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.

The Mars Exospheric Neutral Composition Analyser (MENCA) aboard the Indian Mars Orbiter Mission (MOM) is a quadrupole mass spectrometer which provides in situ measurement of the composition of the low‐latitude Martian neutral exosphere. The altitude profiles of the three major constituents, i.e., amu 44 (CO2), amu 28 (N2 + CO), and amu 16 (O) in the Martian exosphere during evening (close to sunset terminator) hours are reported using MENCA observations from four orbits of MOM during late December 2014, when MOM's periapsis altitude was the lowest. The altitude range of the observation encompasses the diffusively separated region much above the well‐mixed atmosphere. The transition from CO2 to O‐dominated region is observed near 270 km. The mean exospheric temperature derived using these three mass numbers is 271 ± 5 K. These first observations corresponding to the Martian evening hours would help to provide constraints to the thermal escape models.

No abstract

No abstract

No abstract