The Oxygen Release Instrument: Space Mission Reactive Oxygen Species Measurements for Habitability Characterization, Biosignature Preservation Potential Assessment, and Evaluation of Human Health Hazards

We describe the design of an instrument, the OxR (for Oxygen Release), for the enzymatically specific and non-enzymatic detection and quantification of the reactive oxidant species (ROS), superoxide radicals (O2•−), and peroxides (O22−, e.g., H2O2) on the surface of Mars and Moon. The OxR instrument is designed to characterize planetary habitability, evaluate human health hazards, and identify sites with high biosignature preservation potential. The instrument can also be used for missions to the icy satellites of Saturn’s Titan and Enceladus, and Jupiter’s Europa. The principle of the OxR instrument is based on the conversion of (i) O2•− to O2 via its enzymatic dismutation (which also releases H2O2), and of (ii) H2O2 (free or released by the hydrolysis of peroxides and by the dismutation of O2•−) to O2 via enzymatic decomposition. At stages i and ii, released O2 is quantitatively detected by an O2 sensor and stoichiometrically converted to moles of O2•− and H2O2. A non-enzymatic alternative approach is also designed. These methods serve as the design basis for the construction of a new small-footprint instrument for specific oxidant detection. The minimum detection limit of the OxR instrument for O2•− and O22− in Mars, Lunar, and Titan regolith, and in Europa and Enceladus ice is projected to be 10 ppb. The methodology of the OxR instrument can be rapidly advanced to flight readiness by leveraging the Phoenix Wet Chemical Laboratory, or microfluidic sample processing technologies.


Introduction
On Earth, the production of reactive oxygen species (ROS) in soils is typically associated with the relatively high abundance of O 2 (g) in the atmosphere [1]. In other solar system environments, or space environments beyond our solar system, where O 2 (g) exists only in trace amounts (e.g., Mars [2], the Earth's Moon [3,4], Europa [5], Saturn's rings [6], interstellar clouds [7]) the production and accumulation of ROS is not precluded.
Even in planetary environments lacking O 2 (g), ROS can be produced by many well-known natural processes, for example, environments containing H 2 O, CO, and/or CO 2 [8]. On the Moon (and presumably on Mars), ROS can be generated by the interaction of H 2 O ice with cosmic rays [9]. Experiments indicate that Lunar (and presumably Martian) dust can generate hydroxyl free radicals ( • OH) via the Fenton reaction as demonstrated with Lunar simulants [10] and Fe-rich silicate The justification for this type of instrument is supported by the results of the Viking Mars mission. In 1976, the Viking Lander performed biological experiments designed to detect extant life in Martian regolith. The reactivity of the Martian regolith was first indicated by the release of O 2 in the Gas Exchange Experiment (GEX), and the decomposition of organics, contained a culture media, in the Labeled Release (LR) experiment [23][24][25]. In the GEX, up to~770 nmoles O 2 (g) was produced from regolith samples (1 cm -3 ) upon humidification or wetting. The persistence of O 2 (g) release from samples that were heated to 145 • C for 3 h and then cooled prior to wetting or humidification, ruled out a biological explanation of the GEX results [23,26]. In the Viking LR, up to~30 nmoles 14 C labeled gas, presumed to be CO 2 , was released after regolith samples (0.5 cm -3 ) were wetted with an aqueous solution containing 14 C-labeled organics [27,28]. The release of 14 C-labeled gas in the LR was eliminated by heating the sample to 160 • C for 3 h and then cooling prior to the addition of the labeled aqueous organics. These results lead to the conclusion that the Martian surface material contains more than one type of reactive oxidants [23]. Metal salts of O 2 •− were among the earliest proposed explanations for the thermally stable agent responsible for O 2 (g) release in the GEX. In the case of the LR, peroxide was among the earliest explanations proposed for the thermally liable agent responsible for 14 CO 2 release. In addition to the possible presence of metal salts of O 2 •− , it has been proposed that O 2 •− is generated on Martian dust and regolith surfaces by a UV-induced mechanism [29]. Such a mechanism for O 2

•−
photo-generation has also been shown with Mars analog Mojave and Atacama regolith [1]. More recently, high levels of regolith perchlorate (ClO 4 − ) were directly detected at the Phoenix landing site [30]. Following up on this result, the presence of ClO 4 − at the Viking landing sites was inferred [31], and its presence at the Martian equator verified by the Sample Analysis at Mars (SAM) instrument on Mars Science Laboratory (MSL)-based on thermal analyses [32]. While the stability of ClO 4 − under the conditions of the GEX and LR preclude it as a direct explanation for these experiments, it has been suggested that ClO 4 − radiolysis products reproduce the major aspects of both experiments [25]. The form of the trapped O 2 , in particular, derived from ClO 4 − radiolysis, was not identified, and it has been suggested that some fraction may exist as superoxide radical or peroxide [25,33]. This suggestion was confirmed by the finding that both of these oxidants are generated-together with • OH-by γ-ray exposure of ClO 4 − (mixed in Mars salt analogs) upon water wetting [34]. Preceding this finding, other possible oxidants present on the surface of Mars have been reviewed in detail [35]. In the context of instrument development for in situ analysis, it is useful to note that the expected concentration of oxidants, as inferred from the Viking Biology Experiments, is at the parts per million level ( Table 1 in [36]). Given the poorly understood nature and distributions of oxidants in Martian and Lunar regolith, there is a need for the development of flight instruments for their specific identification and quantification. The only flight instrument previously built for the quantitative, although non-specific, in situ determination of oxidants was the Mars Oxidant Experiment (MOX) instrument [36] as the United States contribution to the failed Soviet Union's Mars '96 mission. That instrument would have exposed materials-sensors to the Martian regolith and monitored their reaction with oxidants over time. Materials included various metallic (e.g., Al, Ag, Pb, Au) and organic layers (e.g., L-and D-cysteine).
Metal Peroxides/Hydroperoxides Release H 2 O 2 by the Following Reactions [37] Me-salts of O 2

Principle of Operation of the OxR (for Oxygen Release) Instrument
The OxR instrument for the detection and quantification of O 2 •− and O 2 2− addresses priorities for human exploration of Mars and the Moon as highlighted in the NASA plan to "Explore Moon to Mars" which will use the Moon as "a testbed for Mars [ . . . ] and beyond." [45]. Our approach is to quantitatively convert peroxides and superoxide radicals into O 2 (g), which can then be detected easily, precisely, and with very high sensitivity. The principle of the OxR instrument is based on the enzymatic conversion of the dismutation and hydrolysis products of superoxide radicals (O 2 •− adsorbed on released by the hydrolysis of metal peroxides) to O 2 , followed by quantitative detection using an O 2 electrode. The OxR instrument design includes a sealable, temperature-and pressure-controlled sample chamber. The chamber is equipped with an O 2 -sensor, and inlets for the sequential dispensing of three reagents, after each of which the concentration of released O 2 is measured. The two enzymatic reagents (Cu/Zn-superoxide dismutase, SOD, and catalase, CAT) used are stored in a solid form separated from their aqueous solvents (to withstand cosmic radiation exposure). The third reagent, acetonitrile (ACN), is separately stored at any temperature above its melting point (−46 • C). The enzymic reagents are mixed with their solvents right before use either by storing them in separate reagent crucibles (analogous to those used in the Wet Chemistry Laboratory of the 2007 Phoenix Mars Scout Lander mission), or in (commercially available) dual-chamber pre-fillable syringes (one chamber for storing the enzyme reagent in solid form, and the other for its solvent, to be mixed upon piston movement), and their sequential dispensing in the chamber.  [46,47], and for the Moon [48]).

Enzyme-Based ROS Specificity of the OxR Instrument
We  Table 2.
•− ); additional details for their hydrolysis/dismutation are presented in reactions 1-3, and in Figure 1 step a.
Release of O 2 via CAT-catalyzed decomposition of H 2 O 2 [44], resulting from I, step 2, and/or II, step 2: , which is equal to the second dismutation reaction product H2O2 (= ½ xH2O2). In a subsequent step b, the addition of catalase (CAT) causes the additional release of O2 (via the CAT-catalyzed decomposition of H2O2, the second product of O2 •− dismutation), which is also recorded (as reading Adism/CAT = ¼ xO2 plus the already released ½ xO2; see Treatment B in text). If there are also metal O2 2− or free H2O2 present in regolith (represented as yH2O2 moles), these are simulated in Figure 1 by the addition of 40 nmoles H2O2 (in step c). These peroxides will also be decomposed by the CAT (added in step b) to ½ yO2, and in this case, the total released O2 will be recorded in step c as reading Adism/CAT (the sum ½ xO2 + ¼ xO2 + ½ yO2).

OxR Assay Simulation Verification on Mars-Analog Regolith
The OxR assay was performed using a semi-sealed liquid-phase O2 electrode with known concentrations of O2 •− and H2O2, and in the presence/absence of Mars-like regolith from the Mojave (CIMA volcanic field) and the Atacama deserts. The assay was further validated on commercial sources of metal salts of O2 •− (KO2) and O2 2− (Na2O2, CaO2, MgO2) in the presence of CO 2− and ClO4 − these are simulated in Figure 1 by the addition of 40 nmoles H 2 O 2 (in step c). These peroxides will also be decomposed by the CAT (added in step b) to 1 ⁄2yO 2 , and in this case, the total released O 2 will be recorded in step c as reading A dism/CAT (the sum 1 ⁄2xO 2 + 1 4 xO 2 + 1 ⁄2yO 2 ).

OxR Assay Simulation Verification on Mars-Analog Regolith
The OxR assay was performed using a semi-sealed liquid-phase O 2 [49]. To validate the OxR assay enzymatic reactions 1 and 7 (in Table 2) in the Clark-type O 2 electrode chamber, the following treatments were performed (data are shown in Figure 1 Table 2): The molar concentrations of xO 2 •− and yH 2 O 2 are then estimated using the following mathematical equations, derived by appropriately combining the molar equations A dism and A dism/CAT : The released O 2 concentrations (A dism and A dism/CAT ) during the OxR assay (shown in Figure 1) matched the concentrations predicted by the stoichiometry of each of the assay reactions 1 and 7. Indeed, when the values A dism (corresponding to 24 nmoles from step a) and A dism/CAT (corresponding to Simulation of cosmic radiation effect on the OxR assay enzymic reagents: Another consideration for the OxR instrument is whether its enzymatic reagents SOD and CAT would be functional upon exposure to cosmic radiation levels expected during missions to Mars, the Moon, and possibly icy satellites Jupiter and Saturn. To address this question for Mars and Moon, cosmic radiation simulation experiments were performed [49], where solid SOD and CAT were exposed to γ-radiation at a dose range comparable to that which would be received during a space mission. Activities were also determined for these enzymes in various concentrations (% v/v) of ACN since 100% ACN is used to wet the regolith sample to quantitatively purge out any unknown source trapped O 2 . The SOD retained functional activity after exposure to a γ-radiation dose of 6 Gy (an equivalent to the cosmic radiation dose received from 38 round trips to Mars [53]). The CAT specific activity was unaffected up to~3 Gy (equivalent to 19 round trips to Mars, and many more trips to the Moon) after which it decreased linearly to 40% (of its unexposed activity) at 6 Gy. SOD activity was unaffected in up to 50% ACN, while CAT activity decreased in a manner that matched the ACN concentration. For example, an initial CAT specific activity of~3 U µg −1 at 0% ACN decreased by 50-fold at the maximum tested 50% ACN. This result indicates that for an OxR assay based on 3 U µg −1 CAT at 0% ACN in laboratory testing, the CAT concentration should be increased 50-fold (i.e., 150 U µg −1 ) plus a margin for flight, if a 50% ACN concentration is optimum for instrument implementation.

The Potential of the OxR Assay for a Field-Deployable Instrument
The OxR assay can be extended to the search of possible metal supero/peroxidant cycles in terrestrial and extraterrestrial ecosystems. We expect that the full instrument can be packaged in 1 U (i.e., CubeSat sized at 10 cm/side) using a reaction chamber scheme with an  The OxR assay can be extended to the search of possible metal supero/peroxidant cycles in terrestrial and extraterrestrial ecosystems. We expect that the full instrument can be packaged in 1 U (i.e., CubeSat sized at 10 cm/side) using a reaction chamber scheme with an O2-sensor (to monitor the enzymatic release of O2 from O2 •− and O2 2− in a regolith sample during interaction (under constant mixing) with SOD and CAT, as illustrated and described in Figure 2.

Figure 2.
Diagrammatic principle of an OxR assay-based field instrument for the identification/quantification of regolith superoxide radicals and peroxides (shown as xO2 •− and yH2O2 moles, respectively): Regolith sample is subjected to the following released O2 recording procedures. In step 1, the regolith is wetted with anhydrous ACN to flush out loosely bound O2 (designated zO2 moles; enclosed in dotted squares) for (i) canceling out any background O2 and (ii) measuring it as coming from unidentified sources. In step 2, SOD is administered in the instrument chamber dissolved in K-phosphate-DTPA buffer (pH = 7.2) at an equal (at least) to ACN volume (resulting in at least 50% ACN), and the released O2 (enclosed in solid-line square, which results from the group of metal O2 •− via SOD-catalyzed dismutation of their hydrolysis product O2 •− , together with H2O2) is recorded by the chamber O2 sensor as reading Adism. In a subsequent third step, K-phosphate-DTPA-

Non-enzymatic OxR instrument version:
We have also developed a non-enzymatic OxR assay for cases where enzymatic stability may be insufficient (e.g., missions to Titan, Europa, and Enceladus) or when the required long-term −20 • C SOD and CAT storage is not possible. Moreover, some future rovers may long outlast their expected life times (as past ones have done), and for whichever rover carries an OxR instrument the reagent enzymes may degrade over the years, whereas the inorganics may be more durable. A non-enzymatic version of the OxR instrument is based on the following reagents, which we have preliminarily tested successfully (data not shown): In place of SOD, CuSO 4 , MnCl 2 , and MnSO 4 can be used: 1. CuSO 4 (at 0.1 to 300 µM) and MnCl 2 (at 0.1 to 100 µM)) [58,59]; MnCl 2 dismutates O 2 •− as effectively as SOD does [59].
Concluding, the principle of the OxR assay can be used for the development of an instrument for the detection of planetary and terrestrial O 2 •− and O 2 2− with the following considerations: 1. OxR assay enzymes SOD and CAT are used in excess; they are sufficient when used even in the amount of a few activity units.
2. SOD and CAT are stored (below −20 • C for long-term storage) separate from their aqueous solvents, and are mixed right before administration. This can be accomplished by storing them, for example, in two separate reagent crucibles (analogous to those used in the WCL instrument of the 2007 Phoenix Mars Scout Lander mission [63]), or in (commercially available) dual-chamber pre-fillable syringes (one chamber for storing the enzyme and one for its solvent, to be mixed upon piston movement), followed by their sequential dispensing in the instrument's regolith chamber (under continuous mixing of its reagents).
3. The instrument can use solid state electrochemical or optical sensing O 2 -electrodes of high sensitivity. There are commercially available O 2 probes (e.g., sensor type PSt6, by PreSens Precision Sensing GmbH, Regensburg, Germany) that are based on the luminescence quenching by O 2 , and are sensitive enough to measure O 2 at much lower concentrations (~1 nmole O 2 cm -3 regolith) than that (775 nmoles) detected by the GEX [26]. For example, the typical detection limit of the PreSens sensor PSt6 is 0.002% O 2 , with 1 ppb and 0.5 ppm for aqueous and gaseous O 2 , respectively. The PreSens Precision Sensing O 2 probes come either as needle-type optical fiber probes (with a tip size < 50 µm, protected, e.g., inside a stainless-steel needle), or as implantable probes (with a tip size of < 50 to 140 µm, while the outer diameter ranges from 140 µm to 900 µm). Therefore, O 2 sensing by the OxR instrument with solid-state sensors can be done in both gaseous and liquid phase. Regarding released O 2 partition between liquid and headspace in the sample chamber, underestimation of the released reactive O 2 due to such exsolution can be addressed by either adding an extra gas phase O 2 sensor, or by the calculation of the partition between liquid and gas phase at the set pressure and temperature.
4. Respective ACN and SOD reagent process steps 1 and 2 are omitted in testing water samples (e.g., from Enceladus and Europa) by the OxR instrument, because O 2 •− dismutates to H 2 O 2 and O 2 under aqueous conditions (see reaction 1, Table 1). In such an application, the first step of the OxR instrument will record O 2 of unknown origin (for instrument calibration) in a melted ice sample. Following this step, CAT will be administered to convert to O 2 any present H 2 O 2 . This will be the only peroxidant specifically determined by the OxR instrument in the (melted) ice samples from the surface or plums of Enceladus and Europa.

Implementation of the OxR Instrument
One approach for implementing the OxR assay for field instrument construction is to keep it compatible with the Wet Chemistry Laboratory (WCL) that flew as part of the Phoenix lander mission to Mars [30]. The WCL (Figure 3) consists of a lower beaker containing sensors designed to analyze the chemical properties of the regolith and an upper actuator assembly for adding regolith, water, reagents, and stirring [63]. The WCL sensor set included an O 2 electrode, pressure sensor, and thermocouple needed for the OxR assay. A key part of the WCL system is the storage of liquid and dry reagents. Our prototype design uses a reagent dispenser assembly similar to WCL (which uses five crucibles to store the reagents to be dispensed). We will use the following three crucibles: A crucible for dispensing into the beaker the anhydrous ACN to wash out from regolith any background O 2 (for recording its level).
A crucible divided into two compartments to store the SOD enzyme and its solvent (for recording O 2 released from the dismutation of regolith superoxide radicals; H 2 O 2 will also be released by this dismutation) separately.
A crucible divided into two compartments to store the CAT enzyme and its solvent (for recording O 2 released from superoxide radical-derived H 2 O 2 , and that derived from regolith peroxides) separately. The automated and sequential dispensing of the reagents is critical to the success of the prototype. A diagram of the Phoenix system is shown in Figure 4. The reagent dispenser assembly will be coupled to the construction and operational testing of the beaker (reaction cell). A diagram of the WCL reaction cell is shown in Figure 5. In contrast to the complex array of sensors in the WCL on Phoenix [63], we have only O2 sensors, temperature, and pressure. Joining the reagent dispenser assembly and the reaction cell completes the prototype. However, the WCL instrument uses a 25-cc chamber to analyze 1 cc of regolith. For some missions, this is an important issue and motivates microfluidics approaches. An alternative instrument construction approach will be based on the microfluidic transport/delivery technology [64][65][66][67][68][69], already developed by the R.A. Mathies's Space Sciences Laboratory at Berkeley University. The chip for microscopic fluid transport between the components of an instrument such as OxR (e.g., reagent storage capsules, regolith sample chamber with O2/temp/pressure sensors, and waste reservoirs), is analogous to digital electronic processors, and all that is needed is a change in the order of operations conducted by the device [66]. All macroscopic reagent volumes are contained within stainless steel bellows expanded or contracted by externally applied N2 gas. The OxR instrument chip can be constructed as a scaled-down version (e.g., a 200 gr, 2 Watts, 10x10x10 cm package) of the Enceladus Organic Analyzer (EOA) chip ( Figure  6).
The OxR instrument prototype will be tested in a laboratory setting and results compared to standard laboratory procedures, followed by field testing in the Mojave Desert. This has been a continuing test site for our studies [1], and, thus, we have a deep knowledge base of the site and the expected results providing a convenient basis for prototype testing. The automated and sequential dispensing of the reagents is critical to the success of the prototype. A diagram of the Phoenix system is shown in Figure 4. The reagent dispenser assembly will be coupled to the construction and operational testing of the beaker (reaction cell). A diagram of the WCL reaction cell is shown in Figure 5. In contrast to the complex array of sensors in the WCL on Phoenix [63], we have only O 2 sensors, temperature, and pressure. Joining the reagent dispenser assembly and the reaction cell completes the prototype. However, the WCL instrument uses a 25-cc chamber to analyze 1 cc of regolith. For some missions, this is an important issue and motivates microfluidics approaches. An alternative instrument construction approach will be based on the microfluidic transport/delivery technology [64][65][66][67][68][69], already developed by the R.A. Mathies's Space Sciences Laboratory at Berkeley University. The chip for microscopic fluid transport between the components of an instrument such as OxR (e.g., reagent storage capsules, regolith sample chamber with O 2 /temp/pressure sensors, and waste reservoirs), is analogous to digital electronic processors, and all that is needed is a change in the order of operations conducted by the device [66]. All macroscopic reagent volumes are contained within stainless steel bellows expanded or contracted by externally applied N 2 gas. The OxR instrument chip can be constructed as a scaled-down version (e.g., a 200 gr, 2 Watts, 10x10x10 cm package) of the Enceladus Organic Analyzer (EOA) chip ( Figure 6).
The OxR instrument prototype will be tested in a laboratory setting and results compared to standard laboratory procedures, followed by field testing in the Mojave Desert. This has been a continuing test site for our studies [1], and, thus, we have a deep knowledge base of the site and the expected results providing a convenient basis for prototype testing.
The OxR instrument prototype will be tested in a laboratory setting and results compared to standard laboratory procedures, followed by field testing in the Mojave Desert. This has been a continuing test site for our studies [1], and, thus, we have a deep knowledge base of the site and the expected results providing a convenient basis for prototype testing.  [63]. Figure 4. Diagram of the reagent dispenser assembly with crucibles ready for deployment [63].

Studies with the OxR Instrument
The OxR instrument can have the following potential applications: Identification of the ROS O2 •− and O2 2− , on the Moon and Mars, with extension to future missions to Jupiter's satellite Europa and Saturn's Enceladus and Titan.
Monitor the levels of ROS for astronaut health and safety, given that O2 •− can become biotoxic (via conversion of Fe 3+ /Cu 2+ to Fe 2+ /Cu + , which, via the Fenton-reaction with the other ROS O2 •− , will generate the highly biotoxic free radical • OH [44]). Moreover, measuring dust/silica-induced ROS generation is crucial for the evaluation of possible health hazards [18] in future manned missions to Mars and Moon.
Identify mineral deposits rich in ROS to be used as an O2g source for human consumption. O2g can be easily produced on a large scale due to the following ROS reactions: O2 •− is converted to O2g by mixing with (i) H2O (also releasing H2O2), or (ii) Fe 3+ or Cu 2+ [44]. O2g can be produced from O2 2− (e.g., H2O2 also released from reaction (i)) by mixing with MnO2 [56] or silver, platinum, lead, ruthenate, or RuO2 [57].
Identify O2 •− /O2 2− on the metal parts of manned space vehicles/stations. ROS can be generated by a combination of O2g (in vehicle) with cosmic radiation [1]. O2 •− and O2 2− have implications for exploration because they can: (i) cause corrosive oxidative deterioration of space vehicles/stations, (ii) pose a serious risk for oxidative modification of stored foods, making them unsafe for astronauts, (iii) compromise astronaut health due to their well-known biotoxic effects [44].

Studies with the OxR Instrument
The OxR instrument can have the following potential applications: Identification of the ROS O2 •− and O2 2− , on the Moon and Mars, with extension to future missions to Jupiter's satellite Europa and Saturn's Enceladus and Titan.
Monitor the levels of ROS for astronaut health and safety, given that O2 •− can become biotoxic (via conversion of Fe 3+ /Cu 2+ to Fe 2+ /Cu + , which, via the Fenton-reaction with the other ROS O2 •− , will generate the highly biotoxic free radical • OH [44]). Moreover, measuring dust/silica-induced ROS generation is crucial for the evaluation of possible health hazards [18] in future manned missions to Mars and Moon.
Identify mineral deposits rich in ROS to be used as an O2g source for human consumption. O2g can be easily produced on a large scale due to the following ROS reactions: O2 •− is converted to O2g by mixing with (i) H2O (also releasing H2O2), or (ii) Fe 3+ or Cu 2+ [44]. O2g can be produced from O2 2− (e.g., H2O2 also released from reaction (i)) by mixing with MnO2 [56] or silver, platinum, lead, ruthenate, or RuO2 [57].

Studies with the OxR Instrument
The OxR instrument can have the following potential applications: (i) cause corrosive oxidative deterioration of space vehicles/stations, (ii) pose a serious risk for oxidative modification of stored foods, making them unsafe for astronauts, (iii) compromise astronaut health due to their well-known biotoxic effects [44]. Identify locations on Mars and the Moon with low ROS levels which may be indicative of the high potential for biosignature (e.g., [70,71]) preservation. Of particular upcoming interest is the Dragonfly mission to Titan by NASA (launched in 2026, and landing in 2034), which will search for evidence of prebiotic chemical processes on the surface of Titan [72].
The instrument is also applicable to terrestrial research, with indicative studies being: (i) O 2 •− /O 2 − association to microorganisms' oxidative stress in extreme desert environments, with extension to life's origin [1,73]; (ii) health hazard implications from measuring ROS-reactivity of (a) volcanic ash (due to • OH generation) [74], and (b) pyrites (from O 2 /H 2 O 2 /surface-bound ferric iron-induced • OH generation during pyrite oxidation) in coal mining regions [75].

Conclusions
We have developed a sensitive assay for the use in a future Oxygen Release (OxR) instrument for the detection of ROS, with potential applications to the Mars, Moon, Europa, Titan, and Enceladus missions. The instrument can support the exploration of the Moon (including monitoring of astronaut health hazards), exploration on Mars, Moon, and Titan, and terrestrial studies. The OxR instrument is based on the selective and specific enzymatic decomposition of supero/peroxidants to O 2