13-6 Europa is covered with a smooth layer of ice that may cover a worldwide ocean

Europa, the second of the Galilean satellites, is the smoothest body in the solar system. There are no mountains and no surface features greater than a few hundred meters high (Figure 13-10). There are almost no craters, indicating a young surface that has been reprocessed by geologic activity. The dominant surface feature is a worldwide network of stripes and cracks (Figure 13-11). Like Io, Europa is an exception to the general rule that a small world should be cratered and geologically dead (see Section 7-6).

Figure 13-11: RIVUXG
Europa’s Fractured Crust False colors in this Galileo composite image emphasize the difference between the linear ridges and the surrounding plains. The smooth ice plains (shown in blue) are the basic terrain found on Io.
(NASA/JPL)
Figure 13-10: RIVUXG
Europa Dark lines crisscross Europa’s smooth, icy surface in this false-color composite of visible and infrared images from Galileo. These lines are fractures in Europa’s crust that can be as much as 20 to 40 km (12 to 25 mi) wide. Only a few impact craters are visible on Europa, which indicates that this satellite has a very young surface on which all older craters have been erased.
(NASA/JPL/University of Arizona)

Europa’s Surface: Icy but Active

Geology on Europa is based on solid ice and liquid water, not rock and molten magma

Neither Voyager 1 nor Voyager 2 flew very near Europa, so most of our knowledge of this satellite comes from the close passes made by Galileo. (Both Figure 13-10 and Figure 13-11 are Galileo images.) But even before these spacecraft visited Jupiter, spectroscopic observations from Earth indicated that Europa’s surface is almost pure frozen water (see Figure 7-4). This observation was confirmed by instruments on board Galileo, which showed that Europa’s infrared spectrum is a close match to that of a thin layer of fine-grained water-ice frost on top of a surface of pure water-ice. (The brown areas in Figure 13-10 show where the icy surface contains deposits of rocky material from meteoritic impacts, from Europa’s interior, or from a combination of these sources.)

The purity of Europa’s ice suggests that water is somehow brought upward from the moon’s interior to the surface, where it solidifies to make a fresh, smooth layer of ice. Indeed, some Galileo images show what appear to be lava flows on Europa’s surface, although the “lava” in this case is mostly ice. This idea helps to explain why Europa has very few craters (any old ones have simply been covered up) and why its surface is so smooth. Europa’s surface may thus represent a water-and-ice version of plate tectonics.

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Although Europa’s surface is almost pure water-ice, keep in mind that Europa is not merely a giant ice ball. The satellite’s density shows that rocky material makes up about 85 to 90 percent of Europa’s mass. Hence, only a small fraction of the mass, about 10 to 15 percent, is water-ice. Because the surface is icy, we can conclude that the rocky material is found within Europa’s interior.

Europa is too small to have retained much of the internal heat that it had when it first formed. But there must be internal heat nonetheless to power the geologic processes that erase craters and bring fresh water to Europa’s surface. What keeps Europa’s interior warm? The most likely answer, just as for Io, is heating by Jupiter’s tidal forces. The rhythmic gravitational tugs exerted by the orbital resonance of Io, Ganymede, and Europa deform Europa’s orbit, causing variations in the tidal stresses from Jupiter that make Europa flex. But because Europa is farther from Jupiter, tidal effects on Europa are only about one-fourth as strong as those on Io, which may explain why no ongoing volcanic activity has yet been seen on Europa.

Some features on Europa’s surface, such as the fracture patterns shown in Figure 13-11, may be the direct result of the crust being stretched and compressed by tidal flexing. Figure 13-12 shows other features, such as networks of ridges and a young, very smooth circular area, that were probably caused by the internal heat that tidal flexing generates. The rich variety of terrain depicted in Figure 13-12, with stress ridges going in every direction, shows that Europa has a complex geologic history.

Figure 13-12: R I V U X G
Europa in Close-up This high-resolution Galileo image shows a network of overlapping ridges, part of which has been erased to leave a smooth area and part of which has been jumbled into a rugged patch of terrain. Europa’s interior must be warm enough to power this complex geologic activity.
(NASA/JPL)

Among the unique structures found on Europa’s surface are ice rafts. The area shown in Figure 13-13a was apparently subjected to folding, producing the same kind of linear features as those in Figure 13-12. But a later tectonic disturbance broke the surface into small chunks of crust a few kilometers across, which then “rafted” into new positions. A similar sort of rafting happens in Earth’s Arctic Ocean every spring, when the winter’s accumulation of surface ice breaks up into drifting ice floes (Figure 13-13b). The existence of such structures on Europa suggests that there is a subsurface layer of liquid water or soft ice over which the ice rafts can slide with little resistance.

Figure 13-13: R I V U X G
Moving Ice on Europa and Earth (a) Some time after a series of ridges formed in this region of Europa’s surface, the icy crust broke into “rafts” that were moved around by an underlying liquid or plastic layer. The colors in this Galileo image may be due to minerals that were released from beneath the surface after the crust broke apart. (b) Europa’s ice rafts are analogous to ice floes created when pack ice breaks up, as in this spacecraft view of part of the Canadian arctic.
(a: NASA/JPL; b: USGS and NASA)

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But questions remain: Is Europa’s heat mostly gone, leaving behind a frozen interior today? Is there a large subsurface ocean, or just thin pockets of liquid water? To answer these questions, we return to measurements of magnetic fields, which act like faithful messengers from a planet’s interior.

An Underground Ocean?

Magnetic field measurements made by the Galileo spacecraft have provided key evidence for a large saltwater ocean within Europa. Unlike Earth or Jupiter, Europa does not seem to create a steady magnetic field of its own. But as Europa moves through Jupiter’s intense magnetic field, electric currents are induced within the satellite’s interior, just as they are around Io (see Section 13-5), and these currents generate a weak but measurable induced magnetic field. (The strength and direction of this induced magnetic field varies, and can even reverse within hours, as Europa moves through different parts of the Jovian magnetosphere; these quick reversals could not occur if Europa generated its field by itself.)

To explain these observations, there must be an electrically conducting fluid beneath Europa’s crust—a perfect description of an underground ocean of water with dissolved minerals. (Pure water is a very poor conductor of electricity, so electrically charged particles, such as dissolved minerals, would make the ocean water conducting.) The dissolved minerals might be the familiar salt found in Earth’s saltwater oceans, but the actual minerals are not yet known.

If some of this water should penetrate upward to Europa’s surface through cracks in the crust, it would vaporize and leave the dissolved minerals on the surface. In fact, these minerals might explain the dark linear features in Figure 13-11.

By combining measurements of Europa’s induced magnetic field, gravitational pull, and oblateness, scientists estimate that Europa has a frozen ice crust about 10 km thick on top of a liquid water ocean about 100 km deep (Figure 13-14). Beneath the ocean, models suggest a rocky mantle surrounding a metallic core some 600 km (400 mi) in radius. Since water is known to partially dissolve some of the minerals that make rock, contact with this mantle would be the source of the ocean’s electrical conductivity.

Figure 13-14: R I V U X G
Europa’s Ocean While small compared to Europa’s overall size, its ocean is still enormous. Estimated at about 100 km deep, it would be over 10 times deeper than Earth’s oceans and contain more than twice the volume of water.
(NASA/JPL)

Interestingly, Europa also has an extremely thin atmosphere of oxygen (producing an atmospheric pressure only a trillionth of Earth’s). We saw in Section 9-5 that oxygen in Earth’s atmosphere is produced by plants through photosynthesis. But Europa’s oxygen atmosphere is probably the result of ions from the solar wind and Jupiter’s magnetosphere striking the satellite’s icy surface. These collisions break apart water molecules, liberating atoms of hydrogen (which escape into space) and oxygen.

The existence of a warm, subsurface ocean on Europa, if proved, would make Europa one of the few worlds in the solar system other than Earth with liquid water. Liquid water on Europa would have dramatic implications. On Earth, water and warmth are essentials for the existence of life. Perhaps single-celled organisms have evolved in the water beneath Europa’s crust, where they would use dissolved minerals and organic compounds as food sources. In light of this possibility, NASA has taken steps to prevent biological contamination of Europa. At the end of the Galileo mission in 2003, the spacecraft (which may have carried traces of organisms from Earth) was sent to burn up in Jupiter’s atmosphere, rather than remaining in orbit where it might someday crash into Europa. An appropriately sterilized spacecraft may one day visit Europa and search for evidence of life within this exotic moon.

CONCEPT CHECK 13-6

If the orbital resonance of Io, Europa, and Ganymede contributes to the heating of their interiors, why isn’t Europa hot enough to have volcanism like Io?

CONCEPT CHECK 13-7

Why does an induced magnetic field coming from Europa indicate a large subsurface ocean?