Stringent upper limit of CH 4 on Mars based on SOFIA/EXES observations

Discovery of CH 4 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 4 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 4 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 ∘ ) 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 4 lines because it reduces the impact of telluric CH 4 on Martian CH 4 , and allows us to use CH 4 lines in the 7.5 μ m band which has less contamination. However, our results show no unambiguous detection of Martian CH 4 . The Martian disk was spatially resolved into 3 × 3 areas, and the upper limits on the CH 4 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 4 on Mars is sporadic and/or localized if the process is present.