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2 Natural Influx and Cross-Contamination
Pages 21-25

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From page 21...
... While the dominant mass is in objects exceeding 10~4 kg, there are significant excesses in mass of particles ranging roughly iThe numbers of particles that constitute the interplanetary debris complex roughly follow a differential power law in diameter with an exponent of about -3.5 (which is also a theoretically derived exponent for a population of colliding objects in collisional equilibrium)
From page 22...
... 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.
From page 23...
... Therefore, while most of the observational evidence about the contribution of cometary materials to Earth concerns cometary dust (and the rare impact by an intact comet nucleus) , there is the possibility that significant cometary material is contributed by objects of intermediate sizes.
From page 24...
... Although the satellites are small, and cratering ejecta is readily lofted into Mars orbit, much larger velocities would have to be achieved for material from Phobos or Deimos to escape Mars's gravitational well and enter independent heliocentric orbits and join the interplanetary debris complex. If Phobos and Deimos are, as they may well be, gravelly or rubbly bodies, ejecta velocities may be too low to reach Mars escape velocity because loosely consolidated bodies absorb energy more efficiently than compact bodies through reduction of porosity.
From page 25...
... 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)

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