Superoxide will decompose to hydrogen peroxide that reacts with water to produce oxygen gas (O₂) and carbon dioxide (CO₂)

The result of formate oxidation, as well as production of O₂ and CO₂ with addition of water in the Viking Lander experiments opened the possibility of metabolism and subsequent biology on Mars. Here a model for abiotic reactions by superoxide-derived hydrogen peroxide to explain the results of the Viking Labeled Release (LR) and Gas Exchange (GEx) is proposed.

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Title

Preliminary findings of the Viking gas exchange experiment and a model for Martian surface chemistry

Authors

Oyama, VI and Berdahl, BJ and Carle, GC

Abstract

Oxygen and CO2 were evolved from humidified Martian soil in the gas exchange experiment on Viking Lander 1. Small changes in N2 gas were recorded. A model of the morphology and a hypothesis of the mechanistics of the Martian surface are proposed.

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Radiation-damaged perchlorate salts (ClO4-) release oxygen gas, and the product, hypochlorite (ClO^-), reacts with amino acids.

Here, two major results from the Viking metabolism experiments were shown to have abiotic explanations: the release of O₂ gas during the Viking Gas exchange experiment (GEx), and the production of oxidized ¹⁴C label gas from a labeled aqueous mixture of organics (including amino acids) during the Viking Labeled Release experiment (LR). Hydrogen peroxide and/or superoxide was previously proposed to explain these results, but these oxides do not explain thoroughly the mixed results of the Viking metabolism data. The discovery of perchlorate that undergoes heavy gamma-ray bombardment on Mars led these researchers to look into radiolysis perchlorate products and found that hypochlorite (ClO^-), chlorine dioxide (ClO₂), and oxygen gas will form. Perchlorate is typically highly stable, by ionizing radiation causes the production or much more reactive compounds. The ClO^-, once formed, is much more reactive and will indeed react with the organic alanine under heated conditions identical to the LR experiment and form N- chloroalanine that will decompose through a first order reaction mechanism. The oxygen gas (O₂) formed as a result of perchlorate (ClO₄^-) decomposition to O₂ remains trapped within the salt, but with water additions and heat identical to the GEx experiments, is released at 0.83 % per mole perchlorate. These are levels very similar to what  was  observed  in  the  initial  Viking experiments. 

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Title

Perchlorate radiolysis on Mars and the origin of martian soil reactivity

Authors

Quinn, Richard C and Martucci, Hana FH and Miller, Stephanie R and Bryson, Charles E and Grunthaner, Frank J and Grunthaner, Paula J

Abstract

Results from the Viking biology experiments indicate the presence of reactive oxidants in martian soils that have previously been attributed to peroxide and superoxide. Instruments on the Mars Phoenix Lander and the Mars Science Laboratory detected perchlorate in martian soil, which is nonreactive under the conditions of the Viking biology experiments. We show that calcium perchlorate exposed to gamma rays decomposes in a CO2 atmosphere to form hypochlorite (ClO(-)), trapped oxygen (O2), and chlorine dioxide (ClO2). Our results show that the release of trapped O2 (g) from radiation-damaged perchlorate salts and the reaction of ClO(-) with amino acids that were added to the martian soils can explain the results of the Viking biology experiments. We conclude that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments.

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Active decomposition of organic species occurs in Mars analog soils by nonbiological processes.

The driest place on Earth is the Yungay area, a part of the Atacama Desert in Peru, where DNA reaches levels below the limit of detection and extremely trace organic material is found. This site was used as an analog for Mars to reproduce incubation experiments done by the Viking Lander that indicated oxidation of formate at the rates expected for biological metabolism. Here, biological (L- alanine and D-glucose) and nonbiological (D- alanine and L-glucose) isomers were used to measure rates of oxidation of formate in this analog site to better understand the results of the Viking experiments. No DNA was recovered from the Yungay area, indicating that the dry limit of microbial survival was passed. However, some formate oxidation was observed in the Yungay soil samples and there were equal rates of use of biological and nonbiological isomers that was 3x higher than observed in blank controls. In areas of the Atacama with DNA recovery, formate oxidation increased, as expected, and the biases towards use of biological isomers also increased as expected. Control experiments indicated the possibility of hydrogen peroxide or sodium peroxide being the chemical oxidant for formate.

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Title

Mars-like soils in the Atacama Desert, Chile, and the dry limit of microbial life

Authors

Navarro-Gonz{\'a}lez, Rafael and Rainey, Fred A and Molina, Paola and Bagaley, Danielle R and Hollen, Becky J and de la Rosa, Jos{\'e} and Small, Alanna M and Quinn, Richard C and Grunthaner, Frank J and C{\'a}ceres, Luis and others

Abstract

The Viking missions showed the martian soil to be lifeless and depleted in organic material and indicated the presence of one or more reactive oxidants. Here we report the presence of Mars-like soils in the extreme arid region of the Atacama Desert. Samples from this region had organic species only at trace levels and extremely low levels of culturable bacteria. Two samples from the extreme arid region were tested for DNA and none was recovered. Incubation experiments, patterned after the Viking labeled-release experiment but with separate biological and nonbiological isomers, show active decomposition of organic species in these soils by nonbiological processes.

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Perchlorate can be activated with galactic cosmic rays that bombard Mars to form chlorate anions that are significantly more reactive with carbon compounds

A laboratory study investigated the reactivity of perchlorate using magnesium perchlorate hexahydrate (Mg(ClO4)2·6H2O) in a contamination-free ultra-high-vacuum (UHV) stainless steel chamber under Viking-lander Mars site temperatures and pressures. Here, the authors were able to find that with energetic electrons identical in radiation and damaging power to  galactic cosmic ray electrons were able to cause destruction of the perchlorate anion to form chlorate and atomic oxygen. Further, the formed chlorate can then react with multiple carbon compounds under temperatures found on Mars and under the conditions used in the Viking lander labeled- release experiments.

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Title

Mechanistic studies on the radiolytic decomposition of perchlorates on the Martian surface

Authors

Turner, Andrew M and Abplanalp, Matthew J and Kaiser, Ralf I

Abstract

Perchlorates—inorganic compounds carrying the perchlorate ion (ClO4-)—were discovered at the north polar landing site of the Phoenix spacecraft and at the southern equatorial landing site of the Curiosity Rover within the Martian soil at levels of 0.4–0.6 wt%. This study explores in laboratory experiments the temperature-dependent decomposition mechanisms of hydrated perchlorates—namely magnesium perchlorate hexahydrate (Mg(ClO4)2·6H2O)—and provides yields of the oxygen-bearing species formed in these processes at Mars-relevant surface temperatures from 165 to 310 K in the presence of galactic cosmic-ray particles (GCRs). Our experiments reveal that the response of the perchlorates to the energetic electrons is dictated by the destruction of the perchlorate ion (ClO4-) and the inherent formation of chlorates (ClO3-) plus atomic oxygen (O). Isotopic substitution experiments reveal that the oxygen is released solely from the perchlorate ion and not from the water of hydration (H2O). As the mass spectrometer detects only molecular oxygen (O2) and no atomic oxygen (O), atomic oxygen recombines to molecular oxygen within the perchlorates, with the overall yield of molecular oxygen increasing as the temperature drops from 260 to 160 K. Absolute destruction rates and formation yields of oxygen are provided for the planetary modeling community.

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Martian perchlorate or other oxychlorines are present at the original Viking landing sites and react abiotically with potentially Martian organic carbon to form chlorobenzene.

Initial pyrolysis-gas chromatograph mass spectrometer (GCMS) results from the 1979 Viking lander experiments found no organic compounds on Mars other than chlorobenzene. The observed chlorinated compound was considered contamination from terrestrial dichloromethane, which was used as part of a cleaning solution for the GCMS ovens. However, the more recent discovery of abundant Martian perchlorate has prompted further investigation of the source of the chlorinated compound detected during the original Viking GCMS experiments. Researchers looked to see if chlorobenzene could form from pyrolysis with perchlorate, as well as explore the possibility of the reaction occurring with Martian organic carbon. They found that the temperature at which chlorobenzene release occurred was much higher than a terrestrial source of chlorinated carbon compound such as  dichloromethane. Instead, the temperature 

range of chlorobenzene release overlapped with the temperature range where decomposition of oxychlorine compounds occurs. This suggests that the chlorobenzene signal measured in the original Viking experiments originated from Martian chlorine sources. Additionally, this data suggest that due to the temperature range where the chlorobenzene signal was observed, there is potential for a Martian carbon source reacting with Martian oxychlorine species during pyrolysis.

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Title

Identification of Chlorobenzene in the Viking Gas Chromatograph-Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars

Authors

Melissa Guzman,Christopher P. McKay,Richard C. Quinn,Cyril Szopa,Alfonso F. Davila,Rafael Navarro-González,Caroline Freissinet

Abstract

Motivated by the recent detection of chlorobenzene by the Sample Analysis at Mars instrument suite on the Curiosity rover, and the identification of its carbon source as indigenous to the martian sample, we reexamined the original, microfilm preserved, Viking gas chromatograph-mass spectrometer data sets. We found evidence for the presence of chlorobenzene in Viking Lander 2 (VL-2) data at levels corresponding to 0.08–1.0 ppb (relative to sample mass), in runs when the sample was heated to 350°C and 500°C. Additionally, we found a correlation between the temperature dependence of the chlorobenzene signal and the dichloromethane signal originally identified by the Viking gas chromatograph-mass spectrometer team. We considered possible sources of carbon that may have produced the chlorobenzene signal, by reaction with perchlorate during pyrolysis, including organic carbon indigenous to the martian parent sample and instrument contamination. We conclude that the chlorobenzene signal measured by VL-2 originated from martian chlorine sources. We show how the carbon source could originate from the martian parent sample, though a carbon source contributed from instrument background cannot yet be ruled out.

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UV-activated perchlorate accelerates the oxidation of methane to produce carbon dioxide at a rate 5x higher than without perchlorate. 

Previous gas-phase chemistry calculations for the lifetime of methane (CH4) on Mars indicated a length of ~300 years. Yet, there is a distinct absence of detectable organic carbon on the Martian surface. Here, the authors experimented with magnesium perchlorate (Mg(ClO4)2•6H2O) surfaces exposed to UV light at wavelengths identical to the galactic cosmic rays that bombard the Martian surface. It was found that UV rays alone will accelerate the decomposition of methane (CH4), and cause formation of carbon dioxide (CO2), carbon monoxide (CO). In addition, UV rays will decompose perchlorate to chlorate (ClO3-) and volatile chlorine oxides. Lastly, the production rates for CO2 and CO on UV- decomposed perchlorate are accelerated by a factor of 2.5 and 4.5. 

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Title

Reaction of methane and UV-activated perchlorate: Relevance to heterogeneous loss of methane in the atmosphere of Mars

Authors

Zhang, Xu and Berkinsky, David and Markus, Charles R and Chitturi, Sathya R and Grieman, Fred J and Okumura, Mitchio and Luo, Yangcheng and Yung, Yuk L and Sander, Stanley P

Abstract

The absence of significant detectable signatures of organic molecules in the atmosphere and on the surface of Mars is a major unsolved puzzle. One possible explanation is that perchlorate-rich Martian soils, activated by solar ultraviolet (UV) radiation, create an environment favorable for the rapid oxidation of organics such as alkanes (including methane or CH4). In this paper, we measured product formation rates from the methane-perchlorate-UV system at room temperature. Our results show that magnesium perchlorate (Mg(ClO4)2•6H2O) surfaces exposed to UV light at wavelengths reaching the Mars' surface accelerate the decomposition of methane (CH4), resulting in the formation of carbon dioxide (CO2), carbon monoxide (CO), and volatile chlorine oxides. The production rates for CO2 and CO on UV-activated perchlorate surfaces are accelerated by a factor of 2.5 and 4.5, respectively, compared to those in the absence of perchlorate. In addition, with UV radiation exposure, perchlorate (ClO4−) decomposes to chlorate (ClO3−) and chlorine oxides. These results are incorporated into a simple box model to estimate the near-surface atmospheric methane lifetime. The model gives a lower bound of the lifetime on the order of hours to days, substantially shorter than ~300 yrs. calculated from methane loss by gas-phase chemistry alone.

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