Using ground-penetrating radar, NASA's Perseverance rover has detected underground remains of an ancient river delta on Mars in some of the oldest evidence yet obtained showing how water once ​flowed on the surface of Earth's planetary neighbor.

Researchers said the six-wheeled rover revealed geological features up to 35 meters underground while traversing ‌6.1 kilometers of terrain inside Jezero Crater, an area in the Martian northern hemisphere believed to have been flooded with water and home to an ancient lake basin long ago.

Perseverance identified layered sediments and eroded surfaces indicative of a delta environment, a large-scale fan-shaped deposit of sediment formed at the location where a river enters a larger body of water like a lake. They ​estimated that the now-buried delta dates to about 3.7 to 4.2 billion years ago. Mars, like Earth, formed roughly 4.5 billion years ago, meaning this ​delta existed relatively early in its history.

The researchers said this delta predated a similar nearby surface feature called the Western Delta that dates to about 3.5 to 3.7 billion years ago.

The rover's RIMFAX instrument sends radar pulses downward and records pulses bouncing back off underground features, allowing a ​three-dimensional mapping of the subsurface. The new findings were based on RIMFAX's deepest data collected to date, obtained from September 2023 to February 2024 over a span of 250 ​Martian days.

Because water is considered crucial to the possibility of past life on Mars, the mounting evidence of its wet past is of particular interest. Mars, now cold and desolate, long ago possessed a thicker atmosphere and warmer climate, allowing for liquid water on its surface.

"From the features mapped by RIMFAX, we believe that Jezero Crater hosted an ancient water-rich environment, capable of biosignature ​preservation that existed prior to the formation of Jezero's Western Delta," said Emily Cardarelli, a planetary scientist at the University of California, Los Angeles, a member of the Perseverance science team and lead author of ​the research published on Wednesday in the journal Science Advances.

A biosignature refers to chemical or physical evidence indicating past or present life.

On Earth, river deltas are places that concentrate sediments and create ‌niches favorable ⁠to microbial life.

Chinese scientists last year said ground-penetrating radar employed by China's ​Zhurong rover detected subsurface evidence of what look like sandy beaches from the shoreline of an ocean that may have existed ​on the northern plains of Mars.

"Over time, we've seen more and more evidence for liquid water on the Martian surface at various rover landing sites, areas we've traversed to, as well as from orbital imagery. We have seen channels where water may have flowed, crater lakes where water once ponded, and deltaic sediments deposited as rock outcrops and now as buried remnants, with this (research) paper," Cardarelli ​said.

"Mars is diverse, and each rover mission reveals another piece of its puzzling past and the early development of our rocky neighbor," Cardarelli said.