Europa’s subsurface oceans are thought to contain twice as much water as all of Earth’s oceans combined. Measurements of the moon’s magnetic field also hinted that this water might be salty, suggesting it is in contact with a rocky seabed — a potential source of life-giving energy.
If the liquid ocean really is there, it could make Europa an especially promising place to look for life. But many mysteries remain about this moon, from the thickness of the ice shell to the details of what happens beneath it.
NASA’s Europa Clipper mission, which is due to launch in 2024 and arrive at Jupiter in 2030, would confirm whether Europa does indeed have a subsurface ocean capable of supporting life.
Enceladus
Europa’s appeal in the search for life is matched, and even exceeded, by Enceladus. The moon of Saturn shares a few key characteristics with Europa, including an icy shell that shows hints of hiding a liquid water ocean beneath it. Like Europa, the subsurface oceans of Enceladus show signs of making contact with a rocky seabed, which could provide energy for life forms in those waters. But where Enceladus outdoes Europa is in the accessibility of its ocean for study.
Enceladus’ south pole is covered in a series of fissures through which plumes of water have been observed shooting up into space. This makes sampling the moon’s subsurface ocean much easier than it would be to sample water hidden below tens of kilometers of icy crust, which might be the case with Europa.
In fact, NASA’s Cassini Saturn orbiter flew through Enceladus’ plumes several times between 2008 and 2015, adapting one of its instruments to analyze the composition of the water. Within the plume, Cassini detected a variety of organic molecules including phosphorus, one of the ingredients for amino acids that has never previously been found in extraterrestrial oceans.