Preservation of organic matter by reactive iron species on Mars relevant to Mars Sample Return

Background: Both NASA's Mars 2020 Perseverance rover and ESA's ExoMars rover Rosalind Franklin will be looking for signs of potential life on Mars. Perseverance launched in July 2020 and will land at Jezero crater in February 2021. Jezero crater once held a lake and has delta-like deposits at the mouth of inflow channels as prioriy area for its investigations. Perseverance will kick-off the Mars Sample Return effort by collecting and caching samples for return to Earth by subsequent missions. Rosalind Franklin will launch in 2022 and land at Oxia Planum on Mars. This area is characterized by wide-spread clay mineral deposits and includes subtle fluvial channels and delta-like deposits as well. Rosalind Franklin id equipped with a drill to collect samples from up to 2 m depth to look for organic material preserved from the hars radiation environment on Mars' surface.
Iron minerals play a key role in the sequestration and preservation of carbon. Over 20% of organic carbon in sediments on Earth is directly bound to reactive iron phases (Lalonde et al. 2012), also dubbed the Rusty Carbon Sink. The organic matter-iron mineral complexes mutually stabilize each other against organic matter degradation and mineral transformation, whereby the iron species involved often occur in nanoparticulate and X-ray amorphous forms (Schröder et al. 2016). Upon diagenesis, however, the redox properties of iron facilitate the oxidation of organic matter (Posth et al. 2013; Schröder et al. 2016; Tan et al. 2020). On that basis, a good target for the search for potential organic biosignatures would be rich in nanophase iron phases and X-ray amorphous minerals. Aqueously altered rocks and soils investigated in Gusev crater and at Meridiani Planum on Mars contain abundant nanophase iron oxides (e.g. Morris et al. 2019), and 20-60 wt% of minerals in fluvio-lacustrine deposits in Gale crater are X-ray amorphous and this amorphous phase is rich in iron (e.g. Rampe et al. 2017, 2019).
Similar deposits should be targeted in Jezero crater and at Oxia Planum to look for organic biosignatures. However, the difficulty is to recognize deposits rich in reactive iron phases (and therefore lacking a diagenetic or metamorphic overprint). The Mars Exploration Rovers Spirit and Opportunity who landed at Gusev crater and Meridiani Planum, respectively, had Mössbauer spectrometers as their principle mineralogical tool, while the MSL Curiosity rover at Gale crater used XRD. Perseverance and Rosalind Franklin will use Raman spectrometers for mineralogical analyses.
Objectives:
- Investigate sediments and sedimentary rocks of different ages, which contain reactive iron mineral phases (i.e. lack diagenetic or metamorphic overprint) using XRD, Mössbauer and Raman spectroscopy
- Develop Mössbauer spectroscopy as a tool to be used for the analysis of returned samples
Methodology: We will identify and collect a series of sediments and sedimentary rocks rich in reactive iron mineral phases with a range of different ages. These will be investigated for their mineralogy using XRD, Mössbauer and Raman Spectroscopy, plus XRF for elemental analysis and SEM for spatial charactiersation. We will measure organic carbon content and quantify specific carbon compounds. Different Mössbauer spectroscopy applications - transmission and low-temperature measurements; non-destructive backscattering applications; synchrotron-based applications - will be used and optimized with a view to apply the to the investigation of returned samples.