Four-billion years old spherule beds revealed by Perseverance on the outer rim of Jezero crater

Jezero crater is a 45-km diameter impact crater, formed during the Early-Middle Noachian period, ~3.9 Ga, on the northwest rim of the Isidis Planitia within the highland crust of Mars of the Nili Fossae region. Rocks excavated by the impact, thus pre-dating Jezero crater, potentially >4.0Ga, were investigated by the Perseverance rover on the rim of the crater. The outer Jezero rim displays a light-toned, layered unit informally named Witch Hazel Hill, which has been analyzed by Perseverance in locations named Broom Point and Sallys Cove. There, the SuperCam Remote Micro-Imager (RMI) and Mastcam-Z cameras revealed rocks with spherical granules, which hereafter we refer to as “spherules”, a term used here as purely descriptive. At Broom Point, we analyzed the largest number of spherule-bearing targets, among which two clasts and two bedrock targets have been analyzed thoroughly. The spherules are ~2-mm of mean diameter in all targets, are closely packed, and represent >90% of the granules. In one of the targets, they are partly broken and piled up by an energetic process. The elemental composition derived SuperCam is basaltic, close to that of the surrounding bedrock. However, the featureless infrared reflectance spectra lack signatures of hydration, and are interpreted as glasses, in agreement with their shiny surface on images. In contrast, the surrounding rocks display hydration features linked to the presence of sulphates and phyllosilicates. At Sally’s Cove, 50 m away to the north, spherules are scattered along laminae of the bedrock. They display a mean diameter (<0.5 mm) too small for SuperCam individual analysis. While no proximity science was possible at Broom Point, Sallys Cove was favourable for a chemical analysis by the PIXL instrument. The composition of the eight spherules analysed there show rims distinct from the interior, and diverse compositions ranging from plagioclase-rich to pyroxene-rich. On Earth, spherule-bearing rocks can be found in impact, volcanic or sedimentary rocks. The chemical characteristics of Jezero rim’s spherules do not favour sedimentary concretions such as those observed at Meridiani Planum. A volcanic context would reasonably explain the presence of spherical clasts such as accretionary lapilli produced by explosive volcanism. Nevertheless, the homogeneity of the spherule size and their well-defined sphericity is frequent for impact spherules observed on Earth at the K-Pg boundary for which spherules were created by droplets of melt ejected to several thousands of km. The basaltic, anhydrous composition is consistent with such a hypothesis, although it does not fully rule out volcanic fire fountains. Yet, at Sallys Cove, the variable compositions of spherules measured by PIXL are difficult to explain in a volcanic context, which assumes homogeneous compositions. Hence, we currently favour the presence of these spherules from impact ejecta. If this hypothesis was confirmed, the sample collected at this location could represent a unique opportunity to analyse impact processes at the surface of a terrestrial planet in the early history of the solar system.