2000

An Ocean on Ganymede?

Between 1995 and 2003, NASA’s Galileo Jupiter orbiter conducted a grand orbital investigation of the giant planet’s atmosphere, magnetic field, moons, and rings. The spacecraft’s trajectory was designed so that it would make close flybys of the Galilean satellites Io, Europa, Ganymede, and Callisto in order to study their surface features up close, but also to measure their masses and gravity fields by monitoring the way that they slightly deflected Galileo’s orbit.

Over the course of the mission, Galileo made six close flybys of Ganymede, the solar system’s largest moon. Gravity data indicated that the interior of Ganymede is differentiated—that is, segregated into a dense core of rock and iron, a lower-density (presumably icy) mantle, and an outer, icy crust. The biggest surprise from the flybys, however, was the discovery that Ganymede has its own magnetic field, embedded within the strong magnetosphere of Jupiter. Ganymede is the only moon in the solar system known to have its own magnetosphere.

Ganymede’s magnetic field is thought to be generated by the same process that generates Earth’s magnetic field: a spinning, partially molten core of conductive iron. Decay of radioactive elements in the satellite’s interior, and some tidal heating from gravitation interactions with Europa and Io, provide the heat to keep the core molten. By studying Ganymede’s magnetic field, Galileo scientists were able to determine that the satellite’s mantle is also electrically conductive. The simplest explanation, given the likely icy composition of the mantle and the strong internal heat sources, is that Ganymede has a deep layer of salty, liquid water—a subsurface ocean—about 125 miles (200 kilometers) beneath the icy crust. Similar Galileo observations of Callisto also revealed tentative evidence for a subsurface liquid-water ocean, though less deep, on that moon as well. Could these oceans, or Europa’s subsurface ocean, harbor life?

While compelling, the Galileo results don’t prove that there is an ocean on Ganymede or Callisto. That proof, as well as proof of an ocean on Europa, would have to come from future missions, perhaps future orbiters and landers, that will have to use radar sounding and perhaps deep drills to settle the issue for sure.

Artist’s conception cutaway view of the interior of Jupiter’s largest moon, Ganymede, showing the hypothesized deep subsurface ocean of liquid water above a lunar-size core of rock and metal. The ocean is hypothesized to exist based on gravity and magnetic field data from NASA’s Galileo Jupiter orbiter mission.