Did You Know?
Not all moons are dry and dusty like ours. Jupiter's Europa has a liquid ocean under an icy crust.
Decades ago, science fiction offered a hypothetical scenario: What if alien life were thriving in an ocean beneath the icy surface of Jupiter’s moon Europa? The notion pulled Europa out of obscurity and into the limelight where it has remained, stoking the imaginations of people both within and outside the science community who fantasize about humans discovering life beyond Earth. That fantasy, however, may be grounded in reality.
From ground-based telescopes, scientists knew that Europa's surface is mostly water ice, and scientists have found strong evidence that beneath the ice crust is an ocean of liquid water or slushy ice. In 1979 the two Voyager spacecraft passed through the Jovian system, providing the first hints that Europa might contain liquid water. Then ground-based telescopes on Earth, along with the Galileo spacecraft and space telescopes, have increased scientists’ confidence for a Europan ocean.
Scientists think Europa’s ice shell is 10 to 15 miles (15 to 25 kilometers) thick, floating on an ocean 40 to 100 miles (60 to 150 kilometers) deep. So while Europa is only one-fourth the diameter of Earth, its ocean may contain twice as much water as all of Earth’s oceans combined. Europa’s vast and unfathomably deep ocean is widely considered the most promising place to look for life beyond Earth. A passing spacecraft might even be able to sample Europa’s ocean without landing on the moon’s surface because it is possible that Europa’s ocean may be leaking out into space.
While no plumes were observed while the Galileo spacecraft was in the Jupiter system in the 1990’s, more recent observations from telescopes such as Hubble Space Telescope, as well as a reanalysis of some data from the Galileo spacecraft, have suggested that it is possible that thin plumes of water are being ejected 100 miles (160 kilometers) above Europa’s surface. In November 2019, an international research team led by NASA announced they had directly detected water vapor for the first time above Europa’s surface. The team measured the vapor using a spectrograph at the Keck Observatory in Hawaii that measures the chemical composition of planetary atmospheres through the infrared light they emit or absorb.
If the plumes do exist, and if their source is linked to Europa’s ocean, then a spacecraft could travel through the plume to sample and analyze it from orbit, and it would essentially be analyzing the moon’s ocean (the Cassini spacecraft performed this feat at Saturn’s moon Enceladus, which is known to have an ocean spraying into space). Even if Europa isn’t ejecting free samples into space, a 2018 study concluded that samples of Europa’s ocean could get frozen into the base of the moon’s ice shell, where the ice makes contact with the ocean. As the ice shell distorts and flexes from tidal forces, warmer and less-dense ice would rise, carrying the ocean samples to the surface where a spacecraft could analyze it remotely, using infrared and ultraviolet instruments, among others. Scientists could then study the material’s composition to determine whether Europa’s ocean might be hospitable for some form of life.