Perseverance reveals ancient rivers beneath Jezero Crater

NASA’s Perseverance rover was primarily sent to investigate an ancient lakebed in Jezero Crater. Fresh measurements from beneath the surface now suggest that the history of water on the Red Planet began much earlier in this area than expected – and was more complex, dynamic and hospitable to life than earlier models had indicated.

A rover, a crater and an awkward question

Perseverance has been travelling across Jezero Crater since touching down in February 2021. Years before its arrival, researchers had identified features from orbit that closely resembled a dried-up river delta. The assumption was that a lake once occupied the crater, supplied by a river system flowing in from the surrounding landscape.

The rover’s earliest findings fitted that scenario extremely well. Perseverance detected carbonate-rich rocks on the crater floor, consistent with an ancient lake environment. High-resolution cameras also revealed sediments deposited in layers around the delta’s edge – strong evidence for a calm basin that received fine material over an extended period.

New data from depths of up to 35 metres now indicate that an even older river network lies concealed beneath the delta layers.

That raises an important question: have we viewed this region’s water history as too brief and too straightforward?

Ground-penetrating radar on Mars: how Perseverance “sees” below ground

To tackle precisely this kind of mystery, the rover was equipped with an instrument more commonly associated with ground surveys or archaeology: ground-penetrating radar, or GPR. On Perseverance, the instrument is called RIMFAX.

Its operating principle is remarkably simple:

• A transmitter sends high-frequency electromagnetic waves into the Martian ground.
• When a wave encounters a boundary – for example, between sand and denser rock – part of the signal is reflected.
• A receiver aboard the rover records these returning waves.
• The travel time of the signals can then be used to reconstruct the depth of the layers.

Depending on the frequency used, radar can reach from only a few decimetres to several dozen metres below the surface. As Perseverance drives, it scans a subterranean cross-section, rather like a medical CT scan drawn horizontally across the landscape.

Hidden structures: ancient rivers beneath the delta

Researchers have now examined these radar readings in detail. Measurements taken along the outer edge of Jezero Crater reveal distinct, repeating patterns below ground, far beneath the visible delta formations.

The signals point to ancient channels, packages of layers and inclined sediment bodies that strongly resemble buried river courses on Earth.

Specialists’ interpretations point to several possible scenarios:

• a meandering river with broad bends that repeatedly shifted its channel;
• an extensive alluvial fan, where water from the highlands washed material into the crater; or
• a braided network of many river channels, similar to braided rivers in polar or mountainous areas.

Which explanation is correct in every detail remains unresolved. What is clear, however, is that a large-scale, active river system existed before the delta complex visible today was formed. The delta so prominent in images from Mars orbiters appears to be only the most recent chapter in a far longer history of water.

The timescale: wet conditions began much earlier than expected

Placed in the context of Martian geology, the finding becomes particularly significant. The newly identified structures are attributed to the early Noachian. This period dates to roughly 4.2 to 3.7 billion years ago, making it a very early phase in the planet’s evolution.

By contrast, the visible delta in western Jezero Crater appears considerably younger. It is likely to have developed around the transition from the late Noachian to the Hesperian, approximately 3.7 to 3.5 billion years ago.

Era	Approximate period	Significance for Jezero
Early Noachian	4.2–3.7 billion years ago	Radar structures, ancient river networks, early water phase
Late Noachian / early Hesperian	3.7–3.5 billion years ago	Visible delta in Jezero Crater, younger lake

This substantially extends the period during which liquid water was present in this region. Rather than a single “wet interlude”, the data portray several phases in which rivers were active and deposited sediment.

What this means for the search for life

If water flowed for longer and during multiple episodes, the likelihood that habitable niches developed increases. Long-lived water-bearing systems may have created chemical gradients that microorganisms could use, much as they do on Earth in river deltas, thermal springs and coastal areas.

The longer a region remains wet, the more time potential life has to emerge, adapt and leave traces in the rock.

Perseverance is deliberately collecting rock samples from different layers, with the aim of bringing them to Earth one day through a sample-return mission. The new radar results now help place those samples within a broader timeline. A drilled sample from an underground layer becomes substantially more valuable if it can be shown to belong to a very early river phase.

For astrobiology, this creates something like a shortlist of especially promising sediments: areas where several water phases lie on top of one another may be particularly rich in preserved signatures, whether organic molecules, mineral structures or textures that point to microbial mats.

Why comparisons with Earth are useful

For decades, geophysicists and sedimentologists have used ground-penetrating radar on Earth to reconstruct former river systems. In desert landscapes, beneath farmland and in coastal deltas, it can reveal buried channels and ancient shorelines without excavating a single metre.

Many of the patterns now recognised beneath Jezero are strikingly similar to terrestrial examples. Inclined packages of layers may, for instance, represent riverbanks that gradually migrated sideways. Lens-shaped bodies often suggest former channels or side branches that were later filled in.

The comparison never makes an interpretation completely certain, but it gives researchers a sound foundation. They test on Earth: “What does a buried meander look like in radar data?” – and then cautiously apply that experience to Mars. Without such a reference, this would scarcely be possible.

A brief guide to key terms

What makes ground-penetrating radar so useful

Ground-penetrating radar responds to differences in the electrical properties of materials. Dry, loose sand allows waves to travel differently from denser rock that may contain clay. Frozen water or salt deposits can also produce pronounced contrasts.

Mars adds another factor: its atmosphere is thin, while the surface is exceptionally dry and often heavily compacted. This can increase radar range because less energy is lost to moisture. At the same time, interpreting the readings remains difficult, as comparison data from boreholes or exposed rock sections are limited.

Delta, alluvial fan and braided river – what is the difference?

For readers who do not work with sedimentology every day, here is a short guide:

• Delta: Forms where a river enters a still body of water. The flow slows down and sediments are deposited in fan-shaped formations.
• Alluvial fan: A debris fan that usually develops at the mouth of a valley. Sudden changes in flow spread gravel, sand and silt across a broad fan.
• Braided river: A network of many shallow channels that constantly shift position. It is typical of regions with high sediment loads and variable water flow.

On Mars, these environments can no longer be observed directly; they can only be reconstructed from preserved sedimentary structures. The radar data suggest that Jezero may have passed through several such environments over time.

What comes next for Perseverance and the history of water

The rover is gradually moving into higher, younger layers and examining the delta complex from different elevations. Throughout this work, the ground-penetrating radar remains active almost continuously. Every new profile adds to the three-dimensional image of the subsurface.

In the coming years, scientific teams expect to develop a kind of “stratigraphy in 3D”: a digital model showing when individual river channels were active, when lake levels rose or fell, and when prolonged dry periods may have taken hold. These time-resolved maps could ultimately also reveal whether particular water phases were especially favourable for the formation of organic traces.

At the same time, new questions are emerging. Are there similar, previously hidden river systems in other craters? Can a global pattern be identified to show whether Mars remained wet across large areas for a long time – or only in a few regional oases? Future missions are likely to make even greater use of subsurface radar to find these missing pieces of the puzzle.

There is also a practical side effect for the prospect of a crewed mission to Mars. Anyone hoping to build stations or even infrastructure there one day must know where ancient sediments, voids or unstable deposits are located. Perseverance’s new view beneath the surface will aid not only scientific research, but also the long-term planning of safe sites.