Challenges in searching for hydrocarbons in CRISM-IR data

After having observed  some absorptions around 3.3 μm band in some CRISM spectra, we begun a study in the range between 3.2 and 3.4 μm to exploit the high spatial resolution of CRISM spectrometer (~18 m/pixel) to look for methane or other C-H absorptions on Mars surface. Concerning methane, we were searching for high concentrations, comparable to the “methane spikes” concentrations detected by Curiosity on Mars surface and the methane plumes detected in Mars atmosphere from ground telescopes. The search for absorptions around 3.3 μm was carried out fitting the spectra of selected CRISM datasets with the MGM function in the 3.2-3.4 μm range.  From the MGM fit we obtained a map of the absorption depths. By this depth map, aside rare, suspected, absorptions, a spectral artifact was highlighted.  Therefore, we chose to consider spectra with absorptions around 3.3 μm not clearly related to known and unknown artifacts, and band depth values greater than 4*standard deviation of the depth map. We used the Planetary Spectrum Generator tool to find the relation between the absorption depths at 3.3 μm and methane concentration. We finally discuss the rare interesting spectra both as potentially true absorptions and as a still unknown artifact.