NASA‘s Perseverance rover has detected what appears to be an ancient river delta buried more than 35 meters beneath the Martian surface — older than the visible delta scientists have studied since the rover landed in Jezero Crater in 2021.
The discovery comes from RIMFAX, the rover’s ground-penetrating radar instrument, which fired radar waves into the ground every 10 centimeters of travel during a campaign running from September 2023 to February 2024 — spanning more than 250 Martian sols. Those signals bounced back from boundaries between rock, sediment, and ice layers, allowing researchers to construct a vertical cross-section of the Martian crust.
The survey focused on a geological zone called the Margin unit, a broad deposit sitting between the crater’s western fan and its inner rim. Its rocks proved unusually transparent to radar, letting signals penetrate deeper than anywhere else previously scanned in Jezero — more than 1.75 times deeper than measurements taken on the crater floor or the overlying Delta units. Accounting for surface topography, the team estimates the Margin unit’s true thickness at 85 to 90 meters.
Layered geometry points to flowing water
What the radar found at depth was not featureless rock. The data showed parallel sediment layers tilting toward the center of the Jezero basin at angles of three to 15 degrees — a geometry geologists call clinoforms, the signature of sediment building outward into standing water as a river deposits material. The images also captured rollover points, the transitions between the angled foreset layers and the flat bottomset layers where finer sediment fans out at the base of a delta. Those features, the research team says, indicate repeated episodes of sediment deposition over an extended period, not a single flood event.
“We saw really high complexity in the subsurface,” said Emily L. Cardarelli, an astrobiologist at the University of California Los Angeles who led the team interpreting the RIMFAX data, according to the report.
Carbonate rock and the search for ancient life
The Margin unit’s chemistry adds a layer of interest. It is rich in magnesium carbonates — one of the primary reasons scientists selected Jezero Crater as a landing site. On Earth, carbonate rock preserves biological signatures with unusual fidelity. Cardarelli pointed to the Cliffs of Dover as an example: carbonate formations that contain abundant fossilized material. A buried delta within carbonate-rich rock, subjected to repeated water flow, fits the profile of an environment where microbial life could have taken hold.
“I think it’s a promising place to look for signs of biosignatures at depth,” Cardarelli said. “Microbial life could have potentially developed in those types of environments.”
The subsurface delta predates the crater’s visible Western Delta, the fan-shaped feature that has anchored Perseverance’s surface science since landing. Its existence suggests Jezero Crater experienced a longer and more complex hydrological history than the surface geology alone implies — with layers of that history now locked tens of meters underground, beyond the reach of any instrument currently on Mars.
Photo by Zelch Csaba on Pexels
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