The Lake St. Martin impact structure (Manitoba, Canada): A simulated rover exploration of a sulfate-bearing impact crater

摘要

The exploration of Mars can benefit from rover-like exploration of terrestrial analogue sites. We undertook such a study of the Lake St. Martin (LSM) impact structure in Manitoba, Canada. The site consists of intracrater evaporite/gypsum deposits, a granitic impact-melted and -shocked central uplift, carbonate-rich impact melt rocks, and poorly-stratified fluvio-lacustrine conglomerates ("red beds") that contain clasts of all of these lithologies. LSM may be relevant to Jezero crater landing site on Mars because it is also characterized by intra-crater fluviolacustrine activity. Exploration of the site was driven by prioritized science goals and undertaken using, initially, panoramic color imaging of two landing sites from which a heterogeneous science team, consisting of undergraduates and grade 10-12 high school students under the supervision of a senior mentor, selected regions and targets of interest for more detailed investigation and sample triage (in the context of sample return) using closeup imagery, reflectance, and Raman spectroscopy. Targets of interest were subsequently analyzed off-site using Xray diffractometry, X-ray fluorescence, wet chemistry and additional reflectance and Raman spectroscopy. It was found that reflectance spectroscopy and Raman spectroscopy were complementary because they generally identified different minerals and phases. Reflectance spectra of the surfaces of whole rocks often differed from bulk powders of the same samples in terms of spectral influence of Fe oxyhydroxides, overall reflectance, and absorption band depths. Powdering of samples generally reduced the spectral contributions of the weathered surface and Fe oxyhydroxide grain coatings, allowing the identity of the "bulk rock" to be better determined. Raman spectra of powdered bulk samples were often able to identify one or more specific mineral phases, but induced fluorescence reduced the number of identifiable Raman peaks. The deployment also provided valuable operational experience for current and next-generation planetary researchers. Evidence consistent with impact cratering and associated hydrothermal alteration was suggested by the field data. Overall, the deployment demonstrated that multiple analytical techniques are highly complementary in exploring and characterizing an impact crater hosting evaporites, conglomerates, and impact melt/shock products.