from 10-9 to 10-7 kg, 10-4 to 10-3 kg, and, especially 105 to 107 kg (Ceplecha, 1997). The processes that govern the production and losses of material within the interplanetary debris complex, including redistribution of material among the bodies and the planets that are sources of meteorites, depend on these characteristics of the size distribution.

Associated with the size distribution of particles and objects in the interplanetary debris complex are characteristic lifetimes against collisional destruction. In general, the smaller a particle, the shorter the time it lasts before being destroyed or physically removed from the system. In a biological context, an important criterion is whether material within a body is protected from solar and galactic cosmic rays. Generally speaking, the interiors of bodies larger than approximately 1 meter may be shielded for appreciable periods. However, in some cases (e.g., cometary dust liberated from a comet that is very near Earth) the lifetimes of very small particles before impact may be short enough to preclude sterilization of potential biological entities by radiation.

In general, the meteoroids and dust in the interplanetary debris complex are short-lived. Large objects move according to Keplerian and chaotic orbital dynamics, affected by perturbations from distant, massive bodies. Once in Earth-crossing orbits, they last for time periods (e.g., 106 to 107 years) that are very short compared with the age of the solar system before encountering a planet, impacting the Sun, or being ejected from the solar system primarily by Jupiter's gravity. Particles meters in size and smaller are pushed around by the solar wind and radiation forces (e.g., Yarkovsky effects) and are swept up by the Sun and the terrestrial planets or else driven beyond the terrestrial planet zone (Burns, 1987).

All of these short-lived objects must be resupplied from long-lived "parent bodies" that have survived in "storage locations." Large comets and asteroids, and still larger planets and planetary satellites, serve as parent bodies for such materials so long as they are away from regions with high impact rates or are in stable orbits that do not cross the orbits of other planets and thus serve as reservoirs of fresh material. Their smaller cousins may also serve as parent bodies provided that they have been stored in locations, such as the Oort Cloud, where the volume density and velocities of other objects are low enough that collisions are rare and less destructive. The chief locations of parent bodies that contribute to the interplanetary debris complex are the main asteroid belt, located between 2.2 and 3.2 AU from the Sun; the Trojan asteroids, at 5 AU; the Kuiper Belt (and associated Scattered Disk), ranging out several tens of astronomical units beyond Neptune's orbit; the Oort Cloud, a spherical halo of comets weakly gravitationally bound to the distant Sun and extending part way to the nearest stars; and the major planets and satellites of the solar system.

The smaller debris is resupplied from these parent bodies predominantly by exogenic and endogenic processes. First, hypervelocity impacts among the components of the interplanetary debris complex or impacts of asteroids and comets onto the surfaces of planets and satellites produce sprays of ejected material from the target bodies (dust, boulders, and so on), which launch the debris away from the impact point, some fraction of it often exceeding the target's escape velocity. Second, active processes on bodies (especially sublimation of ices near comet surfaces that carries away surficial grains, but also powerful volcanic processes on bodies like Io) drive fine material away from the parent object, thus incorporating it into the interplanetary debris complex.

This chapter assesses the parent objects and the modes of delivery to Earth of material produced by these processes, prior to its being removed from the interplanetary debris complex by processes of (primarily collisional) physical destruction or dynamical removal from the complex. Inasmuch as interplanetary debris encounters other planetary bodies and other bodies within the interplanetary debris complex, effectively "contaminating" them, material received on—or brought back to—Earth from one body may contain material originally derived from another body.

Analysis of lunar soils suggests that a small percentage are of extralunar origin, as small meteoroids become incorporated into the lunar regolith. In addition, meteorites have occasional (but not extremely rare) visible fragments of other meteorite types embedded within them, implying that relatively intact lithic fragments from one parent object can be delivered as a result of sampling a different parent body (Zolensky et al., 1996). Cross-contamination is not universal. It may be difficult or impossible to derive by natural processes materials contained deep within large bodies, located deep within the gravity fields of large planets, or associated with bodies far from Earth and in orbits that are not easily converted into Earth-crossing orbits. It may be that such materials do not reach "any" of the other bodies that reach Earth. These issues are addressed below.



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