First In Situ Evidence Of Marc Nonthermal Exosphere

Dedicated in situ measurements of Mars' exospheric Ar density by Neutral Gas Ion Mass Spectrometer/Mars Atmosphere and Volatile Evolution (NGIMS/MAVEN) highlight profiles displaying two slopes altitude variation. Below 230 km, the Ar density is characterized by scale height of 13.5 +/- 1.7 km and above 400 km by scale height of 156 +/- 25 km. A comparison with the model HELIOSARES suggests that the most probable origin of the Ar nonthermal component is the collisional interaction, below the exobase, between the atmospheric Ar and the nonthermal O atoms produced by the dissociative recombination of O-2(+). These measurements therefore provide a new approach to constrain one of the main sources of Mars' atmospheric escape. They also lead to set up an upper limit on the efficiency of pickup ion sputtering at Mars. The comparison with HELIOSARES suggests that NGIMS detection sensitivity might allow a positive detection of this mechanism if more measurements at high altitude deep in the nightside are performed.