An ancient meteor strike on Mars has revealed the first direct evidence of how warm liquid water may have shaped a habitable underground environment.

Images taken by NASA’s Mars Reconnaissance Orbiter have revealed the first carbonate rocks found inside Leighton Crater at a level that was once buried 6 kilometers below the red planet’s surface.

Carbonates hold carbon dioxide and can form readily in the presence of water, but have previously been found only in a few scattered locations on Mars.

This first-time discovery of carbonates in an underground location points to a warmer epoch in the ancient martian past with more atmospheric carbon dioxide, as well as ancient seas. The carbonates also turned up alongside silicate minerals and clays that suggest the presence of hydrothermal systems — similar to the deep sea vents on Earth.

“This discovery doesn’t really hint at life, but it does reveal a very strong candidate for a habitable environment, perhaps the best discovered so far,” said Paul Niles, a planetary geologist with the NASA Johnson Space Center in Houston.

The ancient rocks may date back as far as 4 billion years, according to Joseph Michalski, a planetary geologist at the Planetary Science Institute in Arizona. That means they don’t necessarily reflect the martian underground as it exists today, but they do point to processes that could make the Mars subsurface a habitable place.

Researchers have long looked to the underground environment in hopes of finding martian life, because cold, dry conditions coupled with ultraviolet radiation on the surface of Mars makes existence up above unlikely.

“The subsurface environment provides a warm, stable environment that should be conducive for life to evolve given our limited understanding of that process,” Niles said in an e-mail.

The research is detailed in the Oct. 10 issue of the journal Nature Geosciences.