ar X iv : a st ro - p h / 03 04 28 9 v 1 1 5 A pr 2 00 3 The Mid - Infrared Spectrum of the Zodiacal and Exozodiacal Light

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 µm and a wide range of orientations relative to the Sun (solar elongations 68 • –113 •) and the ecliptic (plane to pole). The temperature in the ecliptic ranged from 269 K at solar elongation 68 • to 244 K at 113 • , and the polar temperature, characteristic of dust 1 AU from the Sun, is 274 K. The observed temperature is exactly as expected for large (> 10µm radius), low-albedo (< 0.08), rapidly-rotating, grey particles 1 AU from the Sun. Smaller particles (< 10 µm 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 µm radius range. In addition to the continuum, we detect a weak excess in the 9–11 µm 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 µm silicate feature, dirty crystalline olivine provides the red wing of the silicate feature (and a bump at 11.35 µm), 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 ∼ 1 µm) 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. – 2 – To compare the properties of zodiacal dust to dust around other main sequence stars, we reanalyzed the exozodiacal light spectrum for β 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 …