Europa

Europa is the smallest of the four Galilean moons and one of the most scientifically compelling worlds in the Solar System. Beneath its bright, smooth, crack-riven ice shell — 15 to 25 km thick — lies a global ocean of liquid salt water that may contain twice as much water as all of Earth’s oceans combined.

Europa’s ocean sits in contact with a rocky seafloor, potentially producing the chemistry — warmth, water, rock-water interaction — that many astrobiologists consider the minimum requirements for life. Tidal heating from the Laplace resonance with Io and Ganymede keeps the subsurface warm enough for liquid water. Hubble observations (2013-2016) detected water-vapor plumes erupting from the southern hemisphere, and a reanalysis of Galileo flyby data has found evidence of a plume passage in 1997, suggesting these eruptions may carry ocean material to the surface where a future mission could sample it.

Europa has been observed up-close by Voyager 1 & 2, Galileo (which made eight flybys between 1995 and 2003), Juno (2022 flyby at 352 km), and most intensively will be surveyed by NASA’s Europa Clipper — launched 14 October 2024, arriving in 2030 to conduct ~50 close flybys.