1,1,- 1LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France
- 2NASA/GSFC, University of Maryland, Greenbelt, MD, USA
- 3Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO 80304, USA.
Argon is an inhert gas that can be used to measure different areas of thermal populations and non-thermal populations in the upper atmosphere of Mars. In this work, a statistical survey over 7 years' worth (2018-2024) of MAVEN/NGIMS measurements during Mars Year 35 – 37 (25,000+ standard survey orbits + 268 dedicated high-cadence argon orbits (1,100+) has been produced that provide reliable measurements from 150 km periapsis altitudes and up to 1200 km in the extended exosphere.
The thermal populations of argon in the thermosphere (150 – 300 km) have a pronounced diurnal variation with respect to solar-driven expansion. Temperatures derived from the scale height technique show a range of 150 K (dawn minimum around LST 3 - 6) to 300 K (afternoon maximum around LST 14 - 16) which results in a factor of 2 diurnal amplitude. Dayside temperatures in the thermosphere correlate strongly with EUVM Lyman-A irradiance as observed by EUVM indicating that the Ar thermosphere reacts to changing heliocentric as well as the Solar Cycles 24 – 25 transition with variations of 100 - 200 K, which indicates that dynamical effects (tides and gravity waves) also occurs.
At higher altitudes (above 400 km), Ar exhibits a coherent response to variations in UV flux but experience reduced effective scale heights which is likely due to collisional damping. Increased EUV results in downward displacement of the O₂⁺ production layer into denser portions of the thermosphere, thereby increasing the column density above it and thermally stimulating the upward-moving suprathermal Ar resulting in more frequent collisions of the hot O and thus a partial thermalization.
On the night side, high-altitude argon approaches NGIMS detection limits (~10 cm⁻³), while terminator passes reveal that the scale height temperature drastically inreases, which could be an indicator of a different suprathermal process than recombinative dissociation. Organized by SZA, altitude regime (thermal/transition/suprathermal), local time, season (Ls), latitude, and solar forcing, these results establish argon as a robust diagnostic of thermospheric dynamics, suprathermal production physics, terminator transport asymmetries, and baseline conditions for quantifying non-thermal neutral escape pathways at Mars.
Figure: Averaged distribution of the Ar corona at Mars
How to cite: Steichen, V., Leblanc, F., Chaufray, J.-Y., Benna, M., Nenon, Q., Modolo, R., and Curry, S.: A Statistical Survey of the Martian Argon Exosphere Using MAVEN Data, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-417, https://doi.org/10.5194/epsc2026-417, 2026.
