A long-standing mystery about the presence of hydrogen peroxide (H2O2) on Jupiter’s icy ocean moon Europa may be closer to being solved.
Hydrogen peroxide forms as a byproduct when energetic particles break apart water molecules, leading to the recombination of OH radicals — highly reactive molecules with unpaired electrons.
H2O2 was first observed on Europa by the Galileo Near Infrared Mapping Spectrometer, a scientific instrument aboard NASA’s Galileo Jupiter orbiter that was designed to study the composition and surface features of the gas giant’s moons and atmosphere using infrared light. Later, the James Webb Space Telescope (JWST) noticed elevated levels of hydrogen peroxide in unexpected areas on the Jovian satellite.
Lab studies predicted that higher concentrations of hydrogen peroxide would be found in Europa’s colder polar regions — but JWST observations showed the opposite, detecting higher levels in the moon’s warmer equatorial regions. These areas, known as chaos terrains, are marked by broken blocks of surface ice that appear to have shifted, drifted and refrozen.
“Europa’s peroxide distribution does not follow the temperature dependence predicted for pure water ice,” wrote the team in their paper. Lab studies consistently show that colder ice has more H2O2, while warmer ice has less.
In a new study, scientists report that they have noticed higher levels of carbon dioxide (CO2) in the chaos terrains alongside elevated levels of H2O2. This is probably the result of CO2 escaping Europa’s subsurface ocean through cracks in the ice, the researchers say.
The team therefore wondered if the presence of CO2 might be changing the ice’s chemistry.
“Could the presence of CO2 drive the enhanced peroxide production in Europa’s chaos regions, signaling a surface composition more conducive to the formation of this radiolytic oxidant?” they wrote in their paper. “Supporting this hypothesis are preliminary experiments on irradiated H2O-CO2 ice mixtures that show increased H2O2 yields compared to pure water ice.”
To find a definitive answer, they “simulated the surface environment of Europa inside a vacuum chamber by depositing water ice mixed with CO2,” Bereket Mamo, a graduate student at The University of Texas at San Antonio and a contractor with the Southwest Research Institute, said in a statement. “We then irradiated this ice mixture with energetic electrons to see how the peroxide production changed.”
The experiment confirmed what the team had suspected: Even small amounts of CO2 in water ice can greatly boost hydrogen peroxide production at temperatures similar to those on Europa’s surface, helping to explain the unexpected JWST observations.
This occurs because CO2 molecules behave as “molecular scavengers,” grabbing hold of any stray electrons produced when radiation hits the water ice. By capturing these electrons, the CO2 helps protect hydrogen peroxide from being broken apart by further impacts or reactions.
“Synthesis of oxidants like hydrogen peroxide on Europa’s surface is important from an astrobiological point of view,” said study co-author Richard Cartwright, from the Johns Hopkins University Applied Physics Laboratory. “In fact, an entire NASA mission, the Europa Clipper, is en route to the Jovian system right now to explore the icy moon and help us understand Europa’s habitability.
“Our experiments provide clues to better understand JWST Europa observations and serve as a prelude to upcoming close-range investigations by Europa Clipper and ESA’s [the European Space Agency] JUICE spacecraft,” Cartwright added.
The new study was published in the Planetary Science Journal on Monday (July 21).
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