On the oxidation ability of the NASA Mars‐1 soil simulant during the thermal volatilization step: Implications for the search of organics on Mars

Abstract

The search for organic molecules on Mars has been a major goal in planetary science. Viking performed the first analyses of the Martian soil in 1976, but was unable to detect organics at the ppb level using the thermal volatilization (TV) method. Three decades later, the Phoenix lander conducted analyses of Martian soil samples by TV, and found the release of CO2 from 400 to 680°C that was attributed to Mg or Fe carbonate, adsorbed CO2, or organic molecules. We have previously reported that high levels of iron species present in the soil efficiently oxidize the organics to CO and CO2 by TV. Here we explore in detail the oxidation ability of the organic‐free NASA Mars‐1 soil simulant during TV in molecular hydrogen or in the presence of stearic and mellitic acids. Our results imply that there are two sources of strong oxidizers in palagonite soils: (1) hydroxyl radicals originating from the dehydroxylation of the silica layer matrix at 320°C to 600°C and (2) oxygen atoms released by the reduction of iron(III) to iron(II) species at 320°C to 600°C and of iron(II) to metallic iron at 850–1150°C. These strong oxidizers completely decompose stearic or mellitic acids to carbon dioxide when they are present at low levels (<0.05% wt) in the NASA Mars‐1 soil simulant. Since organics are expected to be present at very low levels on Mars, future space missions utilizing TV will face the challenge of determining if any CO2 released is of inorganic or organic origin.

Magnesium sulfate as a key mineral for the detection of organic molecules on Mars using pyrolysis

“…On Mars, magnesium sulfate could be mixed with others minerals which also favor the decomposition of organic molecules during pyrolysis, such as perchlorates and iron oxides [ Iniguez et al , ; Navarro‐Gonzalez et al , ; Schulten and Leinweber , ]. In the analog samples we studied, trapped organic molecules are completely released at 700°C under SAM‐like pyrolysis conditions.…”

“…For a process such as oxidation, the number of organics by‐products detected decreases with the concentration of organic molecules initially present in the sample. At low levels, organic compounds may be entirely transformed into inorganic by‐products (e.g., mainly CO 2 ), or be present below an instrument's detection limit [ Iniguez et al , ; Navarro‐Gonzalez et al , ; Steininger et al , ]. The total amount of organic carbon at the Mars surface is expected to be at a trace level.…”

Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars

“…In computer simulations it was assumed that Martian soil contains 0.5% of absorbed water and 0.5% of hydroxyl groups present in the mineral surfaces that are released as hydroxyl radicals (OH) during the heating process at temperatures ≥200°C. Freshly ground basaltic minerals produce sufficient quantities of hydrogen peroxide (H 2 O 2 ) when immersed in liquid water [ Hurowitz et al , 2007] or hydroxyl radicals when heated [ Iñiguez et al , 2009], to explain the oxidizing characteristics of Martian soil samples collected by the Viking Landers. Reaction of two hydroxyl radicals leads to the formation of water and molecular oxygen according to reactions 9 and 7 in Table 2.…”

“…However, it is important to emphasize that the second step, namely, the GC‐MS analyses operated as designed in both the Viking Landers. Future missions will have to incorporate additional analytical methods to avoid the oxidation of Martian soil by TV due to iron oxides [ Navarro‐González et al , 2006, 2009; Iñiguez et al , 2009] and perchlorate. Considering that the Viking Landers did detect the presence of organics in the soil, we suggest that an in situ life detection mission to Mars is essential in the near‐future.…”

The Moon as a Recorder of Organic Evolution in the Early Solar System: A Lunar Regolith Analog Study

“…Space weathering would gradually break up an organic-bearing meteorite on the surface of the regolith, with the result that a portion of the organic material would become dispersed within the lunar regolith, with direct contact between organic material and lunar minerals. The minerals of the lunar regolith may have properties that accelerate the destruction or reconfiguration of organic materials when heated, or otherwise result in organic products different to those where the organic matter is heated in isolation, as has been investigated for the martian regolith (e.g., Iñiguez et al, 2009).…”