It's possible that the quest for life on Mars has just become much more difficult.
While rovers like Curiosity and Perseverance probe the surface in search of signs of prehistoric life, new data suggests we might need to go far deeper to uncover them. Any traces of amino acids from a possible habitable period on Mars are probably buried at least 2 meters (6.6 feet) below.
That's because Mars receives a significantly greater dosage of cosmic radiation on its surface than Earth does due to its weak atmosphere and lack of magnetic field. We are aware of both this and the fact that cosmic radiation annihilates amino acids.
Today, we also know that this process occurs on extremely brief geological timeframes because to experimental evidence.
"Our results suggest that amino acids are destroyed by cosmic rays in the Martian surface rocks and regolith at much faster rates than previously thought," according to physicist Alexander Pavlov of NASA's Goddard Space Flight Center.
"Current Mars rover missions drill down to about 2 inches (around 5 centimeters). At those depths, it would take only 20 million years to destroy amino acids completely. The addition of perchlorates and water increases the rate of amino acid destruction even further."
In reality, cosmic radiation is a major risk for Mars exploration. On average, a person on Earth receives 0.33 millisieverts of cosmic radiation annually. That yearly exposure on Mars may exceed 250 millisieverts.
This intense radiation may penetrate rock, ionizing and destroying any organic molecules it comes into contact with. It is produced by solar flares and other powerful natural phenomena like supernovae.
It is believed that Mars formerly had a considerably thicker atmosphere and a worldwide magnetic field similar to that of Earth. There is also ample evidence that seas, rivers, and lakes formerly existed on the Martian surface in the form of liquid water.
Due to a number of factors, it is possible that Mars once at a time may have supported life.
The existence of amino acids is one indicator that Mars may be habitable. These chemical substances are some of the most fundamental components of life, not biosignatures.
Asteroid Ryugu and the atmosphere of Comet 67P are two examples of space rocks that contain amino acids, which are the building blocks of proteins. They aren't a surefire indication of life, but discovering them on Mars would be another more hint that life may have once been there.
Pavlov and his colleagues devised an experiment to assess the durability of these substances in order to better understand the chances of discovering traces of amino acids on the Martian surface.
They combined amino acids with mineral mixtures made of silica, hydrated silica, or silica plus perchlorates (salts), intended to resemble Martian soil, then sealed the mixtures in test tubes that replicated the Martian atmosphere at a range of Mars-like temperatures.
The scientists next exposed the samples to ionizing gamma radiation to simulate the amount of cosmic radiation that would have been exposed to Mars' surface during an estimated 80 million years. The soil simulants were not used in earlier tests; just the amino acids were. The amino acids' expected lifetime may have been miscalculated as a result.
"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," explains Pavlov.
"It turns out that the addition of silicates and particularly silicates with perchlorates greatly increases the destruction rates of amino acids."
This implies that any amino acids present on Mars' surface earlier than around 100 million years ago have likely been blasted into nothing.
The few millimeters that Curiosity and Perseverance can dig down to on Mars' surface are unlikely to contain amino acids given that the planet's surface hasn't supported life as we know it for much longer than that - billions of years as opposed to millions of years.
Although both Mars rovers have discovered organic material, this cannot be considered proof of life because the molecules might have been created by non-biological processes. Additionally, the team's study suggests that those molecules may have undergone considerable ionizing radiation modification after being formed.
The study team may be onto something, according to additional findings. It is true that occasionally stuff travels to Earth from beneath the Martian surface. Even amino acids have been discovered there.
"We did identify several straight-chain amino acids in the Antarctic Martian meteorite RBT 04262 in the Astrobiology Analytical Lab at Goddard that we believe originated on Mars (not contamination from terrestrial biology), although the mechanism of formation of these amino acids in RBT 04262 remains unclear," according to NASA Goddard astrobiologist Danny Glavin.
"Since meteorites from Mars typically get ejected from depths of at least 3.3 feet (1 meter) or more, it is possible that the amino acids in RBT 04262 were protected from cosmic radiation."
However, finding out more may need us to wait until we have more powerful excavating equipment on Mars.
The research has been published in Astrobiology.
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