Observation of Suprathermal Argon in Mars Exosphere

The outermost region of a planetary atmosphere, called the exosphere, is the tenuous region where the mean free path of the particles is much larger compared to the scale height. The altitude over which the atmospheric density decreases by about a factor of 2.7 compared to its previous level is called the scale height. In the upper atmosphere of a planet, this depends on the mass of the species, temperature of the species, and the gravity of the planet. This region is being explored in-situ by MENCA (Mars Exospheric Neutral Composition Analyser) experiment on-board the Mars Orbiter Mission (MOM). MENCA has discovered 'hot' (suprathermal) Argon in the exosphere of Mars. The words 'hot' as well as 'suprathermal' indicate that they are more energetic compared to the thermal populations and hence their kinetic temperatures are higher. These observations were conducted during December 2014, when periapsis of MOM was closest to the Mars.   

The presence of hot Argon atoms is revealed by the changes in its scale height in certain orbits. The increase in scale height indicates an increase in exospheric temperature, which in normal conditions is about 270 K while on orbits when hot argon is seen, the exospheric temperature is observed to be greater than 400 K. This increase in temperature is because of the significant presence of suprathermal neutral Argon atoms in the Martian exosphere. The discovery has important implications in the context of understanding the energy deposition in the Martian upper atmosphere and will help understand why the Martian atmospheric escape rates are higher than what was believed previously.

MENCA is a mass spectrometer, developed at Space Physics Laboratory of Vikram Sarabhai Space Centre at Thiruvananthapuram. MENCA is providing in-situ measurements of the neutral composition of the exosphere of Mars. MENCA has provided several measurements of the composition of the key species of the Martian neutral exosphere. Observations pertaining to the evening time exosphere of Mars, has already been published on May 02, 2016.

The above result has been published recently in the American Geophysical Union (AGU) journal Geophysical Research Letters vol. 44, 2017. doi:10.1002/2016GL072001.