This artist’s impression shows NASA’s Cassini spacecraft flying through a cloud of presumed water flowing from the surface of Saturn’s moon Enceladus. Credit: NASA
NASA scientists have found out amino acidspotential indicators of life, could survive near the surface Europe and Enceladus, the moons Jupiter and Saturn respectively.
Experiments suggest that these organic molecules can withstand radiation just below the ice, making them available for future robotic landers without deep drilling.
Exploring the potential for life on icy moons
Jupiter’s moon Europa and Saturn’s moon Enceladus have evidence of oceans beneath their icy crusts. The 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 ice despite the harsh radiation on these worlds. If robotic landers are sent to these moons to look for signs of life, they wouldn’t have to dig very deep to find amino acids that have survived alteration or destruction by radiation.
“Based on our experiments, the ‘safe’ sampling depth for amino acids on Europa is almost 8 inches (about 20 centimeters) in the high latitudes of the posterior hemisphere (the hemisphere opposite to the direction of Europa’s motion around Jupiter) in an area where the surface has not been disturbed much by impacts meteorites,” said Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, lead author of a paper on the research published July 18 in the journal. Astrobiology. “Subsurface sampling is not required to detect amino acids on Enceladus—these molecules survive radiolysis (decay by radiation) anywhere on Enceladus’ surface less than a tenth of an inch (less than a few millimeters) from the surface.”
Dramatic clouds, large and small, spray water ice and vapor from many locations along the famous “tiger stripes” near the south pole of Saturn’s moon Enceladus. Credit: NASA/JPL/Space Science Institute
The cold surfaces of these nearly airless moons are likely uninhabitable due to radiation from both high-speed particles trapped in their host planet’s magnetic fields and powerful deep space events such as exploding stars. However, both have oceans beneath their icy surface that are heated by the tides from the gravitational pull of the host planet and neighboring moons. These subsurface oceans could harbor life if they have other needs such as energy supply 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 proxies for 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 life on Earth uses them as building blocks for proteins. Proteins are essential to life because they are used to make enzymes that speed up or regulate chemical reactions and to build structures. Amino acids and other compounds from the subsurface oceans could be brought to the surface by geyser activity or by the slow swirling motion of the ice crust.
This view of Jupiter’s icy moon Europa was taken by JunoCam, the public view camera aboard NASA’s Juno probe, during the mission’s close flyby on September 29, 2022. The image is a composite of the second, third, and fourth JunoCam images 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 mils 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 visible here. Europa’s surface is criss-crossed by faults, ridges, and belts that have erased terrain older than about 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 evaluate 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, at various doses. Because the oceans can 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 consider potential mixing of material from meteorites or the interior with the surface ice.
Implications for future space missions
The experiments provided key data for determining the rates at which amino acids break down, called radiolysis constants. With these, the team used the age of the ice surface and the radiation environment of Europa and Enceladus to calculate the depth of the boreholes and the locations where 10 percent of the amino acids would survive radiolytic destruction.
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 break down the amino acids, as their mere alteration or degradation is enough to make it impossible to determine if they 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 experiment to test the survival of amino acids mixed with dust.
The team found that the amino acids degraded faster when mixed with dust, but slower when they came from microorganisms.
This image shows experimental samples placed in a specially designed Dewar vessel, which will shortly be filled with liquid nitrogen and placed under gamma radiation. Note that the flame-sealed tubes are wrapped in cotton cloth to hold them together, as the tubes float in the liquid nitrogen and begin to float in the dewar, interfering with proper radiation exposure. Credit: Candace Davison
“Slow speed amino acid the destruction of biological samples under conditions similar to the surface of Europa and Enceladus will strengthen the case for future life detection measurements using the Europa and Enceladus landing missions,” said Pavlov. “Our results suggest that the rate of potential degradation of organic biomolecules in the silica-rich regions of both Europa and Enceladus is higher than in pure ice, and therefore potential future missions to Europa and Enceladus should be cautious in sampling silica-rich sites on both ice moons.”
A potential explanation for why amino acids in bacteria survived longer involves the ways in which ionizing radiation alters molecules — directly by breaking their chemical bonds or indirectly by creating reactive compounds nearby that then alter or break down the molecule of interest. It is possible that the bacterial cell material protected the amino acids from reactive compounds produced by the radiation.
Reference: “Variable and large losses of diagnostic biomarkers after simulated cosmic ray exposure in a clay-carbonate-rich environment Mars Analog Samples” by Anaïs Roussel, Amy C. McAdam, Alex A. Pavlov, Christine A. Knudson, Cherie N. Achilles, Dionysis I. Foustoukos, Jason P. Dworkin, S. Andrejkovičová, Dina M. Bower, 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 Internal Funding Program through the Fundamental Laboratory Research work package at Goddard, and NASA Astrobiology NfoLD award 80NSSC18K1140.