Subsurface radar observations acquired by NASA’s Perseverance rover inside Jezero Crater have revealed a previously unknown buried river-delta system beneath the western fan deposits of the basin. The discovery comes from measurements made by the rover’s Radar Imager for Mars’ Subsurface Experiment (RIMFAX), which recorded inclined sedimentary layers and channel-like structures extending tens of meters below the surface. These reflections represent an earlier generation of deltaic deposition that predates the exposed delta landforms mapped from orbit before the mission began.

Radar observations reveal a buried delta

Perseverance traversed the western margin of Jezero Crater during an extended campaign between September 2023 and early 2024. Throughout this traverse, the rover’s RIMFAX instrument continuously transmitted radar pulses into the subsurface while recording returning reflections from layered materials below the surface.

Scientists later combined these radar profiles into a coherent subsurface stratigraphic model. The reconstructed geometry revealed inclined reflectors that dip toward the center of the crater basin. Such structures closely resemble foreset bedding produced during delta formation on Earth. In addition, the radar signals outlined channel-like features and stacked sediment lobes consistent with repeated sediment transport by flowing water.

Importantly, these reflections extend more than 35 meters beneath the present surface. Their depth confirms that the structures belong to an earlier depositional phase rather than the exposed delta deposits currently visible in orbital imagery and rover observations.

Jezero Crater: A long-lived lacustrine basin

Jezero Crater formed more than 3.7 billion years ago following a large impact event. Later, surface runoff breached the western crater rim, delivering sediment into the basin. Orbital observations conducted before the Mars 2020 mission identified a preserved delta at this entry point. That structure strongly influenced NASA’s decision to select Jezero as the landing site for Perseverance.

Scientists interpreted the exposed delta as evidence that the crater once hosted a standing lake capable of trapping fine sediments and preserving chemical signatures of its environment. However, the newly detected buried delta demonstrates that the hydrologic history of the basin extends further back in time than previously recognized.

Instead of representing a single depositional episode, the crater margin now appears to preserve stacked deltaic sequences formed during separate intervals of fluvial activity. This interpretation suggests that lake conditions developed repeatedly rather than during one isolated event.

Repeated episodes of sediment transport

The geometry of the radar reflections provides insight into how sediments accumulated within the crater. Inclined reflectors mark the progradation of delta fronts toward the basin interior. Meanwhile, nearly horizontal layers correspond to topset deposits formed along the upper delta surface.

These structures outline a classic delta architecture similar to that observed in terrestrial river-lake systems. Their thickness indicates sustained delivery of sediment over extended intervals rather than deposition during a single flood.

Some radar profiles also reveal truncation surfaces separating stacked sediment packages. These boundaries likely formed during periods of erosion between episodes of deposition. Such features support the interpretation that Jezero experienced multiple hydrologic cycles involving changing water levels and sediment supply conditions.

In addition, layered materials detected along the crater margin differ from the main delta deposits. These units may represent shoreline environments that developed as lake levels fluctuated through time.

Evidence for multiple phases of surface water activity

The presence of two superposed delta systems within Jezero Crater has important implications for understanding early Martian climate evolution. Surface water cannot remain stable under present Martian atmospheric conditions. Therefore, each preserved delta marks a period when the environment supported flowing liquid water.

The buried delta indicates that such conditions existed earlier than the formation of the exposed western fan deposits. Later, additional sediment entered the crater during a second phase of river activity, forming the younger delta mapped from orbit.

This sequence extends the duration of hydrologic activity recorded at the site. It also suggests that environmental conditions favorable for river flow developed more than once during the crater’s history.

Because delta formation requires sustained sediment transport through stable channels, these structures record extended rather than short-lived wet intervals. Consequently, the stratigraphy preserved in Jezero strengthens the case for recurring surface water during the late Noachian and early Hesperian periods.

Clear skies!

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