On a crisp night in 2011, a faint pulse of radio waves shot from the Mojave Desert toward Jupiter’s moon Europa, 630 million kilometers away. For over a decade, scientists at NASA’s Goldstone Solar System Radar kept sending these 3.5-centimeter signals, patiently listening for echoes that would reveal what lies beneath Europa’s frozen crust. What they found—confirmed through 13 years of observations and captured by the NSF Green Bank Telescope—was a radar signature unlike any seen on rocky planets: a brilliant, diffuse reflection suggesting clean, porous ice that scatters radio waves in a phenomenon known as the coherent backscatter opposition effect. This isn’t just a quirk of physics—it’s a window into one of the solar system’s most promising habitats for life.

Europa, a shimmering moon slightly smaller than Earth’s own, has long captivated scientists because of its hidden ocean, believed to hold more liquid water than all of Earth’s seas combined. But surface images only hint at what’s below. Radar, as UCLA graduate student Tunhui (Tina) Xie explains, “delves below what is easily seen,” probing the structure and purity of the ice shell that shields that ocean from space. The new study, the most extensive Earth-based radar campaign of Europa to date, shows the moon reflects radar with an albedo far higher than most planetary bodies—so bright that it suggests radio waves are bouncing repeatedly within a deep, transparent, and surprisingly clean ice layer before returning to Earth.

Using a bistatic setup—Goldstone transmitting and both Goldstone and the Green Bank Telescope receiving—the team measured how the radar echo changed with viewing angle. They discovered something unexpected: Europa’s radar brightness remained nearly constant across different angles, indicating the backscatter peak is broader than previously thought. This places a new constraint on how deep the radio waves can travel before being absorbed—critical data for interpreting upcoming measurements from NASA’s Europa Clipper, set to arrive in the 2030s. The stability of the radar signal over 13 years also confirms that Europa’s icy shell has consistent properties over time, reinforcing confidence in both ground-based and future spacecraft observations.

While the moon’s radar signature stayed largely uniform as it rotated, a subtle hint emerged: the trailing hemisphere may be slightly brighter in one polarization state. Though not statistically definitive, this could point to surface alterations caused by Jupiter’s intense magnetosphere, where charged particles bombard the ice, potentially altering its texture or chemistry. As Will Armentrout of the NSF NRAO notes, these ground-based radar techniques are sharpening the tools for future exploration. With new technologies in development, the Green Bank Telescope could soon offer even more powerful planetary radar capabilities—helping us peer deeper than ever into alien ice.