Ground-Based Radar Pings Reveal Europa’s Icy Secrets Ahead of Clipper Arrival

Thirteen years of persistent radar pulses fired at Jupiter’s enigmatic moon have produced the clearest picture yet of its frozen exterior. The data confirm that Europa scatters radio waves with a distinctive vigor unseen on rocky bodies across the solar system. But the observations do more than refine old measurements. They hand scientists a practical map for the instruments soon to arrive in orbit.

And the timing could hardly be better. NASA’s Europa Clipper, carrying the REASON ice-penetrating radar, is already en route after its 2024 launch. It will reach Jupiter in 2030. The new ground-based results, drawn from NASA’s Goldstone Solar System Radar transmitter and the NSF Green Bank Telescope receiver, fill a 30-year gap since the last major radar campaign in the late 1980s and early 1990s. National Radio Astronomy Observatory reported the findings just days ago.

The campaign ran from 2011 through 2024. Researchers bounced 3.5-centimeter radio waves off Europa hundreds of times. They collected the echoes with Green Bank. Tunhui Xie, a UCLA graduate student, and Jean-Luc Margot, her advisor, led the analysis. Their dataset spans a far wider range of Europa’s rotational phases than previous efforts. “More numerous and cover a much broader rotational phase of Europa,” Xie explained during a presentation at the American Astronomical Society’s 248th meeting.

What emerged is striking. Europa’s radar albedo sits far higher than that of the Moon, Mars or any terrestrial planet. Its echoes display strong circular polarization and a coherent backscatter opposition effect. These traits point to multiple scattering inside clean, porous ice. Radio waves bounce repeatedly among tiny voids, cracks and irregularities before returning to Earth. The brightness holds steady even as viewing angles shift. That consistency sets a fresh limit on how deeply such signals can probe before they diffuse into noise.

The scattering resembles “a hallmark of multiple scattering inside clean, porous ice,” the NRAO statement noted. Xie put it another way. “Radar delves below what is easily seen, because radio waves can penetrate into the ice and carry information about its internal structure and purity.” The work reinforces earlier findings yet sharpens them with modern precision. No major changes appear across longitudes or between leading and trailing hemispheres, though one polarization hints at slightly higher brightness on the trailing side.

These Earth-based results arrive alongside a separate breakthrough from NASA’s Juno spacecraft. During a 2022 close flyby, Juno’s Microwave Radiometer measured thermal emission from Europa at multiple frequencies. The data yield an average conductive ice-shell thickness of 29 kilometers, plus or minus 10 kilometers. That figure assumes pure water ice. Steve Levin, Juno project scientist at NASA’s Jet Propulsion Laboratory, described the meaning plainly.

“The 18-mile estimate relates to the cold, rigid, conductive outer-layer of a pure water ice shell,” Levin said. NASA published the account in late January. If a warmer convective layer sits beneath, total ice thickness grows. Add modest salt and the estimate shrinks by roughly five kilometers. The Juno team also found evidence of centimeter-scale scatterers—cracks, pores or voids—reaching hundreds of meters below the surface. Their volume fraction is low. They are unlikely to serve as major conduits linking surface chemistry to the ocean below.

Scott Bolton, Juno principal investigator, captured the broader stakes. “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.” The Nature Astronomy paper appeared online in December 2025. It discriminates for the first time between thin-shell models, once favored by some, and the thicker versions now supported by data. A 29-kilometer barrier means any exchange between surface oxidants and the subsurface ocean takes longer. That slower mixing could shape the prospects for life in the hidden sea.

Yet the radar work from Earth offers something immediate and tactical. It defines exactly how Europa’s near-surface ice will behave when Clipper’s REASON instrument begins its own sounding in 2030. REASON operates at different frequencies. It is designed to pierce deeper, potentially all the way to the ice-ocean boundary if the shell is not too thick. The ground-based campaign shows that shallow scatterers will complicate those signals. It also quantifies the moon’s high reflectivity and diffuse character. Future interpretations will rest on this baseline.

Will Armentrout, a scientist at the National Radio Astronomy Observatory, sees expanding possibilities. “Future planetary science and space flight missions, like NASA’s Europa Clipper, could benefit from this type of radar science. 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.” The bistatic setup—transmit from Goldstone in California, receive at Green Bank in West Virginia—proved robust across more than a decade. It required careful coordination. Atmospheric effects, Jupiter’s synchrotron radiation and the moon’s own slow spin all had to be accounted for.

Scientists have debated Europa’s ice thickness for decades. Early models swung between a few kilometers and tens of kilometers. Thin ice raised hopes of easy access to the ocean and any chemistry churning there. Thick ice suggested greater isolation. The Juno microwave data tilt the scales toward the thicker side while leaving room for nuance. The radar echoes add texture to the top few hundred meters. Together they portray an ice shell that is cold and rigid near the surface, riddled with small-scale imperfections, yet largely transparent to longer radio wavelengths.

Clipper will fly past Europa dozens of times. Its full instrument suite includes cameras, spectrometers, a magnetometer and REASON. The radar will map the shell’s three-dimensional structure, hunt for pockets of liquid water within the ice, and search for the ocean interface. Success depends on understanding the very scattering properties now quantified from Earth. Without the 13-year dataset, Clipper scientists would face greater uncertainty in separating true subsurface reflections from surface clutter.

Recent modeling studies reinforce the value of these observations. Papers published in 2024 and 2025 explore how salt layers or convective processes might appear to ice-penetrating radar. One analysis shows that salt deposits formed by freezing of briny reservoirs could produce distinct echoes detectable by both Clipper and the European Space Agency’s JUICE mission, which arrives at Jupiter in 2031. Another constrains the conductive portion of the shell using sparse echoes. The ground-based radar results provide the empirical anchor those models need.

The combined picture is still incomplete. No one yet knows the total ice thickness with certainty. Thermal models, gravity data and magnetic induction measurements will add pieces. But the recent radar and microwave campaigns mark a genuine advance. They replace decades of conjecture with hard numbers on scattering behavior and conductive-layer depth. Europa no longer looks quite so opaque to human instruments.

So the next few years promise sharper insight. Clipper’s data will be interpreted against this fresh Earth-based standard. The question of whether the ocean communicates readily with the surface—and whether that communication could sustain habitable conditions—gains clearer parameters. The ice is thicker than many once hoped. Its upper layers scatter signals in complex ways. Yet the tools to peer through it have never been better calibrated.

Researchers continue to mine the full 13-year archive. Additional publications may refine the latitude dependence or polarization details. Upgrades to Green Bank’s radar system could extend the work to other icy moons. For now, the message is practical. Planetary scientists have a sharper sense of what to expect when the spacecraft radar arrives. The echoes from Earth have done their job. They have prepared the way.