Carbonated ultramafic igneous rocks in Jezero crater, Mars.

The Perseverance rover landed in Jezero crater on Mars, which once contained a lake of liquid water. We report the rock properties encountered by Perseverance during a 10-kilometer traverse extending over 400 meters in elevation, from beneath Jezero's western sedimentary fan to the upper crater rim. These rocks consist of coarse-grained olivine, magnesium and iron carbonates, silica, and phyllosilicates, including some of the oldest materials exposed within Jezero. We infer that these rocks formed by olivine accumulation in an igneous system of layered intrusions, followed by exposure to water and carbon dioxide, which caused extensive carbonation of the silicate minerals. Aqueous alteration was more pronounced at lower elevations. Higher-elevation exposures on the crater rim appear similar to olivine-rich rocks distributed over the wider Nili Fossae region. Editor's summary: Jezero crater on Mars once contained an ancient lake. The Perseverance rover landed inside the crater and has driven from its floor to its rim. Williford et al. investigated the rocks encountered by Perseverance around the edge of the crater using imaging and compositional measurements. They found that the rocks have an igneous origin (i.e. they result from volcanism) and were later modified by reactions with liquid water containing dissolved carbon dioxide. This aqueous alteration is most extensive at lower altitudes, which is consistent with a lake that varied in depth over time; however, the alteration also extends above the top of the lake into the surrounding region. —Keith T. Smith INTRODUCTION: The Mars 2020 Perseverance rover landed in Jezero crater, Mars, which contained a lake of liquid water about 3.5 billion years ago. After exploring the igneous rocks of the crater floor, Perseverance ascended to the crater rim. Previous orbital spectroscopy indicated that the composition of these rocks is dominated by olivine and carbonate and that they were of unknown formation process and alteration history. It had been hypothesized that these rocks were deposited along the shoreline of an alkaline lake. Alternatively, they might be igneous in origin, part of a regional olivine- and carbonate-bearing system of rocks that extends outside the crater and on the crater floor. RATIONALE: Over more than 10 km of traversed terrain extending 400 m in elevation, the rover explored and sampled rocks of the Margin unit, located on and inside the western rim of the crater. The location of the Margin unit along the inner crater rim is at an elevation consistent with that of the highest water level of the ancient lake. Rover instruments, including cameras and spectrometers, were used to investigate the geology and stratigraphic relations and to determine the rock compositions. The rover's drill was used to abrade rock surfaces prior to some compositional measurements and collect three samples for potential return to Earth. RESULTS: The rocks of the Margin unit have millimeter-scale granularity, commonly comprising olivine cores rimmed by Fe- and Mg carbonates, with minor amounts of phyllosilicates (including serpentine minerals). The intergranular matrix is composed of variably hydrated silica and contains dispersed ~100-μm Ti-bearing chromite grains. Layering was observed on several scales, with fewer complex sedimentary-like structures. There is no evidence for the systematic structural and grain size variations that would be produced by large-scale sedimentary deposition. The observed structures are similar to those in layered intrusive igneous settings on Earth. Systematic variations in the olivine forsterite content and Ti/Cr ratios of chromites are consistent with fractional crystallization from an evolving magma. Rocks encountered at lower elevations (below the maximum lake level) have undergone more extensive aqueous alteration than exposures encountered at higher elevation on the crater rim. The latter rocks show primary igneous textures, more olivine, and less carbonate and silica. The western sedimentary fan and the Neretva Vallis inlet channel eroded (and thus formed after) the Margin unit. CONCLUSION: We infer that the Margin unit rocks formed by accumulation of olivine crystals in a layered intrusive igneous system. These rocks were later variably altered in the presence of liquid water and carbon dioxide to form carbonates, phyllosilicates, and silica. The higher abundance of alteration minerals in lower-elevation targets indicates that the Jezero lake system enhanced the carbonation process but was not the sole driver. These carbonated igneous rocks on Mars record interactions between magmatic processes, a CO2-rich early atmosphere, and surface liquid water. Perseverance investigates the Margin unit.: A Navigation Camera image showing coarse-grained, polygonally fractured Margin unit rocks in the foreground. The overlying sedimentary fan is in the middle distance, and the southeastern Jezero crater rim is in the background. The rover's robotic arm, instrument turret, and drill are visible. [ABSTRACT FROM AUTHOR]