Stringent Upper Limit of CH4 on Mars Based on SOFIA/EXES Observations S
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A&A 610, A78 (2018) https://doi.org/10.1051/0004-6361/201730903 Astronomy & © ESO 2018 Astrophysics Stringent upper limit of CH4 on Mars based on SOFIA/EXES observations S. Aoki1,2,3, M.J. Richter4, C. DeWitt4, A. Boogert5,6, T. Encrenaz7, H. Sagawa8, H. Nakagawa3, A. C. Vandaele1, M. Giuranna9, T. K. Greathouse10, T. Fouchet7, A. Geminale9, G. Sindoni9, M. McKelvey6, M. Case4, and Y. Kasaba3 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. Key words. planets and satellites: atmospheres 1. Introduction So far, the remote-sensing observations of Martian CH4 has been investigated mainly by four groups, two from The presence of CH4 in the Martian atmosphere has led to spacecraft-borne observations (Geminale et al. 2008, 2011; much discussion since it could be a signature of ongoing Fonti & Marzo 2010), and two from ground-based observa- and/or past biological and/or geological activities on Mars. After tions (Mumma et al. 2009; Krasnopolsky 2012; Villanueva et al. preliminary detections from ground-based observations being 2013). The spacecraft-born PFS measurements showed the vari- reported in scientific meetings (Mumma et al. 2003), the first ations of CH4 amounts depending on season, location, and detection of CH4 on Mars was published from the observa- local time on Mars (Geminale et al. 2008, 2011). In partic- tions by the Planetary Fourier Spectrometer (PFS) onboard the ular, an enhancement of CH4 (∼60 ppb) over the north polar Mars Express (MEx) spacecraft (Formisano et al. 2004). The cap during the northern summer was reported, which implied mean abundance of CH was found to be ∼10 ppb. In the 4 the possible presence of a CH4 reservoir associated with the same year, detection of CH4 by ground-based observations with polar cap (Geminale et al. 2011). In contrast, Fonti & Marzo the Canada-France-Hawaii Telescope (CFHT)/Fourier Trans- (2010) analyzed the data obtained with another spacecraft-born form Spectrometer (FTS) was also published (Krasnopolsky instrument, Thermal Emission Spectrometer (TES) onboard et al. 2004). These discoveries of CH4 on Mars were remark- Mars Global Surveyor (MGS), and found substantially differ- able because its source could be either biological activity (e.g., ent spatial and seasonal distributions, with peak abundance near subsurface micro-organisms) and/or hydrothermal activity (e.g., 70 ppb over low-latitudes (Tharsis, Arabia Terra, and Elysium). serpentinization, Atreya et al. 2007). Identification of the source Meanwhile, the other two groups investigated CH4 on Mars of CH4 is very valuable for advancing not only planetary science using high-resolution, infrared spectrographs on ground-based but also future life explorations on Mars. facilities. Mumma et al.(2009) found extended plumes of Article published by EDP Sciences A78, page 1 of9 A&A 610, A78 (2018) CH4 (∼40 ppb) during the northern summer over low-latitude The Echelon-Cross-Echelle Spectrograph (EXES) onboard regions from IRTF/CSHELL observations performed on 11– the Stratospheric Observatory for Infrared Astronomy (SOFIA) ◦ ◦ 13 January 2003 (Ls = 122 ), 19–20 March 2003 (Ls = 155 ), is uniquely capable of performing a sensitive search for CH4 ◦ and 29 May 2005 (Ls = 220 ) – but they found no CH4 on from Earth. Through the entire spectral range, the strongest CH4 16 January using Keck/NIRSPEC nor 26 February 2006 using lines are located at 3.3 µm and 7.5 µm. Figure1 shows the terres- ◦ ◦ −1 IRTF/CSHELL near the vernal equinox (Ls = 357.3 and 17.2 ). trial and Martian spectra around the CH4 lines at 3038.498 cm −1 The same group reported no detection of CH4 on 6 Jan- (3.291 µm) and 1327.0742 cm (7.535 µm), that are simulated uary 2006 using IRTF/CSHELL and Keck/NIRSPEC, nor with for the IRTF/CSHELL and SOFIA/EXES observations, respec- VLT/CRIRES in 2009 and Keck/NIRSPEC in 2010 (Villanueva tively. As shown in Fig.1, one of the advantages of the 7.5 µm et al. 2013). They derived an upper limit of 7 ppb from those band is less contamination of minor terrestrial lines such as 13 non-detection observations, which is generally smaller than the CH4 or O3 even though the intrinsic intensities of the CH4 seasonal variations reported by the spacecraft-borne measure- lines at 3.3 µm and 7.5 µm are comparable. However, the Mar- ments groups. By contrast, Krasnopolsky(2012) claimed the tian CH4 line in this 7.5 µm band cannot be accessed from the detection of CH4 (0–20 ppb) over Valles Marines using ground- ground-based observations because the Doppler shift (due to based IRTF/CSHELL observations performed on 10 February Martian motion with respect to Earth) of the line frequency 2006, just 28 days after the Villanueva et al.(2013) observations, is about half of that of 3.3 µm and thus the Martian lines are where no CH4 had been observed over the same region (upper buried in the strong absorption of terrestrial CH4. Encrenaz et al. limit 7.8 ppb). (2005) attempted to search for CH4 on Mars using IRTF/TEXES In short, these remote-sensing observations suggest a signif- in the mid-infrared spectral range, however, they could not use icant variability of CH4 in space and time. The transport and/or the strongest lines because of the deep terrestrial CH4 absorp- diffusion times of days to weeks causes rapid plume disper- tion. This imposed a limitation on their data (derived upper sal, while the loss of global methane over a span from March limits were 20 ppb in the morning side and 70 ppb in the evening 2003 to January 2006 requires a lifetime less than four Earth side). The situation changes drastically for SOFIA thanks to the years and perhaps as short as 0.6 Earth years (Mumma et al. higher altitude (∼13.7 km). With SOFIA, Martian CH4 lines at 2009). In contrast, the standard photochemical models showed the 7.5 µm band can be measured in the wing of the terrestrial that the lifetime of CH4 in the Martian atmosphere is about lines, which makes this air-borne facility unique compared to 300–600 years (Lefèvre & Forget 2009), and, as a consequence, any ground-based facilities including those located at the sum- CH4 should be uniformly distributed in the atmosphere. This mit (∼4 km in the altitude) of Mauna Kea. In order to detect discrepancy between the observed variability and the model pre- the narrow Martian CH4 lines located at the wings of the deep diction has led to much debate on the reliability of the previous terrestrial line, high spectral resolution is essential. The EXES remote-sensing observations. The reason behind such a debate instrument realizes high spectral resolution of ∼90 000, and it is that the detected signal of CH4 is very weak and the obser- largely improves the chances to detect CH4 lines although the vations had a large uncertainty because of contamination of Martian lines are not fully resolved. One of the disadvantages at terrestrial lines and the need for a high signal-to-noise ratio. The 7.5 µm is that the signals depend on the thermal contrast between previous ground-based observations used lines in the P-branch the surface and atmosphere on Mars because the CH4 line is or R-branch of 3.3 µm band. The widths of these lines (half formed in the thermal emission regime.