Organic matter should be present on Mars from the infall of exogenous objects such as comets, chondritic meteorites, and interplanetary dust particles, and possibly also from indigenous biological or geochemical synthesis on early Mars. It has been estimated that long term infall has delivered up to ~ 104 g/m2yr of organic matter and probably includes; amino acids, nucleobases, alkylbenzenes, naphthalene and higher polycyclic aromatic hydrocarbons, carboxylic acids, and complex insoluble organic matter. Biology would expand on abiotic molecular sources to produce more complex molecules, and more specifically, organic molecules which would provide chemical markers of past or present life. These molecules, or the slightly altered compounds (fragments) produced by via diagenesis or interactions with radiation, are referred to as chemical biomarkers or just biomarkers . A variety of such compounds or fragments (e.g., hopanoids, lipids, chlorophyll, histidine, RNA/DNA) have been identified as being priority targets for life detection.
Given that exogenous infall should be present on Mars, it was surprising when neither the Gas Chromatograph-Mass Spectrometer (GC-MS) on board the 1975 Viking Mars landers or the Thermal and Evolved-Gas Analyzer (TEGA) on the 2007 Phoenix Mars Lander detected any organic compounds in the martian surface samples [1,2]. However, when the Wet Chemistry Lab (WCL) onboard the Phoenix performed the first wet chemical analysis of the martian soil, in addition to determining a variety of soluble salts, the analyses revealed ~0.6 wt% perchlorate (ClO4-), most likely in the form of Ca/Mg(ClO4)2 [3-5]. The widespread occurrence of ClO3- and ClO4-, referred to as oxychlorines (ClOx), have subsequently been confirmed by their detection in the martian meteorites EETA79001 and Tissint [6,7] and by the Sample Analysis at Mars (SAM) instrument on the Curiosity rover . The presence of the ClOx in the martian soil explains why no organic compounds could be detected by either the Viking GCMS or the Phoenix TEGA. Even if they had been present, they would have been combusted before reaching the mass spectrometer.
Under Mars ambient conditions ClO4- and ClO3- can be photochemically produced on Cl-bearing mineral surfaces, most likely due to silicate (SiO2) and/or metal-oxides acting as photocatalysts to generate radicals such as O2-, which can then react with chloride . During this process (Figure 1) several ClOx intermediates such as hypochlorite (ClO-), chlorite (ClO2-), chlorate (ClO3-), and chlorine dioxide (ClO2) gas, as well as radicals such as ClO-, O2-, ‧OCl, ‧Cl, and ‧OH are also likely generated [9-11]. These intermediate products can alter or destroy organic compounds, with only highly refractory and/or those well protected from UV surviving. Some of the intermediaries may be even more destructive than direct UV-driven reaction processes because they are not limited by regolith screening, and through diffusion, cryoturbation, or impact reworking, could over time reach to greater depths (e.g., chlorine dioxide gas  by diffusion). Understanding the formation of oxychlorines and the processes by which they or their intermediates destroy or alter organic compounds is key to understanding the preservation and detection of biomarker compounds. We have recently shown the fragmentation of tryptophan when exposed to UV in the presence of an oxychlorine [10A].
Figure 1. Reactive oxychlorines and radicals are produced, which along with UV irradiation, can alter or destroy biomarker molecules .
In addition to the oxidation of organics by oxychlorines, radiation in various forms is by itself a dominant factor. Even though much higher levels of solar energetic particles (SEP) and galactic cosmic rays (GCR) reach Mars’ surface, their total energy is ~104 times less than the solar UV energy. Exposure of organic compounds to direct UV radiation is one of the main degradation processes on Mars. The thin CO2 atmosphere allows UV with 190<λ<400 nm to reach the surface with a flux of ~1.4×1015 photons/s/cm2. Although the UV at the surface penetrates only millimeters into the regolith, its destructive power is more widely felt due to cryoturbation, wind, and impacts, allowing the regolith to be circulated and exposed to it.
The presence of organic compounds on Mars has now reported after several analyses by the SAM GCMS on the Curiosity rover. In 2015, the initial results indicated the presence of chlorobenzene at 150-300 ppb and C2-C4 dichloroalkanes at 70 ppb and were interpreted as being reaction products of indigenous perchlorate
and organic carbon . In 2018, analysis of 3.5-billion-year-old sample yielded a variety of compounds, including thiophens, aromatics, aliphatic compounds, chloromethanes and chlorobenzene . In 2020, analyses showed the presence of decane (C10H22), undecane (C11H24) and dodecane (C12H26), and suggested that a possible source may be from decomposition of long-chain carboxylic acids (e.g., 10-undecenoic acid) preserved in the mudstone . Even though their origin remains uncertain they could be derived from either abiotic or biotic sources.
The identification of a large number of chlorinated compounds has been of special interest. There are currently three competing hypotheses as to the source of the detected chlorinated organic compounds . These include: (1) terrestrial contamination that was chlorinated during the analysis; (2) martian organic matter that was chlorinated during analysis; and (3) martian organic compounds that were chlorinated prior to analysis by in-situ reactions with oxychlorines, their intermediary formation products, and/or radical species, produced during the formation or breakdown of oxychlorines.
The objectives of our research are to determine the extent of alteration, chlorination, or destruction of organic biomarker molecules under Mars ambient conditions:
(1) during the UV-driven production of oxychlorines from chloride-bearing salts; and
(2) in the presence of existing oxychlorines. We will also, for the first time, use an actual martian sample in the form of sawdust from the EETA79001 Mars meteorite. The EETA79001 sawdust sample has already been received from the NASA-JSC meteorite curation center .
Although there are a number of studies on the effects of UV radiation on a variety of organic compounds identified as biomarkers [16-21], the effects of UV irradiation while oxychlorines are either being produced or present have not been previously studied. The results of our research will be of significance because we will for the first time be determining the extent of alteration or destruction of both biotic and abiotic organic compounds that could be present on Mars if life was present; biomarkers such as amino acids, simple and long-chain carboxylic acids (lipids), PAHs, hopanes, nucleobases, kerogen, and DNA/RNA. Over time, these biomarker molecules would have been exposed to diagenesis, radiation, or reactive oxidizing species, that would have altered or destroyed them. The original biomarker would have most likely been fragmented to new lower mass and simpler biomarkers. The question is: can we piece together these fragments from the original biomarker so that we can identify the parent biomarker with some level of confidence? Unfortunetly, the greater the fragmentation of the original biomarker, the more difficult it becomes to reconstruct it.
Our research, funded by NASA, will advance our understanding of the effects of oxychlorines and their UV-driven production on organic compounds and potential biomarkers on Mars, and thus advance our understanding of the extent of alteration or destruction of these compounds that should be present on Mars. A comprehensive understanding of the interactions between oxychlorine species and organics will impact our insight into the questions of what biomarkers we might expect to find in martian regolith and whether or not they would be chlorinated. Our results will help answer the questions about the presence of organic compounds and extant life on Mars, as well as providing a framework for interpreting the results of future landed Mars missions that search for biomarkers and life.
The Tufts Mars Simulation Chamber (MSC):
The MSC consists of a stainless steel cylindrical chamber with an internal depth of 45 cm and 60 cm diameter (~ 1.3×10-5 cm3). Samples are staged on a 25 cm diameter cold-plate whose temperature is controlled by an FTS RC210C0 recirculating cooler that can maintain temperatures as low as -80°C. A Mars gas mixture (CO2 95.3%, N2 2.8%, Ar 1.8% and O2 0.10%) can be introduced into the chamber at a constant flow rate. Pressure can be maintained between 1000-0.1 mbar monitored by an MKS baratron and controller connected to a scroll pump. Relative humidity (RH) and temperature are monitored by a Lascar EasyLog USB data logger and RH by a Decagon RH Sensor.
A Mars solar intensity spectrum is produced by 400 or 1000W xenon-arc lamps and delivered, via an IR filter, beam turner, and cutoff filters, through a 22 cm fused silica port on top of the chamber. The level of UV radiation (190-400 nm) is monitored using a Stellarnet UV BLUE-Wave Fiber Optic Spectrometer in order to correlate exposure time in the MSC to equivalent Mars UV dosages. Chamber gas composition and gaseous products can be monitored in real-time using an SRS QMS200 Mass Spectrometer.
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