Jupiter's moon Europa has an ice shell about 18 miles thick — and that could be bad news for alien life

A close up of a gray planet in the darkness of space with streaks of brown running across it.

After years of debate, NASA researchers have zeroed in on the thickness of the Europa's ice shell.

We first discovered that the Jupiter moon's surface looked icy in 1979, when Voyager 2 flew by. Another NASA mission, the Galileo Jupiter orbiter, later confirmed the ice shell while it inspected the giant planet, Europa, and other Jovian moons during the 1990s.