperature is exactly as expected for large ( > 10 \mu m radius ) , low-albedo ( < 0.08 ) , rapidly-rotating , grey particles 1 AU from the Sun .
Smaller particles ( < 10 \mu m radius ) radiate inefficiently in the infrared and are warmer than observed .
We present theoretical models for a wide range of particle size distributions and compositions ; it is evident that the zodiacal light is produced by particles in the 10–100 \mu m radius range .
In addition to the continuum , we detect a weak excess in the 9–11 \mu m range , with an amplitude of 6 % of the continuum .
The shape of the feature can be matched by a mixture of silicates : amorphous forsterite/olivine provides most of the continuum and some of the 9–11 \mu m silicate feature , dirty crystalline olivine provides the red wing of the silicate feature ( and a bump at 11.35 \mu m ) , and a hydrous silicate ( montmorillonite ) provides the blue wing of the silicate feature .
The presence of hydrous silicate suggests the parent bodies of those particles were formed in the inner solar nebula .
Large particles dominate the size distribution , but at least some small particles ( radii \sim 1 \mu m ) are required to produce the silicate emission feature .
The strength of the feature may vary spatially , with the strongest features being at the lowest solar elongations as well as at high ecliptic latitudes ; if confirmed , this would imply that the dust properties change such that dust further from the Sun has a weaker silicate feature .

To compare the properties of zodiacal dust to dust around other main sequence stars , we reanalyzed the exozodiacal light spectrum for \beta Pic to derive the shape of its silicate feature .
The zodiacal and exozodiacal spectra are very different .
The exozodiacal spectra are dominated by cold dust , with emission peaking in the far-infrared , while the zodiacal spectrum peaks around 20 \mu m. We removed the debris disk continuum from the spectra by fitting a blackbody with a different temperature for each aperture ( ranging from 3.7 ^ { \prime \prime } to 27 ^ { \prime \prime } ) ; the resulting silicate spectra for \beta Pic are identical for all apertures , indicating that the silicate feature arises close to the star .
The shape of the silicate feature from \beta Pic is nearly identical to that derived from the ISO spectrum of 51 Oph ; both exozodiacal features are very different from that of the zodiacal light .
The exozodiacal features are roughly triangular , peaking at 10.3 \mu m , while the zodiacal feature is more boxy .

Keywords : zodiacal light , infrared observations , interplanetary dust