Perchlorate induced low temperature carbonate decomposition in the Mars Phoenix Thermal and Evolved Gas Analyzer (TEGA)

Authors


Corresponding author: K. M. Cannon, Department of Geological Sciences, Brown University, 134 Brook St., Box 1846, Providence, RI 02912, USA. (kevin_cannon@brown.edu)

Abstract

[1] Simulated Thermal Evolved Gas Analyzer (TEGA) analyses have shown that a CO2 release detected between 400°C and 680°C by the Phoenix Lander's TEGA instrument may have been caused by a reaction between calcium carbonate and hydrated magnesium perchlorate. In our experiments a CO2release beginning at 385 ± 12°C was attributed to calcite reacting with water vapor and HCl gas from the dehydration and thermal decomposition of Mg-perchlorate. The release of CO2 is consistent with the TEGA detection of CO2 released between 400 and 680°C, with the amount of CO2increasing linearly with added perchlorate. X-ray diffraction (XRD) experiments confirmed CaCl2 formation from the reaction between calcite and HCl. These results have important implications for the Mars Science Laboratory (MSL) Curiosity rover. Heating soils may cause inorganic release of CO2; therefore, detection of organic fragments, not CO2 alone, should be used as definitive evidence for organics in Martian soils.

1. Introduction

[2] The NASA Mars Phoenix Lander Thermal and Evolved Gas Analyzer (TEGA) detected a high temperature (∼700°C) CO2 release due to a calcium carbonate phase and a lower temperature CO2 release between 400°C and 680°C from the Wicked Witch soil sample [Boynton et al., 2009]. Possible explanations for the low temperature CO2release include thermal decomposition of Mg or Fe carbonates or a zeolitic-type desorption reaction. However, these explanations are not entirely consistent with the observed TEGA CO2 release. Another possible explanation for the TEGA 400°C to 680°C CO2 release is combustion of organic compounds present in the soil [Boynton et al., 2009]. The perchlorate anion (ClO4) was measured by the MECA Wet Chemistry Laboratory (WCL) and determined to be present in Phoenix soils at ∼0.5 wt % [Hecht et al., 2009]. It has been suggested that soil perchlorates could have oxidized organic compounds in TEGA to CO2, preventing detection of their characteristic mass fragments [Ming et al., 2009]. The identification of Martian perchlorate prompted Navarro-González et al. [2010] to reevaluate the Viking organic analysis, using Mars analog soils spiked with magnesium perchlorate. They concluded that chloromethane, dichloromethane, and CO2 detected by the Viking GCMS were due to the thermal decomposition of soil organics in the presence of magnesium perchlorate. Biemann and Bada [2011] challenged their findings, reiterating [Biemann et al., 1976, 1977] that the origin of the chloromethane and dichloromethane detected by Viking was terrestrial contamination, possibly formed by a reaction between HCl and CH3OH. However, Navarro-González and McKay [2011] claimed that there was no obvious source of HCl inside the Viking GCMS.

[3] We have used a laboratory differential scanning calorimeter (DSC) coupled to a mass spectrometer (MS) to simulate TEGA analysis of a series of calcite and Mg-perchlorate mixtures in an effort to assess the nature and temperature of the endothermic and exothermic reactions and associated evolved CO2gas releases. We have also used powder XRD to assess the mineralogical changes that occur during heating of calcite and Mg-perchlorate mixtures. We propose that the low temperature CO2 release observed during the thermal analysis of Martian soils may be the decomposition of soil carbonates by HCl formed during the thermal decomposition of perchlorate.

2. Experiment

[4] Magnesium perchlorate hexahydrate is the stable phase at ambient Martian and terrestrial surface conditions [Robertson and Bish, 2011]. Reagent-grade magnesium perchlorate hexahydrate (Sigma-Aldrich) and Iceland spar calcite from Chihuahua, Mexico (Ward's) used in this work were crushed and sieved to 53μm–150 μm particle sizes for DSC experiments, and to a <53 μm fraction to provide better resolution for powder XRD. Iceland spar calcite was chosen because it is a natural calcite with minimal impurities and should be a sufficient analog to any natural calcite that may form on Mars. There is no way of knowing the purity of the Martian calcite; however, the calcite used in this work should be adequate in assessing the thermal behavior of Martian calcite under TEGA operating conditions. The DSC particle size fraction reflects similar grain sizes found at the Phoenix landing site [Goetz et al., 2010]. Mixtures of CaCO3 (0.025 mmole) and Mg(ClO4)2•6H2O (0.0015, 0.0133, 0.025, 0.0368 and 0.0485 mmole) were evaluated using a laboratory DSC/MS. The absolute amounts of calcite (0.025 mmole) and magnesium perchlorate hexahydrate (0.0015 mmole) correspond to amounts estimated to be in the Wicked Witch soil sample [Boynton et al., 2009; Hecht et al., 2009].

[5] Laboratory analyses of the calcite/Mg-perchlorate mixtures were conducted in a Setaram MHTC 96 heat flux-1400 differential scanning calorimeter coupled to a Pfeiffer Vacuum ThermoStar GDS 301 T quadrupole mass spectrometer. Experiments were carried out under a 12 mbar N2atmosphere (oil-free vacuum pump) with a flow rate of 1 sccm to simulate TEGA conditions. Samples were heated in an alumina crucible (0.45 cm3) to 1350°C at a rate of 20°C min−1. A reheat to 1350°C 3 hours later was used to provide baseline DSC calorimetric data that was subtracted from the first heating. Gasses evolved during the DSC analysis were determined with the MS by monitoring a predetermined set of individual masses of interest as a function of time (e.g., m/z 18 H2O, 35 Cl, 36 HCl, 44 CO2, 70 Cl2).

[6] Heated powder XRD was performed with an Anton Paar XRK900 heating chamber in a PANalytical X'pert Pro diffractometer using CoKα (Fe filtered) radiation. Calcite (167.2 mg) was mixed with Mg(ClO4)2•6H2O (32.8 mg), which approximates the ratio that was calculated for the Phoenix soil scaled up to 200 mg in order to fill the sample holder. The chamber was held at 12 mbar pressure with a N2 gas flow rate of 1 sccm, the same as the DSC/MS experiments. Scans were collected at 25°C intervals between 25°C and 875°C.

3. Results and Discussion

[7] Multiple endothermic transitions at low temperatures (75°C–250°C) (peaks a1, a2, a3, Figure 1) represent the dehydration of Mg(ClO4)2•6H2O, consistent with the findings of Robertson and Bish [2010]. The sharp exotherm at 495 ± 7°C (peak b) is attributed to the decomposition of magnesium perchlorate and subsequent crystallization of calcium chloride. At higher temperatures the thermal decomposition of calcite (peak c) appears as a broad endothermic transition at 684 ± 6°C. Above this, a series of endotherms (especially peak d) is present and believed to be caused by the decomposition of oxidized calcium chloride phases that were identified using XRD (discussed below). The calorimetric data from the Wicked Witch sample in TEGA most closely resemble the experiment with Phoenix-calculated amounts of sample (0.0015 mmole Mg-perchlorate and 0.025 mmole calcite,Figure 1), with similarly shaped endothermic transitions (peaks c and d) at high temperatures (>700°C). The Wicked Witch data does not show any thermal phase transitions at low temperatures that correspond to the dehydration of magnesium perchlorate. This is likely due to the low mass of Mg-perchlorate in the Wicked Witch soil, such that evidence of dehydration was not likely to have been detected in Phoenix-TEGA. The sharp endotherm in the TEGA data at ∼385°C (peak e), is caused by the nickel composition of the TEGA oven and represents the curie transition point of Ni [Boynton et al., 2009].

Figure 1.

Calorimetric results showing heat flow versus temperature for laboratory magnesium perchlorate (0.0015–0.0485 mmole)/calcite experiments compared to the Phoenix-TEGA Wicked Witch (WW). Downward and upward peaks indicate endothermic and exothermic reactions, respectively. Dehydration of magnesium perchlorate is indicated by a1, a2, and a3. The sharp exotherm at 495 ± 7°C (b) represents decomposition of magnesium perchlorate. A broad endotherm (c) at 684 ± 6°C is calcite thermal decomposition. The sharp endotherm (d) is attributed to oxidation of CaCl2, while the endotherm (e) in the TEGA data represents the curie transition temperature of the nickel ovens.

[8] Two CO2 releases were observed in the laboratory analyses; one low temperature release with an onset of 385 ± 12°C attributed to an inorganic reaction between HCl gas and calcite and a higher temperature CO2 release with an onset of 680 ± 17°C associated with the thermal decomposition of calcite (Figure 2) [cf. Boynton et al., 2009, Figure 2]. Increasing the amount of magnesium perchlorate clearly demonstrates that a reaction between magnesium perchlorate and calcite is responsible for the low temperature CO2 release in the laboratory experiments. The low temperature CO2 release becomes larger with higher amounts of perchlorate, and the high temperature CO2 release becomes smaller (Figure 2). There was a strong linear relationship between the mass of magnesium perchlorate reacted with calcite and the amount of CO2 (in mmole) released at high (y = 0.0237 − 0.396x; R2 = 0.97) and low temperatures (y = 0 + 0.0521x; R2 = 0.99). The low temperature CO2 release was closely associated with the detection of Cl2(m/z 35 and m/z 70 fragments) (496 ± 5°C) and HCl (m/z 35 and 36) (499 ± 9°C) in the 0.025–0.0485 mmole Mg-perchlorate treatments which further demonstrated that HCl was causing low temperature CO2 release (Figure 3). However, the amount of evolved chlorine was indistinguishable against the background Cl2 levels of the mass spectrometer (data not shown) for the lowest perchlorate treatments (0.0015 and 0.013 mmole). The release of both Cl2and HCl gases from Mg-perchlorate decomposition is consistent with the results ofLauer et al. [2009]. HCl may be a result of Cl2 gas reacting with ambient moisture in the system, or it could be released directly from the decomposing magnesium perchlorate that is not completely dehydrated:

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Figure 2.

Evolved CO2gas release versus temperature for the laboratory magnesium perchlorate (0.0015–0.0485 mmole)/calcite experiments compared to the Phoenix-TEGA Wicked Witch sample. Peak 1 is the low temperate CO2release caused by a reaction between Mg-perchlorate and calcite. Peak 2 represents the thermal decomposition of calcite. Dashed line at 385 ± 12°C indicates initial onset of CO2 detection, which is consistent between all experiments and the Wicked Witch results.

Figure 3.

Mass spectrometry data for laboratory experiment with 0.025 mmole CaCO3 and 0.0368 mmole Mg(ClO4)2•6H2O. Gas releases of H2O (m/z 18), CO2 (m/z 44), HCl (m/z 36) and Cl (m/z 35) Cl2 (m/z 70) are plotted versus temperature. Dashed line at 489 ± 9°C indicates the initial detection of both chlorine species, coincident with a change in peak shape in the CO2 profile.

[9] HCl gas has been demonstrated to react with calcite to release CO2 at low temperatures, but the water vapor driven off of the perchlorate salt may also play a role in the low temperature release of CO2. Closer scrutiny of the experiments with higher amounts of magnesium perchlorate revealed that the release of Cl2 and HCl gas (496–499°C) occurs well after the onset of the low temperature CO2 release (385°C) (Figure 3). There is an abrupt change in the shape of the CO2 peak at 489 ± 9°C (dashed line, Figure 3) directly corresponding to the first detection of chlorine. Studies have shown that steam can dramatically reduce the temperature onset of calcite decomposition [e.g., Heller-Kallai et al., 1986; Khraisha and Dugwell, 1991; Wang and Thomson, 1995]. This demonstrates that a low temperature CO2 release is possible independent of the presence of magnesium perchlorate, however it is not expected as is shown here that this mechanism can produce significant amounts of CO2.

[10] There are a number of possibilities that could explain why the ratio of the high:low temperature laboratory CO2release from the Phoenix amounts of calcite and magnesium perchlorate (0.025 mmole and 0.0015 mmole, respectively) is higher than what was observed by TEGA. The ratio of high temperature to low temperature peaks was calculated by integrating the two peak areas with the program OriginPro (V8.5.1, OriginLab) and is 22 in this experiment, but only 2.7 in Phoenix-TEGA (i.e., a greater proportion of the total CO2 was given off in TEGA at low temperatures relative to the laboratory results (Figure 2). Sutter et al. [2012] have demonstrated that the onset temperature for CO2 release from siderite and magnesite in simulated TEGA conditions are 512 ± 12°C and 580°C, respectively. Since the low temperature CO2 release from the Wicked Witch sample extends from 400–680°C, the thermal decomposition of either of these carbonates could account for the upper half of this CO2 peak. Ming et al. [2009] have shown that perchlorate salts could have masked the signatures of organic compounds in the TEGA mass spectrometer by oxidizing them to produce CO2. This allows for the presence of organics in the Phoenix soil, even though their characteristic mass fragments were not observed. However, it is not in and of itself evidence for the presence of organics, which may not persist on the Martian surface [e.g., Oró and Holzer, 1979; Stoker and Bullock, 1997]. As well, Archer et al. [2012] found that the amount of organic material needed to produce a noticeable CO2 signature in TEGA is probably unreasonable for the Martian surface The quantity of CO2 produced during the thermal decomposition of magnesium perchlorate will depend on the residence time of HCl and Cl2 in the sample ovens, thus instrumentation differences between TEGA and our laboratory experiments may be responsible for the discrepancy in CO2 peak ratios. It is also possible that the measurements of carbonate and perchlorate concentrations from TEGA and WCL are incorrect, such that carbonate was overestimated and/or perchlorate was underestimated. However, the error in these measurements would have to be outside the uncertainties reported in Boynton et al. [2009] and Hecht et al. [2009].

[11] Heated powder XRD demonstrated that a reaction between calcite and magnesium perchlorate was responsible for the low temperature CO2 release in the laboratory experiments. Figure 4shows the evolution of phases in the perchlorate-carbonate system as it is heated, beginning with proportions of CaCO3 and Mg(ClO4)2•6H2O calculated from the Phoenix data. From 75°C to 175°C a series of three dehydration steps produced a tetrahydrate, dihydrate and finally what is identified as anhydrous magnesium perchlorate. The Mg(ClO4)2 was stable until 350°C, when it decomposed to form periclase (MgO). Calcium chloride began to form (375°C) immediately after this. These temperatures in the XRD coincide with the initial release of CO2 in the DSC/MS experiments. However, the exotherm identified as magnesium perchlorate decomposition occurred higher in the DSC/MS, at ∼500°C. This may be due to differences in heating rate or particle sizes, as the DSC was ramped continuously while the XRD experiments included an equilibration period every 25°C. As well, a smaller grain size was used in the XRD experiments, which is expected to lower the onset temperature of chemical reactions. Above 450°C CaCl2 was oxidized to form the exotic oxides Ca4Cl6O and then Ca5Cl8O. By 700°C all the calcium phases in the system had been entirely oxidized and/or melted to produce lime. The decomposition of these Ca-Cl-O phases may be responsible for the endothermic transitions seen at high temperatures in the calorimetric data (peak d inFigure 1) and the second Cl2 and HCl releases (Figure 3) The decomposition of a phase such as Ca4Cl6O could account for the previously unidentified peak d in the Phoenix-TEGA data (Figure 1).

Figure 4.

XRD results for the temperature evolution of the Mg-Ca-Cl-O-H system. The beginning of each solid line represents the first appearance of a phase, and the end indicates the last appearance. Mg phases are shown in red, and Ca phases in blue.

4. Conclusion

[12] The detection of HCl gas released from the thermal decomposition of magnesium perchlorate at 499 ± 9°C, and its reaction with calcite to produce calcium chloride and release CO2 is a clear demonstration of the thermal interaction between CaCO3 and Mg(ClO4)2•6H2O. This reaction occurs in the same temperature range as the unidentified low temperature release of CO2 detected in TEGA. Hydrated magnesium perchlorate may have contributed directly to this CO2 release by reacting with a carbonate phase and/or indirectly by the release of water vapor into the system. Other possible perchlorate species, KClO4 and NaClO4, are not likely involved in this reaction because their thermal decomposition does not evolve Cl2 or HCl [Markowitz and Boryta, 1960; Devlin and Herley, 1987]. Calcium perchlorate [Ca(ClO4)2] can provide some HCl but predominantly decomposes to form CaCl2 [Marvin and Woolaver, 1945] and thus would not be expected to be a significant factor in any low temperature CO2 release from carbonate in the Phoenix soil. Thus, if the low temperature CO2release in TEGA was a result of a perchlorate-carbonate reaction, it is implied that a large proportion of the perchlorate is magnesian. The heated XRD results demonstrate that HCl gas evolved from Mg-perchlorate decomposition reacts readily with calcite to form calcium chloride. A number of oxidized Ca-Cl-O phases were identified at high temperatures, and the thermal decomposition of these phases may explain previously unidentified thermal transitions in the Wicked Witch TEGA sample. The pattern of two CO2releases and their temperature ranges in our results are generally consistent with data from the Wicked Witch TEGA sample, however the high-temperature CO2 release in TEGA extends to much higher temperatures, and the ratio of CO2 evolved at high to low temperatures is much lower (2.7) than in our experiments (22). A variety of scenarios may explain the CO2 ratio discrepancy, including differences between the laboratory and flight instruments, and Mg or Fe carbonates or organic molecules that are possibly present in the Phoenix soil. However, inorganic reactions involving calcite, magnesium perchlorate and an additional source of water vapor could account for all CO2releases detected by Phoenix-TEGA. We believe this is the best explanation for the low temperature CO2release observed by TEGA because there is evidence for Ca-carbonate, perchlorate, and water in the Phoenix soil. Results from this work will have implications for evolved gas analysis experiments to be carried out by the Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory Curiosity rover [Mahaffy et al., 2012], when it searches for organics in Gale Crater soil and rock. Mg-perchlorate in the presence of carbonates would likely produce a similar signature to our results in the SAM instrument with two distinct CO2 releases. This could be misinterpreted as evidence for organic compounds at the surface. Therefore, only detection of organic fragments, and not CO2, should be used as definitive evidence for organics on Mars.

Acknowledgments

[13] Funding for this work was provided by the Lunar Planetary Institute Summer 2010 Intern program to K. Cannon, by the Mars Data Analysis Program to B. Sutter, D. W. Ming, and W. V. Boynton (grant NNX10AQ22G), and by the NASA Astrobiology: Exobiology and Evolutionary Biology grant (NNX09AM93G) to R. Quinn. The authors would like to thank Ron Peterson and the two anonymous reviewers whose comments significantly improved this paper.

[14] The Editor thanks two anonymous reviewers for assisting in the evaluation of this paper.

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