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R. M. Killen (University of Maryland), D. C. Boice (Southwest Research Institute)
The surface composition of various planetary bodies has been measured by spectrometers in the infrared, UV, x-ray and gamma-ray region of the spectrum (e.g. Deep Space 1, NEAR, Odyssey), and others are planned in the near future (CONTOUR, Messenger, Bepi-Colombo). The extent to which the surface composition reflects the composition of the crust or regolith as a whole is an important issue. For example, the observtions with the x-ray and gamma-ray spectrometers onboard the NEAR spacecraft indicated that the extreme surface of Eros is depleted in sulfur [1]. The surface of the Moon, although depleted in sulfur, is enhanced in most volatiles. It is important to understand the reasons for these extreme surface anomalies. We have considered selective vaporization of volatiles on airless bodies due to impact vaporization. We use the planar impact approximation to hyper-velocity impact [2], and we scale the critical pressure for vaporization by the enthalpy of the species in question [3]. On small asteroids, we assume that all of the vaporized material escapes. However, the situation on bodies such as the Moon and Mercury is more complicated. On these bodies, we determine the rate of escape by assuming that the vapor velocity distributions are either Maxwell-Boltzmann or Weibull. This analysis will show that the volatile depletion is related to the enthalpy of vaporization on asteroids and also to the gravitational potential for the Moon, and to the magnetospheric configuration for Mercury. Thus the surface composition must be understood in the context of complex interactions between the surface and the space environment.
[1] Nittler, T. et al., (2001) Meteoritics and Planet. Sci. 36, 1673 - 1695. [2] Melosh, H.J. (1989) Impact Cratering: A geologic process, Oxford Press. [3] Killen, R.M. and D.C. Boice (202) Proceed. ACM, Berlin, Germany.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.