Observation of suprathermal argon in the exosphere of Mars

Abstract

The altitude profiles of argon-40 (Ar) in the Martian exosphere are reported using Mars Exospheric Neutral Composition Analyser aboard Indian Mars Orbiter Mission (MOM) from four orbits during December 2014 (Ls = 250°–257°), when MOM's periapsis altitude was the lowest. The upper limit of Ar number density corresponding to this period is ∼5 × 105 cm−3 (∼250 km), and the typical scale height is ∼16 km, corresponding to an exospheric temperature of ∼275 K. However, on two orbits, the scale height over this altitude region is found to increase significantly making the effective temperature >400 K. Neutral Gas and Ion Mass Spectrometer observations on the Mars Atmosphere and Volatile Evolution mission also indicate that the change in slope in Ar density occurs near the upper exosphere (around 230–260 km). These observations indicate significant suprathermal CO2 and Ar populations in the Martian exosphere. Significant wave-like perturbations are observed but only on certain days when suprathermal population is seen. Pickup ion-induced heating is discussed as the other viable source.