Synthetic spectra around the CH4 line at 3038.498 cm−1 (left) and at 1327.0742 cm−1 (right). The former spectral region was used for the previous observations performed by IRTF/CSHELL, Keck/NIRSPEC, and VLT/CRIRES (Mumma et al. 2009; Villanueva et al. 2013), and the latter spectrum was used for the SOFIA/EXES observations. The black and blue curves in the left-hand figure represent the transmittance with/without the main CH4 line due to terrestrial atmosphere calculated from US standard atmosphere above 4 km (which is the altitude of Mauna Kea Observatory) with airmass = 1.4 (which is the minimum airmass during the observation on 20 March 2003 by Mumma et al. 2009.). The black and blue curves in the right-hand figure are the transmittances calculated from US standard atmosphere above 14 km (which is the altitude of SOFIA) with airmass = 2.0 (which is the maximum airmass during our observation on 16 March 2016). The purple curve shown in the right figure is the same as black one but from US standard atmosphere above 4 km. The red curves shown in both figures are absorption depth due to 10 ppb of Martian CH4 with the Doppler shift between Mars and Earth being ± 15 km s−1. The green bars shown in the left and right figures represent the spectral resolutions of IRTF/CSHELL and VLT/CRIRES, and that of SOFIA/EXES, respectively.
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