Beyond the main asteroid belt lie the giant planets Jupiter and Saturn and their impressive retinues of moons. What chances for life are out here in the frozen wastes of the solar system? At first glance, you’d imagine none. The planets themselves have no solid surface (unless you go thousands of miles down), and their moons are bitterly cold, with surface temperatures of minus 160 C or less. Life as we know it needs liquid water (plus some source of energy and carbon-containing substances), and there might seem no hope of finding water so far from the Sun.
But, as early as the 1970s, theorists had begun to speculate that some of the larger moons of Jupiter, especially Europa, might have their interiors warmed up through tidal heating. This process involves flexing of the moon as it moves around its orbit and interacts gravitationally with nearby satellites and Jupiter itself. The result might be a subsurface ocean of liquid water.
These suspicions were confirmed by images and other data sent back by the Galileo probe during its eight-year exploration of the Jovian system from 1995 to 2003. The evidence is now compelling that Europa has a large saltwater ocean, hidden under several kilometers or more of ice. Subsurface oceans might also exist on two of Jupiter’s other moons, Ganymede and Callisto, on Saturn’s Titan and Enceladus, and on Neptune’s Triton. In fact, such bodies of water could be incredibly common throughout the galaxy. The big question is: are they capable of supporting life?
A step toward answering this could be taken by landing a spacecraft on Europa and sampling some of the material that appears to have welled up through cracks in the icy crust from the ocean below. Analysis of the rusty brown chemicals staining the surface of Europa might provide us with the first clues as to the habitability of the dark, watery depths below. Later on, it might be possible to send a more sophisticated mission to drill or melt through the ice and release instruments into the ocean to hunt for anything that might live down there.
At a meeting of NASA’s Outer Planet Analysis Group (OPAG) in March 2012, various options were considered for a Europa-targeted mission, including a multiple fly-by craft and an orbiter plus lander. Meanwhile, the European Space Agency has selected the Jupiter Icy Moon Explorer (JUICE) as a candidate for its first L-class mission, scheduled for launch in 2022 and arrival at Jupiter in 2030. JUICE will eventually orbit Ganymede (2033)—the largest moon in the solar system, and another candidate for a subsurface, possibly habitable, ocean—having flown by Europa and Callisto a number of times.
Astrobiologists are also extremely interested in Titan, the biggest moon (by far) of Saturn and second largest moon in the solar system (after Ganymede). Titan is the only moon in the solar system to have a dense atmosphere and a liquid in a stable state on its surface. This liquid is not water, however, but methane. Instead of a water cycle, Titan has a hydrocarbon cycle involving methane and ethane. Has the complex brew of organic chemicals that floats in the moon’s atmosphere or settled on the surface nurtured primitive organisms? Is there life in the subsurface, watery ocean that might also exist on Titan? The European Huygens probe, carried by Cassini, has already landed on Saturn’s big moon, sending back remarkable images during its descent and on the surface. Various plans have been suggested for a future craft to land on one of the methane lakes or seas on the moon, or investigate the surface buoyed by a balloon. But, for now, these remain only tantalizing possibilities.