Juno measures oxygen production on Europa

by Martijn Luinstra

Using NASA’s Juno spacecraft, scientists have calculated the rate of oxygen production on Jupiter’s moon Europa — the first time any spacecraft had directly measured charged oxygen and hydrogen particles from the moon’s atmosphere. From these measurements, the team determined the rate of oxygen production to be much lower than estimated in previous studies.

Europa, Jupiter’s fourth largest moon, has a rocky interior and a surface made of water ice. Measurements of the moon’s magnetic field, collected by NASA’s Galileo mission, provided evidence for the presence of a salty ocean beneath the ice sheets. Ocean worlds, like Europa, are the prime candidates in the search for extraterrestrial life in our solar system.

“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 lead author Jamey Szalay of Princeton University in New Jersey.  “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.”

On Sept. 29, 2022, Juno observed Europa when it flew by at an altitude of 353 kilometers. It used its Jovian Auroral Distributions Experiment (JADE) instrument to measure charged particles that trailed the moon in its orbit around Jupiter.

Illustration showing charged particles impacting Europa’s surface, splitting water molecules from the moon’s icy crust into hydrogen and oxygen molecules. The inset depicts how these oxygen molecules could migrate to the ocean below. (Credit: NASA/JPL-Caltech/SWRI/PU)

“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,” explained Szalay. “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.”

With Juno’s observations, the team determined the rate at which hydrogen was produced at the moon’s surface. The process that splits the water molecules in the ice produces hydrogen and oxygen molecules in a two-to-one ratio. From this, the scientists estimate the total oxygen production on Europa at around 12 kilograms per second.

“Before Juno, previous estimates ranged from a few kilograms per second to over 1,000 kilograms per second,” said Szalay. “The findings unambiguously demonstrate oxygen is continuously produced in the surface, just a good bit lower than we expected.”

The team believes that some of the produced oxygen could find its way through the ice sheet into the oceans below, where it could potentially support life. However, they estimate the rate at which this happens to be a lot lower than suggested in previous studies.

Juno performed the measurements as part of its extended mission. The spacecraft was launched in 2011 and arrived at Jupiter in 2016. After Juno completed its prime mission in July 2021, NASA authorized a mission extension to 2025, or until the spacecraft’s end of life.

“We designed JADE to measure the charged particles that create Jupiter’s auroras,” said co-author Frederic Allegrini of Southwest Research Institute (SwRI) in San Antonio, Texas. “Flybys of Europa were not part of the primary Juno mission. JADE was designed to work in a high-radiation environment but not necessarily Europa’s environment, which is constantly bombarded with high levels of radiation. Nonetheless, the instrument performed beautifully.”

Through its extended mission, Juno contributes to the goals of future missions to the Jovian system. The authors of the new paper state that their findings have important implications for the European Space Agency’s Jupiter Icy Moons Explorer (Juice) mission and NASA’s upcoming Europa Clipper.

Illustration showing the orbits for Juno’s prime and extended missions. (Credit: NASA/JPL-Caltech/SwRI)

Juice, which launched on April 14, 2023, is scheduled to arrive at Jupiter in 2031. To get there, it will fly by Earth and Venus multiple times to perform so-called gravity assist maneuvers. The first of which, an Earth-Moon flyby, is set to happen this summer.

At Jupiter, Juice will study Europa and two other ocean-bearing moons, Ganymede and Callisto, with its 10 science instruments. During two flybys, Juice will measure the thickness of Europa’s ice crust and scout locations for potential future exploration.

Europa Clipper is currently being tested ahead of its launch on a SpaceX Falcon Heavy, scheduled for October 2024. After it arrives in 2030, the spacecraft will use its nine science instruments to study Europa’s habitability during 49 flybys. The current study’s authors suggest that the observations made with one of these instruments, the Plasma Instrument for Magnetic Sounding instrument, are affected by the new findings.

Together, these missions will help scientists understand ocean worlds and how they could potentially support life.

“Europa is a fascinating object because scientists are confident a liquid ocean exists in its interior,” said co-author Robert Ebert of SwRI. “Water is important for the existence of life and can be found in or on objects with varying characteristics. Europa is a good place to search for water within our solar system.”

Szalay et al.’s study was published in the journal Nature Astronomy on March 4.

(Lead image: Europa imaged by Juno during its flyby on Sept. 26, 2022. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill)

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