Mars' crust could support life with radiation.

Underground, radioactive elements decay water molecules and produce ingredients that fuel underground life. This process, known as radiolysis, has kept isolated bacteria, gaps filled with water, and rock pores on Earth for millions to billions of years. According to a study published in Astrobiology, radiolysis could have fed microbial life in the subsoil of Mars.

Dust storms, cosmic rays, and solar winds damage the surface of the red planet. But underground, a certain life could take shelter. "The environment with the best chance of habitability on Mars is the subsurface," says Jesse Tarnas, a planetary scientist at NASA's Jet Propulsion Laboratory and the new study’s lead author. Examining the subsurface of Mars could help scientists determine if life could have survived - and the best subsurface samples available today are Martian meteorites that crashed on Earth.

Tarnas and colleagues evaluated the grain size, mineral composition, and abundance of radioactive elements in Martian meteorites and estimated the porosity of the Martian crust using satellite and rover data. They plugged these attributes into a computer model that simulated radiolysis to see how efficiently the process would have generated hydrogen gas and sulfates: chemical ingredients that can power the metabolism of underground bacteria. The researchers report that if water were present, radiolysis in the Martian subsoil could have kept microbial communities going for billions of years, and possibly even today.

Scientists have already investigated the radiolysis of Mars, but this marks the first estimate using Martian rocks to quantify the habitability of the sub-surface planet. Tarnas and his colleagues also evaluated the potential richness of life in the Martian underground and found that as many as a million microbes could exist in a kilogram of rock. (Geobiologists found comparable densities in the ground below ground.)

The most liveable meteorite samples analyzed appeared to be of the rocky type called regolith breccia. "It is believed that they originate from the southern highlands of Mars, which is the oldest land on Mars," says Tarnas.

Underground life, as described by this research, would require water — and it remains unknown if groundwater exists on the planet, says Lujendra Ojha, a planetary scientist at Rutgers University, who was not involved in the study. Determining whether the Martian crust contains water will be an important next step, but this investigation helps to motivate that search, Ojha says: "Where there is groundwater, there could be life."

This article originally appeared under the headline "Alien Underground" in Scientific American 325, 2, 18 (August 2021)