Distributions of hot oxygen for Venus and Mars
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Abstract:
Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O 2 + are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO 2 in the Mars atmosphere is probably less than 10 Myr.Keywords:
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Attention is given to noble gases in planetary atmospheres, the photochemistry of the stratosphere of Venus, the chemistry of metastable species in the Venusian ionosphere, the Venus ionosphere at grazing incidence of solar radiation, disappearing ionospheres on the nightside of Venus, and the observed composition of the ionosphere of Venus. Other investigations considered are concerned with the predicted electrical conductivity between 0 and 80 km in the Venusian atmosphere, sulfuric acid vapor and other cloud-related gases in the Venus atmosphere, the composition and vertical structure of the lower cloud deck on Venus, amorphous sulfur as the ultraviolet absorber on Venus, and polarization studies of the Venus UV contrasts. A description is provided of topics related to temporal variability of ultraviolet cloud features in the Venus stratosphere, zonal mean circulation at the cloud level on Venus, the influence of thermospheric winds on exospheric hydrogen on Venus, and an analysis of Venus gravity data.
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On 20 May NASA launched Pioneer Venus Orbiter, the first of a pair of unmanned craft designed to make a detailed scientific study of Venus. The second craft, Pioneer Venus Probe, will be launched in late summer/early autumn but due to its faster journey will arrive at Venus in December only a few days after Orbiter.
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