CHEMICAL BIOSIGNATURE PRESERVATION IN THE IRON MOUNTAIN MASSIVE SULFIDE DEPOSIT: IMPLICATIONS FOR BIOSIGNATURE DETECTION ON MARS WITH THE CURIOSITY ROVER

An ideal biosignature preserves both molecular and physical evidence of the organism(s) of interest. Here, we document the detection of molecular biosignatures (fatty acid methyl esters [FAMEs]) co-occurring with previously documented physical biosignatures (mineralized microbial filaments) in iron oxides ranging in age from modern to 1000s of years old from the Iron Mountain gossan (oxidized sulfide deposit), CA. Gossans, dominated by iron oxides, have been proposed as Martian environmental analogs. Organic molecules are thermodynamically unstable in the presence of iron oxides, but may be preserved depending on how they are hosted in mineralogically diverse sediments. Additionally, lipids are known to be well preserved molecular biomarkers over geologic time due to their ability to survive oxidative stress.

The detection of FAMEs in these iron oxide rock samples used tetramethylammonium hydroxide (TMAH): MeOH (25%) thermochemolysis at 600°C followed by gas chromatograph mass spectrometry (GCMS). A similar approach will be used by the Sample Analysis at Mars instrument suite (SAM) onboard the Curiosity rover. SAM has two TMAH: MeOH wet chemistry cups to search for organic molecules such as FAMEs in the sediments of lower mound of Mt. Sharp. To understand the effectiveness of the SAM TMAH experiment, we compared results of the SAM-like approach with TMAH to a more recently developed approach using thermal desorption at 350°C after addition of an alternative reagent, trimethylsulfonium hydroxide (TMSH).

Rock samples were broken open and sampled from their interior under organically clean conditions. Powdered rock samples were reacted with either TMAH or TMSH. Using TMAH, FAMEs detected in modern iron oxides included n-C₁₀, ₁₂, ₁₄, ₁₅, ₁₆, _16:1, ₁₇, ₁₈, _18:1, ₂₀. FAMEs detected in older (100s-1000s of years old) iron oxides included n-C₈, ₁₄, _{16, 18}. Using TMSH, FAMEs detected in modern iron oxides included the same as detected with TMAH, plus n-C_{18:2, 22}. FAMEs detected in older iron oxides included n-C₁₀, ₁₂, ₁₄, ₁₆, ₁₈, _18:1. The majority of these FAMEs are microbial biomarkers. Similar TMSH and SAM-like TMAH thermochemolysis results indicate that SAM is capable of detecting FAME biomarkers. Moreover, these results demonstrate the FAME biomarkers may be preserved over longer timescales in iron oxides on Mars.