Europa: Ice world with a secret ocean
by Cynthia Phillips
- Europa is a large, icy moon of Jupiter
- It likely has a subsurface ocean with more water than all Earth's oceans combined
- Europa could have all the key ingredients for life
Europa is an icy moon of Jupiter, located about five times farther from the sun than we are here on Earth. Europa is close in size to Earth's moon, but its surface appearance is quite different. Our moon is an old, cratered body with very limited endogenic - that is, internally-generated - geologic activity. Europa, on the other hand, has a bright, icy surface covered with cracks, ridges and disrupted areas, with only a handful of large craters visible. Europa's wealth of endogenic features and lack of craters means that this enigmatic moon is almost certainly geologically active, and that geologic processes are constantly changing and modifying the surface to remove all but the youngest impact craters from view.
Europa's surprising degree of geologic activity results from the configuration of the Jupiter system. Europa is one of the four large, so-called "Galilean" moons of Jupiter - in order from closest to Jupiter, they are Io, Europa, Ganymede, and Callisto. As these four moons orbit Jupiter, the inner three are in a resonance - close approaches of the moons to each other tweak the orbits such that every time Ganymede orbits Jupiter once, Europa orbits twice, and Io orbits four times. This resonance keeps the three moons in stable orbits, but also gives the orbits a slight non-circular shape. The resonance is the key to the whole story, because without it, the orbits would naturally evolve into a circular configuration in which the distance from the moon to the planet it orbits does not change. In the case of these three resonant Galilean moons, however, their distance from Jupiter changes as each moon orbits the planet. Jupiter's strong gravity raises tides on the surfaces of the moons that vary in height because of the moons' changing distance from Jupiter. The moons are flexed and pulled, and heating occurs as a result.
This "tidal heating" is greatest at Io because of its close proximity to Jupiter, and it makes Io the most volcanically active body in the solar system. Europa, the next moon out, probably receives sufficient heating to have maintained a subsurface ocean layer of liquid water over the entire age of the solar system. Ganymede, farther out, also receives some heating and may have a subsurface ocean that is much more deeply buried than Europa's. Ganymede's surface shows some signs of endogenic activity, but large portions of its surface are covered with ancient cratered terrain, suggesting not much activity from within for a very long time. Callisto does not take part in the resonance and thus has a near-circular orbit and essentially no tidal heating. Callisto is an old, cratered world that bears the strongest resemblance to Earth's moon of the four Galilean moons.
Europa's surface geology is dominated by tectonic features formed by the movement of the crust, such as cracks and ridges. As the surface is stretched and pulled by tidal forces, cracks form, which seem to transform into ridges or broad bands over time. The details of the processes that create these features are still somewhat mysterious, but a variety of ideas have been proposed for their formation, including several that rely on the action of a subsurface ocean below.
One of the major unanswered questions about Europa is just how deep below the surface a liquid water ocean layer is located. We know from gravitational measurements obtained by the Galileo spacecraft in the late 1990s that Europa's outermost layer with the density of water, is about 50 to 90 miles (80 to 150 kilometers) thick. Unfortunately the spacecraft's measurements were not detailed enough to tell how much of that water is frozen water ice and how much of it is liquid. Estimates for the thickness of Europa's outer icy layer range from a few miles (kilometers) to a few tens of miles (kilometers).
Scientists are particularly excited about Europa because it may have all the needed "ingredients" for life - liquid water, the right chemical elements, and a source of energy to support metabolism. It is unlikely that life in Europa's ocean would be able to perform photosynthesis, given that the sun is 25 times fainter at Jupiter than it is here on Earth and little sunlight is likely to penetrate more than a meter or so through ice. However, radiation due to Jupiter's strong magnetic field could create many useful chemical products on Europa's surface. In time, geological processes could cause these materials to be mixed down into the subsurface and eventually the liquid ocean, perhaps serving as an energy source for microbial life that might possibly live there.
Future missions could map the surface of Europa at high resolution, as well as use other techniques to study the intriguing moon's subsurface structure and composition. Such data might reveal locations where water is close to the surface and more accessible. These locations would be attractive as sites for a future Europa lander that could try to understand whether life could exist beneath the ice.
Dr. Cynthia Phillips is a planetary geologist based at the SETI Institute, in Mountain View, CA, who has devoted much of her career to studying the icy moons of the outer solar system. Her favorite place is Europa.