Using NASA's Juno spacecraft, scientists have calculated the rate of oxygen production on Jupiter's moon Europa, the first time a spacecraft has directly measured charged oxygen and hydrogen molecules from the moon's atmosphere. From these measurements, the team determined that the rate of oxygen production was 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, have provided evidence of a salty ocean beneath the ice sheets. Ocean worlds, such as Europa, are prime candidates in the search for extraterrestrial life in our solar system.
“When NASA's Galileo mission flew by Europa, it opened our eyes to the complex and dynamic interaction that Europa has with its environment. Juno has brought new ability to accurately measure the composition of charged particles emitted from Europa's atmosphere,” said lead author Jamie Salai of Princeton University in New Jersey. “But what we didn't realize was that Juno's observations would give us strict limits on the amount of oxygen produced at Europa's icy surface.”
On September 29, 2022, Juno observed Europa when it flew by at an altitude of 353 kilometers. The Jovian Auroral Distributions Experiment (JADE) instrument was used to measure charged particles left behind by the Moon in its orbit around Jupiter.
Illustration showing the impact of charged particles on Europa's surface, splitting water molecules from the moon's icy crust into hydrogen and oxygen molecules. The inset depicts how these oxygen molecules can migrate to the ocean below. (Credit: NASA/JPL-Caltech/SWRI/PU)
“Europa is like a snowball slowly losing its water in a flowing stream. Except, in this case, the stream is a liquid of ionized particles being swept up into Jupiter by its extraordinary magnetic field.” When these ionized particles collide with Europa, they break up a molecule of water ice. A single molecule on the surface to produce hydrogen and oxygen. Somehow, the entire ice shell is constantly being eroded by waves of charged particles sweeping away at it.
Through Juno's observations, the team determined the rate of hydrogen production on the moon's surface. The process that splits water molecules in ice produces hydrogen and oxygen molecules in a two-to-one ratio. From this, scientists estimate the total oxygen production in Europe at about 12 kilograms per second.
“Before Juno, previous estimates ranged from a few kilograms per second to more than 1,000 kilograms per second,” Szalay said. “The results show unequivocally that oxygen is continuously produced at the surface, which is slightly less than we expected.”
The team believes that some of the oxygen produced could find its way across the ice sheet into the oceans below, where it could support life. However, they estimate that the incidence of this is much lower than suggested in previous studies.
Juno made the measurements as part of its extended mission. The spacecraft was launched in 2011 and arrived at Jupiter in 2016. After Juno completed its main mission in July 2021, NASA allowed the mission to be extended until 2025, or until the end of the spacecraft's life.
“We designed JADE to measure the charged particles that make up Jupiter's aurora,” said co-author Frederic Allegrini of the Southwest Research Institute (SwRI) in San Antonio, Texas. “Flybys of Europa were not part of Juno's primary mission. JADE was designed to operate in a high-radiation environment but not necessarily Europa's environment, which is constantly bombarded with high levels of radiation. However, the machine performed admirably.
Through its extended mission, Juno contributes to the goals of future missions of the Jovian system. The authors of the new paper point out that their findings have important implications for ESA's Jupiter Icy Moons Explorer (Juice) mission and NASA's upcoming Europa Clipper.
Illustration showing the orbits of Juno's main and extended missions. (Credit: NASA/JPL-Caltech/SwRI)
Juss, which launched on April 14, 2023, is scheduled to reach Jupiter in 2031. To get there, it will fly around Earth and Venus several times to perform so-called gravity assist maneuvers. The first, a flyby of the Earth and the Moon, is scheduled to take place this summer.
On Jupiter, Goss will study Europa and two other ocean-bearing moons, Ganymede and Callisto, using his 10 scientific instruments. During two flybys, Goss will measure the thickness of Europe's ice crust and scout sites for possible future exploration.
Europa Clipper is currently being tested ahead of its launch on the SpaceX Falcon Heavy, scheduled for October 2024. After arriving in 2030, the spacecraft will use its nine science instruments to study Europa's habitability during 49 flights. The authors of the current study point out that observations made using one of these instruments, the Magnetic Sounding Plasma Instrument, are influenced by the new findings.
Together, these missions will help scientists understand ocean worlds and how they can support life.
“Europa is a fascinating body because scientists are confident there is a liquid ocean 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 different properties. Europa is a good place to look for water within our solar system.”
Szalay et al.'s study was published in Nature Astronomy on March 4.
(Main image: Europa imaged by Juno during its flyby on September 26, 2022. Image credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill)
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