Photochemical escape of atomic C and N on Mars: clues from a multi-instrument MAVEN dataset

¹ School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, PR China
e-mail: cuijun7@mail.sysu.edu.cn
² Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, PR China
³ Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei, PR China
⁴ Space Science Institute, Macau University of Science and Technology, Macau, PR China
⁵ Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, PR China

Received: 30 June 2018
Accepted: 8 November 2018

Abstract

Context. Photochemical escape of hot atoms is crucial to the long-term evolution of the Martian climate. For atomic C and N, photochemical escape is primarily driven by photodissociation (PD) of CO and N₂.

Aims. Combining the Mars Atmosphere and Volatile Evolution (MAVEN) measurements of atmospheric neutral densities and solar EUV/X-ray irradiance, we perform a state-of-the-art analysis of atomic C and N escape on Mars.

Methods. For each MAVEN orbit, we calculated the hot C and N production rates in the dayside Martian upper atmosphere via PD, from which the escape rates are estimated using a simplified technique to parameterize the respective escape probabilities taking into account multiple collisions with ambient neutrals.

Results. The mean C and N escape rates are 1 × 10²⁴ s⁻¹ and 9 × 10²⁴ s⁻¹, appropriate for low to moderate solar activity conditions, and thermospheric PD makes a larger contribution to the total N escape than to the total C escape. The above differences highlight the importance of nascent energy, with more energetic nascent escaping atoms able to survive collisions with ambient neutrals more easily, thus extending down to deeper regions of the atmosphere. Solar cycle variation in C and N escape is revealed by our analysis, whereas solar zenith angle variation is absent for both species. These results could be explained by the fact that the production of nascent escaping atoms responds to varying solar illumination angle at low altitudes where the escape probability is negligible, but responds to varying level of solar EUV/X-ray irradiance at high altitudes where the atmosphere is essentially collisionless.