X-ray amorphous material that is variably Mg/Fe/Si-rich and Al-poor and that likely contains secondary alteration products is prevalent in Gale crater sediments and rocks (15-73 wt.%). However, the structure and origin of these materials and their implications for past environmental conditions remain unknown. In this study, we use transmission electron microscopy and synchrotron microprobe analyses to examine Mg/Fe/Si-rich and Al-poor ultramafic soils from the warm Mediterranean climate Klamath Mountains of California and cold subarctic climate Tablelands of Newfoundland, Canada to help interpret environmental conditions during the formation of chemically similar X-ray amorphous material in Gale crater, Mars. Primary glass is absent from the Klamath Mountains and Tablelands materials; secondary X-ray amorphous material includes globular amorphous silica and chemically heterogeneous nanospherical amorphous material and nanocrystalline phases. Globular amorphous silica is only present in soils that undergo extensive periods of cyclic freezing. Fe-containing X-ray amorphous material from the subarctic Tablelands is significantly richer in Mg and Si than X-ray amorphous material from the warmer Klamath Mountains. Fe-rich nanocrystallites contain more Mg and Si in the subarctic Tablelands but are more highly Fe-enriched in the warmer Klamath Mountains. Potential secondary nanocrystalline phyllosilicates are only observed in the warmest examined soil in the Klamath Mountains. These characteristics – the presence or absence of amorphous silica, the chemical composition of X-ray amorphous material, the abundance and composition of Fe-rich nanocrystallites, and the presence or absence of secondary phyllosilicates - provide helpful identifiers to interpret past environmental conditions during the formation of X-ray amorphous material on Mars.

The NASA Mars 2020 Perseverance rover mission will collect a suite of scientifically compelling samples for return to Earth. On the basis of orbital data, the Mars 2020 science team* identified two notional sample caches to study (1) the geology of Jezero crater, collected during the prime mission and (2) the ancient crust outside of Jezero crater, collected during a possible extended mission. Jezero crater geology consists of well-preserved, Early Hesperian to Late Noachian deltaic and lacustrine deposits sourced from a river system that drained Noachian terrain. The crater floor comprises at least two distinct units of sedimentary or volcanic origin whose relationship to the deltaic deposits is presently unclear. Remotely-sensed data reveal signatures of carbonate+olivine and clay minerals within crater floor and crater margin units. Samples from within Jezero that comprise the prime mission notional sample collection thus include: crater floor units; fine- and coarse-grained delta facies, the former with potential to preserve organic matter and/or biosignatures, the latter to possibly constrain the type and timing of sediment deposition; chemical sediments with the potential to preserve biosignatures; a sample of crater rim bedrock; and at least one sample of regolith. The region of southern Nili Planum, directly outside the western rim of Jezero crater, is geologically distinct from Jezero crater and contains diverse Early or even Pre-Noachian lithologies, that may contain records of early planetary differentiation, magnetism, paleoclimate and habitability. The notional sample collection from this region will include: layered and other basement rocks; megabreccias, which may represent blocks of (pre-)Noachian crust; basement-hosted hydrothermal fracture fill; olivine+carbonate rocks that are regionally significant and may be related to units within Jezero crater; and mafic cap unit rocks. The samples described are notional and may change with ongoing surface investigations. However, the samples we anticipate collecting align well with community priorities for Mars exploration, addressing geologic diversity, potential ancient biologic activity on Mars, planetary evolution, volatiles, and human health hazards. *Many other Mars 2020 team members were involved in this planning