Vertical Structure and Variability of Argon in Mars’ Exosphere from High-Resolution NGIMS Data

Argon (Ar) is a chemically inert noble gas used as a tracer to investigate transport processes and energy deposition in the Martian upper atmosphere. Its high atomic mass makes it sensitive to both thermal and non-thermal mechanisms, while its chemical stability ensures that its distribution reflects physical processes occurring in the thermosphere. NGIMS, the Neutral Gas and Ion Mass Spectrometer aboard NASA’s MAVEN spacecraft, has been measuring neutral species including Ar since late 2014, enabling systematic monitoring of the Martian atmosphere from ~150 to over 1000 km in altitude. First studies of Ar densities reflect that below 350 km the population is dominated by thermal processes, whereas above this altitude, detected Ar is primarily attributed to suprathermal components (Mahaffy et al., 2014; Leblanc et al., 2019).

Over its seven years of operation, MAVEN has collected more than 25,000 orbits with NGIMS in neutral mode, including over 1,100 orbits dedicated to high-cadence observations of argon. These targeted campaigns increase vertical resolution and sampling near periapsis and provide enhanced sensitivity up to 1200km. The compiled dataset spans a broad range of local times, solar zenith angles (SZA), seasons (solar longitudes), and latitudes, offering extensive spatial and temporal coverage.

This study investigates the dependence of Ar density on SZA using both standard and high-cadence NGIMS measurements. The resulting profiles indicate systematic variations with SZA, particularly in the 300–1000 km altitude range. Ar densities tend to decrease with increasing SZA, with the strongesat gradients observed above the nominal exobase.

A subset of high-cadence profiles is also examined in greater detail to contextualize vertical structure in terms of MAVEN's orbital geometry, local time, latitude, and season. Particular attention is given to trajectories passes occurring near the morning and evening terminators. These profiles are analyzed individually to assess variability in Ar density structure and its possible relationship to viewing geometry and seasonal conditions. In several cases, localized enhancements in Ar density are detected at altitudes above 700 km. The origin of these features remains under investigation but may be consistent with the influence of waves or perturbations propagating upward from lower atmospheric layers. The apparent asymmetry between dusk and dawn sectors, as previously noted in global maps, is also investigated through this targeted approach.