A River Older Than the One We Could Already See
When NASA targeted Jezero Crater as the landing site for the Perseverance rover, the reasoning was straightforward: orbital images clearly showed a fan-shaped river delta, the kind of sedimentary deposit that forms when a river carrying sediment flows into a standing body of water. But new data from Perseverance's RIMFAX ground-penetrating radar instrument has revealed something that orbital cameras could never see: a second, older river delta buried tens of meters beneath the one geologists have been studying on the surface.
Published in the journal Science Advances, the findings indicate that Mars was an even wetter and more geologically complex world during its ancient past than the visible geology suggests.
How RIMFAX Sees Underground
The Radar Imager for Mars Subsurface Experiment fires low-frequency radio waves into the ground with each 10-centimeter advance of the rover's wheels. When those waves hit a boundary between materials of different densities, some of the signal is reflected back to the surface antenna. By analyzing the timing, strength, and character of these echoes, scientists construct a two-dimensional vertical cross-section of the subsurface, analogous to a geological sonogram.
During 78 traverses across the floor of Jezero Crater, the rover collected radar soundings penetrating to depths of more than 35 meters, roughly twice as deep as RIMFAX had imaged in previous campaigns. At that depth, the instrument revealed steeply inclined reflectors — the classic radar signature of cross-bedded sedimentary layers laid down by a flowing river that was depositing material in pulses as it built a delta into a lake.
A Deeper and Older Water System
The geometry of the buried reflectors is telling. The cross-bedded layers dip at angles consistent with foresets — the inclined strata that build up on the leading edge of an advancing delta. This architecture is distinct from the near-horizontal reflectors that characterize the overlying, better-known delta deposits, suggesting the two systems formed at different times and possibly under different hydrological regimes.
Lead author Emily Cardarelli and her colleagues conclude that the buried deposit predates the surface Western Delta by a significant margin. Their interpretation is that Jezero experienced at least two distinct phases of river and lake activity, separated by a period of erosion or deposition that buried the older delta under volcanic or eolian material.
The age estimates for both systems place them in the Noachian and early Hesperian periods, roughly 3.5 to 4 billion years ago, when Mars is thought to have had a thicker atmosphere capable of sustaining liquid water for geologically meaningful timescales.
Implications for the Search for Life
River deltas are among the most attractive targets for astrobiology because they concentrate and preserve organic material transported from a broad watershed. If ancient microbial life existed in the Jezero watershed, the sediment deposited in the delta's foreset beds would have been among the best places for that evidence to be entombed and shielded from radiation.
The buried delta is not directly accessible to Perseverance's surface instruments — drilling to 35 meters is well beyond current rover capability — but its discovery changes the calculus of Mars Sample Return. The rover has been collecting core samples from the visible delta and crater floor; this new finding suggests that deeper subsurface samples might represent even older and better-preserved potential biosignature repositories.
The Jezero results echo radar findings from China's Zhurong rover at Utopia Planitia, which detected buried coastlines possibly representing a vast ancient ocean. Together, the two datasets build a picture of a Mars that was hydrologically active across multiple latitude bands and geological epochs, strengthening the case that the planet was habitable long enough to potentially support life.
This article is based on reporting by Mashable. Read the original article.
