III. Solar System Dust

Interplanetary dust particles (IDP) are evident in the optical and IR surface brightness of zodiacal light easily observed from the Earth. The total IR-emitting (effective) cross-sectional area of grains between 1 and 2 AU from the Sun is about 2 x 1020 cm². The grains characterizing the IR emission near Earth's orbit have typical sizes of 10-100 µm. The surface area given above thus corresponds to a mass of roughly 10¹⁸ - 10¹⁹ g for a density of 3 g cm^-3, equivalent to a single solid body only 5-10 km in radius. The dust in the 1-2 AU annulus has an areal surface density (vertical optical depth) of ~ 1 x 10^-7, increasing slowly toward the Sun. These characteristics will be referred to here as a "1-zodi" cloud (Good 1994; Backman et al. 1997)

Figure 1:
Model cross-section of the solar system zodiacal dust cloud showing a "wedge" shape consistent with grain drift inward from the asteroid belt under the influence of PR drag. Densities at the latitude extrema are enhanced, reflecting orbits inclined to the mean plane executing simple harmonic motion along the vertical axis.The inner cutoff represents termination of the model calculation rather than a real discontinuity in the cloud.

Table 1 gives the COBE DIRBE team's results (Kelsall et al. 1998) for albedo and emissivity of the zodiacal grains at 1-3 AU. The albedo in the near-IR is about twice that in the visible. Absolute emissivity should equal 1 - Albedo.

Table 1: IDP Albedos and Emissivities

The bolometric luminosity ratio L_dust/Lsun is ~ 2 x 10^-8 for the 1-2 AU annulus and is estimated to be about 1 x 10^-7 for the entire zodiacal cloud. The latter value depends strongly on the dust density and density gradient close to the Sun. The luminosity of the hottest dust may not be a crucial issue in interferometric detection of exozodiacal systems and extrasolar planets because a central null intended to block stellar emission will also block the inner reaches of exozodiacal clouds. COBE and IRAS observations have little to say about the amount of dust at r < 0.9 AU. There are, however, constraints on the variation of dust density from 1 AU to 0.3 AU which imply that the volume density varies roughly as r^-1.3 (Leinert et al. 1981). If the geometry of the cloud is a "wedge" (Figure 1; constant opening angle subtended at the Sun, scale height 'h' proportional to 'r'), then that volume density gradient is equivalent to face-on surface density varying as r^-0.3. We do not know the position of the inner cutoff of the dust distribution, although it may correspond simply to the vaporization temperature of silicate grains. Observations close to the Sun during eclipses show the zodiacal dust merging smoothly into the F-corona, continuing in to 3 solar radii (0.015 AU) with volume density slope flattening slightly to r^-1.0 (Mann et al. 1996). The details of dust density beyond 2-3 AU are also not well known. There is a result from Pioneer 10 observations that the radial decrease of zodiacal light intensity is about r^-2.5 in the visible, equivalent to an r^-1.5 volume density law (Hanner et al. 1976). Table 2 gives estimated polar zodiacal light surface brightnesses as if viewed from various positions out to 3.5 AU based on Pioneer, Helios, and COBE DIRBE data.

Table 2: Polar Zodiacal Surface Brightness versus R

A patch of the zodiacal cloud in our solar system at 1 AU with diameter 0.3 AU (viewed face-on) would have the same brightness as the planet Earth. This would be roughly true for both IR thermal emission and optical scattered light to the extent that the material has approximately the same albedo and temperature as Earth. Detecting earth-like planets around other stars in the face of this emission thus requires spatial resolution with this scale as well as strategies for distinguishing moving emitters from "fixed-pattern" brightness.