Thermospheric X-Ray and Euv Heating by the Young Sun on Early Venus and Mars |
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Authors: | Helmut Lammer Yuri N Kulikov Herbert I M Lichtenegger |
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Institution: | (1) Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria;(2) Polar Geophysical Institute (PGI), Russian Academy of Sciences, Khalturina Str. 15, 183010 Murmansk, Russia |
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Abstract: | In view of the low H2O abundance in the present Venusian and Martian atmospheres several observations by spacecraft and studies suggest that both
planets should have lost most of their water over the early active period of the young Sun. During the first Gyr after the
Sun arrived at the Zero- Age-Main-Sequence high X-ray and EUV fluxes between 10 and 100 times that of the present Sun were
responsible for much higher temperatures in the thermosphere-exosphere environments on both planets. By applying a diffusive-gravitational
equilibrium and thermal balance model for investigating radiation impact on the early thermospheres by photodissociation and
ionization processes, due to exothermic chemical reactions and cooling by CO2 IR emission in the 15μm band we found expanded thermospheres with exobase levels between about 200 km (present) and 2000
km (4.5 Gyr ago). The higher temperatures in the upper atmospheres of both planets could reach “blow-off” conditions for H
atoms even at high CO2 mixing ratios of 96%. Lower CO2/N2 mixing ratio or higher contents of H2O vapor in the early atmospheres could have had a dramatic impact from the loss of atmosphere
and water on both planets. The duration of this phase of high thermal loss rates essentially depended on the mixing ratios
of CO2, N2, and H2O in the early atmospheres and could have lasted between about 150 and several hundred Myr. |
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Keywords: | thermospheric heating exospheres water loss young Sun Venus Mars |
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