III.A.2 Dust Sources

Approximately 50% (15-75%) of IDP in the 10-100 µm size range come from the asteroid belt, and 10% (5-25%) of the asteroidal dust comes from 3 known asteroid collision families representing ongoing interactions of debris from significant collisions in the past (Dermott et al. 1996a; Durda and Dermott 1997). The scale height of dust near the Earth indicates at least some of the local dust is cometary (Liou et al. 1995). The relative contributions of these and other sources are not well known.

The Trojan asteroids (Jupiter Lagrangian population) may also be a significant source of collision debris drifting past Earth toward the Sun because their encounter times are orders of magnitude shorter than for main belt asteroids. The Trojan encounter velocities are not as large, however, so their fragment size spectrum will probably have a different shape than for main belt debris and the proportion of grains in the size range producing substantial mid-IR emission is unpredictable.

Comet dust grains almost certainly have different characteristic sizes, compositions, and albedos from asteroid dust, and thus may behave differently in response to the various forces discussed above. Perihelion passage of a large comet such as P/Halley releases grains with surface area of order 10^-5 of that in the entire zodiacal cloud. If the PR destruction time scale of typical cometary grains is assumed to be roughly 10⁵ yr (as for asteroidal grains) then P/Halley may contribute 1% of the equilibrium zodiacal dust population during its (maximum) life of 10⁵ yr ~ 10³ orbits.

Lower and upper limits on the Kuiper Belt dust parent body (comet nuclei) population (Cochran et al. 1995 and Backman et al. 1995, respectively) imply that the surface density of collisionally-produced KB dust is comparable to that in the inner solar system zodiacal cloud. These grains should drift toward the Sun via PR drag. However, for them to make a noticeable contribution to the inner solar system dust population, they must survive: a) collisions with interstellar grains, which may be especially important for larger and more slowly-drifting KB grains that are exposed to bombardment for longer intervals, and b) gravitational perturbation by the Jovian planets that can trap grains in resonances and/or eject them from the solar system; again, this should be more important for slowly-drifting large grains. Both effects are the subject of present modeling efforts (e.g. Liou et al. 1996). Gauging their efficiency is important because without these effects the relative densities indicate that a significant fraction of the inner solar system dust could actually be originally from the KB (Flynn 1996; Backman et al. 1997).

Interstellar grains are expected to be a significant source of the smallest particles in the outer solar system. Ulysses detected ISM dust in the outer solar system coming from the direction of the solar motion through the galaxy (Grün et al. 1994). In-situ measurements indicate that ISM dust surface density is only of order 0.1% of IDP density at 1 AU (Grogan et al. 1996).