Europa, one of Jupiter's moons, and Enceladus, Saturn's moon, have evidence of oceans beneath their icy surfaces. A NASA experiment suggests that if these oceans support life, traces of that life in the form of organic molecules (e.g., amino acids, nucleic acids, etc.) can survive just beneath the surface ice despite the harsh radiation on these worlds. If robotic landers are sent to these moons to look for signs of life, they would not need to dig deep to find amino acids that have survived alteration or destruction caused by radiation.

“Based on our experiments, the 'safe' sampling depth for amino acids on Europa is nearly 20 centimeters at high latitudes of the trailing hemisphere (the hemisphere opposite the direction of Europa's movement around Jupiter) in areas where the surface is not significantly disrupted by meteorite impacts,” said Alexander Pavlov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, lead author of the study published on July 18 in the journal Astrobiology. “Subsurface sampling is not necessary for detecting amino acids on Enceladus - those molecules will survive radiolysis (decomposition due to radiation) anywhere on Enceladus's surface less than a few millimeters from the surface.”

The cold surfaces of these nearly airless moons are likely uninhabitable due to radiation from high-speed particles trapped in the magnetic fields of their parent planets and powerful deep space events, such as star explosions. However, both moons have oceans beneath their icy surfaces warmed by tidal forces from the gravitational pull of their parent planet and neighboring moons. These subsurface oceans could support life if they contain other necessary conditions, such as energy sources and elements and compounds used in biological molecules.

The research team used amino acids in radiolysis experiments as potential representatives of biomolecules on icy moons. Amino acids can be created by life or non-biological chemistry. However, finding certain types of amino acids on Europa or Enceladus would be a potential sign of life because terrestrial life uses them as building blocks for proteins. Proteins are essential for 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 subsurface oceans could be brought to the surface by geyser activity or slow ice mixing.

To assess the survival of amino acids on these worlds, the team mixed amino acid samples with ice chilled to about -196 Celsius in sealed, airless vials and bombarded them with gamma rays, a type of high-energy light, at various doses. Since the oceans may contain microscopic life, they also tested the survival of amino acids in dead bacteria in ice. Finally, they tested amino acid samples in ice mixed with silicate dust to account for potential mixing of materials from meteorites or the interior with surface ice.

The experiments provided key data for determining the degradation rates of amino acids, called radiolysis constants. Using these data, the team used the age of the icy surface and the radiation environment on Europa and Enceladus to calculate the drilling depth and locations where 10 percent of amino acids would survive radiolytic destruction.

Although experiments to test the survival of amino acids in ice have been done before, this is the first time that lower doses of radiation that do not completely degrade amino acids have been used, as mere alteration or degradation is sufficient to prevent determining whether they are potential signs of life. This is also the first experiment to use 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 degrade faster when mixed with dust but more slowly when they come from microorganisms.

“Slow degradation rates of amino acids in biological samples under Europa and Enceladus surface conditions increase the chances for future life detection measurements by lander missions to Europa and Enceladus,” said Pavlov. “Our results show that degradation rates of potential organic biomolecules in silicate-rich regions on Europa and Enceladus are higher than in pure ice, so potential future missions to Europa and Enceladus should be cautious when sampling silicate-rich locations on both icy moons.”

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

Further research in this area can help better understand how these processes relate to potential signs of life on Europa and Enceladus. This includes future experiments that will simulate even more precise conditions on and beneath the surface of these moons. Additionally, research will expand to analyze other organic molecules that could be key to identifying signs of life.

The research was supported by NASA under award number 80GSFC21M0002, NASA's Planetary Science Division Internal Scientist Funding program through the Fundamental Laboratory Research work package at Goddard, and NASA's Astrobiology NfoLD award 80NSSC18K1140.

Source: National Aeronautics and Space Administration