Despite a hostile environment, there is evidence for organic material at several locations on Mars. The Sample Analysis at Mars instrument onboard the Mars Science Laboratory (MSL) Curiosity Rover detected chlorinated hydrocarbons indigenous to the ~3.5‐Ga lake sediments at Yellowknife Bay, Gale crater (Freissinet et al., 2015 ). Other aliphatic, aromatic, and sulfur‐bearing hydrocarbons that were detected in lake sediments studied at Pahrump Hills are thought to be derived from larger macromolecular structures that aided their preservation (Eigenbrode et al., 2018 ). Other potential indications of indigenous organic matter are indirect and are observed as thermal evolution of CO 2 and CO observed during Sample Analysis at Mars' evolved gas analysis and are in part attributed to decarboxylation and decarbonylation of organics in samples from diverse rocks and aeolian sediments in Gale crater (Ming et al., 2014 ; Sutter et al., 2016 ), which may also explain CO 2 observed by the Viking GCMS (Biemann et al., 1976 ). Chloromethanes (Navarro‐González et al., 2010 ) and chlorobenzene (Guzman et al., 2018 ) detected in Viking data may also be Martian carbon signals, but terrestrial sources have not been definitively excluded. Additionally, reduced carbon phases including graphite, polycyclic aromatic hydrocarbons, and macromolecular phases with diverse synthesis mechanisms have been detected in Martian meteorites (Steele et al., 2016 ). These observations together indicate that Martian organic matter is present in different preservation states but the sources and alteration processes affecting these organics are not well constrained.

Detecting organic material is a central goal of the Mars Exploration Program (MEPAG, 2015 ), but this goal is complicated by evidence for oxidizing Martian soils (Oyama & Berdahl, 1977 ; Zent & McKay, 1994 ). Various oxidizing species have been proposed to explain the high soil reactivity, including superoxide ions (Yen et al., 2000 ), hydroxyl and oxygen radicals (Benner et al., 2000 ), and perchlorate intermediates (Carrier & Kounaves, 2015 ; Quinn et al., 2013 ). These reactive species likely form by interactions with ionizing radiation. The lack of a magnetic field and thin atmosphere exposes the Martian surface to high doses of ionizing electromagnetic (e.g., ultraviolet [UV] and γ) and particle (i.e., proton, neutron, and higher mass atoms) radiation that can oxidize organic matter directly or through reactions with secondary oxidants (Hassler et al., 2014 ; Patel et al., 2002 ).

The preservation and detection of organic material on Mars is critical to our understanding of Mars' ability to host life throughout time. Chemical signatures encoded in organic material provide information on ancient ecologies if life existed in the past and, in the absence of life, provide clues about the environmental and thermal history of Mars.

1.2 Alteration of Organic Material by Ionizing Radiation

Unlike Earth, Mars lacks a magnetic field and is therefore bombarded by ionizing radiation that can alter organics. Previous work has employed a range of UV, γ rays, and X‐rays to study the effect of ionizing radiation on organic material. Of these, the majority of studies concerning organic matter preserved on Mars have focused on the effect of UV radiation. Studies on the survivability of amino acids (Poch et al., 2013; Stalport et al., 2009; ten Kate et al., 2005), carboxylic acids (Stalport et al., 2009), PAHs, and urea (Poch et al., 2014) exposed to varying doses of UV radiation revealed that long‐term exposure to UV radiation (<10 years on Martian surface) completely degraded organic material. Stalport et al. (2009) also found that exposure to UV led to the formation of metastable compounds that survived on longer time scales than the initial compounds.

Stalport and colleagues' work supports an idea originally introduced by Benner et al. (2000) to explain the lack of organics detected by the Viking missions. Benner et al. predicted that UV‐driven, Fenton reactions of meteorite‐delivered organics would produce organic acids as a metastable intermediate. Although traditionally an aqueous reaction, Fenton reactions degrade organic material by producing OH radicals from H 2 O 2 in the presence of an iron catalyst. Hydroxyl radicals then react with organic material, likely via H abstraction, to form new degradation products (Pignatello et al., 2006). The predicted organic acids are largely nonvolatile and were likely invisible to the Viking gas chromatograph‐mass spectrometer. Laboratory studies have shown that the rate of formation of organic acids outpaces their destruction by a factor of 103, suggesting these molecules can persist on long time scales (Benner et al., 2000; Lamrini et al., 1998). Based on an influx of 2.4 × 108 g/year of reduced carbon and 1‐m mixing depth, Benner et al. predicted organic acid concentrations of ≈500 ppm. While Benner et al.'s estimate did not account for processes that remove organic acids, to fall below Viking detection limits (approximately tens of ppb) over 99% of organic acids would have to be destroyed. Benner's hypothesis was further supported by more recent work that suggests solid oxalic acid is stable on the surface of Mars and may be present in reinterpreted Viking, Phoenix, and MSL data (Applin et al., 2015).

Benner et al. invokes Fenton chemistry that requires exposure to UV radiation (3–100 eV), which can only penetrate the first several millimeters of Martian soil (Cockell & Raven, 2004). In contrast to UV, other forms of ionizing radiation, such as galactic cosmic and solar rays, are predicted to penetrate up to 2 m of Martian soil (Hassler et al., 2014; Pavlov et al., 2012). Understanding how organic matter in rocks, regolith, and aeolian sediments is altered by ionizing radiation below the Martian surface (2–200 cm deep) is crucial to interpreting data from current and future missions. In situ samples analyzed by the MSL and Viking missions were collected at depths of 5–10 cm, and the ExoMars rover will have the ability to drill up to 2 m (Grotzinger et al., 2012; Oyama & Berdahl, 1977; Vago et al., 2006).

At depths relevant to MSL (~5 cm), contributions from solar rays are minimal (Hassler et al., 2014; Pavlov et al., 2012). Galactic cosmic rays are primarily composed of high‐energy protons (87%) that span energies from 10 to 1,000 MeV (Benton & Benton, 2001). Their high energy allows them to cleave bonds indiscriminately within the Martian soil profile creating a highly reactive environment. Studies investigating the effects of γ radiation on macromolecular organic material have found destruction of organic material via cross‐linking, oxidation, and bond scission accompanied by increases in aromaticity of remaining organic material (Brown & Weiss, 2003; Court et al., 2006; Schäfer et al., 2009). Similar results were reported for nucleobases that were destroyed after exposure to γ radiation (Ertem et al., 2017). From these results, it is unclear if metastable species can form or persist below the Martian surface (2–200 cm) or if organic material would be completely destroyed.

Here, we report metastable products formed from radiolysis of macromolecular organic species exposed to high‐energy protons (200 MeV). We irradiated mixtures of recalcitrant organic material and Mars‐relevant minerals for cumulative doses up to 500 kGy, representing ~6.6 million years at the surface of Mars. The focus of this study was to determine if organic acids are produced due to interactions with high‐energy protons. Using these data, we aim to determine the likely metastable products below the Martian surface and constrain how long these products persist in different mineral matrices.