Perseverance RIMFAX Reveals an Ancient Buried Delta on Mars

On a distant world populated solely by robots, NASA’s Perseverance rover is investigating the lonely floor of a crater. This dry Martian terrain was once part of a river system billions of years ago.

New findings show that the Jezero Delta on Mars is not the only surviving trace of the substantial water that once crossed the planet’s surface. Perseverance’s RIMFAX instrument has examined the ground beneath Jezero crater more deeply than before, uncovering an extensive delta system created by flowing water long before the delta currently being explored by the rover.

The result suggests that water travelled across Mars’s surface for far longer than surface features alone indicate, with significant consequences for assessments of the planet’s ancient habitability.

"Overall, RIMFAX elucidates a broader fluvial system than what was observed from orbit, and indicates an extended window of fluvial deposition, aqueous alteration, and habitable conditions than previously envisioned at Jezero crater," geomicrobiologist Emily Cardarelli of the University of California, Los Angeles told ScienceAlert.

"RIMFAX has revealed an earlier subsurface deltaic environment under the present-day delta, thereby extending the period of potential habitability for Jezero back further in time."

Mars and its ancient water history

Years of intensive investigation have established that Mars was not always the dry, rust-coloured dusty planet seen today. Evidence from landscapes carved by water to minerals that could have formed only in liquid water demonstrates that water once flowed there in abundance.

This evidence also prompts further questions. A key issue for habitability is the length of time for which liquid water remained at Mars’s surface. A longer period would have provided a wider opportunity for microbes to develop, which scientists consider the most probable type of life to have existed on Mars.

Mars’s landscape has generally remained exceptionally well preserved over billions of years because it has not experienced the tectonic and weather conditions found on Earth. The Jezero Delta being studied by Perseverance is believed to be roughly 3.7 billion years old, originating from the late Noachian to early Hesperian period.

Yet this was also an era when Mars is known to have possessed surface water, and flowing water at the surface encourages greater erosion and the deposition of sediment.

RIMFAX reveals a buried Jezero Delta

Scientists have been puzzled by the origin and development of certain mineral deposits in Jezero crater, particularly the carbonate- and olivine-rich unit called the Margin. To determine how the Margin unit formed, the team used Perseverance’s RIMFAX ground-penetrating radar to search below the surface for evidence.

Across 78 traverses conducted from September 2023 to February 2024, Perseverance repeatedly made ground-penetrating radar observations along a route measuring about 6.1 kilometres. The readings reached depths of more than 35 metres.

When the researchers assembled the observations, they revealed a concealed deltaic landscape beneath the surface.

"The moment we saw the Sol 909 radargram, we realized that this unit was more transparent to the radar than other units that we had seen before. As we continued driving on the Margin unit, we saw deeper and deeper into the subsurface down to up to 35 meters," Cardarelli said.

"The Sol 1052 radargram was particularly exciting to see because we began to see complex features at depth that we hadn't seen before!"

The radar observations showed numerous rock layers continuing far underground. Their inclined arrangement resembles patterns commonly produced on Earth when sediment settles from water as it enters a broad basin.

The team further detected lobes and channels compatible with creation by running water, as well as scours, backseats, and buried boulders.

"These are common features for river systems to develop, though the preservation is not always guaranteed because river systems are dynamic," Cardarelli said.

Although radar can investigate only tens of metres beneath the ground at an individual site, scientists can reconstruct a substantially thicker deposit by combining readings from Perseverance’s whole traverse.

The Margin unit and Mars habitability

The combined dataset indicates that the Margin may be as much as 90 metres thick. It appears to have resulted from repeated depositional episodes, separated by signs of some erosion. Using Jezero crater’s geological setting, the researchers concluded that the area supported a working delta system as early as the Noachian, around 4.2 to 3.7 billion years ago.

"We estimate that the Margin unit has a true thickness or actual vertical extent of at least 85 to 90 meters," Cardarelli said.

"The features we document range in size from submeter to hundreds of meters in length."

Together, the findings indicate that water on Mars was not limited to a short-lived interval. Instead, the planet underwent several phases in which flowing water altered and sculpted its surface. This longer history of water expands the possible timeframe in which life could have arisen.

"This work also may have implications for the preservation of potential biosignatures and habitability in the subsurface of Jezero crater," the researchers write.

"Fine-scale internal structures could preserve mineral compositions and geochemical conditions of past water-related events and may have once provided past habitable conditions."

The research has been published in Science Advances.