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 (