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

• Published in: Journal of geophysical research. Planets 2021-07, Vol.126 (7), p.n/a
• Main Authors: Campbell, J. D., Schmitt, B., Brissaud, O., Muller, J‐P.
• Format: Article
• Language: English
• Subjects: Absorption; Analogs; Astrobiology; Astrophysics; Carbon dioxide; CO2 ice; Dust; Earth and Planetary Astrophysics; Experiments; Grain size; Ice; Imaging spectrometers; Laboratories; Laboratory experiments; Mars; Mars dust; Mars surface; Organic chemistry; PAHs; Polar caps; Polycyclic aromatic hydrocarbons; Radiation; Regolith; Remote sensing; Satellite-borne instruments; Satellites; Sciences of the Universe; SNC meteorites; South Polar Residual Cap; South Pole; Spectra; spectroscopy; Subglacial lakes; Sublimation; Surface chemistry; Ultraviolet radiation; Water ice
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2020JE006595

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 µm 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 ∼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 µm, and absorption features at 1.14 and most sensitively, at 1.685 µm. 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. Plain Language Summary Carbon molecule chains, or organics, are considered important in the search for life, as all life that we know of on Earth is made from carbon. A particular type of organic, PAHs, that are really common on Earth and throughout space, have never been found on Mars and it is likely they are broken down too quickly on the Martian surface by the Sun's radiation to be discovered using space satellites. However, some areas of Mars move and change, and expose dust. One such area is the Martian South Polar Cap. This study uses laboratory experiments to simulate expected condition