Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0705w&link_type=abstract
American Astronomical Society, DPS meeting #28, #07.05; Bulletin of the American Astronomical Society, Vol. 28, p.1082
Astronomy and Astrophysics
Astronomy
Scientific paper
Many objects in the outer Solar System---asteroids, comets, many satellites, the Uranian ring system---are rich in organic matter, giving them a reddish to dark-grey color. This material could have been either accreted directly from the solar nebula or produced in situ later from simple hydrocarbons. The post-accretional chemistry is driven by numerous sources of irradiation: solar uv photons, solar wind charged particles, charged particles trapped in planetary magnetospheres, decay of radionuclides, and cosmic rays. At increasing doses an initially bright, hydrocarbon ice-rich surface reddens then darkens to a nearly grey color. Although several researchers have reported on the degree of processing experienced by icy surfaces from irradiation [1,2], numerical errors caused major overestimates in their conclusions. Also, static surfaces were generally assumed. Impacts from dust and larger particles will churn and erode the regolith, so it is not a simple matter to calculate the degree of processing of an exposed surface. In a recent analysis of Iapetus, the effects of both radiation and impacts were taken into account to estimate the rate of organic matter production on/near its surface [3]. This is generalized and adapted to planetesimals in the outer Solar System: comets and Kuiper Belt objects. Cosmic rays are incapable of producing anything like the 10 m radiation crust as previously hypothesized [1,2], but under the right circumstances a very thin crust only a few millimeters thick may be produced on astronomically short timescales under the combined influence of solar radiation and light surface gardening. This does not change our conclusion that radiation darkening is not an effective process on Iapetus [3]. References: [1] Strazzulla, Icarus 67, 63--70, 1986. [2] Thompson et al., J. Geophys. Res. 92, 14933--14947, 1987. [3] Wilson and Sagan, Icarus 122, 92--106, 1996.
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