The Hot Atom Coronae of Venus, Mars and the Earth: Current Understanding and Challenges

The exospheres of the terrestrial planets (Venus, Mars, and the Earth) are very different in composition and extent as well as the amount of interaction they experience with the solar wind. In this paper we discuss new results from the analysis of the Pioneer Venus Orbiter Ultraviolet Spectrometer data taken over more than a solar cycle. These observations, in particular those in the atomic oxygen solar resonance line at 130.4 nm, show the response of the planetary ionosphere to changing levels of solar flux and the degree to which the exosphere varies in density. We also report a new analysis of data from the ultraviolet spectrometers on the Mariner Mars missions: these data are nearly thirty years old. We report the detection of a hot oxygen corona here as well. Hot, or non-thermal, oxygen in the Earth's atmosphere is much more difficult to observe. Yet it holds valuable clues to thermospheric processes. We will summarize the differences between the exospheres of the three terrestrial planets and what we still may learn from another aeronomical mission to Mars or Venus.