Long-chain alkanes preserved in a Martian mudstone

Abstract

Organic molecules preserved in ancient Martian rocks provide a critical record of the past habitability of Mars and could be chemical biosignatures. Experiments conducted by the Sample Analysis at Mars instrument onboard the Curiosity rover have previously reported several classes of indigenous chlorinated and sulfur-containing organic compounds in Gale crater sedimentary rocks, with chemical structures of up to six carbons. Here, we report the detection of decane (C₁₀H₂₂), undecane (C₁₁H₂₄), and dodecane (C₁₂H₂₆) at the tens of pmol level, released from the Cumberland drilled mudstone sample, using a modified SAM analytical procedure optimized for the detection of larger organic molecules. Laboratory experiments support the hypothesis that the alkanes detected were originally preserved in the mudstone as long-chain carboxylic acids. The origin of these molecules remains uncertain, as they could be derived from either abiotic or biological sources.

Keywords: Curiosity rover; Mars; organic molecules.

Fig. 1.

Evolved major gases versus temperature as detected by SAM-evolved gas analysis (EGA) of the drilled CB mudstone sample in the second heating step (ODb) of the opportunistic derivatization analysis. The gray boxes show the four temperature cuts selected to send the gases to the hydrocarbon trap for subsequent GCMS analysis. The dashed red line corresponds to the SAM hydrocarbon trap temperature during the run. Identification of the chemical species was performed from specific ions produced by electron impact in the ion source of the mass spectrometer. The characteristic mass over charge (m/z) value for these specific ions is given for each chemical species. BSW: 1,3-bis(1,1-dimethylethyl)-1,1,3,3-tetramethyldisiloxane, known as “bisilylated water.” TFMA: trifluoro-N-methylacetamide. m/z 32 is mainly a contribution from SO₂—no O₂ was detected. The rationale for the selection of the four temperature cuts is to detect the following: Cut#1 Derivatization products from MTBSTFA and its byproducts sent to the hydrocarbon trap Cut#2 Organics released from FeSO₄ thermal decomposition Cut#3 Organics released from MgSO₄ thermal decomposition + suspected release of thiophene Cut#4 Possible refractory organic matter fragments.

Fig. 2.

(A) Identification of decane, undecane, and dodecane in the ODb chromatogram. Reconstructed chromatograms from m/z 77 × 2 + m/z 91 × 5 + m/z 57 × 50 + m/z 71 × 50. The decane, undecane, and dodecane detected in ODb Mars run (orange) were not present in the subsequent OD blank experiment (gray) carried out under identical experiment conditions. The retention times are compared with the C₁₀–C₃₀ n-alkanes analyzed in the SAM TB (blue), with GCMS conditions similar to those used by the SAM flight model on Mars (SI Appendix, section 4). (B) The mass spectrum of decane from the SAM GCMS data (orange) is compared to the one of n-decane in the NIST database. This spectrum also is in better agreement than those of branched C₁₀ alkanes. The lack of decane, undecane, and dodecane in the procedural blank and the correlation of the retention times and mass spectra of the SAM GCMS peaks with SAM TB and NIST confirm that the CB sample is the source of decane, undecane, and dodecane in the ODb GCMS run.

Fig. 3.

Decarboxylation of long-chain carboxylic acids observed in laboratory experiments. Laboratory experiments of (A) 35 °C·min⁻¹ EGA of undecanoic acid 5 wt % on montmorillonite. Specific m/z are displayed to correspond to CO₂ (m/z 44), decane (m/z 142), and undecanoic acid (m/z 186). (B) 35 °C·min⁻¹ EGA of pure undecanoic acid blue) and undecanoic acid 5 wt % on montmorillonite (green). *: dimerization product (C) 35 °C·min⁻¹ pyrolysis-GCMS analysis of undecanoic acid 5 wt % on montmorillonite. The x axis represents the retention time of the compounds onto the GC capillary column. Specific m/z are displayed to correspond to generic alkanes (m/z 57) and undecanoic acid (m/z 186). Experimental conditions are described in SI Appendix, section 9.