Changes in the thermosphere and ionosphere of Mars from Viking to MAVEN

We compare Viking and Mars Atmosphere and Volatile EvolutioN mission (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) observations of the thermosphere and ionosphere of Mars in order to test predictions of large variations in conditions over the solar cycle and with season. Substantial differences exist between the Viking observations at solar minimum and near aphelion and the MAVEN NGIMS observations at moderate solar activity and near perihelion. Differences in the O/CO2 ratio, the O + ionospheric peak, ion densities at high altitude, and neutral and ion scale heights can be attributed to differences in solar activity and season, but the relative importance of solar activity and season for these differences was not established. Current models do not explain the observed differences in the mixing ratios of N, NO, and O2. These results place new constraints on models of how the thermosphere and ionosphere of Mars vary over the solar cycle and with season.