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Europa

Europa is the smallest of Jupiter's Galilean moons, at 1941 miles (3140 km) in diameter; it is slightly smaller than our own Moon. Europa completes one orbit around Jupiter in 3.55 days, at an average distance of 416,000 miles (670,000 km). Europa participates in a three-way, 1:2:4 orbital resonance with Jupiter's other inner Galilean moons (Io and Ganymede). In other words, for each orbit of Ganymede, Europa orbits Jupiter twice, and Io orbits Jupiter four times. Like the other Galilean moons, Europa's rotation is tidally locked with Jupiter, so Europa always shows the same face toward the giant planet.



smoothest objects in the Solar System. It is mostly made of water ice, and displays long streaks and cracks which run for up to several thousand kilometers across its surface. Like Io, Europa displays very few impact craters, so something must be erasing them. The Galileo orbiter photographed evidence that Europa's icy crust is in motion, like the sea ice floating on the Earth's polar oceans. Europa is too dense to be entirely composed of ice, however, and underneath the ice is a thick layer of silicate rock, and (probably) an iron core. Feature names on Europa recall Celtic gods and mythic places, and the Europa myth.

The same tidal forces which power Io's volcanoes also exist inside Europa, although to a lesser degree. These forces heat up Europa's interior enough to melt the ice. Europa's icy crust may be covering an ocean of liquid water up to 100 kilometers deep. Europa is thought to have twice as much liquid water as the Earth! For this reason, there is a possibility that life may exist on Europa, in the darkness under the icy crust, much as life exists in the Earth's undersea volcanic vents. Spacecraft missions to Europa are being planned to investigate this possibility.

The Galileo orbiter was deliberately flown into Jupiter after completing all of its mission objectives in 2003 to avoid any possibility of colliding with Europa and contaminating its surface.

The Jovian satellite Europa is, like Mars, a world for which we have a special fascination. It has about the same size and density as Earth's Moon, but the similarities end there. Europa's surface is among the brightest in the solar system, a consequence of sunlight reflecting from an icy surface. The exotic coloring, texture, and detail visible on that surface continue to pose significant challenges for planetary geologists. There is much we don't understand - and much yet to learn.

What we do know about Europa stems from centuries of telescopic observation - beginning with Galileo's discovery of four Jovian moons in 1610 - combined with spacecraft data and sophisticated computer models. Most of this moon's "leading" hemisphere (facing forward along its orbit) is covered with water frost. But the trailing hemisphere is markedly darker and redder, apparently coated with magnetospheric ions (mostly sulfur derived from Io) that overtake Europa as they orbit around Jupiter.



Spacecraft observations suggest that Europa consists of a metallic core about 750 miles (1,250 km) across, a rocky mantle, and an ice-and-water "crust" about 100 miles (150 km) thick. To have these distinct layers, at some stage of its evolution Europa must have been molten throughout. During its molten phase, dense, metallic elements sank and formed the core, while lighter compounds floated to the surface, leaving medium-density "rocky" materials in the middle.

The lighter compounds were mostly water, forming a global ocean whose exterior froze to form a crust of ice. It's not known if the entire ocean froze, or if the surface has since frozen to some depth over a body of liquid water. Europa has a weak magnetic field, and some planetary scientists believe it arises not in Europa's metallic core but rather induced by the passage of Jupiter's field through a salty (and thus conductive) liquid layer below the surface. Another possibility is that the ocean froze solid and then remelted from the bottom up.

Whatever its true interior structure, Europa holds a unique place in the family of solar-system objects. This moon displays a mix of characteristics that make it something of a transitional world between the rocky planets of the inner solar system and the predominantly ice-rich satellites of the outer solar system. Like neighboring Io, Europa also experiences interior heating from tidal forces and could be geologically active today.



The surface has two distinct types of terrain. Bright plains appear to be the basic surface type, from which many of the other terrains derive. In some places these plains consist of multiple sets of ridges and grooves. Mottled terrain, as the name suggests, includes an irregular patchwork of darker zones, with irregular margins grading into the bright plains. The smallest patches are about 30 miles (50 km) across, and the largest are 10 times bigger.

Scientists now suspect that mottled terrain forms in response to tectonic forces. For example, a region on the trailing hemisphere of Europa named Conamara Chaos consists of bright plains that have been broken into plates with intervening areas of chaotic terrain. Many of the plates have been moved into new positions, and some appear to be tilted - as if they being submerged into the darker, low-lying chaotic material surrounding them.

These features, along with others seen elsewhere on Europa, suggest upward warping of the crust by force applied from below. Several causes are possible, such as buoyant, low-density masses rising toward the surface or intrusions of some "molten" material (a water-ice slush perhaps) that becomes fluidized below the surface. Europa has another characteristic type of geologic feature: long sets of ridges and grooves that crisscross the surface. Some have a curved shape, while others cut across one another. Researchers suspect that these lineae (Latin for lines) form in response to periodic tidal flexing of Europa's crust, which repeatedly creates fractures that allow ice or liquid water to emerge from below. This mechanism could explain the multiple parallel structures seen on many of the ridge sets.


All this cracking and movement of Europa's crust has led to interesting and even provocative speculations. The heat generated by tidal flexing, together with radioactive decay from rocky matter deeper down, might be sufficient to melt part of the ice layer and provide a source of liquid water. And while there's been no direct observation of organic materials on Europa, such compounds have almost surely been delivered by comets and meteorites.

Therefore, all three conditions for biologic development - organic molecules, liquid water, and a source of energy - would appear to exist beneath the icy crust of Europa. However, meeting these simple requirements does not mean that the "magic spark" ever occurred to spawn organic evolution and perhaps life itself. Still, the prospect is tantalizing enough to justify the high priority for its further exploration, and the European Space Agency proposes to launch a spacecraft called Jupiter Icy Moon Explorer (JUICE) in 2022.

[Adapted from a chapter by Ronald Greeley in The New Solar System (Sky Publishing and Cambridge University Press).]