An image taken by NASA's Perseverance rover. Image: NASA/JPL-Caltech/ASU/MSSS

Life might have existed on Mars billions of years ago, when the planet was wetter and warmer. This is why NASA’s Perseverance rover is tasked with collecting samples that might contain the fossilized remains of any ancient Martians, if they existed. 

However, even if Martian fossils once littered the planet’s surface, those traces have likely been obliterated by eons of cosmic radiation, according to the first experiment that has ever mixed amino acids, the building blocks of life, with simulated Martian soil. 

A team led by Alexander Pavlov, a space scientist at NASA’s Goddard Space Flight Center, found that amino acids located in the top 10 centimeters (four inches) of the Martian surface would likely be annihilated by the bombardment of cosmic rays, which are high-energy particles emitted by stars and other powerful space objects, within a timespan of 20 million years. 

As a result, missions may need to dig at least two meters (seven feet) into the red planet’s surface to find intact ancient lifeforms on Mars. This depth is well beyond the range of Perseverance, which is only equipped to drill a few inches into the Martian soil.

“Mars can be degraded by exposure to cosmic rays that can penetrate to a depth of a few meters,” Pavlov and his colleagues said in a study published on Friday in the journal Astrobiology. “Our experimental results suggest serious challenges for the search of ancient amino acids and other potential organic biosignatures in the top [two meters] of the martian surface.”

Earth is mostly shielded from this radiation by its robust atmosphere and a protective magnetic field, whereas Mars lost both of these features early in its history, leaving it vulnerable to the damaging effects of these outer space pressures. Scientists have known about the dangers of radiation on Mars for many decades, not just to any Martian lifeforms but also for human crews that might endeavor to visit the planet. 

Pavlov and his colleagues have now demonstrated that two compounds that are common on Mars, silicates and perchlorates, exacerbate the deterioration of amino acids far more rapidly than previously known, creating a major barrier to the survival of extant lifeforms as well as the preservation of extinct organisms. The team reached this conclusion by combining different amino acids with silicates and perchlorates in fake Martian soil, known as simulant, in conditions mimicking the thin atmosphere and cold temperatures on Mars. 

​​“Our work is the first comprehensive study where the destruction (radiolysis) of a broad range of amino acids was studied under a variety of Mars-relevant factors (temperature, water content, perchlorate abundance) and the rates of radiolysis were compared,” Pavlov said in a NASA statement released Monday. “It turns out that the addition of silicates and particularly silicates with perchlorates greatly increases the destruction rates of amino acids.”

Unfortunately, the experiment suggests that any preserved fossils on Mars are likely buried well beyond the reach of the surface missions that are operating there right now. NASA’s InSight mission, a stationary lander that is currently winding down its mission, is equipped with an instrument that was designed to dig several meters into the surface, but this component wasn’t able to gain traction in the landing site and ultimately conked out at about an inch of depth.

That said, Perseverance and other surface missions might be able to find sites where underground layers have been brought to the surface by recent impacts with small meteorites. These craters and their ejecta might contain materials that were protected from cosmic rays for billions of years, making them relatively pristine and providing an alternate strategy for detecting any fossils on Mars. 

Regardless, future missions that aim to search for life on the red planet may need to consider bringing equipment that can reach deeper into its soil to look for signs of a once-habitable, or possibly inhabited, world. Underground excavations can be complicated here on Earth, let alone on an extraterrestrial planet, but the possibility of finding fossilized aliens on a nearby world may be alluring enough to justify the cost and effort.