Using recent data from NASA’s Juno orbiter, scientists have uncovered new details on the thickness and structure of Europa’s surface ice shell, which encases the Jovian moon’s subsurface ocean. For planetary scientists and astrobiologists, Europa’s ocean makes the moon one of the most likely places for life to exist outside of Earth.
Since entering orbit around Jupiter in July 2016, Juno has conducted extensive studies into Europa and Jupiter’s two other icy moons: Callisto and Ganymede. Juno’s new Europa measurements used the orbiter’s Microwave Radiometer instrument, and are the first to discriminate between the thick and thin ice models that scientists have theorized for the thickness of Europa’s ice shell.
Europa is slightly smaller than Earth’s moon, and understanding the structures and thickness of its surface ice shell will help scientists determine the moon’s inner workings. Furthermore, the measurements can inform theories on the habitability of the moon.
The Microwave Radiometer (MWR) instrument used in the study is designed to perform observations of Jupiter’s atmosphere below the planet’s swirling cloud tops. However, when Juno performs a flyby of a moon, the instrument can also collect data on surface compositions and characteristics. The MWR consists of six antennas that measure electromagnetic waves emitted at frequencies within the microwave range, specifically 600 MHz, 1.2 GHz, 2.4 GHz, 4.8 GHz, 9.6 GHz, and 22 GHz.
For the new Europa study, the MWR was activated on Sept. 29, 2022, when Juno performed a flyby of Europa, bringing it to within 360 km of the moon’s surface. The maneuver enabled MWR to observe half of Europa’s surface and “see” below the ice shell, measuring its temperature at different depths. Analysis of MWR data after the flyby showed that the shell, on average, is around 29 km, or 18 miles, thick.
“The 18-mile estimate relates to the cold, rigid, conductive outer-layer of a pure water ice shell. If an inner, slightly warmer convective layer also exists, which is possible, the total ice shell thickness would be even greater. If the ice shell contains a modest amount of dissolved salt, as suggested by some models, then our estimate of the shell thickness would be reduced by about three miles,” explained lead author Steve Levin, a Juno project scientist at NASA’s Jet Propulsion Laboratory in California.
The MWR measurements suggest that the thick shell model is the most accurate, meaning that oxygen, nutrients, and other ingredients necessary for life would have to travel a longer route from the surface to the subsurface ocean.
Infographic showing the Juno spacecraft. (Credit: NASA)
What’s more, the MWR data revealed that irregularities in Europa’s surface — such as cracks, pores, and voids — can scatter the microwaves being measured by the MWR. These “scatterers” behave similarly to how ice cubes scatter light, and are estimated to be only a few inches in diameter but stretch hundreds of feet into the moon’s surface.
Some theories suggested that these scatterers could serve as pathways to the subsurface ocean. However, the data used in the new study revealed that the scatterers don’t reach far enough into the ice to reach the ocean.
“How thick the ice shell is, and the existence of cracks or pores within the ice shell, are part of the complex puzzle for understanding Europa’s potential habitability. They provide critical context for NASA’s Europa Clipper and the ESA JUICE spacecraft — both of which are on their way to the Jovian system,” said Juno principal investigator Scott Bolton of the Southwest Research Institute in Texas.
Artist’s impression of the Europa Clipper in the Jupiter system. (Credit: NASA)
Juno reached the end of its second mission extension in September 2025, but continues to operate and perform regular flybys of Jupiter. The orbiter’s most recent flyby occurred on Jan. 23, and its 81st flyby of Jupiter is scheduled for Feb. 25.
Meanwhile, Jupiter is set to receive two new moon-studying spacecraft in the coming years: NASA’s Europa Clipper, which launched in October 2024, and the European Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE), which launched in April 2023. While JUICE’s mission is to study Europa, Ganymede, and Callisto, Europa Clipper specifically targets Europa to determine whether its subsurface ocean can support life. The new measurements of Europa’s ice sheet, as well as previous studies into the icy moons using Juno data, will inform future science operations and mission planning for JUICE and Europa Clipper.
Levin et al.’s results were published in Nature Astronomy on Dec. 17.
(Lead image: Europa imaged by Juno. Credit: NASA)
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