Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars

Abstract

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater’s sedimentary delta, finding the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Fe-Mg carbonates along grain boundaries indicate reactions with CO 2 -rich water, under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks were stored aboard Perseverance for potential return to Earth.

An olivine cumulate outcrop on the floor of Jezero crater, Mars

“…Our identifications of olivine and pyroxene minerals are supported by stoichiometric analysis of the PIXL data and are consistent with observations by other Perseverance instruments ( 5 ). We found that the beige equant grains have molar (Mg+Fe)/Si ≈ 2 and total cations of 2.995 to 3.025 per four oxygens (table S2), which is consistent with olivine’s chemical formula, (FeMg) 2 SiO 4 .…”

“…The intergrain reddish-brown materials contain silicates, Fe–Mg carbonate, and Fe–Mg–Ca sulfate, which are secondary materials from alteration. The presence of carbonates was also indicated by other instruments on Perseverance ( 5 ) and consistent with orbital observations. We focused our analysis on the primary minerals.…”

“…Images of Séítah formation rocks taken by the Perseverance rover have shown centimeter- to decimeter-scale layering and abundant dark, millimeter-scale angular grains, reported in a companion paper ( 5 ). These observations were consistent with any of the above hypotheses for the regional unit’s origin.…”

“…A light-toned, olivine- and carbonate-bearing rock unit exposed on the floor of Jezero has been informally named the Séítah formation (Fig. 1C) ( 3 – 5 ). Rocks similar to Séítah in morphology and mineralogy extend beyond the rim of Jezero; they span >70,000 km 2 from inside Isidis Planitia ( 6 ), to the neighboring regions of Northeast Syrtis ( 7 ), Nili Fossae ( 8 ), and Libya Montes (Fig.…”

“…( E ) Mastcam-Z image showing more detail of the Dourbes patch. ( F ) A portion of the abraded patch shown by the autofocus and context imager (ACI) of the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument on Perseverance rover ( 5 ). Image sources are listed in table S7.…”