The discovery of abundant nickel in a once waterlogged region of Mars offers yet another hint that the red planet may once have offered suitable conditions for life.
In Neretva Vallis, an ancient channel that once carried water into the Jezero Crater delta, researchers found nickel in concentrations higher than ever seen before in the bedrock of Mars. Placed in its broader geological context, the metal offers clues about the chemical history of the region and adds a new piece to the puzzle of the planet's past habitability.
"While nickel has been detected on Mars before, this is our strongest detection to date outside of iron-nickel meteorites found on the Martian surface," planetary scientist Henry Manelski of Purdue University told ScienceAlert.
"Generally, nickel is a trace element on the surfaces of Earth and Mars because the vast majority of it migrates into the planets' cores during their formation. The substantial amount we have detected on the surface places unique constraints on how these rocks formed and were subsequently altered."
Nickel isn't exactly scarce on Mars, but it's usually found in fragments of meteorites scattered across the surface.
In 2024, as NASA's Perseverance rover rolled across the long-dry Neretva Vallis, it came across some unusual rocks, including an unusually pale section of exposed bedrock that scientists named Bright Angel.
Bright Angel was found to contain some curious features that are often associated with microbial activity on Earth, including iron-sulfide minerals similar to pyrite – a mineral commonly found in microbe-rich environments – and organic compounds.
As part of its operations, Perseverance collected composition data on many rocks across Neretva Vallis, which Manelski and his colleagues pored over to look for clues about how the rocks formed. It was during this analysis that a peculiarly strong signal of nickel emerged.
Of 126 sedimentary rocks and eight rock surfaces studied by Perseverance, the researchers found 32 with nickel concentrations up to 1.1 percent by weight. But it's what else was in those rocks that starts to bring the story together.
"Nickel-rich iron-sulfide is observed on Earth in ancient sedimentary rocks. Iron sulfide weathers easily in oxygen-rich environments, so its presence in ancient terrestrial rocks is one line of evidence used to demonstrate that Earth's early atmosphere was once very oxygen-poor," Manelski explained.
"This is in stark contrast to another environment where nickel is often found on Earth: laterites, which are highly weathered ancient soils. Observing nickel in iron-sulfide suggests these rocks likely formed in a reducing (oxygen-poor) environment."
The presence of these minerals also points to a dynamic watery environment. The rocks of Neretva Vallis appear to have been shaped by flows of water moving through the sediments, driving chemical reactions over time.
The researchers believe that nickel may have been delivered as part of a meteorite, then dissolved and redistributed by water. But here's where it gets interesting: On Earth, nickel is an essential element for many organisms, including microbes.
The concentrations of nickel the researchers found suggest that it may have been available for use by living organisms (although they make no claims that any life was around to tap into it).
The rocks analyzed by Perseverance also showed the presence of organic compounds, molecules that contain carbon, the element on which all life on Earth is based. Carbon can form in many non-biological ways, of course, but, like water, it's something that life as we know it cannot do without.
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"As we search for evidence of life on ancient Mars, it is useful to draw parallels to life on ancient Earth. Life around 3.5 to 4 billion years ago – the approximate age of Jezero Crater – was dominated by anaerobic microbes," Manelski said.
"Our detection of high nickel abundances directly adjacent to our first discovery of organic carbon and macroscopic zones of reduced sulfur suggests nickel was bioavailable. This further supports the idea that the ingredients for life were present on ancient Mars."
The findings also raise questions about when these conditions existed. The rocks of Neretva Vallis may be younger than other parts of Jezero Crater, suggesting that potentially habitable environments on Mars were not limited to its earliest history.
"Our finding of a seemingly habitable environment for ancient microbial life implies that our search for biosignatures in ever older rocks could be somewhat misplaced," Manelski said, "and we should remain open-minded to exciting discoveries wherever our rovers explore."
The findings have been published in Nature Communications.