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L.J. Paxton (The Johns Hopkins U. APL)
There are a variety of physical processes which are capable of producing atoms or molecules with a non-thermal energy distribution. While these “hot” atoms or molecules may possess a relatively small amount of excess energy that difference can be important. Non-thermal atoms are important as the source of extended planetary coronae and as a mean of altering the photochemistry of atmospheric trace constituents. The observational consequences of these processes are varied. Some processes can actually lead to escape producing observable consequences for light atom isotopic ratios. The most notable example of this is the depletion of 14N relative to 15N from the dissociative recombination of 28N2+ and 29N2+ in the Mars upper atmosphere (e.g. Fox and Hac, 1997). Others processes lead to the production of an extended corona which can be observed using remote sensing or in situ measurement techniques. For example, an extended dual scale height hydrogen corona was detected around Venus from Mariner 5 (Barth et al., 1967). In this paper we review the photochemical sources for hot O in the exospheres of Mars and Venus and compare the predictions of a Monte Carlo simulation to data. We will focus on the observational evidence in ultraviolet measurements: Pioneer Venus Orbiter Ultraviolet Spectrometer (Venus) and Mariner 9 Ultraviolet Spectrograph (Mars). We report here a positive detection of a signal in the atomic oxygen resonance line at 130.4 nm in the Mariner 9 data and an extension of the work done by Paxton and Anderson (1992) for Venus. \\ Barth, C.A., et al., Science, 158, 1675, 1967.\\ Fox, J.L., and A. Hac, J. geophys. Res., 102, 9191, 1997.\\ Jakosky, B.M., et al., Icarus, 111, 271, 1994.\\ Luhmann, J.G., and J.U. Kozyra, J. geophys. Res., 96, 5457, 1991.\\ Paxton, L.J., and D.E. Anderson, Ultraviolet Remote Sensing of Venus and Mars, Venus and Mars: Atmospheres, Ionospheres, and Solar Wind Interactions, Geophysical Monograph 66, eds. J.G. Luhmann, M. Tatrallyay, and R.O. Pepin, pp. 113-190, 1992\\
This work supported by NSF grant AST-9816237.