Europa’s Ice Shell Secrets Revealed by 13-Year Ground Radar Study
Featured image: [Composite of Jupiter’s moon Europa with radar data overlays; credit: NASA/JPL-Caltech/NSF/GBO]
For 13 years, scientists bounced radar signals off Jupiter’s icy moon Europa using two of Earth’s most powerful radio facilities. The result is the most detailed ground-based radar portrait of the moon ever assembled, and it confirms something remarkable: Europa’s surface reflects radio waves more like a mirror than a typical planetary body, a finding that strengthens the case for a liquid water ocean hidden beneath the ice.
The study, presented at the American Astronomical Society’s 248th meeting in June 2026, used NASA’s Goldstone Solar System Radar in California as the transmitter and the National Science Foundation’s Green Bank Telescope in West Virginia as the receiver. The bistatic configuration allowed researchers to measure how Europa’s icy surface reflects radar signals across a wide range of rotational orientations, providing coverage far exceeding the previous such campaign conducted between 1987 and 1991 using the now-collapsed Arecibo Observatory.
Three key findings
The team reported three principal results. First, Europa’s radar albedo, or brightness, is much higher than other planetary bodies, consistent with the earlier Arecibo-era measurements. Second, the moon’s icy surface reflects radar signals in a highly specular, mirror-like manner, unlike the diffuse scattering seen from most solar system objects. Third, the study confirmed the coherent backscatter opposition effect, a phenomenon that produces high radar reflectance when signals pass through pure water ice.
That third finding carries particular weight. The coherent backscatter effect is a signature of extremely pure water ice, and its presence at Europa, Ganymede, and Callisto strengthens the evidence that all three Galilean moons harbor subsurface liquid water oceans. The radar signal is effectively bouncing off the ice-water interface or scattering coherently within ice grains of unusual purity.
“Future planetary science and space flight missions, like NASA’s Europa Clipper, could benefit from this type of radar science,” said Will Armentrout of the Green Bank Observatory, a co-author of the study. “As the Green Bank Telescope’s radar capabilities evolve, with new technologies currently under development, we’re looking forward to providing even more radar capabilities for the scientific community.”
What lies beneath
Europa, slightly smaller than Earth’s Moon, has long been considered one of the most promising targets in the search for life beyond Earth. Its subsurface ocean, confirmed by Galileo spacecraft magnetometer data between 1995 and 2003, contains an estimated twice the water of all Earth’s oceans combined.
The ice shell covering that ocean has been the subject of intense debate for decades. Estimates ranged from less than one kilometer (0.5 miles) to tens of kilometers thick. In January 2026, NASA’s Juno mission provided the first definitive measurement: approximately 29 kilometers (18 miles) average thickness, based on microwave radiometer data collected during Juno’s close flyby of Europa in September 2022.
Steve Levin, Juno project scientist at NASA’s Jet Propulsion Laboratory, cautioned that the figure reflects the conductive outer layer. “The 18-mile estimate relates to the cold, rigid, conductive outer-layer of a pure water ice shell,” he said. If a warmer convective inner layer exists, the total shell would be thicker. If the ice contains modest dissolved salt, the thickness would be reduced by roughly 5 kilometers (3 miles), well within the measurement’s uncertainty.
Juno also detected small scatterers in the near-surface ice, irregularities such as cracks and pores no bigger than a few centimeters in radius, extending to depths of hundreds of meters. Their small size and shallow depth suggest they are unlikely to serve as pathways for nutrients traveling between the surface and the ocean below, an important constraint for assessing Europa’s habitability.
Scott Bolton, Juno principal investigator at the Southwest Research Institute, emphasized the significance. “The thickness of Europa’s icy shell and the existence of cracks or pores within the ice shell are two crucial pieces of the puzzle for understanding Europa’s potential habitability,” he said. “They provide critical new information relevant to the further study of Europa by NASA’s Europa Clipper and the European Space Agency’s Juice, both on their way to the Jovian system.”
Clipper on the way
NASA’s Europa Clipper, the largest planetary spacecraft ever built by the agency, launched in October 2024 and is scheduled to arrive at Jupiter in April 2030. Its four-year science campaign will include 49 flybys of Europa, with closest approaches dipping to just 25 kilometers (16 miles) above the surface.
Among its nine instruments is REASON, the Radar for Europa Assessment and Sounding: Ocean to Near-surface. Built by the University of Texas Institute for Geophysics, REASON carries dual-frequency ice-penetrating radar capable of sounding from 300 meters to 30 kilometers depth. During a Mars gravity-assist flyby in March 2025, the instrument was tested successfully, returning 60 gigabytes of data.
“We got everything out of the flyby that we dreamed,” said Don Blankenship, REASON principal investigator. “The goal was to determine the radar’s readiness for the Europa mission, and it worked. Every part of the instrument proved itself to do exactly what we intended.”
The Goldstone and Green Bank study provides a ground-based complement to Clipper’s orbital radar campaign. While REASON will map Europa’s ice shell structure directly from orbit, the ground-based data offers a long-term baseline extending back to the 1980s, allowing scientists to track any changes in surface properties over decades.
ESA’s Juice mission, which launched in April 2023 and will arrive at Jupiter in July 2031, will add two close Europa flybys and 12 Callisto flybys to the campaign, providing additional radar depth soundings and compositional data from different orbital perspectives.
Why Europa matters
Europa’s ocean has likely been liquid for roughly four billion years, sustained by tidal heating from Jupiter’s powerful gravity. That longevity, combined with chemical energy sources at the seafloor and a surface that may deliver oxidants into the ice, makes it one of the most plausible habitats for extraterrestrial life in the solar system.
The new ground radar data adds a critical piece to that picture. Pure water ice at the surface, a mirror-like reflective layer, and a shell thickness now constrained by direct measurement all help scientists build the models they will test when Clipper and Juice begin their science campaigns at the end of the decade.
As Armentrout put it, ground-based radar is not a replacement for orbital missions, but a complement that can operate continuously across decades. With Clipper still four years from Jupiter and Juice flying an even longer trajectory, the 13-year campaign from Goldstone and Green Bank has given planetary scientists their sharpest Earth-based view of Europa’s secrets yet.