Perseverance’s radar revealed ancient subsurface river delta on Mars

Perseverance rover continues to surprise scientists. Just a few years after landing in Jezero Crater in 2021, the vehicle equipped with advanced ground-penetrating radar discovered something that could change our knowledge of Mars' past: an ancient river delta hidden tens of meters below the surface. This discovery suggests that Mars not only had temporarily wet conditions, but maintained a stable, life-supporting aquatic environment for a long period of time.

The discovery made with the help of the RIMFAX (Radar Imager for Mars Subsurface Experiment) instrument opens new possibilities in the search for traces of ancient microbial life. A team led by astrobiologist Emily L. Cardarelli from the University of California, Los Angeles analyzed radar data collected during a four-month measurement campaign and found evidence of a complex river system that functioned billions of years ago. The thing is, this hidden delta is located below the famous Western Delta, which scientists have been observing for years from orbit — which means Mars was visited by at least two separate phases of water activity.

This discovery not only redefines Mars' geological history, but also drastically increases the chances of finding biomarkers — physical or chemical traces of ancient life. If microbiological organisms existed on the Red Planet, they needed exactly the conditions we now see in Jezero's subsurface.

Radar penetrates Martian layers

RIMFAX is an extraordinary device in its operation. With each 10-centimeter movement of Perseverance, the radar sends electromagnetic waves into the ground. When these waves hit boundaries between different types of rocks, ice, or sediment layers, part of the signal bounces back. By analyzing the delay time and intensity of these reflections, scientists can reconstruct a two-dimensional, vertical cross-section through Mars' crust — something like an ultrasound, but for a planet instead of a patient.

What makes RIMFAX a particularly valuable tool is its ability to penetrate deep soil layers. During a measurement campaign lasting from September 2023 to February 2024 — that is, for more than 250 Martian days (sols) — Perseverance traveled through a geological area known as the Margin unit. This unit is an extensive deposit adjacent to the inner rim of the crater's entrance valley, occupying the space between the western deltaic sediments and the crater's rim.

What made this area particularly promising for research is its mineral composition. The Margin unit is rich in magnesium carbonates — a substance that has been one of the main reasons why scientists chose Jezero Crater as the landing site for Perseverance from the very beginning. On Earth, carbonates exhibit a remarkable ability to preserve chemical traces of life. "You could think of, for example, the Cliffs of Dover, which are entirely composed of limestone — they contain a huge amount of fossils," Cardarelli explains.

When the team analyzed RIMFAX data, they discovered something extraordinary: the rock forming the Margin unit was exceptionally transparent to radar waves. This homogeneous material with low signal loss allowed the radar to penetrate deeper than in any other previously studied region of Jezero Crater. Measurements reached below 35 meters — approximately 1.75 times deeper than at the crater floor or in the overlapping delta layers. Taking into account surface topography, the team estimates that the actual thickness of the Margin unit is at least 85-90 meters.

Layer geometry revealing a watery past

The real discovery, however, turned out to be the highly structured geometry of geological features that the radar observed at these depths. When geologists examine a cross-section of a river delta on Earth, they see distinct characteristic features that tell the story of water flow, sediment deposition, and changes in water level. Exactly this kind of structural layering was shown by RIMFAX data found below the Margin unit — geometric features ranging in size from tens of centimeters to hundreds of meters.

"We observed really high complexity in the subsurface," says Cardarelli. The radar images revealed parallel layers tilted toward the center of the Jezero basin at angles from three to 15 degrees. These broad, laterally continuous lines are a classic signature of what geologists call clinoforms — sediment layers that build outward into water as a river deposits material, creating underwater ramps.

The process is fascinating from the perspective of physics and geology. When a fast-flowing river carrying sand and gravel suddenly hits the calm, deep waters of a lake, it loses its kinetic energy and drops its load. The heaviest sediments quickly settle to the lake bottom, forming flat, horizontal layers called topsets. As sediment continues to accumulate and shift further into the lake, it eventually reaches a critical angle at the edge of existing sediments and cascades down the underwater slope, forming slanted layers called foresets. At the very bottom of the lake, finer sediments spread out into horizontal bottomsets.

The RIMFAX images apparently captured these transitions, known as rollover points, scattered throughout the Martian crust. These rollover points, the team believes, are a characteristic hallmark of a dynamic river environment that not only delivered large amounts of material once, but experienced many distinct episodes of continuous deposition over a long period of time. Now the hidden river delta looks remarkably similar in scale and structure to ancient deltaic environments preserved here on Earth.

Martian water history lasts longer than we thought

This subsurface architecture changes the perspective on the history of Jezero Crater's wet past. Radar data from Perseverance's 6.1-kilometer traverse clearly shows that the Margin unit physically lies beneath the Western Delta rocks. In geology, the principle is simple: what lies at the bottom is usually older.

If Cardarelli's interpretation is correct, it means that long before the Martian river system carved the massive Western Delta we see from orbit today, a completely different river system had already built a huge delta in exactly the same place. This hidden delta formed during the Noachian period — an era of Martian history when the planet was much warmer and wetter, lasting approximately 4.2 to 3.7 billion years ago. The presence of these deep sediment layers suggests that early Mars was not only temporarily wet, but likely maintained consistent, long-lasting conditions that allowed for systematic transport and deposition of enormous amounts of sediment over extensive geological timescales.

This discovery has profound implications for our understanding of Mars' evolution. If the planet maintained wet conditions for a significant part of the Noachian period, and then again through episodes that led to the formation of the Western Delta, then Mars had many opportunities for life to develop. Each of these water episodes could have provided a time window for the evolution of microbial life under favorable conditions.

Alternative explanations await verification

Although the interpretation of the delta hypothesis is convincing, Cardarelli herself and her team acknowledge that an ancient river delta, while very likely, is only one of several hypotheses that could explain the RIMFAX data. Scientists must be careful — the history of science is full of examples where initial interpretations turned out to be wrong.

The first alternative is the possibility that the geological features detected by RIMFAX were created as a result of magmatic processes. "We're talking about volcanic activity — pyroclastic events and volcanic ash fall," Cardarelli explains. The layers her team found could be solidified lava and ash that fell from distant volcanic eruptions. On Mars, where volcanism was a dominant geological force for a long time, this scenario is entirely possible.

Another idea suggested by the team is the possibility that the slanted layers could be remnants of an ancient lake shore. The team also considered a scenario in which the Margin unit was simply an area in front of a glacier, where streams of melting water deposited material washed from the ice, creating broad, layered plains. This process, known as outwash plains, is well known from polar areas on Earth.

"But I think what led us to favor the fluvial, deltaic hypothesis is simply the number and scale of the features observed and their complexity," says Cardarelli. Her leading explanation is also the one that is most favorable for potential Martian life. If microbial life ever existed on Mars, it needed stable, long-lasting aquatic environments. A huge delta system flowing into the crater lake should essentially provide the right combination of nutrients, water, and chemical energy.

Biochemical conditions favorable to life

The environment of a river delta flowing into a lake is particularly interesting for an astrobiologist like Cardarelli. River deltas on Earth are extraordinarily rich ecosystems, where different conditions converge: fresh water from the river, salty lake water, nutrient-rich sediments, various sources of chemical energy. These are places where life flourishes in various forms.

On Mars, if such a river delta truly existed for a long period of time, it could have been an ideal place for the development of microbial life. Organisms could draw energy from various sources: iron oxidation, sulfur reduction, fermentation of organic compounds. The magnesium carbonates present in the Margin unit could protect and preserve traces of this life for billions of years — exactly what scientists are looking for.

"We know there are potential signs of past microbial life on the surface of Jezero Crater," says Cardarelli. "Now we see that there was a water history there for quite a long time." This combination — long-lasting water conditions plus the presence of carbonates — makes Jezero one of the most promising places in the entire Solar System to search for traces of ancient life.

Just the beginning of RIMFAX discoveries

Something that is particularly intriguing about this discovery is the fact that it represents only the tip of the iceberg. Cardarelli's study was based on data from Perseverance's 6.1-kilometer traverse. But here's the catch — the rover has 40 kilometers of RIMFAX data that have not yet been fully analyzed. This means that potentially an enormous amount of new discoveries await us.

"And we have 40 kilometers of data, so please wait for upcoming RIMFAX articles," Cardarelli said. "We have more to say about this area. There are many stories to tell." This statement suggests that the Perseverance team is confident that many significant discoveries still await them. Perhaps more hidden deltas, perhaps other geophysical anomalies, perhaps direct evidence of past biological activity.

RIMFAX research also has implications for future missions to Mars. If Mars' subsurface contains such complex geological structures and potential biomarkers, then future missions may need more advanced tools for deep drilling and sampling. Current vehicles, like Perseverance, can only observe from the surface and dozens of meters deep. To actually find fossils or traces of life, it may be necessary to send deep drilling equipment to Mars.

Consequences for astrobiology and the search for extraterrestrial life

This discovery changes the paradigm in astrobiology. For years, scientists focused on searching for life in places where water is visible — on the surface, in craters, in river valleys. But now we know we must also look deeper. If hidden river deltas existed on Mars, there may also be other hidden geological structures that favored life — underground lakes, hydrothermal vents, perhaps even a deep biosphere.

The Perseverance discovery also suggests that Mars had a more complicated water history than we previously believed. Instead of a simple model where Mars was wet for a short time and then dried up, we now see a picture of a planet with multiple episodes of water activity spread over time. Each of these episodes could potentially have supported life.

This also has implications for the search for life on other planets and moons. If life could survive on Mars in the subsurface, it could also survive on Europa (Jupiter's moon), Enceladus (Saturn's moon), or other worlds with hidden oceans. Astrobiologists will now be more inclined to search for life not only on surfaces, but also deep beneath them.

The research by Cardarelli and her team, published in the prestigious journal Science Advances in 2026, represents a milestone in our understanding of Mars and the potential for extraterrestrial life. But as Cardarelli herself suggests, this is just the beginning. The coming years will undoubtedly bring more fascinating discoveries from RIMFAX and other Perseverance instruments. Mars' subsurface is slowly revealing its secrets — and each discovery brings us closer to the answer to one of the greatest questions in science: are we alone in the Universe?