The search for life on Mars may have gotten a bit more complicated.
As robbers like Curiosity and Perseverance scour the surface for traces of ancient life, new evidence reveals that we may have to dig much deeper to find them. Any evidence of amino acids left over from a time when Mars was habitable is likely buried at least 2 meters (6.6 feet) underground.
That’s because Mars, with its lack of magnetic field and thin atmosphere, is subject to a much higher dose of cosmic radiation on its surface than Earth. We know this, and we know that cosmic rays destroy amino acids.
Thanks to experimental data, we now also know that geologically this process takes place on very short time scales.
“Our results suggest that amino acids are being destroyed by cosmic rays in the surface rocks and regolith of Mars much faster than previously thought,” says physicist Alexander Pavlov from NASA’s Goddard Space Flight Center.
“Current Mars rovers missions drill to about 5 centimeters deep. At those depths, it would only take 20 million years to completely destroy amino acids. The addition of perchlorates and water increases the rate of amino acid destruction even further.”
Cosmic rays are actually a major concern for Mars exploration. An average person on Earth is exposed to about 0.33 millisievert of cosmic rays per year. on Mars, that annual exposure could be over 250 millisieverts†
This high-energy radiation, from solar flares and energetic events such as supernovas, can penetrate rock and ionize and destroy any organic molecules it encounters.
Mars was once thought to have a global magnetic field and a much thicker atmosphere, just like Earth. There is also evidence – much of it – that liquid water once sat on the surface of Mars in the form of oceans, rivers and lakes.
This combination of features suggests that Mars could have been habitable (maybe) repeatedly) in the past.
One sign that could indicate the habitability of Mars is the presence of amino acids. These organic compounds are not biosignatures, but some of the most basic building blocks of life.
Amino acids combine to form proteins and have been found in space rocks, such as asteroid Ryuguand the atmosphere of Comet 67P† So they’re not a definitive sign of life, but finding them on Mars would be another clue that life might one day emerge.
Pavlov and his team wanted to better understand the likelihood of finding evidence of amino acids on the surface of Mars, so they designed an experiment to test the hardiness of these compounds.
They mixed amino acids with mineral mixtures designed to simulate Martian soils, consisting of silica, hydrated silica or silica, and perchlorates (salts), and sealed them in test tubes that mimicked the Martian atmosphere, at various Martian-like temperatures.
The team then irradiated the samples with ionizing gamma rays, to mimic the dose of cosmic rays expected to hit the surface of Mars over a period of about 80 million years. Previous experiments destroyed only the amino acids, without the ground simulants. This may have given an inaccurate amino acid lifetime.
“Our work is the first comprehensive study examining the destruction (radiolysis) of a wide range of amino acids under a variety of Mars-relevant factors (temperature, water content, perchlorate abundance) and comparing the rates of radiolysis,” Pavlov says†
“It turns out that the addition of silicates and especially silicates with perchlorates greatly increases the destruction rate of amino acids.”
That means all the amino acids on the surface of Mars are likely long gone, irradiated into thin air earlier than about 100 million years ago.
Given that the surface of Mars hasn’t been hospitable to life as we know it much longer, billions of yearsinstead of millions – the few inches that curiosity and persistence can dig for probably won’t accumulate amino acids.
Both the robbers to have found organic material on Mars, but since the molecules may have been produced by non-biological processes, they cannot be considered evidence of life. In addition, the team’s research shows that those molecules may have changed significantly since their formation by ionizing radiation.
There is also other evidence to suggest that the research team is on to something. Occasionally, material from beneath the surface of Mars actually makes its way to Earth. Amino acids have even been found in it.
“We identified several straight-chain amino acids in the Astrobiology Analytical Lab at Goddard in the Antarctic Mars meteorite RBT 04262 that we believe to have originated on Mars (no contamination from terrestrial biology), although the mechanism of formation of these amino acids in RBT 04262 remains unclear,” says astrobiologist Danny Glavin from NASA Goddard.
“Since Mars meteorites are typically ejected from depths of at least 1 meter (3.3 feet) or more, it’s possible that the amino acids in RBT 04262 were shielded from cosmic rays.”
We may have to wait until we have more hardcore digging tools on Mars to find out more.
The research was published in Astrobiology†
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