Juno Measures How Much Oxygen is Being Produced by Europa

If the periodic table listed the elements in order of their importance to life, then oxygen might bully its way to the top. Without oxygen, Earth’s complex life likely would not exist. So when scientists detect oxygen on another world, they turn their attention to it.

During Juno’s ambitious mission to the Jovian system, it performed some flybys and observations of some of the Jovian moons. One of those moons, Europa, is a prime target in the search for life because of its subsurface ocean. It became an even more important target when scientists realized that the icy moon was producing oxygen.

We can’t see it with our organic eyes, but Europa’s surface is under bombardment. Not by rocky objects, which do strike occasionally, but by energetic particles. Europa is in a perilous position so close to giant Jupiter, and the planet makes its presence known.

Jupiter’s enormously powerful magnetic field sends a constant stream of charged particles at Europa. The much smaller moon has no defence. When those particles strike Europa’s icy surface, they split water molecules apart and produce hydrogen and oxygen.

“Europa is like an ice ball slowly losing its water in a flowing stream. Except, in this case, the stream is a fluid of ionized particles swept around Jupiter by its extraordinary magnetic field,” said JADE scientist Jamey Szalay from Princeton University in New Jersey. “When these ionized particles impact Europa, they break up the water-ice molecule by molecule on the surface to produce hydrogen and oxygen. In a way, the entire ice shell is being continuously eroded by waves of charged particles washing up upon it.”

Szalay is the lead author of new research published in Nature Astronomy. The research is “Oxygen production from dissociation of Europa’s water-ice surface.”

One of Juno’s instruments is JADE, the Jovian Auroral Distributions Experiment. JADE can detect and measure ions and electrons, primarily in Jupiter’s aurora and magnetosphere regions. In September of 2022, Juno came to within 354 km (220 miles) of Europa. During that flyby, JADE measured the hydrogen and oxygen ions created by the particles bombarding the moon.

Scientists have known about the bombardment and the hydrogen and oxygen since the days of the Galileo mission. But Juno’s updated instruments are providing new insights into the phenomenon.

“Back when NASA’s Galileo mission flew by Europa, it opened our eyes to the complex and dynamic interaction Europa has with its environment. Juno brought a new capability to directly measure the composition of charged particles shed from Europa’s atmosphere, and we couldn’t wait to further peek behind the curtain of this exciting water world,” said Szalay. “But what we didn’t realize is that Juno’s observations would give us such a tight constraint on the amount of oxygen produced in Europa’s icy surface.”

Knowing oxygen is being produced and knowing how much are two different things. It’s possible that some of this oxygen is making its way back down through the ice into the warm, salty ocean that probably exists there. If enough oxygen makes its way into the water, it’s one more factor in favour of life.

An unending stream of charged particles from Jupiter strikes Europa’s icy surface, splitting frozen water molecules into oxygen and hydrogen molecules. This has been going on for billions of years. If some of that oxygen has worked its way into the moon’s subsurface ocean, then it boosts the chances that life could exist there. NASA/JPL-Caltech/SWRI/PU

This new research is refining scientists’ understanding of how much oxygen is being produced on Europa. While previous research into Europa’s oxygen was based on models, these results are based on Juno’s measurements.

“Europa’s atmospheric composition had never been directly sampled, and model-derived oxygen production estimates ranged over several orders of magnitude,” the authors write in their research. “Here, we report direct observations of H2+ and O2+ pickup ions from the dissociation of Europa’s water-ice surface and confirm these species are primary atmospheric constituents.”

Previous research arrived at disparate estimates of the icy moon’s oxygen production. Some said only a few kilograms of oxygen are produced per second, while others range as high as 1000 kilograms per second. But thanks to Juno’s direct sampling, this research puts the amount of oxygen produced on Europa at 12 kg (26 lbs) per second.

Quantifying the amount of oxygen produced is a critical part of understanding the moon and its environment. It’s also a critical part of understanding the moon’s potential habitability. But for the oxygen to do any life-enabling work, it has to find its way through the ice into the ocean. Does it?

When charged particles strike Europa’s surface, they split water molecules apart. The lighter hydrogen floats away into space, but the oxygen stays behind. If the oxygen somehow makes its way to the ocean, it could provide chemical energy for microbial life. Image Credit: NASA

When the particles dissociate the water molecules into hydrogen and oxygen, the hydrogen is so light that it escapes Europa’s gravity. But the oxygen is heavier. It sticks around and forms part of Europa’s thin, tenuous atmosphere, making it one of only a handful of Solar System moons to have an atmosphere and one of an even smaller number of worlds with oxygen.

Multiple studies show that the released oxygen doesn’t all remain in the atmosphere. Research published in 2022 shows that oxygen can make it through the ice and down into the ocean. It’s all because of the moon’s ‘chaos terrain.’

Image of Europa’s ice shell, taken by the Galileo spacecraft, of fractured “chaos terrain.” In this terrain, cracks, ridges, and plains are all jumbled together. Scientists think that this terrain allows surface oxygen to penetrate the ice and make its way into the subsurface ocean. Image Credit: NASA/JPL-Caltech

Europa’s chaos terrain covers about one-quarter of the moon’s surface. Its exact cause is uncertain but is likely related to heating and melting taking place underneath it. The chaos terrain might sit over the top of lakes of melted brine contained in the moon’s icy shell. These lakes aren’t directly connected to the ocean but can drain into them.

Some of these lakes may be only 3 km (1.9 miles) below the surface, according to the 2022 research. The authors say that the surface oxygen can mix with the water in these lakes, which then drains into the ocean. If that’s the case, then the ocean may contain enough oxygen to support microbial life.

This figure from the 2022 research helps explain how oxygen could make it through the ice and into Europa’s ocean. Some of the O2 is released into the moon’s atmosphere, but most of it returns to the icy regolith and is trapped in bubbles. The bubbles are the dominant near-surface reservoir for oxidants. Over thousands of years, the bubbles can make their way down to the ocean. Image Credit: Hesse et al. 2022.

“Our research puts this process into the realm of the possible,” said the lead author of the 2022 research. “It provides a solution to what is considered one of the outstanding problems of the habitability of the Europa subsurface ocean.”

We don’t know if this process takes place on Europa yet. But the oxygen has to go somewhere, so the whole idea is very intriguing.

These new findings are all a result of Juno’s extended mission. The extended mission sent Juno through Europa’s torus, the ring-shaped cloud of ions around the moon so that JADE could take these important measurements.

This figure from the research illustrates Juno’s path through Europa’s torus during the spacecraft’s extended mission. The inset diagram shows how charged particles split H2O molecules into hydrogen and oxygen. The hydrogen escapes into space, but the heavier oxygen sinks back to the surface. Image Credit: Szalay et al. 2024.

“Our ability to fly close to the Galilean satellites during our extended mission allowed us to start tackling a breadth of science, including some unique opportunities to contribute to the investigation of Europa’s habitability,” said Scott Bolton, Juno’s principal investigator from the Southwest Research Institute in San Antonio. “And we’re not done yet. More moon flybys and the first exploration of Jupiter’s close ring and polar atmosphere are yet to come.”

Juno will keep gathering data until its extended mission ends in 2025 or until the spacecraft stops functioning. The extreme radiation at Jupiter is slowly eroding the spacecraft’s electronics, though they’re inside a protective titanium vault.

The Europa Clipper will be the next mission to the Jovian system. It’s focused on Europa and should tell us more about the moon’s oxygen and potential habitability when it reaches the system in 2030.