ALTERATION HISTORY OF ALUMINUM-RICH ROCKS AT JEZERO CRATER, MARS: INSIGHTS FROM DEEPLY WEATHERED TERRESTRIAL PALEOSOLS

The Mars 2020 Perseverance Rover discovered several thousand small, light-toned rock fragments, or float rocks, that are spectrally and compositionally distinct from bedrock so far observed on the mission. Some of these float rocks have a diagnostic spectral signature of a hydrated kaolin-group mineral (e.g., kaolinite, halloysite), and are enriched in Al₂O₃ (~17 - 30 wt. %). Others show spectral evidence of metakaolinite and spinel-group minerals, as well as Al₂O₃ to ~45 wt. % and over 1 wt. % Ni. Most all appear to contain trace Ca and Mg sulfates. One hypothesis for these rocks is that they are pieces of a Noachian (~3.7 billion-year-old) deep weathering profile (e.g., paleosol) that was altered by different degrees of heating, possibly from impact. Deciphering the origin of these float rocks is challenging because they are fully removed from their geological context. To begin contextualizing these float rocks with analog materials from Earth, we compare the geochemistry and spectral properties of hydrated Jezero float rocks to deeply weathered paleosols from the Eocene. We hypothesize that some of these float rocks are similar to terrestrial materials that experienced intense and prolonged pedogenic alteration under chemically reducing conditions, followed by burial and lithification.

We used images and data from the SuperCam and Mastcam-Z instruments onboard Perseverance rover for comparison with previously published geochemical and mineralogical data of terrestrial paleosols. These comparisons focused on early Eocene (55 Ma) kaolinitic Ultisol paleosols from Blacks Beach, La Jolla, San Diego, CA that formed on mafic to felsic conglomerate with estimated mean annual precipitation of 1500 ± 300 mm. Jezero float rocks show spectral and geochemical similarities to terrestrial paleosol horizons that are rich in kaolinite and low in Fe (e.g., the “pallid zone” of bauxite). However, either pedogenic or hydrothermal alteration forms kaolinite and distinguishing between the two requires outcrop. One potential source of these rocks are light-toned, layered megabreccia blocks nearby in the Jezero crater rim that have a spectral signature of kaolinite. Future exploration of the crater rim by Perseverance includes targeting a kaolinite-bearing outcrop, so it is possible that upcoming observations will further reveal the alteration history of these enigmatic float rocks.