Photoelectrons as a tracer of planetary atmospheric composition: Application to CO on Mars

Photoelectrons are an extensively studied component of planetary ionospheres which have been frequently used as a diagnostic of ambient magnetic fields. We show in this study that they also provide information on atmospheric composition via the altitude variation of photoelectron intensity at specific energies. Such an idea is applied to Mars for which a large observational sample of photoelectrons is available from the Solar Wind Electron Analyzer measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN). Our analysis reveals a strong decline in photoelectron intensity from 160 to 200 km, but this trend is restricted to a narrow energy range of 10-15 eV. By employing analytical yield spectrum calculations, we derive the model variations based on different choices of the background atmosphere. We find that the presence of CO is crucial for reproducing the observed variations. This allows atmospheric CO densities to be estimated from photoelectron measurements, which are a factor of 3-7 lower than the published densities based on the MAVEN Neutral Gas and Ion Mass Spectrometer measurements and in better agreement with existing model results. In general, the usefulness of photoelectron intensity at a specific energy as a tracer of atmospheric composition relies critically on the species-dependent inelastic electron impact cross section at this energy. Plain Language Summary On each solar system body with a substantial atmosphere, the ionization of atmospheric neutrals by solar photons produces photoelectrons at a typical energy of several eV to several hundred eV. These electrons have caught extensive research interests for decades because their spatial distribution has been recognized as a good tracer of ambient magnetic fields. In this study, we propose that photoelectrons could also be used as a tracer of atmospheric composition. Such an idea is applied to Mars, showing that the change of photoelectron flux with altitude at 10-15 eV relies critically on the distribution of atmospheric CO. When applied to other planets, the combination of the atmospheric species and photoelectron energy involved may vary, depending on the detailed characteristics of inelastic collisions between electrons and neutrals. Our idea provides a potentially useful constraint on the abundance of key atmospheric species when it cannot be measured accurately by a mass spectrometer, a situation often encountered in modern planetary missions.