Jupiter's moon Europa has much less free oxygen than previously thought, reducing the odds that life could have evolved on this distant orb.

Europa, long considered one of the solar system's best bets for harboring extraterrestrial life, witnessed a flyby by NASA's Juno space probe in 2022, giving scientists the opportunity to test scientific models that predict the levels of free oxygen and hydrogen near the moon's surface.

Beneath its frozen crust, Europa has an internal liquid ocean of salt water, possibly including volcanic activity similar to hydrothermal vents in Earth's oceans. The combination has attracted scientists to consider it a strong candidate for harboring extraterrestrial life, but the presence of the necessary free oxygen has been the subject of debate.

Frozen water on Europa's surface is split into oxygen and hydrogen by a bombardment of radiation from space. Scientists have used remote sensing and modeling to predict the amount of free oxygen (molecular O2), but these estimates of the rate of oxygen production varied widely. Before Juno's transit of Europa, model-driven estimates for the total Europagenic O2 source ranged over two orders of magnitude from 5 to 1,100 kg per second.