Far away, alone in a crater on a planet inhabited only by robots, NASA's Perseverance rover explores a dry landscape that was once a river system billions of years ago.
According to a new discovery, however, the Jezero Delta on Mars is not the sole remnant of abundant water that once flowed across the surface. Perseverance's RIMFAX instrument has now probed deeper than ever beneath the Jezero crater, revealing a vast delta system fed by flowing water that existed long before the one the rover now explores.
In turn, this indicates that water flowed across the surface of Mars for much longer than the surface alone implies – a finding with important implications for the planet's past habitability.
"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," writes a team led by geomicrobiologist Emily Cardarelli of the University of California, Los Angeles.
"This work also improves the chances of finding evidence for past life, as the formation of the present-day Delta Fan may have been rapid."
After many years of dedicated exploration, it has become clear that Mars wasn't always the arid, rusty dustball of a planet it is today. Multiple lines of evidence show that water once flowed abundantly, from water-carved landscapes to minerals that could only have formed in the presence of liquid water.
This raises other questions. One of the most pressing for habitability is how long liquid water persisted on the surface of Mars. A longer timespan offers a larger window for the emergence of microbes that scientists think are the most likely form of life that could have existed on Mars.
In general, the landscape of Mars has been preserved very well for billions of years, since it's not subject to the same tectonic and weather conditions we have here on Earth. The Jezero Delta that Perseverance is exploring is thought to be around 3.7 billion years old, dating back to the late Noachian to early Hesperian period.
However, that time is when Mars is known to have had surface water – and flowing surface water creates the conditions for stronger erosion and sediment deposition.
The formation and evolution of some of the mineral deposits in the Jezero crater have puzzled scientists, specifically a unit rich in carbonates and olivine known as the Margin. To investigate how the Margin unit formed, researchers used Perseverance's RIMFAX to probe deep under the ground for clues.
Over the course of 78 traverses between September 2023 and February 2024, Perseverance repeatedly took measurements with its ground-penetrating radar, collecting data along a trail some 6.1 kilometers (3.8 miles) long. Those measurements penetrated to depths greater than 35 meters (115 feet).
As the researchers began to piece together the data, a hidden deltaic landscape unfolded from the darkness.
The radar data revealed many layers of rock extending deep underground, arranged in sloping patterns that on Earth are typical of sediment settling out of water as it flows into a wide basin. The researchers also identified lobe and channel structures consistent with formation by flowing water.
Although the radar only probes tens of meters underground at any one location, combining those measurements across the entire length of Perseverance's traverse allows scientists to reconstruct a much thicker deposit.
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This combined measurement suggests that the Margin could be up to 90 meters thick, the result of multiple episodes of deposition, with evidence of some erosion between them. Based on the geological context of Jezero crater, the researchers estimated that the region hosted a functional delta system as early as the Noachian, some 4.2 to 3.7 billion years ago.
Taken together, the evidence suggests that Mars did not host water only during a brief period, but experienced multiple phases of water flowing over and shaping its surface. That extended water history opens the window of opportunity for life to have emerged.
"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.