The Detectability Limit of Organic Molecules Within Mars South Polar Laboratory Analogs

A series of laboratory experiments was carried out in order to generate a diagnostic spectrum for polycyclic aromatic hydrocarbons (PAHs) of astrobiological interest in the context of the Martian South Polar Residual Cap (SPRC), to establish PAH spectral features more easily detectable in CO2 ice (mixed with small amounts of H2O ice) than the previously reported absorption feature at 3.29 mu m in order to constrain their detectability limit. There is currently no existing literature on PAH detection within SPRC features, making this work novel and impactful given the recent discovery of a possible subglacial lake beneath the Martian South Pole. Although they have been detected in Martian meteorites, PAHs have not been detected yet on Mars, possibly due to the deleterious effects of ultraviolet radiation on the surface of the planet. SPRC features may provide protection to fragile molecules, and this work seeks to provide laboratory data to improve interpretation of orbital remote sensing spectroscopic imaging data. We also ascertain the effect of CO2 ice sublimation on organic spectra, as well as provide PAH reference spectra in mixtures relevant to Mars. A detectability limit of similar to 0.04% has been recorded for observing PAHs in CO2 ice using laboratory instrument parameters emulating those of the Compact Reconnaissance Imaging Spectrometer for Mars, with new spectral slope features revealed between 0.7 and 1.1 mu m, and absorption features at 1.14 and most sensitively, at 1.685 mu m. Mars regolith analog mixed with a concentration of 1.5% PAHs resulted in no discernible organic spectral features. These detectability limits measured in the laboratory are discussed and extrapolated to the effective conditions on the Mars South Polar Cap in terms of dust and water ice abundance and CO2 ice grain size for both the main perennial cap and the H2O ice-dust sublimation lag deposit.

Automated summary

Laboratory experiments were conducted to generate a diagnostic spectrum for polycyclic aromatic hydrocarbons (PAHs) in the context of the Martian South Polar Residual Cap (SPRC) and to improve the detectability of PAH spectral features in CO2 ice. The study aims to provide data to enhance the interpretation of orbital remote sensing spectroscopic imaging data and records a detectability limit for observing PAHs in CO2 ice with new spectral features revealed between 0.7 and 1.1 μm.