Old rocks exposed on the rim of Jezero Crater preserve a roughly 4-billion-year-old “weather report” from the solar system’s most dynamic era

NASA’s Perseverance Mars rover has discovered that a 245-foot-thick (75-meter) stack of ancient rock on the rim of Jezero Crater was constructed through repeated asteroid impacts. The rover’s science team refers to this layered bedrock sequence as the “Broom Point member,” and it is believed to be more than 3.9 billion years old, qualifying it among the oldest terrain ever examined by a Mars rover.

The findings were published Wednesday in the Journal of Geophysical Research: Planets and provide insight into one of the solar system’s most turbulent periods.

Ken Farley, Perseverance deputy project scientist at Caltech, explained: “On Earth, our earliest geologic history has been fundamentally broken up, deformed, and erased by plate tectonics. Because Mars lacks plate tectonics to recycle its crust, this ancient record remains intact.”

After descending Jezero Crater’s western rim in early 2025, Perseverance examined Broom Point using its scientific instruments. The data revealed six distinct rock types, including breccias—rocks composed of angular fragments—alternating with fine-grained, pulverized rock dust layers. Rock fragments within the breccias contained gas-bubble cavities, indicating previous melting.

Tiny, dark, glassy beads within the layers provided crucial evidence about formation processes. While volcanic activity produces similar beads, their high abundance here points toward asteroid impacts as the primary cause.

Alex Jones, a doctoral candidate in planetary geology at Imperial College London and lead author of the study, noted: “The different rock layers are a record of variable-sized impacts occurring at different distances from where this rock sequence was accumulating.”

Several layers appear to have formed through rapid, ground-hugging debris flows, possibly resulting from interactions with water or ice. When molten rock encounters water or ice, it can generate the powerful fluid-like surges observed in Earth’s geological record.

Some Broom Point layers tilt at angles exceeding 80 degrees—nearly vertical—far steeper than what Jezero Crater’s impact alone could cause. Scientists theorize a two-stage cosmic event shaped the landscape long ago: a massive asteroid impact first created the 1,200-mile-wide Isidis Basin, upending and tilting rock layers. Subsequently, a second asteroid likely struck, forming the 28-mile-wide Jezero Crater, fracturing and uplifting the already-tilted rocks into their current dramatic formations.

To determine when these events occurred, the Perseverance team collected two core samples named “Bell Island” and “Main River.” If returned to Earth by a future mission, laboratory dating could reveal the timing and frequency of early Mars impacts—and by extension, early Earth’s impact history, which plate tectonics has erased.

Jones concluded: “If we can pin down the ages of these layers, it would be like reading a cosmic weather report from 4 billion years ago.”

Journal: Journal of Geophysical Research: Planets
DOI: 10.1029/2026JE009779
Publication Date: July 15, 2026

Source: EurekAlert