In 2008, NASA's Phoenix Lander generated headlines when its thruster exposed subsurface water ice under its landing spot. It then used its robotic arm to dig beneath the surface, where it exposed more ice. Orbiters like Mars Express and the Mars Reconnaissance Orbiter added additional evidence for subsurface ice with radar and imaging. Now, scientists think that the planet may hold vast amounts of water ice under its surface, enough to potentially cover the entire surface of Mars with 1.5 meters of liquid water.
Scientists have wondered whether extant life could persist in briny water pockets associated with all of this ice, and new research shows that the ice itself may hold evidence of ancient life.
All of this ice constitutes a natural deep freeze that could refrigerate microbial amino acids or even microbes themselves for up to 50 million years, according to new research. Ensconced in the ice, the acids or microbes would be protected from harmful cosmic radiation. A future rover equipped with a robotic digging tool could search for it.
The new research is titled "Slow Radiolysis of Amino Acids in Mars-Like Permafrost Conditions: Applications to the Search for Extant Life on Mars" and is published in the journal Astrobiology. The lead author is Alexander Pavlov, a space scientist at NASA's Goddard Space Flight Center.
For Martian bacteria to persist for 50 million years, it has to somehow be protected from deadly radiation. On Earth, cosmic radiation is impeded from reaching the surface by the planet's magnetic field and thick atmosphere. Mars has neither of those, so the radiation bombards the planet's surface unimpeded. "Galactic and solar cosmic rays constantly bombard the martian surface and transform and degrade organic biomolecules over time, eventually destroying chemical evidence of life," the researchers write.
The new research shows that microbes, or at least their organic remains, could be safe from the radiation in the Mars' subsurface water ice.
To reach that conclusion, the researchers placed E. coli bacteria in sealed tubes with pure water. Other E. coli was placed in tubes with water and different components of the Martian regolith, like clay and silicate-based rocks. The samples were then frozen and subjected to bombardment by gamma radiation in a special lab chamber. The temperature in the chamber was lowered to -51 C (-60 F) to mimic the Martian temperature.
The E. coli samples were blasted with the equivalent of 20 million years of cosmic ray exposure then modelled with an additional 30 million years of exposure to reach the equivalent of 50 million years of exposure on the Martian surface. They were then analyzed for the presence of amino acids.
"We studied radiolytic degradation of the individual amino acids (glycine, alanine, and isovaline) in icy mixtures," the authors write. "We found that the radiolytic degradation of amino acids in pure ices was significantly slower than the radiolysis of amino acids in silicates under Mars-like temperatures."
"We found that amino acids in the surface ice on Mars would survive over 50 million years of cosmic ray exposure, which is far greater than the expected age of the current surface ice deposits on Mars," the researchers write in their article.
Interestingly, the water ice samples preserved the amino acids far longer than the Mars-like sediment samples did. 10% of the amino acids in the water ice samples survived the 50 million years of radiation, while the amino acids in the Mars-like sediments were degraded 10 times faster and did not survive. In a montmorillonite sample, the radiation created hydroxyl radicals which accelerated glycine destruction.
"Our calculations indicate that a significant fraction of amino acids in the surface ice on Mars would survive over 50 million years of exposure," the researchers write in their article. "Amino acids from dead biological material (E. coli) dissolved in ice would survive even longer."
These results disagree with previous work by the same group of NASA researchers which showed that amino acids in a 10% water ice/90% Martian soil mixture were destroyed more quickly than amino acids in a sediment-only sample. “Based on the 2022 study findings, it was thought that organic material in ice or water alone would be destroyed even more rapidly than the 10% water mixture,” said lead author Pavlov. “So, it was surprising to find that the organic materials placed in water ice alone are destroyed at a much slower rate than the samples containing water and soil.”
The researchers hypothesized that a film created where ice touches minerals could allow the radiation to reach the amino acids. In pure water ice, no such film can form, protecting the amino acids from the radiation.
“Fifty million years is far greater than the expected age for some current surface ice deposits on Mars, which are often less than two million years old, meaning any organic life present within the ice would be preserved,” said co-author Christopher House, director of the Penn State Consortium for Planetary and Exoplanetary Science and Technology.
These results have implication for future Mars missions. MSL Curiosity is exploring Gale Crater to determine if Mars once had conditions suitable for life. Perseverance is exploring Jezero Crater seeking evidence of ancient life in ancient rocks. But these results imply that a future mission should be searching under the Martian regolith for evidence of ancient bacteria.
“That means if there are bacteria near the surface of Mars, future missions can find it,” House said in a press release.
“There is a lot of ice on Mars, but most of it is just below the surface,” House said. “Future missions need a large enough drill or a powerful scoop to access it, similar to the design and capabilities of Phoenix.”
The Phoenix Lander carried a robotic arm that extended 2.35 m, and had the ability to dig down to 0.5 m below the surface. It gathered samples that were then analyzed by onboard scientific instruments. Phoenix operated for 157 sols, or 161 days, and gathered 131 samples. It also moved rocks out of the way to examine underlying material. Advanced robotic arms and onboard labs are no problem for NASA, so any future mission could duplicate this and provide even better results.
"Based on our experiments, locations with pure ice or ice-dominated permafrost would be the best places to look for recently deposited amino acids on Mars and, thus, should be considered as a target sampling location for future Mars missions searching for extant life," the researchers write in their article.