Scientists have identified eight possible cave openings on Mars that may have been formed by ancient water flows, rather than volcanic activity. These caves in the Hebrus Valles region show signs of past streams and water-related minerals, making them promising sites for evidence of ancient life. Experts suggest such environments could have sheltered life from surface radiation.
Caves carved by water that once flowed beneath Mars's surface could have provided ideal conditions for life on the Red Planet, potentially preserving traces of it today. While Mars features numerous hole-like cave entrances, most are linked to volcanic regions and likely formed by lava flows. However, evidence of karstic caves—formed by water dissolving soluble rock, as seen abundantly on Earth—has been absent until now, despite the planet's ancient watery past.
A team led by Chunyu Ding at Shenzhen University in China has pinpointed eight potential water-formed caves in the Hebrus Valles, a north-western area spanning hundreds of kilometers of valleys and depressions carved by ancient floods. These sites were mapped during NASA's Mars Global Surveyor mission, which orbited the planet from 1997 to 2006. Analysis of spectrometry data from the mission reveals high concentrations of carbonate and sulphate minerals around the cave entrances, substances that typically form in the presence of water.
The researchers also detected signs of ancient streams terminating near the entrances, mirroring patterns observed at Earth's karstic caves. "If you’re looking on a map, you would expect the stream to be on a surface, and then all of a sudden disappear, because the stream water is being pirated by the cave system," says James Baldini at Durham University in the UK.
Daniel Le Corre at the University of Kent, UK, notes that while the mineralogical and geological evidence points to water origins, the caves' appearance resembles known volcanic ones. "I’ve spent an inordinate amount of time looking at the Mars global cave catalogue, and these do look very similar to ones that are known to be volcanic in origin," he says.
If confirmed as water caves, they could be prime locations for astrobiology searches. "In order to have life, you need water and an environment that is sheltered from the intense radioactive bombardment on the surface of Mars," Baldini explains. Unlike volcanic caves, which may lack water involvement, these might harbor stalagmites—protruding rock formations that could serve as time capsules for Mars's ancient climate, recording details like temperature. However, forming such structures requires thousands of years of steady water flow, and dating them would pose significant challenges for future missions, such as rovers or drones.
The findings are published in The Astrophysical Journal Letters (DOI: 10.3847/2041-8213/ae0f1c).