Nitric Oxide Abundance in the Martian Thermosphere and Its Diurnal Variation

As an important species of the Martian thermosphere, NO is chemically and radiatively active. However, its abundance is poorly constrained due to difficulties in both remote sensing and in situ measurements. In this study, we use the Neutral Gas and Ion Mass Spectrometer measurements made onboard the Mars Atmosphere and Volatile Evolution to derive the N(4S), N(2D), and NO abundances in the Martian thermosphere based on time‐dependent odd N chemistry. At a reference altitude of 160 km, our calculations suggest that the NO abundance is maximized in the afternoon whereas the N(4S) and N(2D) abundances maximized in the morning, both driven by the variation of the ambient N2 mixing ratio. The difference in chemical loss time scale implies a strong diurnal variation for NO and N(2D) but a weak variation for N(4S). Plain language summary NO is usually regarded as a good tracer of energy input into the upper atmospheres of terrestrial planets. On Mars, NO is mainly produced via the reaction of atomic N in the excited state with ambient CO2 and lost via the reaction with atomic N in the ground state. Existing investigations of NO abundance in the Martian upper atmosphere are extremely limited due to difficulties in both remote sensing and in situ measurements. In this study, we propose a useful approach to determine the NO abundance on both the dayside and nightside of Mars by combining our knowledge of the odd N chemistry with available neutral and ion density measurements of relevant chemical reactants. On the dayside, we find the NO abundance to be maximized late in the afternoon, which is driven by the variation of N2 mixing ratio in the background atmosphere. Our analysis also allows the diurnal variation of NO to be explored for the first time, revealing a strong day‐night difference by 3 orders of magnitude. This is caused by the fast depletion of NO on the nightside where its production source drops to a minimum level in the absence of solar radiation. Key Points The odd N abundances at 160 km in the Martian thermosphere are calculated with time‐dependent chemistry The NO abundance is maximized in the afternoon whereas the N(4S) and N(2D) abundances maximized in the morning A strong diurnal variation is present for NO and N(2D) but a weak diurnal variation present for N(4S)