The Origins of Long‐Term Variability in Martian Upper Atmospheric Densities

We quantify and interpret the long-term variability of dayside Martian upper thermosphere lower exosphere densities within 180–275 km altitudes. Atmospheric CO2, N2, O, Ar are from NASA Mars Atmosphere Volatile EvolutioN (MAVEN) observations during time period 2015–2020 near solar minimum. These neutral measurements, together with contemporaneous irradiance measurements at Mars, enable disentanglement orbital effect (due to annual Sun-Mars distance change longitude) extreme ultraviolet (EUV) in atmospheric density variations. The relative importance these two effects, which is obtained using a statistical method Dominance Analysis, reveals competition between indirect infrared (via upward coupling middle atmosphere) direct EUV local heating). Our results show that, unlike relatively constant low altitudes then decreases increasing altitude, nearly monotonically increases. effects comparable high (about 240/270/205 for CO2/N2/O). This analysis extended include exospheric mass estimates 400 Global Surveyor Odyssey data, focus on representative cycle phases minimum maximum. It found that km, always key driver regardless phase, while plays minor role greatly enhanced slightly exceeds