No abstract

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( 4 S), N( 2 D), 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( 4 S) and N( 2 D) abundances maximized in the morning, both driven by the variation of the ambient N 2 mixing ratio. The difference in chemical loss time scale implies a strong diurnal variation for NO and N( 2 D) but a weak variation for N( 4 S).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 CO 2 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 N 2 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.

The detection of a water-rich plume erupting from ‘tiger stripes’ near the Enceladus south polar region is an important discovery of geologically active satellites within the Solar system. In this work, we apply an analytical approach to model the plume structure, using as a diagnostic the CO2 distribution extracted from the Cassini Ion and Neutral Mass Spectrometer measurements made during the E14, E17, and E18 flybys. Special focus is placed on the modelling of the spike-like structures by including only sources with substantial contributions to the plume densities. Such a procedure reduces greatly the model complexity and helps to better constrain the source parameters. Both the model source rate and Mach number are found to vary considerably among different sources during the same flyby and also among different flybys for the same source, revealing a complicated spatio-temporal variability in the plume structure. Our analysis suggests a total escape rate of (1.0–7.1) × 1026 s−1 for CO2 and (2.4–6.6) × 1028 s−1 for H2O, in broad agreement with previous estimates. Of particular interest is our identification of a tentative correlation between the Mach number and thermal brightness, of which the latter is a tracer of the geological activity of the emission source. Such a relation should be able to provide more insights into the nature of geyser emission along the ‘tiger stripes’.