Icarus Notes (2025) https://doi.org/10.1016/j.icarus.2025.116923
M. Escudero-Jiménez, M.A. López-Valverde, J. Peralta, T. Moya, A. Belmonte, A. Brines, I. Thomas, G. Villanueva, G. Liuzzi, E. Knutsen, S. Viscardy, F. González-Galindo, A. Modak, B. Funke, J.J. López-Moreno, J. Rodríguez, R. Sanz, F. Daerden, L. Trompet, A.S.J. Khayat, A.C. Vandaele
Here we update the search for two trace species of high interest in the Martian atmosphere, methane (CH4) and carbonyl sulfide (OCS), using solar occultation (SO) measurements from the NOMAD spectrometer on board the Trace Gas Orbiter (TGO). Neither species has been detected in previous analyses of TGO’s dedicated SO instruments, ACS and NOMAD. Our approach focused on NOMAD-SO data, assessing in detail the calibration uncertainties to quantify systematic error components, and investigating if vertical averaging of spectra to reduce random noise would enhance the signal-to-noise ratio (SNR). The latter step required careful data cleaning with precise radiative transfer modeling and should improve upon earlier searches that analyzed spectra individually.
We examined 19,330 NOMAD SO spectra targeting CH4 and OCS, from diffraction orders 134 and 129, corresponding to 76 and 142 vertical scans, respectively. These scans span a wide range of latitudes and seasons over three Martian years (April 2018 to November 2022, MY34-MY36). No CH4 or OCS absorption features were detected and our composite vertical profiles (5–50 km tangent altitude) yielded stringent upper limits. For methane, the 1-sigma upper limits typically lie around 0.5 ppbv, reaching 0.2 ppbv at some altitudes and locations. Our methane limits were primarily constrained by systematic uncertainties, particularly the residual baseline shape, and are therefore relatively higher than those previously reported in Knutsen et al. (2021), who processed the spectra differently to remove this effect. For OCS, our composite profiles indicate concentrations below 5 ppbv at low altitudes and below 2 ppbv in some cases, also at the 1-sigma significance level.
Further improvements will require additional calibration refinements, ideally through better characterization of instrument behavior under thermal variations. As a practical step, narrowing the analyzed spectral intervals can reduce continuum uncertainties, provided contaminating features are absent. For methane features in diffraction order 134, such narrowing lowers uncertainties by a factor of three, getting closer to the most stringent limits previously reported (Knutsen et al., 2021).
Panels a,b,c: Example of vertical averaging of 19 atmospheric transmittance spectra from scan 20190419_062950, within a 6-km interval around a tangent altitude of 15 km. Panel b: All 19 spectra shifted to a common continuum (red dashed line). Panel c: Averaged spectrum (thick black line) and standard deviation (1σ, blue shade); green dashed lines represent ±3σ intervals around the continuum; the brown line shows the best fit obtained in this work with contributions by CO2 and H2O, and the purple solid line shows the same but with the addition of CH4 with a VMR = 0.4 ppbv. Panels (d) and (e): vertical profiles of upper limits for the selected dataset of 142 scans for CH4 and 76 for OCS. Map’s colors are arbitrary and indicate point concentration.