High‐resolution spectroscopy of Mars at 3.7 and 8 μm: A sensitive search for H₂O₂, H₂CO, HCl, and CH₄, and detection of HDO

Abstract

We report the first sensitive upper limits for H 2 O 2 , H 2 CO, and HCl, a sensitive upper limit for CH 4 , and a measured column abundance for HDO. A combination of the Kitt Peak National Observatory (KPNO) 4‐m telescope and Fourier transform spectrometer with the Goddard Space Flight Center (GSFC) postdisperser was used to observe low‐latitude regions of Mars in spectral ranges of 2650–2800 cm −1 and 1229–1237 cm −1 with resolving power v /δ v =2.7×10 5 and 1.2×10 5 , respectively. The main spectral features are lines of isotopic CO 2 and HDO. A radiation transfer code has been developed which divides the atmosphere into 30 layers and the Voigt profile of each line in each layer into 60 intervals. This code couples the reflected solar and thermal radiations. A search for H 2 O 2 at 1229–1237 cm −1 results in a 2‐σ upper limit of 30 ppb, close to the predictions of recent photochemical models. Our 2‐σ upper limit for HCl (<2 ppb) precludes any significant chlorine chemistry in the atmosphere. Our value for methane (70±50 ppb) is consistent with, but does not improve upon, earlier results from Mariner 9 infrared interferometer spectrometer (IRIS). Our 2‐σ upper limit for H 2 CO (3 ppb) is much smaller than recently claimed abundances. The observed HDO lines result in an abundance of (6.06±0.5)×10 16 cm −2 , which, when compared with that of H 2 O measured by the Viking orbiters at the same season and latitudes, yields an enrichment in the D/H ratio of a factor of 5.5±2 relative to Earth. The given uncertainty is mainly caused by possible variations of Mars' atmospheric water abundance.