Compositionally and density stratified igneous terrain in Jezero crater, Mars

Abstract

Before Perseverance, Jezero crater's floor was variably hypothesized to have a lacustrine, lava, volcanic airfall, or aeolian origin. SuperCam observations in the first 286 Mars days on Mars revealed a volcanic and intrusive terrain with compositional and density stratification. The dominant lithology along the traverse is basaltic, with plagioclase enrichment in stratigraphically higher locations. Stratigraphically lower, layered rocks are richer in normative pyroxene. The lowest observed unit has the highest inferred density and is olivine-rich with coarse (1.5 millimeters) euhedral, relatively unweathered grains, suggesting a cumulate origin. This is the first martian cumulate and shows similarities to martian meteorites, which also express olivine disequilibrium. Alteration materials including carbonates, sulfates, perchlorates, hydrated silicates, and iron oxides are pervasive but low in abundance, suggesting relatively brief lacustrine conditions. Orbital observations link the Jezero floor lithology to the broader Nili-Syrtis region, suggesting that density-driven compositional stratification is a regional characteristic.

Fig. 1.. Perseverance rover traverse and compositions over the first 286 sols of the mission.

(A) Traverse is shown as a light gray line starting at the OEB landing site. Inset: Traverse location relative to a larger portion of Jezero crater floor. Locations of SuperCam VISIR spectral (blue circles) and LIBS chemistry observations (red squares) as well as abrasion patches (black circles) are indicated. Names of targets mentioned in this work and sol numbers are also indicated. The topographic profile follows the white line and has been extracted from the Mars Reconnaissance Orbiter High-Resolution Imaging Science Experiment (MRO/HiRISE) digital elevation model (DEM) sampled every 100 m. The dip angles are from (25). Vertical exaggeration (“Ex”) is 3×. (B to D) Elemental abundances versus sol number for bedrock observations. Scatter in the data is due to the influence of individual mineral grains at or near the size of the laser beam (~350 μm; see Methods). The solid line is a 31-point running average, showing trends. Overhead images in (A) are from MRO/HiRISE. NASA/JPL-Caltech/ASU.

Fig. 2.. Rock texture of the units observed during the traverse.

Textures are shown at Mastcam-Z and Navcam scales (left side) and SuperCam RMI scales (right side); hand-drawn lines at far right indicate potential grain boundaries for several targets. (A and B) Target Peppermint (Sol 46), a Máaz fm paver near the OEB landing site. The tangential sunlight highlights grains with angular shapes (nearly cubic for one of them) reaching up to 3 mm in size (white arrows). (C and D) Rock texture at Artuby ridge, which displays a subhorizontal parallel-layer fabric with poor continuity from one layer to the other. The target Grasse (Sol 175) displays a granular texture with millimeter-size grains with planar arrangements of platy cumulus minerals. Grains appear subangular to subrounded. (E and F) Target Cine (Sol 204) at the Bastide outcrop in Séítah (fig. S2) showing centimeter-thick layers, using Gaussian color stretch for the RMI image. The clean ledges (E) have relatively fresh surfaces dominated by angular grains (1 to 2 mm) suggestive of euhedral crystals (F). White arrows indicate euhedral grain shapes; yellow arrows indicate interstitial fill with noneuhedral shapes. (G to H) Target Content (Sol 238), defining the Content mb within the Séítah fm, showing pitted rocks in continuity with layered outcrops. Image designations are described in text S10 and provided in table S6.

Fig. 3.. Molar ratios showing contrasting compositions sampled by SuperCam’s 350-μm laser beam.

(A) Plagioclase- and silica-rich compositions observed in the Máaz fm pavers and Ch’al-like rocks and later in the Content mb of the Séítah fm. In (A), the points consist of mixtures of plagioclase, represented by the onboard andesine target (black squares in top left) with pyroxene, represented by the diopside (high-calcium pyroxene), and enstatite and ferrosilite (low-calcium pyroxene) targets. A few iron oxide points scatter up and to the right of the main group. (B) Observations from Artuby ridge, which clusters around the diopside standard (but are the Fe-rich variety of augite), with trends toward ferrosilite and andesine, although there is a clear lack of points near the andesine standard. Also shown in (B) are the Séítah observations, most of which trend between the olivine and pyroxene standards, with a small amount of scatter partway in the direction of plagioclase. Both panels show the mean compositions of each unit in larger symbols. SEMs are shown in fig. S7. The mean single-observation precision and accuracy are represented on each panel as superposed dark and light crosses, respectively.

Fig. 4.. Representative spectra showing the diversity of VISIR and Raman spectral features.

(A) VISIR reflectance spectra observed in the Séítah (“Penne,” second from top) and Máaz fms (all other plots). (B) Laboratory spectra of pure minerals (see text S7). Spectra are offset for clarity. A few targets have nearly featureless spectra [e.g., “Hedgehog” in (A)], but the mean signature of the Máaz fm [(A), bottom] is consistent with the presence of hydrated surfaces. The spectral features best exemplified in targets “Tsosts’idts’áadah,” “Bidziil,” Bellegarde, and Guillaumes are potentially explained by contributions of oxyhydroxide, Fe-phyllosilicate, monohydrated sulfate or perchlorate, and gypsum or an Al-OH or Si-OH phase, respectively (see the text S7 for band attribution). The “Máaz tgt.” spectrum is from the namesake of the fm and is typical of targets with strong dust coatings. The Penne spectrum was collected in Séítah and is consistent with the presence of olivine and traces of a Mg-OH phase and/or a carbonate (e.g., magnesite). (C) Raman spectrum of sodium perchlorate observed on one point in the Bellegarde abrasion patch, compared with a laboratory spectrum of sodium perchlorate (69).

Fig. 5.. Pyroxene quadrilateral.

(A) Pyroxene compositions observed in Máaz fm (red squares) and Séítah (purple triangles). The range of Mg numbers of Séítah olivines is shown below (see fig. S8). Superposed error bars show mean precision (dark lines) and accuracy (light lines). (B) Comparison with pyroxene composition found in martian meteorites. The Máaz fm contains augite and Fe-rich pyroxenes, similar to basaltic shergottite meteorites, whereas the Séítah fm contains pyroxenes more enriched in Mg, similar to poikilitic and olivine-phyric shergottites. End members and compositions are indicated as follows: Di, diopside; Hd, hedenbergite; Wo50, wollastonite 50%; Pig, pigeonite; Cl-en, clinopyroxene enstatite; Cl-fs, clinopyroxene ferrosilite; En, enstatite; Fs, ferrosilite; Fo, forsterite; Fa, fayalite. Meteorite data in (B) are from references given in reference file S1.

Fig. 6.. Elemental compositions indicating the relative absence of chemical weathering by aqueous leaching of soluble elements.

Trends in the direction of the blue arrows, above the dashed line from the iron-magnesium corner to the ideal plagioclase composition on the left, would indicate leaching of mobile elements represented at the base of the triangle. Observations of onboard standards (63) are plotted along with the compositions of observed points. Dark error bars indicate precision; light error bars indicate mean accuracy (62).

Fig. 7.. Stratigraphic concept and mean elemental composition trends.

(A) Conceptual view of the current stratigraphic positions of Máaz, Artuby, and Séítah. The outcropping portion of Artuby and the SW portion of Séítah adjacent to it show dipping strata in exposure and in RIMFAX radargrams. Artuby was not seen to be exposed on the NE side of Séítah. RIMFAX data indicated dipping strata on that side of Séítah too. Máaz is inferred to be overlying both Artuby and Séítah. All units were emplaced after the fm of Jezero crater and well above the base of the crater. (B) Elemental trends progressing from Máaz to Artuby to Séítah. This plot shows, on a logarithmic scale, a systematic decrease in felsic elements (SiO₂, Al₂O₃, Na₂O, and K₂O) with decreasing proposed original stratigraphic elevation. CaO (dashed line), which occurs in both plagioclase and orthopyroxene, is highest at Artuby due to its enrichment in augite. Error bars indicate SEMs where large enough to be seen, for comparison between points (see Methods).

Fig. 8.. Formation scenarios.

(A) Cumulate formed from stratification of a single melt in which olivine phenocrysts were segregated by gravity in a magma body, which subsequently solidified. All of the rocks observed by SuperCam were produced within this igneous body in this scenario. (B) Cumulate plus lava flow(s) scenario, initiated with the fm of Séítah and Artuby as a cumulate as in (A). The depleted upper portions of the body were removed and subsequent lava flow(s) produced Máaz. (C) Third scenario in which Artuby was also produced by a relatively viscous lava flow before emplacement of Máaz by later and more evolved lava. Triangles at the right indicate overall composition and density trends.