Stringent upper limit of CH4 on Mars based on SOFIA/EXES observations

Abstract

Discovery of CH<SUB>4</SUB> in the Martian atmosphere has led to much discussion since it could be a signature of biological and/or geological activities on Mars. However, the presence of CH<SUB>4</SUB> and its temporal and spatial variations are still under discussion because of the large uncertainties embedded in the previous observations. We performed sensitive measurements of Martian CH<SUB>4</SUB> by using the Echelon-Cross-Echelle Spectrograph (EXES) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) on 16 March 2016, which corresponds to summer (Ls = 123.2<SUP>∘</SUP>) in the northern hemisphere on Mars. The high altitude of SOFIA ( 13.7 km) enables us to significantly reduce the effects of terrestrial atmosphere. Thanks to this, SOFIA/EXES improves our chances of detecting Martian CH<SUB>4</SUB> lines because it reduces the impact of telluric CH<SUB>4</SUB> on Martian CH<SUB>4</SUB>, and allows us to use CH<SUB>4</SUB> lines in the 7.5 μm band which has less contamination. However, our results show no unambiguous detection of Martian CH<SUB>4</SUB>. The Martian disk was spatially resolved into 3 × 3 areas, and the upper limits on the CH<SUB>4</SUB> volume mixing ratio range from 1 to 9 ppb across the Martian atmosphere, which is significantly less than detections in several other studies. These results emphasize that release of CH<SUB>4</SUB> on Mars is sporadic and/or localized if the process is present.