Possible influence of Martian surface mineralogy on the detectability of atmospheric trace gases - mid-infrared simulation results

The paper focuses on the influence of the optical properties of Martian surface minerals on remotely detected gaseous components of the Martian atmosphere, when the spectrometer receives a combined signal from the Martian soil and atmosphere. Our considerations are primarily concerned with the detectability of methane, but the problem may also apply to other trace gases. Detections of methane in the Martian atmosphere have been reported from Mars Express (orbiting Mars), the Curiosity rover on the Martian surface, and from Earth. Its presence in the Martian atmosphere is being questioned today. The reason for these doubts is that both spectrometers onboard ExoMars Trace Gas Orbiter have not yet detected any methane in the Martian atmosphere using the very sensitive solar occultation method. The solar occultation method is unable to probe the lowest layers of the atmosphere at mid-latitudes, and so, its presence in this part of the atmosphere is assumed to be due to its possible source in the ground, as suggested by some works.

This paper considers whether the spectral characteristics of the soil may hinder the remote detection of methane. One of the examples discussed in the article relates to the possible observation of methane over mineralogical surfaces that may be the source of this gas. The examples of other surface mineralogical compositions are also discussed. The series of numerical simulations carried out in the region of the strong methane absorption band and the examples where the optical properties of the surface change the shape and contrast of this absorption band are shown. The codes used provide estimates of the spectral reflectance/emittance and total radiance of the Martian surface and atmosphere in the mid-infrared spectral region. The surface covered by dust was described by the reflectance and emittance calculated from n,k using Mie and Hapke theories or known from laboratory measurements. The different concentrations of atmospheric trace gases were taken into account.