In speculating about extraterrestrial life in the Solar System, astronomers have long considered Jupiter’s system of satellites an interesting target to investigate.  The idea that there might be something extraordinary going on in that neighborhood was probably fused most successfully to the popular imagination by Stanley Kubrick’s classic 1968 film 2001: A Space Odyssey, the script for which was co-written by Arthur C. Clarke.  However, getting even unmanned spacecraft there in reality has been an incredibly difficult and exciting challenge for space scientists and engineers.

A Shiny, Discolored Billiard Ball

Twenty years before Juno’s much heralded insertion into orbit around Jupiter just weeks ago, Galileo was the first spacecraft to visit the Jovian system for an extended stay.  Among the revealing and even startling images it sent back were pictures of Jupiter’s moon Europa – a shiny, mostly white orb bearing extensive reddish-brown discoloration that looks almost like sand trapped in ice.  A network of striations (called lineae) crisscross the moon as if something had struck the sphere with sufficient force to crack but not shatter it altogether.

The trailing hemisphere of Europa as imaged by the Galileo spacecraft. (Image credit: NASA/JPL/DLR)

Astronomers have learned an enormous amount about Europa since the Galileo mission’s eight-year sojourn.  It is comprised primarily of silicate rock and is covered entirely by a water-ice crust.  Indications are that it likely has an iron-nickel core.  The apparent lineae that extend all around the moon are actually covered by ice, which makes Europa’s surface the smoothest of any Solar System object thus far observed.  Surrounding it is a tenuous atmosphere comprised mostly of oxygen.

What Lies Beneath?

An obvious question for analysts is what lies beneath that icy crust?  The most intriguing hypothesis is that the ice covers a vast subsurface ocean of liquid water – an ocean that might contain a volume of water two-to-three times that of Earth.  According to this theory, the water is maintained in a liquid form by heat produced by tidal flexing resulting from Europa’s gravitational interaction with Jupiter and its other moons.  Such flexing would drive geological activity akin to plate tectonics on Earth.

Reddish bands on Europa’s surface contain hydrated salts and (possibly) magnesium sulfate or sulfuric acid. (Image credit: NASA/JPL-Caltech/SETI Institute)

Assuming that the idea of abundant, subsurface, liquid water holds up – and there have been indications that it will – then it is only natural for scientists to conjecture about the possibility of life existing somewhere in that reservoir.  Perhaps, some think, life might have arisen along the inner surface of Europa’s icy crust (similar to bacteria and algae in polar regions on Earth).  Others think it more likely that life forms might occupy a niche close to hydrothermal vents on the subsurface ocean’s floor, much as they do in such extreme conditions on Earth.  Interestingly, geneticists from Heinrich Heine University, in Germany, recently published research in Nature Microbiology concluding that all life on Earth descended from a microbial common ancestor that arose near such deep-ocean vents some four billion years ago.

Returning for a Closer Look?

Many scientists favor giving high priority to exploratory missions focused on Europa to investigate the possibility that life forms exist (or have existed) there.  Some have even gone so far as to suggest the moon as a possible site for human colonization at some point far in the future.  More practically, NASA and the European Space Agency currently plan separate missions to Europa – but neither mission is expected to launch for several years or to land a probe on the moon.  NASA’s spacecraft, however, is expected to orbit Jupiter and fly by Europa dozens of times and to descend to an altitude of only 16 miles above the frozen surface.

Artist’s rendition of NASA’s planned Europa mission spacecraft. (Image credit: NASA/JPL-Caltech)

Bonus Video!

This is one of our newest SpaceRip videos exploring the exciting mission of Juno! But for the scientists in charge of the mission, the unknowns and dangers are frightening.