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Fig. 28. Timescale for delivering 100 eV per p = 16 atom as a function of depth in water ice (density 1000 kg m-3) at three heliocentric distances. The dashed horizontal line at the top marks the age of the Solar system. The wavelength of visible photons is marked by Aopt at the bottom. Damaged layers thicker than Aopt are likely to have significant effect on the reflected light spectrum. Replotted from [23]

results in a gradient in the flux of energetic particles. The figure shows that the Kuiper belt objects at ^40 AU in fact exist in a relatively benign radiation environment. Solar wind particles quickly irradiate a surface skin ^100 A thick (in 104 year) but damage to 0.1 |m takes a considerable fraction of the age of the Solar system. At 85 AU, close to the recently detected termination shock (where the Solar wind decelerates as it impacts the heliopause from the inside) the flux of energetic particles is increased and total damage occurs to depths of ^10~4 m on billion-year timescales. In the open interstellar medium, the damage can reach depths in ice m on the same timescale.

What does all this mean? First of all, the timescales for irradiation damage (Fig. 28) are vastly longer than those for the production of a rubble mantle (Fig. 26). I conclude that irradiation mantles should not be found on any object whose past life has allowed the possibility of mass loss and, so, of rubble mantle formation. Objects in the outer Solar system are too cold to sublimate water and so remain as candidates for irradiation mantling. Perhaps the ultrared matter (S' > 25%/1000A) that appears to be a unique feature of the KBOs and of some Centaurs, is irradiated mantle material. Consistent

Fig. 29. Possible styles for the destruction of irradiation mantle. On the left, billions of years of exposure to energetic particles on a frigid surface has created an irradiation mantle (black) on a nucleus that is otherwise pristine (shaded). At the onset of sublimation-driven mass-loss, the irradiation mantle could be buried (middle) or cracked and ejected by gas drag (right), the exposed surface of the nucleus being replaced by a rubble mantle consisting of excavated, unirradiated matter in both cases

Fig. 29. Possible styles for the destruction of irradiation mantle. On the left, billions of years of exposure to energetic particles on a frigid surface has created an irradiation mantle (black) on a nucleus that is otherwise pristine (shaded). At the onset of sublimation-driven mass-loss, the irradiation mantle could be buried (middle) or cracked and ejected by gas drag (right), the exposed surface of the nucleus being replaced by a rubble mantle consisting of excavated, unirradiated matter in both cases with this inference is the observation that ultrared matter does not survive approach to the sun within the orbit of Jupiter [71,79], corresponding to the heliocentric distance inside which water begins to sublimate and the timescale for rubble mantle formation becomes short (Fig. 26). The mode of destruction of the irradiation mantle is not clear, however. The mantle could still be present but buried beneath a recently deposited rubble mantle consisting of (less red) debris excavated from beneath the ~1m thick irradiated layer. Or it could be ejected by gas drag at the onset of strong sublimation inside AU (Fig. 29).

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