Life on Mars? Ancient bacteria may have survived for 280 million years

NASA and the European Space Agency (ESA) aim to get Mars samples back to Earth at some point in the 2030s thanks to their incredibly ambitious Mars Sample Return mission.

When they do, they should look carefully for ancient sleeping bacteria, a new study published in the journal Astrobiology explains.

The new paper highlights the fact that we may find living microbial life in red planet samples, as it may have been lying dormant under the surface for eons. This is because bacteria could survive just under Mars' surface for much longer than previously thought.

Dormant life on Mars

The new findings prove that certain strains of bacteria can survive even in Mars' harsh environment, drawing up the possibility that future crewed missions could contaminate the red planet and bring Martian bacteria back to Earth.

Crucially, the discovery also means that future Mars drilling missions, including ExoMars and the Mars Life Explorer, are more likely to find bacterial life two meters below the red planet's surface than previously thought.

"Our model organisms serve as proxies for both forward contamination of Mars, as well as backward contamination of Earth, both of which should be avoided," Michael Daly, a professor of pathology at Uniformed Services University of the Health Sciences (USU) and member of the National Academies' Committee on Planetary Protection, who led the study, explained in a press statement.

The study proves that microbial life could be found today only a short distance below Mars' surface. Still, its authors also caution that bacteria from Earth could hitch a ride aboard a future Mars mission, causing irreversible problems in the pursuit of scientific knowledge.

“We concluded that terrestrial contamination on Mars would essentially be permanent — over timeframes of thousands of years,” said Hoffman, a senior co-author of the study. “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.”

Simulating Mars' harsh environment

In order to reach their new findings, the scientists first set out to simulate the harsh -80 degrees Fahrenheit (-63 degrees Celsius) conditions of the red planet's mid-latitude regions. If the freezing conditions aren't bad enough, Mars is constantly bombarded by cosmic radiation. These factors have long been thought to make the red planet's surface inhospitable.

The researchers first determined the ionizing radiation survival limits of microbial life. They then placed six different types of Earth bacteria and fungi in a freezing, dry space to simulate Mars' surface and zapped them with gamma rays and protons to mimic space radiation.

“There is no flowing water or significant water in the Martian atmosphere, so cells and spores would dry out,” Hoffman said. “It also is known that the surface temperature on Mars is roughly similar to dry ice, so it is indeed deeply frozen.”

Based on their experiment, the researchers found that some of Earth's microorganisms could potentially survive on Mars for hundreds of millions of years. They also singled out one microbe, called Deinococcus radiodurans, stating that it is particularly capable of surviving Mars' environment. During the experiments, it could outlast Bacillus spores, which can survive for millions of years on Earth. The researchers measured the bacterias' resistance to radiation via the buildup of manganese antioxidants in the bacteria cells, which they measured using advanced spectroscopy techniques.

The scientists also exposed the bacteria to smaller doses of radiation to simulate the amount they would receive only a few meters under the Martian surface. Ultimately, they found that just 10 centimeters below the Martian surface, Deinococcus radiodurans could live for 1.5 million years. Ten meters underground, meanwhile, it could live a staggering 280 million years. That means a similar bacteria that evolved when flowing water still existed on Mars' surface could still be dormant below the red planet's surface.

“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,” Daly explained. “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.”