Juno Measures The Rate Of Oxygen Production At Europa

By Keith Cowing
Press Release
March 4, 2024
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Juno Measures The Rate Of Oxygen Production At Europa
Water ice on the surface of Europa is dissociated by radiolysis to form O2 and H2. These gases can migrate both inwards towards the subsurface ocean or escape the surface by thermal desorption or direct sputtering to form its atmosphere. The lighter H2 occupies a more extended region than heavier O2, which remains closer to the surface. A portion of the neutrals in the atmosphere are ionized and picked up by the magnetospheric plasma. Juno observes these PUIs, with the relative abundances driven by the various processes described here. The radiolysis dissociation inset was adapted from ref. 2. Particles shown are O2 (blue), H2 (pink) and H2+ (grey). — NASA/Nature Astronomy

Scientists with NASA’s Juno mission to Jupiter have calculated the rate of oxygen being produced at the Jovian moon Europa to be substantially less than most previous studies.

Published on March 4 in Nature Astronomy (open access), the findings were derived by measuring hydrogen outgassing from the icy moon’s surface using data collected by the spacecraft’s Jovian Auroral Distributions Experiment (JADE) instrument.

The paper’s authors estimate the amount of oxygen produced to be around 26 pounds every second (12 kilograms per second). Previous estimates range from a few pounds to over 2,000 pounds per second (over 1,000 kilograms per second). Scientists believe that some of the oxygen produced in this manner could work its way into the moon’s subsurface ocean as a possible source of metabolic energy.

With an equatorial diameter of 1,940 miles (3,100 kilometers), Europa is the fourth largest of Jupiter’s 95 known moons and the smallest of the four Galilean satellites. Scientists believe a vast internal ocean of salty water lurks beneath its icy crust, and they are curious about the potential for life-supporting conditions to exist below the surface.

This illustration shows charged particles from Jupiter impacting Europa’s surface, splitting frozen water molecules into oxygen and hydrogen molecules. Scientists believe some of these newly created oxygen gases could migrate toward the moon’s subsurface ocean, as depicted in the inset image. NASA/JPL-Caltech/SWRI/PU

It is not just the water that has astrobiologists’ attention: The Jovian moon’s location plays an important role in biological possibilities as well. Europa’s orbit places it right in the middle of the gas giant’s radiation belts. Charged, or ionized, particles from Jupiter bombard the icy surface, splitting water molecules in two to generate oxygen that might find its way into the moon’s ocean.

“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.”

a,b, Density of H2+ PUIs directly picked up from Europa’s neutral atmosphere for XEPhiO (a) and ZEPhiO (b). Velocity arrows indicate the plasma velocity vector as determined from proton observations, with the rigid corotation of 104 km s−1. RE ≡ 1,560.8 km is Europa’s radius. Streamlines and associated wake are from an analytic model (Methods). c–e, Fluxes of O2+ and S+ (c), H2+ (d) and H+ (e) from JADE’s TOF product. Horizontal dashed lines indicate the ram energy for O2+ (c) and cutoff energies (c–e) for PUIs assuming rigid corotation (Methods). f, Densities of individual species (orange, black and blue), all ions (dashed black) and electron impact ionization rates (purple, right axis). The altitude is shown underneath c–f. The boundaries of the geometric wake are shown with horizontal grey bars above or below each panel. Juno’s close approach (C/A) was on 2022-272 9:36:29. Each horizontal tick corresponds to 1 min. avg., average; imp., impact. — NASA/Nature Astronomy

Capturing the Bombardment

As Juno flew within 220 miles (354 kilometers) of Europa at 2:36 p.m. PDT Sept. 29, 2022, JADE identified and measured hydrogen and oxygen ions that had been created by the bombarding charged particles and then “picked up” by Jupiter’s magnetic field as it swept past the moon.

“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.”

Juno carries 11 state-of-the-art science instruments designed to study the Jovian system, including nine charged-particle and electromagnetic-wave sensors for studying Jupiter’s magnetosphere.

a–h, Average ion count rates as a function of energy per charge and mass per charge. The diagonal line on each shows the cutoff for locally picked up ions assuming that they are picked up at a rigid corotation speed of 104 km s−1 relative to Europa (Methods). The corresponding 30 s intervals are indicated on the top of i. i,j The same as shown in Figs. 1c (i) and 1e (j). — NASA/Nature Astronomy

“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.”

Oxygen production is one of many facets that NASA’s Europa Clipper mission will investigate when it arrives at Jupiter in 2030. The mission has a sophisticated payload of nine science instruments to determine if Europa has conditions that could be suitable for life.

Now Bolton and the rest of the Juno mission team are setting their sights on another Jovian world, the volcano-festooned moon Io. On April 9, the spacecraft will come within about 10,250 miles (16,500 kilometers) of its surface. The data Juno gathers will add to findings from past Io flybys, including two extremely close approaches of about 932 miles (1,500 kilometers) on Dec. 30, 2023, and Feb. 3, 2024.

H2+ density coloured by XEPhiO such that densities observed upstream from the centre of Europa are blue and are red or orange downstream. The outbound portion of the trajectory, ‘Fit’ in the inset, is compared to a PUI advection solution (Methods). Overlaid curves show PUI densities corresponding to the advection solution for a PUI population from: (1) an ionized neutral atmosphere varying as exp(−h/λ)r−2 where λ = 6,090 ± 890 km (grey), (2) scaled from a comprehensive DSMC atmosphere model (blue)3 and (3) scaled from a sputtered-only model (purple). The grey region at the bottom shows the expected density of Europa-genic H2+ PUIs already incorporated into Jupiter’s magnetospheric plasma. — NASA/Nature Astronomy

More About the Mission

NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Juno mission for the principal investigator, Scott Bolton, of the Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers Program, which is managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. The Italian Space Agency (ASI) funded the Jovian InfraRed Auroral Mapper. Lockheed Martin Space in Denver built and operates the spacecraft.

More information about Juno is available at:

Oxygen production from dissociation of Europa’s water-ice surface, Nature Astronomy (open access)

Astrobiology, Astrochemistry

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