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NASA researchers have found that amino acidspotential indicators of life, could survive near the surface of Europe and Enceladus, moons of Jupiter and Saturn respectively.

Experiments show that these organic molecules can withstand radiation just below the ice, making them accessible to future robotic landers without deep drilling.

Exploring the life potential of icy moons

Jupiter’s moon Europa and Saturn’s moon Enceladus have evidence of oceans beneath their icy crusts. A NASA experiment suggests that if these oceans support life, signatures of that life in the form of organic molecules (eg amino acids, nucleic acids, etc.) could survive just below the surface of the ice despite the harsh radiation of these worlds. If robotic landers are sent to these moons to look for signs of life, they won’t have to dig very deep to find amino acids that have survived alteration or destruction by radiation.

“Based on our experiments, Europa’s ‘safe’ amino acid sampling depth is almost 8 inches (about 20 centimeters) at high latitudes in the hind hemisphere (hemisphere opposite to Europa’s direction of motion around Jupiter) in the zone, where the surface has not been disturbed much by meteorite impacts,” said Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, lead author of a paper on the study published July 18 in the journal. Astrobiology. “Subsurface sampling is not necessary to detect amino acids on Enceladus—these molecules will undergo radiolysis (breakdown by radiation) anywhere on Enceladus’ surface less than a tenth of an inch (below a few millimeters) from the surface.”

Dramatic jets, both large and small, spray water ice and vapor from many places along the famous “tiger stripes” near the south pole of Saturn’s moon Enceladus. Credit: NASA/JPL/Space Science Institute

The cool surfaces of these nearly airless moons are likely uninhabitable due to radiation from both high-velocity particles caught in the host planet’s magnetic fields and powerful events in deep space, such as exploding stars. However, both have oceans beneath their icy surfaces that are heated by the tides of the gravitational pull of the host planet and neighboring moons. These subsurface oceans could harbor life if they had other needs, such as energy supplies as well as elements and compounds used in biological molecules.

Experimental approaches and findings

The research team used amino acids in radiolysis experiments as possible representatives of biomolecules on icy moons. Amino acids can be created by life or by non-biological chemistry. However, finding certain types of amino acids on Europa or Enceladus would be a potential sign of life, as they are used by life on Earth as a component to build proteins. Proteins are essential to life because they are used to make enzymes that speed up or regulate chemical reactions and to create structures. Amino acids and other compounds from the subsurface oceans can be brought to the surface by geyser activity or the slow movement of the ice crust.

This view of Jupiter’s icy moon Europa was captured by JunoCam, the public engagement camera aboard NASA’s Juno spacecraft, during the mission’s close flyby on September 29, 2022. The picture is a composite of the second, third, and fourth images of JunoCam taken during the flyby, as seen from the perspective of the fourth image. North is on the left. The images have a resolution of just over 0.5 to 2.5 miles per pixel (1 to 4 kilometers per pixel). As with our Moon and Earth, one side of Europa always faces Jupiter, and that’s the side of Europa seen here. Europa’s surface is criss-crossed by fissures, ridges, and striations that have eroded terrain dating back some 90 million years. Citizen scientist Kevin M. Gill processed the images to improve color and contrast. Credit: NASA/JPL-Caltech/SwRI/MSSS, Kevin M. Gill CC BY 3.0

To assess the survival of amino acids on these worlds, the team mixed samples of amino acids with ice cooled to about minus 321 Fahrenheit (-196 Celsius) in sealed, airless vials and bombarded them with gamma rays, a type of high-energy light, in varying doses. Because the oceans may host microscopic life, they also tested the survival of amino acids in dead bacteria in the ice. Finally, they tested samples of amino acids in ice mixed with silicate dust to account for the potential mixing of material from meteorites or the interior with the surface ice.

Implications for future space missions

The experiments provide basic data for determining the rates at which amino acids decay, called radiolysis constants. With these, the team used the age of the icy surface and the radiation environment of Europa and Enceladus to calculate the penetration depth and the locations where 10 percent of the amino acids would survive radiolytic decay.

Although experiments testing the survival of amino acids in ice have been done before, this is the first to use lower doses of radiation that do not completely degrade the amino acids, as simply changing or degrading them is enough to make it impossible to determine if are potential signs of life. This is also the first experiment using Europa/Enceladus conditions to assess the survival of these compounds in microorganisms and the first to test the survival of amino acids mixed with dust.

The team found that amino acids break down faster when mixed with dust, but more slowly when they come from microorganisms.

This image shows experimental samples loaded into the specially designed dewar, which will soon be filled with liquid nitrogen and placed under gamma radiation. Note that the flame-sealed test tubes are wrapped in cotton cloth to hold them together, because the tubes become buoyant in liquid nitrogen and begin to float around in the dewar, preventing proper radiation exposure. Credit: Candace Davison

“Slow Amino Levels acid destruction of biological samples under surface conditions similar to Europa and Enceladus support the case for future life-detection measurements from the Europa and Enceladus landers missions,” Pavlov said. “Our results show that the rate of degradation of potential organic biomolecules in the silica-rich regions of both Europa and Enceladus is higher than in the pure ice, and therefore potential future missions to Europa and Enceladus should be cautious in sampling silica-rich sites on both icy moons.

A potential explanation for why amino acids survive longer in bacteria involves the ways in which ionizing radiation changes molecules—directly by breaking their chemical bonds or indirectly by creating reactive compounds nearby that then change or degrade the molecule of interest. It is possible that the bacterial cell material protected the amino acids from the reactive compounds produced by the radiation.

Reference: “Variable and Large Losses of Diagnostic Biomarkers After Simulated Cosmic Radiation Exposure in Clay- and Carbonate-Rich Mars Analog Samples” by Anais Roussel, Amy S. McAdam, Alex A. Pavlov, Christine A. Knudson, Cheri N. Achilles, Dionysis I. Foustoukos, Jason P. Dworkin, S. Andrejkovichova, Dina M. Bauer, and Sarah Stewart Johnson, July 18, 2024, Astrobiology.
DOI: 10.1089/ast.2023.0123

The research was supported by NASA under award number 80GSFC21M0002, the NASA Planetary Science Division Scientist Internal Funding Program through the Goddard Basic Laboratory Research Work Package, and NASA Astrobiology NfoLD Award 80NSSC18K1140.