Exospheric Temperature Estimation and Atmospheric Loss: a Comparative Study of Mars and Venus

Physics – Geophysics

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Mars Atmosphere, Exospheric Temperatures, Ionosphere, Radio Occultation, Atmospheric Loss

Scientific paper

The heating of the thermospheres and the formation of the ionospheres of terrestrial planets like Venus, Earth and Mars are mainly controlled by the solar X-ray and extreme ultraviolet radiation (0.1 - 100 nm). The current estimates of the Martian exosphere temperature (in the order of about 300-350 K) are based on hydrogen Lyman-alpha measurements. There is much evidence however, that these exospheric temperature estimates are too high. The reason for that comes mainly from to the fact that hot neutral hydrogen particles may have contaminated the neutral hydrogen. This study compares the ionospheric data obtained by the Mars Global Surveyor spacecraft with previous ionospheric data obtained by observations of former spacecraft missions. As the various data sets were obtained during different solar activity periods, it was planned to study the behavior of the dependence between the neutral temperature in the thermosphere obtained from a Chapman ionospheric profile near the ionospheric peak, and the solar 10.7 cm radio flux (F10.7). The estimations of this investigation clearly show that the exospheric temperature of Mars does not exceed a value of about 240 K, which is confirmed by inferred data of the Mars Global Surveyor aerobraking experiment. These results were compared to known observations (measurements of ionospheric profiles, F10.7 flux, and neutral gas temperatures at the Exobase level) on Venus. Furthermore the importance of these temperature estimations for the escape of hydrogen from the planets atmosphere are investigated and calculated. This study provides important results in the field of comparative planetology as it helps to obtain a good estimation of the Martian Exobase temperature, which so far (in contrast to Venus) has not been directly measured. This study is performed in the framework of the Mars Express participation of the Institute for Geophysics Astronomy and Meteorology (IGAM) of the Karl-Franzens University Graz/Austria and the Space Research Institute (IWF) of the Austrian Academy of Sciences.

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