Astrobiological potential at a glance: How life-friendly could the ocean that is suspected to lie beneath the ice sheet of Jupiter's moon Europa be? This question is now illuminated by a new assessment of oxygen production on the celestial body, which is based on data from NASA's Juno spacecraft. The potential elixir of life is formed in the ice and could ultimately reach the underlying ocean through displacement processes. However, the chemical splitting of the water ice on the surface produces significantly lower amounts of oxygen than previously expected, the researchers report.
A frosty ball in orbit around Jupiter: At first glance, the moon Europa doesn't seem particularly hospitable to life. But the inconspicuous celestial body has a lot to offer, as studies in recent years have clearly shown. According to the evidence, beneath the moon's thick ice sheet lies an ocean of liquid salt water that is warmed by Jupiter's tidal forces. This brought Europe into the focus of astrobiology: Given the history of the development of life on earth, it seems possible that organisms also emerged in the subglacial water of this small world. In addition to water, a second element also became the elixir of life on Earth: oxygen. This raises the question of how much O₂ could be available for the metabolism of potential organisms in the European ocean.
Potential livelihoods in view
One possible source is oxygen, which is created by high-energy ions from Jupiter: This cosmic radiation splits the water molecules in the ice into oxygen and hydrogen. The lighter hydrogen then mostly escapes into the atmosphere and beyond. However, the heavier oxygen molecules tend to remain on the surface and can be bound in the ice. As a result of shifting processes in Europe's presumably tectonically active ice sheet, the oxygen could eventually reach the subglacial water. “By absorbing the radiation, oxygen is created in the ice, so that the ice shell could act as the lungs of Europe and represent a potential source of oxygen for the ocean,” says first author Jamey Szalay from Princeton University.
But how much oxygen could actually be formed on the surface of the moon? So far there have only been very vague assumptions based on remote sensing data and simulations. But now Szalay and his team have evaluated data from NASA's Juno space probe, which allowed more precise conclusions to be drawn. The probe flew past Europa in 2022 at a distance of just 352 kilometers and was able to detect molecules that came from the moon's atmosphere. This involved significant amounts of charged molecular oxygen and hydrogen. “For the first time, we were able to clearly detect these substances using in-situ measurements, thereby confirming that Europe's atmosphere consists mainly of hydrogen and oxygen molecules,” says co-author Robert Ebert from the University of Texas at San Antonio.
Less oxygen than expected
Based on their data and model simulations, they were then able to draw conclusions about the amount of oxygen that is continuously being formed on Europa's surface. “We have now been able to limit Europe’s total oxygen production to around twelve kilograms per second. The results clearly show that oxygen is continuously produced on the surface – but less than we expected,” says Szalay. Previous estimates ranged up to over 1000 kilograms per second.
This also means that less oxygen could reach the subglacial ocean through displacement processes than previously thought. Exactly what astrobiological consequences could result from this remains unclear. But at least the results contribute to a better understanding of the chemistry on the icy moon Europa and provide starting points for improved models of the possible conditions that could exist in the subglacial ocean. “The results have direct implications for assessments of Europe’s liveability,” says co-author Scott Bolton from the Southwest Research Institute in San Antonio.
Source: Southwest Research Institute, NASA, specialist article: Nature Astronomy, doi: 10.1038/s41550-024-02206-x