Influence the composition of Martian surface on visibility the spectral features of atmospheric trace gases in the Mid-infrared spectra - the results of numerical simulations

Trace gases - important ingredient of planetary atmospheres  interact with other components of the surroundings of the planets and with planetary surfaces, creating a new conditions on the surfaces and in the atmosphere. These changes can be obviously visible in spectrometric measurements. In the case of nadir  measurements, the ground optical information is distorted by the influence of the atmosphere and vice versa when examining the atmosphere, by the influence of the surface. This  fact can cause ambiguity in remotely  detection of both: the composition of the atmosphere and the soil. The  paper is focused on the study the  influence of the optical characteristic of minerals on the surface on  remotely detected  gaseous components of the Martian atmosphere. This influence  is visible in the radiance spectra  as the overlapping of spectral features of reflectance/emissivity on extinction of atmospheric gases. The main question is to what extent the spectra of the soil can obscure the gaseous spectra of the atmosphere. Is it possible that the surface spectrum completely changes the spectral characteristics of atmospheric trace gases?

In the paper the influence of spectral properties of the  surface on intensity and profiles of Martian spectra in the spectral region of absorption bands of methane were carefully examined. An inspiration to start analyzing this issue was  the fact that the excellent NOMAD spectrometer the part of payload of  ExoMars Trace Gas Orbiter has not yet detected methane in the Martian atmosphere. The methane has been detected many times (from the  Martian orbiters and from Earth) and its presence in the Martian atmosphere is rarely questioned today. For this reason, the topic seems  extremely interesting. In the paper the numerical simulations of the measurements of the device such as NOMAD spectrometer are shown.

The elaborated codes provide estimates of the spectral reflectance/emittance and the total radiance of the Martian surface and atmosphere in the  Mid-Infrared spectral range. The surface with various structures were described by the reflectance  of pure serpentynite or it's mixtures with other minerals like olivine and carbonates. The reflectance were calculated from n,k with Mie and Hapke theories or  measured in the laboratory. The various  granulation of the surface and  concentration of atmospheric gases were   taken into account. The surface structure was characterized by particles of various sizes. The physical properties of the atmosphere were characterised in terms of its thermodynamical parameters and absorbing and scattering properties.

One of the discussed examples is the observation of methane over  mineralogical surfaces which may be the source of this gas. At the beginning have been assumed that this gas is emitted from Mg-rich serpentinized rocks. Other mineralogical compositions of the surface were also taken into account. Examples are shown in which the optical characteristics of the surface significantly change the shape and contrast of the absorption band of methane.

Our analysis shows that in the spectral region around 3.32, where there is a strong methane absorption band, it is possible to detect this gas by nadir spectrometric measurements over serpentine-water ice regions on the Martian Surface. But the presence of other substances, e.g. dolomite, significantly changes the nature of the radiance spectrum in the region of interest. We have not found so far that the surface characteristics can lead to the complete disappearance of the methane band.

The paper  is  related  with  stereoscopic camera CASSiS measurements  the a part of payload of  ExoMars Trace Gas Orbiter (TGO) of the ESA mission in which researchers from SRC – PAS are engaged. The Cassis camera  give us the opportunity of analysis the structure, color and locations on the surface of  Mars  possible sources  of  trace gases e.g. methane.