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-- 266 K blackbody fit E

i . .

10 100 Wavelength [micron]

Figure 7. ISOPHOT broad-band photometry and mid-infrared spectrophotometry of the zodiacal light at A — A0 = 90° and p = 0°, using the PIA V7 calibration. For comparison the corresponding DIRBE values are overplotted. All values are colour corrected.

that observed in comets, rules out the presence of an important population of very small silicate grains.

The observed spectrum of the zodiacal light is an integral of dust emission along the line of sight. The volume emissivity depends on the optical properties of the grains, on their size distribution, and on the temperature. Though the infrared observations cannot be used to derive directly these parameters, the spectrum of the zodiacal light provides a very strong constraint on models which use different optical constants and size distributions. The mid-infrared spectral range is very sensitive to dust temperature, because it corresponds to the Wien part of a Planck curve for large particles at 1 AU. The temperature, on the other hand, depends strongly on the wavelength dependent absorption properties of the dust grains.

Reach et al. [35] compared the mid-infrared spectrum of the zodiacal light observed by ISOCAM with synthetic spectra calculated for typical materials like obsidian, olivine, andesite, graphite, magnetite, water ice, and different size distributions. They found that none of the assumed constituents or size distributions could reproduce the observed spectrum. The best results were achieved for andesite with a "lunar" size distribution (derived from lunar micro-crater studies by [16]), and for "astronomical silicates" [37] with an "interplanetary" size distribution [16].

Our motivation to study the variation of the mid-infrared spectrum of the zodiacal light over the sky was to learn if interplanetary dust particles of different origin were distinguishable by their observed properties. It was expected that the spectrum of dust at high

Figure 8. (a) distribution of the 27 ISOPHOT-S spectra over the sky accessible by ISO, assuming a symmetry of the zodiacal light wrt. the sun. The sky was divided into six segments (long-dashed lines), and the individual spectra of each segment were averaged. (b) temperature values derived by Planck-fitting to the average spectra of the segments.

Figure 8. (a) distribution of the 27 ISOPHOT-S spectra over the sky accessible by ISO, assuming a symmetry of the zodiacal light wrt. the sun. The sky was divided into six segments (long-dashed lines), and the individual spectra of each segment were averaged. (b) temperature values derived by Planck-fitting to the average spectra of the segments.

ecliptic latitudes should show similarities to the spectrum of comets of high inclination, while properties of dust close to the Ecliptic Plane should resemble those of the asteroids. Our ISOPHOT-S data taken at 27 different positions over the sky, however, do not show any significant variation in the spectra other than temperature effects. It appears that the origin of the particles cannot be determined from their mid-infrared spectrum, because of the lack of spectral features, and because the present IDC appears to be well mixed.

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