Investigation on the Absorption Bands Around 3.3 μm in CRISM Data

Abstract While analyzing NIR-IR CRISM data on a site in Oxia Planum area, in the view of the ExoMars2022 mission, absorptions around 3.3 µm were noted in some pixel spectra. Absorptions in the range 3.1–3.6 mm are under the spotlight in planetary contexts since hydrocarbons molecules show bands in this range. Consequently, we started to search eventual hydrocarbon compounds on Mars surface, using CRISM data in the range 3.2–3.4 mm. To date, methane has been the unique hydrocarbon detected on Mars. Like other hydrocarbons, methane shows absorptions around 3.3 mm. Hence, we begun the investigation on CRISM data, on that locations where methane was detected and where it could form on the basis of the mineralogy of the specific site. The datasets chosen for this study include some sites of observations in Oxia Planum area, Gale Crater and Nili Fossae area. Beside the study of CRISM IR data, in the event that some observed absorptions were related to methane, we tried to calculate a theoretical lower limit of detection for each observation in CRISM data. This was performed using the Planetary Spectrum Generator, simulating the CRISM spectra of the different sites, with the diverse concentrations of CH4. These simulations served to establish the relation between concentration and methane band depths, as seen by CRISM spectrometer. Then, mapping the Modified Gaussian Model (MGM) fit on CRISM data, we extracted the band parameters of the absorptions in the 3.3 µm spectral region. Aside rare, suspected absorptions, an artifact was highlighted. Therefore, we took measures to avoid, as more as possible, this artifact from the interpretation of the 3.3 mm absorptions: distribution of clusters of pixels not vertically stacked and a rather conservative threshold on depth value. As result, we found interesting clusters of pixels which spectra overcome the imposed threshold. Furthermore, the threshold value varies from one observation to another, in a range between 0.0136–0.0237, that would correspond to a range of theoretical lower limits of concentration between 180 and 600 ppbv. Despite all the precautions, we consider that part of these absorptions could still be an artifact. Nevertheless, the aim of this paper is to show that CRISM data can show theoretically absorptions of hydrocarbons, and methane, in such quantities that in some observations are compatible with the order of the methane spikes effectively detected on Mars. Even if this work does not confirm nor deny the occurrences of methane seepages or hydrocarbons in the investigated images, it shows a possible method for searching hydrocarbons in CRISM data and for assessing a confidence limit in the detection methane band in CRISM data.