Mariner 10 observations of hydrogen Lyman alpha emission from the venus exosphere: Evidence of complex structure |
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| Authors: | PZ Takacs AL Broadfoot GR Smith S Kumar |
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| Institution: | Kitt Peak National Observatory, Tucson, AZ 85726, U.S.A.;Jet Propulsion Laboratory, Pasadena, CA 91103, U.S.A. |
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| Abstract: | The Ultraviolet Spectrometer Experiment on the MARINER 10 spacecraft measured the hydrogen Lyman α emmission resonantly scattered in the Venus exosphere at several viewing aspects during the encounter period. Venus encounter occurred at 17:01 GMT on 5 February 1974. Exospheric emissions above the planet's limb were measured and were analyzed with a spherically symmetric, single scattering, two-temperature model. On the sunlit hemisphere the emission profile was represented by an exospheric hydrogen atmosphere with Tc = 275±50 K and nc = 1.5 × 105 cm?3 and a non-thermal contribution represented by TH = 1250±100 K with nH = 500±100 cm?3. The observations of the dark limb showed that the spherically symmetric model used for the sunlit hemisphere was inappropriate for the analysis of the antisolar hemisphere. The density of the non-thermal component had increased at low altitudes, < 12,000 km, and decreased at high altitudes, > 20,000 km, by comparison. We conclude that the non-thermal source is on the sunward side of the planet. Analysis of the dark limb crossing suggests that the exospheric temperature on the dark side is <125 K if the exospheric density remains constant over the planet; upper limits are discussed. An additional source of Lyman α emission, 70 ± 15 R, was detected on the dark side of the planet and is believed to be a planetary albedo in contrast to multiple scattering from the sunlit side. Our analysis of the MARINER 10 data is consistent when applied to the MARINER 5 data. |
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