From 1987 to 1991, astronomers last measured how Jupiter's icy moons reflect radio waves. Then came a long pause. In 2011–2024, researchers aimed a radar beam at Europa again: this time using the 70-meter antenna at Goldstone and the Green Bank Telescope.

Ice is nearly transparent to radio waves, like clear window glass is to light. By sending a signal and catching its reflection, you can learn what's happening in the upper tens of meters of the icy crust. The radar showed that Europa's leading side (the one facing forward in its orbit) reflects waves differently than the trailing side. It's as if one half of a mirror were frosted and the other polished.

Such a difference hints that the moon's surface is not uniform: in some places the ice is cleaner and denser, in others it's loose or mixed with impurities. Understanding these differences helps reconstruct Europa's history and assess how deep beneath the ice a liquid water ocean might hide.