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 traces of ancient life on Mars. This depth is well beyond the range of Perseverance, which is only equipped to drill a few inches into the Martian soil, the team reported in a study published on Friday in the journal Astrobiology.
“Amino acids are a very important organic biomarker,” said Pavlov in an email. “Terrestrial life in all its incredible diversity is very selective and uses only 22 amino acids despite hundreds of amino acids discovered in nature (meteorites).” In addition, he noted that Earth life favors a particular configuration of amino acids, called enantiomeric excess, meaning that the “discovery of a limited set of amino acids with strong enantiomeric excess can be a good sign of life on the ancient Mars.”
“Hence, we need to understand how long amino acids can survive on Mars and what abundance of the ancient amino acids can be expected at various sampling depths,” Pavlov added.
Earth is mostly shielded from cosmic rays 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. This approach distinguished it from past studies that examined gamma radiation on pure amino acids that are not mixed with Martian surface compounds, which Pavlov said “is not realistic on Mars.”
“It is hard to imagine a scenario where a chunk of pure amino acids would be present in the ancient Martian rock,” Pavlov explained. “Instead, amino acids (if present) will be mixed with some kind of silicates and salts.”
“Hence, we believed that in order to estimate the rate of radiolytic destruction of the amino acids on Mars, we need to expose amino acids mixed with silica and perchlorates,” he continued. “It turns out that mixing of amino acids with silica greatly increases the rate of amino acid degradation during exposure to gamma radiation.”
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.
“Microcraters (impact craters from small impactors) are common on Mars,” Pavlov said. "Small impactors can excavate rocks from several meters of depth. Cosmic rays are significantly reduced by two-meter depth into a rock and do not penetrate at all below four meters. Therefore, an ejecta from such depths would have small exposure time to cosmic rays and thus, may contain the primordial unaltered amino acids from billions of years ago.”
In addition to targeting microcraters, future missions that aim to search for life on the red planet may need to bring 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.
To that end, the researchers hope to build on their findings with new investigations of organic molecule degradation in salts and sulfates, comparisons of degradation rates of different organic molecules, and the decay of biomolecules in ices, among many other topics.
“We have only scratched the surface of this problem,” Pavlov concluded.
Update: This article has been updated to include comments from study lead Alexander Pavlov.