Microbes may have survived for millions of years beneath the Martian surface

By Ashley Strickland, CNN

Ancient bacteria might be sleeping beneath the surface of Mars, where it has been shielded from the harsh radiation of space for millions of years, according to new research.

While no evidence of life has been found on the red planet, researchers simulated conditions on Mars in a lab to see how bacteria and fungi could survive. The scientists were surprised to discover that bacteria could likely survive for 280 million years if it was buried and protected from the ionizing radiation and solar particles that bombard the Martian surface.

The findings suggested that if life ever existed on Mars, the dormant evidence of it might still be located in the planet’s subsurface — a place that future missions could explore as they drill into Martian soil.

While Mars was likely a more hospitable environment for life billions of years ago, including an atmosphere and water on its surface, today the red planet is more like a frozen desert. The planet’s arid midlatitudes have an average temperature of minus 80 degrees Fahrenheit (minus 62 degrees Celsius). And then there’s the constant threat of radiation because Mars has such a thin atmosphere.

“There is no flowing water or significant water in the Martian atmosphere, so cells and spores would dry out,” said study coauthor Brian Hoffman, Charles E. and Emma H. Morrison Professor of Chemistry and professor of molecular biosciences in Northwestern University’s Weinberg College of Arts and Sciences, in a statement. “It also is known that the surface temperature on Mars is roughly similar to dry ice, so it is indeed deeply frozen.”

A research team determined the survival limits of microbial life when it is exposed to ionizing radiation like it might experience on Mars. Then, the team introduced six types of bacteria and fungi found on Earth to a simulated Martian surface environment — all while zapping them with protons or gamma rays to mimic space radiation.

A clear winner emerged called Deinococcus radiodurans. The microbe, nicknamed “Conan the Bacterium” due to its tough nature, seemed perfectly suited to life on Mars.

The bacteria is a polyextremophile, meaning it can survive harsh conditions such as dehydration, acid and cold temperatures. The hardy microbe is one of the most radiation-resistant organisms known to science.

Previous research has found that the bacteria could survive 1.2 million years just beneath the surface of Mars amid the harsh radiation and dry, frozen environment — and outlast some microorganisms known to survive on Earth for millions of years.

The new study determined that when Conan the Bacterium is dried, frozen and buried deep beneath the Martian surface, it could survive 140,000 units of radiation — 28,000 times greater than the level of radiation exposure that could kill a human.

The bacteria, which resembles a pumpkin when viewed beneath a microscope, would likely survive only a few hours on the Martian surface after relentless exposure to ultraviolet light. Conan the Bacterium’s expected survival increased to 1.5 million years just 4 inches (10 centimeters) below the surface, and about 280 million years if the bacteria was 33 feet (10 meters) down.

The journal Astrobiology published a study detailing the findings Tuesday.

The researchers were able to measure how many manganese antioxidants accumulated in the cells of the microorganisms as they were exposed to radiation. The more manganese antioxidants the team found, the more likely the microbe was able to resist the radiation and survive.

Conan the Bacterium’s genomic structure links chromosomes and plasmids together, meaning the cells stay aligned and can repair themselves after radiation exposure. And if a microbe similar to Conan evolved on Mars billions of years ago, when water still existed on the Martian surface, the bacteria’s dormant remnants might just be slumbering deep in the planet’s subsurface.

“Although D. radiodurans buried in the Martian subsurface could not survive dormant for the estimated 2 to 2.5 billion years since flowing water disappeared on Mars, such Martian environments are regularly altered and melted by meteorite impacts,” said study author Michael Daly, a professor of pathology at Uniformed Services University of the Health Sciences and member of the National Academies’ Committee on Planetary Protection, in a statement.

“We suggest that periodic melting could allow intermittent repopulation and dispersal. Also, if Martian life ever existed, even if viable lifeforms are not now present on Mars, their macromolecules and viruses would survive much, much longer. That strengthens the probability that, if life ever evolved on Mars, this will be revealed in future missions.”

The findings have implications for both returning Martian samples to Earth as well as landing crewed missions on Mars.

The Mars Sample Return program, an ambitious program jointly steered by NASA and the European Space Agency, will launch multiple missions to Mars to collect and return samples that were gathered by the Perseverance rover.

The rover team hopes that the rock and soil samples, taken from the site of an ancient lake and river delta in Mars’ Jezero Crater, could determine if life ever existed on the red planet. The samples might even contain microfossils of ancient microbial life.

Additionally, astronauts have the potential to accidentally deliver hitchhiking bacteria from Earth when they land on Mars.

“We concluded that terrestrial contamination on Mars would essentially be permanent — over timeframes of thousands of years,” Hoffman said. “This could complicate scientific efforts to look for Martian life. Likewise, if microbes evolved on Mars, they could be capable of surviving until present day. That means returning Mars samples could contaminate Earth.”

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