It is probable that cross-contamination among objects augments the variety of parent bodies represented in what reaches Earth by natural influx. For example, while outer-belt asteroids cannot be dynamically converted into Earth-crossing orbits, such objects in moderately eccentric orbits do collide with asteroids in the inner asteroid belt, which do communicate with Earth. One could imagine a chain of contamination providing an access route for materials from virtually any body—e.g., ejecta from Europa is encountered by a comet passing through the Jupiter system, which ultimately crashes on the Moon; then such material gets to Earth by excavation in a lunar cratering impact. Obviously, as in this example, the probabilities of occurrence of most multichain contamination routes drop geometrically toward zero, but cannot be totally ruled out in principle. However, to the degree that the apparent safety of the natural influx is relied upon to declare a body safe for sample return, it must be realized that the episodic nature of cross-contamination and delivery processes probably means that exactly zero percent (not just a very small percentage) of material impacting Earth during a finite time (e.g., a century) can be expected to come via such multichain routes. Only the tiniest particles might escape this generalization, and any biological materials are rapidly sterilized by radiation in such small particles.
Earth receives from other bodies in the solar system abundant material that is ejected from such bodies and delivered to the surface of Earth. Because of cross-contamination, small but significant fractions of the delivered materials could have been formed originally on bodies from which Earth does not receive meteorites directly. However, there are some bodies, as well as places on other bodies, from which material would be so difficult to obtain, or would arrive so infrequently, that it is unlikely that Earth has received samples during our lifetimes, even if such material is delivered on rare occasions. For some planetary satellites and small bodies, the uncertainty associated with cross-contamination reduced the degree of confidence in the inherent safety of a sample returned from such bodies.
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