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

J D Campbell,O Brissaud,J‐P Muller,B Schmitt

doi:10.1029/2020je006595

Abstract

AbstractA 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.

Highlights

Our neighbouring planet, Mars, has increasingly been the target of scientific exploration and of particular interest are its geological history, current environmental conditions, and perhaps most importantly, its potential as a host for extraterrestrial life (Fairén et al, 2010)
The North Polar Cap (NPC) is composed almost entirely of water ice, while the longer, colder winter in the higher altitude southern hemisphere means that the central part of the South Polar Cap (SPC) is covered by a permanent layer of CO2 ice (Byrne, 2009), up to 10 metres thick, underlain, and surrounded by, water ice layers known as Polar Layered Deposits (PLD; Piqueux et al, 2008)
This was tested at room temperature with a ~1% mix of Polycyclic Aromatic Hydrocarbons (PAHs) mixture with a transparent powder in order to estimate the minimum amount of PAH required for detection within CO2 ice but outside its absorption bands

Summary

Introduction

Mars, has increasingly been the target of scientific exploration and of particular interest are its geological history, current environmental conditions, and perhaps most importantly, its potential as a host for extraterrestrial life (Fairén et al, 2010). Mars have more recently emerged as an area of scientific interest due to their abundance of water ice and their dynamic nature, especially with the discovery of a possible stable body of liquid water beneath the Martian South Pole (Orosei et al, 2018). The Martian North and South Pole have permanent ice caps that expand throughout their respective winters, and maintain residual caps in the summer. Seasonal cycles of sublimation and deposition of CO2 ice on the SPC are responsible for the formation of ‘Swiss Cheese Terrain’ (SCT). This unique cryomorphology manifests as flattopped mesas with flat floored, quasi-circular depressions that form patterns resembling

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