Compositional fingerprints of past environments in the Martian rock record: Recent in situ discoveries and links to orbital data
Abstract
As on Earth, primary silicate minerals Mars can be transformed to secondary phases through dissolution/precipitation, hydrolysis, leaching, and oxidation reactions. Specific geochemical conditions drive the types of reactions that occur and the products that form, so the investigation of secondary phases and their geologic context on Mars provides valuable clues to past environments. In this talk, we will summarize how data collected by in situ missions over the last $\sim$~7 years has advanced our knowledge of secondary mineral formation on Mars, and how the findings from in situ missions affect our interpretation of orbital spectral and imaging data. Opportunity spent the latter half of its mission exploring the rim of the $\sim$~22 km diameter Endeavour impact crater. Ferric phyllosilicates had been detected from orbit on the crater's rim, which pre-dates the Meridiani Burns formation sulfate-rich sandstones. In situ data showed all of the rocks in Endeavour's rim are roughly basaltic in composition but have key chemical variations that record at least three to four episodes of fluid flow. The oldest rocks exposed in rim are pieces of pre-impact terrain, and some of them are covered by dark veneers enriched in volatile/mobile elements. These veneers are inferred to be one of the carriers of the orbital ferric phyllosilicate signature and interpreted to have formed by post-impact element mobilization via subsurface fluids along fractures. Impact breccias sit atop the pre-impact rocks and have FeO/MnO ratios which cannot be explained by igneous fraction trends alone, suggesting Mn mobilization. Subsurface fluids precipitated salts along the unconformity between the old, pre-impact rocks and the overlying impact breccias. Ground water also interacted with along fractures in the crater's rim and evidenced by boxwork veins associated with Al-phyllosilicates, interpreted to indicate local areas of enhanced leaching. Isochemical alteration from smaller amounts of water also formed Fe3+ clays that contribute to the orbital signature. At the Spirit rover site, there is limited evidence for aqueous alteration in orbital data. The Hesperian-aged basalts in Gusev crater exhibited only minor rinds and coatings. However, small-scale in situ analyses revealed Al phyllosilicates, Fe/Mg carbonates, and diverse igneous rocks in the ancient Columbia Hills. Moreover, opaline silica deposits at Home Plate site indicate volcanic-related fumarolic/spring activity, and salts found in sediments excavated by the rover point to relatively modern water availability in the upper ten's of cm. The Curiosity rover has encountered a diverse suite of fluvial, lacustrine, and eolian sedimentary rocks in Gale crater and its central mound, Mt. Sharp. From orbit, the lower reaches of Mt. Sharp showed hydrated phases including silica, hematite, sulfate, and phyllosilicates without clear stratigraphic ordering that transition up section to hematite-bearing ridge, a Fe,Mg phyllosilicate enriched unit, and a sulfate-bearing unit. In situ data indicate sediments were transported into the basin and experienced early and late stage diagenesis in a variety of environments. Early analyses of lacustrine mudstones were interpreted as evidence for in situ formation of magnetite and phyllosilicate from olivine weathering under circumneutral conditions; numerous early diagenetic dark-toned nodules and veins were present, as were later crosscutting Ca-sulfate filled fractures. Continued ascent up Mt. Sharp into the Murray formation revealed a relatively constant presence of phyllosilicates. In the lower Murray, silica-enriched sediments and fracture halos may represent zones of intensive, moderately elevated temperature alteration and/or deposition of sediments from evolved source rocks. Tridymite is a key indicator phase seen in situ though its interpretation is enigmatic. Stratigraphically higher heterogeneous facies (sandstone, siltstone) show intervals with >50% Ca and Mg sulfates, scattered NaCl, and mud cracks, pointing to episodes with increased aridity and lower lake levels. Towards the hematite-bearing (Vera Rubin) ridge, diagenetic textures and iron-bearing phases such as hematite, jarosite, and akaganeite mark a complex history of diagenesis. Intriguingly in situ age dating show a young age of the jarosite compared to what is thought to be the age of the lacustrine sediments, supporting the interpretation for a late diagenetic origin. Mudstone and sandstone samples drilled from the phyllosilicate unit indeed have the highest smectite concentration, and the erosional texture partly controls the orbital signature. In all formations, late diagenesis indicates ground waters substantially altered the sediments after deposition. In situ measurements have corroborated secondary minerals detections that had were based on absorption features seen in orbital spectral data. However, in situ measurements have also shown that the diversity of the Martian mineralogical record is far greater that what can be seen from orbit, including potentially large tracts of rock whose alteration history is obscured by dust cover. Limitations in the sensing depth and spatial resolution of orbital imagers and spectrometers should be taken into account when trying to draw detailed conclusions about Martian geologic processes, especially in light of the rover evidence that many sites on Mars have complex histories with multiple discrete episodes of diagenesis (for example shown by comparatively young jarosite age) and/or chemical weathering. Orbital and in situ data are highly complementary with one another because of their differences in spatial coverage, and sensitivity to isochemical mineralogical changes. Future exploration of Mars will be enhanced by continuing the practice of integrating in situ and orbital measurements.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.371F