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

¹ Planetary Aeronomy, Belgian Institute for Space Aeronomy, 3 av. Circulaire, 1180 Brussels, Belgium
e-mail: shohei.aoki@aeronomie.be
² Fonds National de la Recherche Scientifique, rue d’Egmont 5, 1000 Brussels, Belgium
³ Department of Geophysics, Tohoku University, Sendai, Miyagi 980-8578, Japan
⁴ Physics Department, University of California, Davis, CA 95616, USA
⁵ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
⁶ Universities Space Research Association, Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, MS 232-11, Moffett Field, CA 94035, USA
⁷ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁸ Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
⁹ Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁰ Southwest Research Institute, Div. ♯ 15, San Antonio, TX 78228, USA

Received: 31 March 2017
Accepted: 20 October 2017

Abstract

Discovery of CH₄ 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 CH₄ 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 CH₄ 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 CH₄ lines because it reduces the impact of telluric CH₄ on Martian CH₄, and allows us to use CH₄ lines in the 7.5 μm band which has less contamination. However, our results show no unambiguous detection of Martian CH₄. The Martian disk was spatially resolved into 3 × 3 areas, and the upper limits on the CH₄ 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 CH₄ on Mars is sporadic and/or localized if the process is present.