Earthlings have often dreamed about life on other planets. Back when people thought life on Mars was possible, Martians were incorporated into a variety of books, cartoons, and comics. Since Venus is Earth’s “sister planet”, many hoped for the possibility of Venusians. Now we know that life on those planets is not possible, but scientists are still searching the universe for signs of habitable planets. Astronomers and scientists have found many extrasolar planets that are in the habitable zones of other solar systems, offering a very promising location for life. However, astronomers have been gathering more data about our own solar system, and we might not have to look beyond our solar system to find other life.
Astronomers have discovered something amazing about several of the jovian moons: they very likely have subsurface salty oceans. Three moons that have these subsurface oceans are large moons of Jupiter: Europa, Ganymede, and Callisto.
Ganymede is a fascinating moon. It is the largest moon in the solar system and is the only moon with its own magnetic field. Scientists studied the aurorae on Ganymede using the Hubble Space Telescope and determined that the fairly stable aurorae indicate a huge salty ocean. However, Ganymede is not the best moon to start looking for life. The ocean exists, but it is far below the moon’s surface, which means it most likely lacks a hydro-thermal system and thus probably lacks life. While Ganymede is still a wonderful place to study, it is not a great place to look for life.
Callisto was originally thought to be a very boring moon, as it was heavily cratered and showed no real signs of geological activity. That changed when the Galileo spacecraft flew by, giving scientists more data that suddenly made Callisto more interesting. Fluctuations in Callisto’s induced magnetic field implied the existence of electrical currents within the moon. With Callisto’s thin atmosphere and rocky surface, these currents are likely to come from a subsurface salty ocean. It’s still unclear whether or not this subsurface ocean can support life.
Europa is probably the most promising of the three moons in terms of supporting life. Europa’s icy surface is young and full of cracks, filled in by dark, icy material, that indicate the surface had been active in the past. When studying some of the linear features on Europa, scientists determined the patterns would fit if the surface moved independently from the rest of the interior. Studies of tidal heating suggest a global subsurface ocean. Like Callisto, fluctuations in Europa’s induced magnetic field indicate electrical currents, likely cause by a salty ocean. Europa’s hot interior could warm the ocean through volcanic vents, providing enough heat to support life.
In the 2020s, the European Space Agency is planning to launch the JUICE (Jupiter Icy Moons Explorer) mission. The spacecraft will visit these moons of Jupiter and study them for more information about their subsurface oceans and the possibility of life.