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 organic compounds 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 been reported by both Curiosity’s SAM/GCMS and Perseverance’s Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC). The organic compounds include benzene, naphthalene, thiophenes, chlorinated aromatics, a variety of chloro-hydrocarbons [12,13]. Most recently, preliminary results from the GCMS showed the possible 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 [14]. It has been suggested that a possible sourcemay be the decomposition of long-chain carboxylic acids (e.g., 10-undecenoic acid). 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 [15].  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 or fragmentation 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, be using 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 [6].

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 fragmentation 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.