Europa and Enceladus, the icy moons of Jupiter and Saturn respectively, are believed to have oceans hidden beneath their icy surfaces. A NASA study indicates that if these oceans can support life, signs of that life—through organic molecules such as amino acids and nucleic acids—might be preserved just below the ice, despite the intense ionizing radiation that permeates these celestial bodies. Robotic landers sent to investigate these moons wouldn’t need to dig deep to discover amino acids that have remained intact against radiation damage.
Europa, the moon of Jupiter, and Enceladus, the moon of Saturn, are thought to have oceans lying beneath their ice crusts. Findings from a NASA study suggest that if these oceans are capable of sustaining life, hints of that life, represented by organic molecules (like amino acids and nucleic acids), could endure just beneath the ice surface despite the extreme radiation found there. Robotic landers deployed to these moons would only need to scrape the surface lightly to locate amino acids that have withstood the effects of radiation.
“According to our experiments, the optimal depth for collecting amino acids on Europa is nearly 8 inches (approximately 20 centimeters) in the high-latitude area of the trailing hemisphere—where surface disturbances from meteorite impacts are minimal,” noted Alexander Pavlov from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. He is the lead author of a recent paper on this research published on July 18 in Astrobiology. “On Enceladus, subsurface sampling isn’t necessary; amino acids persist just below the surface, even less than a tenth of an inch beneath the ice.”
The icy, nearly airless environments of these moons are generally regarded as inhospitable due to radiation from energetic particles trapped in their parent planets’ magnetic fields and from cosmic events such as supernovae. However, the moons contain internal oceans heated by tidal forces from their host planets and neighboring moons. These submerged oceans could potentially host life, provided they possess vital elements and energy sources essential for biological processes.
The research team employed amino acids in radiolysis (radiation breakdown) experiments to represent potential biomolecules on these icy moons. Amino acids can be produced by both living organisms and non-biological processes. However, discovering specific types of amino acids on Europa or Enceladus would hint at the existence of life since these molecules are integral to protein construction in Earthly life. Proteins play crucial roles, acting as enzymes that accelerate and regulate chemical reactions and forming cellular structures. Some amino acids and organic materials from the buried oceans could be propelled to the surface through geysers or the gradual movement of the ice crust.
In their evaluations, the research team combined amino acids with ice that was cooled to around minus 321 Fahrenheit (-196 Celsius), sealed in air-tight vials, and subjected them to gamma-rays—a type of high-energy light—at various intensities. They also examined the resilience of amino acids encapsulated within deceased bacteria frozen in ice. Lastly, they assessed amino acids mixed with silicate dust to factor in the possibility of surface ice and material from meteorites undergoing mixing.
The experiment provided crucial data on how quickly amino acids decompose, specifically their radiolysis constants. Using these constants, the researchers calculated the depth and precise locations on Europa and Enceladus where 10% of the amino acids would likely remain intact despite radiation exposure.
While previous studies have examined amino acid survivability in ice, this research uniquely utilized lower levels of radiation that do not entirely decompose the amino acids. Alterations or degradation of these molecules is typically enough to hinder the ability to ascertain potential biological significance. This is the first study to simulate Europa and Enceladus’ environmental conditions to monitor amino acid stability in microorganisms, as well as the impact of dust on amino acid preservation.
The findings suggested that amino acids weaken more quickly when mixed with dust but degrade at a slower rate when found in bacteria.
“The slow degradation rates of amino acids in biological samples under conditions resembling those of Europa and Enceladus strengthen the argument for future missions aimed at detecting life on these moons,” stated Pavlov. “Our findings reveal that potential organic biomolecules are likely to degrade faster in silicate-rich regions compared to pure ice, suggesting that future explorations should adopt a cautious approach when sampling silica-rich areas on both of these icy satellites.”
One possible reason amino acids last longer within bacteria is due to the protective nature of cellular material against the disruptive effects of ionizing radiation, which can break chemical bonds directly or create reactive molecules nearby that further damage the targeted compounds.
This research was funded by NASA under contract number 80GSFC21M0002, through its Planetary Science Division’s Internal Scientist Funding Program as part of the Fundamental Laboratory Research project at Goddard, included in NASA’s Astrobiology NfoLD award 80NSSC18K1140.