Europa and Enceladus are important targets for the search for evidence of extraterrestrial life in the solar system. However, the surfaces and shallow subsurfaces of these airless icy moons are constantly exposed to ionizing radiation that can degrade chemical biosignatures. Therefore, sampling the icy surfaces in future life-searching missions to Europa and Enceladus requires a clear understanding of the required ice depths where intact organic biomolecules may exist. A team of scientists from NASA and Pennsylvania State University conducted experiments exposing individual biological and abiotic amino acids in the ice to gamma radiation to simulate conditions on these icy worlds.

“Based on our experiments, a 'safe' sampling depth for amino acids on Europa is about 20 centimetres (8 inches) at high latitudes in the trailing hemisphere (the hemisphere opposite the direction Europa moves around Jupiter), in an area where the surface has not been significantly disturbed by meteorite impacts,” said Dr. Alexander Pavlov, a research scientist at NASA's Goddard Space Flight Center.

“Detecting amino acids on Enceladus does not require subsurface sampling; these molecules survive radiolysis (breakdown by radiation) anywhere on Enceladus' surface, within a few millimeters (tenths of an inch) of the surface.”

Dr. Pavlov and his colleagues used amino acids in their radiolysis experiments as representative examples of biomolecules on icy moons.

Amino acids are produced by both living organisms and non-living processes.

But if certain types of amino acids were found on Europa or Enceladus, they could be a sign of life, as they may be used by life on Earth as building blocks of proteins.

Proteins are essential for life because they are used to create structures and to produce enzymes that speed up or control chemical reactions.

Amino acids and other compounds found underground in the ocean could be transported to the surface by geyser activity or the slow churning motion of the ice shell.

To assess the survival of amino acids on these planets, the researchers mixed amino acid samples with ice cooled to minus 196 degrees Celsius (minus 321 degrees Fahrenheit) in sealed, airless vials and exposed them to various doses of gamma rays (a type of high-energy light).

Because the ocean may harbor microorganisms, the researchers also tested the viability of amino acids contained in dead bacteria in the ice.

Finally, the researchers tested samples of amino acids in the ice mixed with silicate dust to see if meteorites or interior materials could be mixing with the surface ice.

This experiment provided vital data for determining the rate at which amino acids break down (called the radiolysis constant).

Using these, the scientists used the age and radiation environment of the icy surfaces of Europa and Enceladus to calculate drilling depths and where 10% of amino acids would survive radiolysis.

While experiments have been done before to test for the survival of amino acids in ice, this is the first to use low doses of radiation that don't completely break down the amino acids – changing or breaking them down would be insufficient to determine whether they were a sign of life.

This is also the first experiment to use Europa/Enceladus conditions to assess the survival of these compounds in microbes, and the first to test the survival of amino acids mixed with dust.

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

“The slow rate of breakdown of amino acids in biological samples under surface conditions like those on Europa and Enceladus strengthens the case for future life detection measurements from lander missions to Europa and Enceladus,” Dr Pavlov said.

“Our results indicate that the decomposition rates of potential organic biomolecules are higher in the silica-rich regions of both Europa and Enceladus than in pure ice. Future missions to Europa and Enceladus should therefore be careful when sampling the silica-rich regions of these icy moons.”

“A possible explanation for why amino acids survive longer in bacteria is the way that ionizing radiation alters molecules, either directly by breaking chemical bonds or indirectly by creating nearby reactive compounds that alter or break down the target molecule.”

“It's possible that the bacterial cellular material protected the amino acids from reactive compounds produced by the radiation.”

Team paper Published in the journal Astrobiology.

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Alexander A. Pavlov others2024. Effects of radiolysis on biological and abiotic amino acids in shallow subsurface ice on Europa and Enceladus. Astrobiology 24(7); doi: 10.1089/ast.2023.0120

This article has been edited based on the original NASA release.