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Europa ( ; or as Greek Ευρώπη) is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei (and possibly independently by Simon Marius), and named after a mythical Phoenicianmarker noblewoman, Europa, who was courted by Zeus and became the queen of Cretemarker. It is the smallest of the four Galilean moons.

At just over in diameter, Europa is slightly smaller than Earth's Moon and is the sixth-largest moon in the Solar System. Though by a wide margin the least massive of the Galilean satellites, its mass nonetheless significantly exceeds the combined mass of all moons in the Solar System smaller than itself. It is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This young surface is striated by cracks and streaks, while craters are relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life. This hypothesis proposes that heat energy from tidal flexing causes the ocean to remain liquid, and drives geological activity.

Although only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The Galileo mission provided the bulk of current data on Europa. A new mission to Jupiter's icy moons, the Europa Jupiter System Mission (EJSM) is proposed for a launch in 2020. Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions.

Discovery and naming

Europa, along with Jupiter's three other largest satellites, Io, Ganymede, and Callisto, was discovered by Galileo Galilei in 1610. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter, in this case Europa, daughter of the king of Tyremarker. The naming scheme was suggested by Simon Marius, who apparently discovered the four satellites independently, though Galileo alleged that Marius had plagiarized him. Marius attributed the proposal to Johannes Kepler.

The names fell out of favor for a considerable time, and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as (a system introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now considered Jupiter's sixth satellite, though it is still sometimes referred to as .

Physical characteristics

Interior of Europa (click for description)

Internal structure

Europa is similar in bulk composition to the terrestrial planets, being primarily composed of silicate rock. It has an outer layer of water thought to be around thick; some as frozen-ice upper crust, some as liquid ocean underneath the ice. Recent magnetic field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. The layer is likely a salty liquid water ocean. The crust is estimated to have undergone a shift of 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.

Surface features

Europa is one of the smoothest objects in the Solar System. The prominent markings crisscrossing the moon seem to be mainly albedo features, which emphasize low topography. There are few craters on Europa because its surface is tectonically active and young. Europa's icy crust gives it an albedo (light reflectivity) of 0.64, one of the highest of all moons. This would seem to indicate a young and active surface; based on estimates of the frequency of cometary bombardment that Europa probably endures, the surface is about 20 to 180 million years old. There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.


Approximately natural color image of Europa by the Galileo spacecraft, showing lineae

Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called ( ). Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are more than across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.

The most likely hypothesis states that these lineae may have been produced by a series of eruptions of warm ice as the Europan crust spread open to expose warmer layers beneath. The effect would have been similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter. Since Europa is tidally locked to Jupiter, and therefore always maintains the same approximate orientation towards the planet, the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to occur at increasingly different orientations the older they are. This could be explained if Europa's surface rotates slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon's surface from its rocky mantle and the effects of Jupiter's gravity tugging on the moon's outer ice crust. Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage. The full revolution of the outer rigid shell relative to the interior of Europa occurs over a minimum of 12,000 years.

Other geological features

Other features present on Europa are circular and elliptical lenticulae (Latin for "freckles"). Many are domes, some are pits and some are smooth, dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below.

One hypothesis states that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in the Earth's crust. The smooth, dark spots could be formed by meltwater released when the warm ice breaks through the surface, and the rough, jumbled lenticulae (called regions of "chaos", for example the Conamara Chaos) would then be formed from many small fragments of crust embedded in hummocky, dark material, appearing like icebergs in a frozen sea.

An alternative hypothesis suggest that lenticulae are actually small areas of chaos, and that the claimed pits, spots and domes are artefacts resulting from over-interpretation of early, low-resolution Galileo images. The implication is that the ice is too thin to support the convective diapir model of feature formation.

Subsurface ocean

Most planetary scientists believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat. The heating by radioactive decay, which is almost the same as in Earth (per kg of rock), cannot provide necessary heating in Europa, because the volume-to-surface ratio is much lower due to the moon's smaller size. Europa's surface temperature averages about at the equator and only at the poles, keeping Europa's icy crust as hard as granite. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is extremely controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the present surface. The different models for the estimation of the ice shell thickness give values between a few kilometers and tens of kilometers.

Two possible models of Europa
The best evidence for the thick ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is predicted that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about deep. This leads to a volume of Europa's oceans of 3 × 1018 m3, slightly more than two times the volume of Earth's oceans.

The thin ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust which behave elastically when affected by Jupiter's tides. One example is flexure analysis, in which the moon's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as . If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.

In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on the moon because of its small but non-zero obliquity. This previously unconsidered kind of tidal force generates so-called Rossby waves that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree, the resonance from Rossby waves would store 7.3 J of kinetic energy, which is two hundred times larger than that of the flow excited by the dominant tidal forces. Dissipation of this energy could be the principal heat source of Europa's ocean.

The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. The field strength at the magnetic equator (about 120 nT) created by this magnetic moment is about one-sixth the strength of Ganymede's field and six times the value of Callisto's. The existence of the induced moment requires a layer of a highly electrically conductive material in the moon's interior. The most plausible candidate for this role is a large subsurface ocean of liquid saltwater. Spectrographic evidence suggests that the dark, reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, since these materials are colorless or white when pure, some other material must also be present to account for the reddish color, and sulfur compounds are suspected.


Magnetic field around Europa.
The red line shows a trajectory of the Galileo spacecraft during a typical flyby (E4 or E14).
Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope, first described in 1995, revealed that Europa has a tenuous atmosphere composed mostly of molecular oxygen (O2). The surface pressure of Europa's atmosphere is 0.1 μPa, or 10−12 times that of the Earth. In 1997, the Galileo spacecraft confirmed the presence of a tenuous ionosphere (an upper-atmospheric layer of charged particles) around Europa created by solar radiation and energetic particles from Jupiter's magnetosphere, providing evidence of an atmosphere.

Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation. Solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's icy surface, splitting water into oxygen and hydrogen constituents. These chemical components are then adsorbed and "sputtered" into the atmosphere. The same radiation also creates collisional ejections of these products from the surface, and the balance of these two processes forms an atmosphere. Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not stick (freeze) like a water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.

Observations of the surface have revealed that some of the molecular oxygen produced by radiolysis is not ejected from the surface. Because the surface may interact with the subsurface ocean (considering the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes.One estimate suggests that, given the turnover rate inferred from the apparent ~0.5 Gyr maximum age of Europa's surface ice, subduction of radiolytically generated oxidizing species might well lead to oceanic free oxygen concentrations that are comparable to those in terrestrial deep oceans.

The molecular hydrogen that escapes Europa's gravity, along with atomic and molecular oxygen, forms a torus (ring) of gas in the vicinity of Europa's orbit around Jupiter. This "neutral cloud" has been detected by both the Cassini and Galileo spacecraft, and has a greater content (number of atoms and molecules) than the neutral cloud surrounding Jupiter's inner moon Io. Models predict that almost every atom or molecule in Europa's torus is eventually ionized, thus providing a source to Jupiter's magnetospheric plasma.

Potential for extraterrestrial life

Europa's unlit interior is now considered to be the most likely location for extant extraterrestrial life in the Solar System. Life could exist in its under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostokmarker. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.

Until the 1970s, life, at least as the concept is generally understood, was believed to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize sugars from carbon dioxide and water, releasing oxygen in the process, and are then eaten by oxygen-respiring animals, passing their energy up the food chain. Even life in the deep ocean far below the photic zone was believed to obtain its nourishment either from the organic detritus raining down from the surface, or by eating animals that in turn depend on that stream of nutrients. A world's ability to support life was thus thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Riftmarker in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers. These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain was a form of bacterium that derived its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubbled up from the Earth's interior. This chemosynthesis revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist. It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats.

While the tube worms and other multicellular eukaryotic organisms around these hydrothermal vents respire oxygen and thus are indirectly dependent on photosynthesis, anaerobic chemosynthetic bacteria and archaea that inhabit these ecosystems provide a possible model for life in Europa's ocean — and as previously noted, on Europa, radiolytically generated free radicals delivered by subduction of surface ice may provide a sufficient source of free oxygen, even in the absence of photosynthesis. The energy provided by tidal flexing drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. While Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source. However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface. Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to habitate on Earth. Alternatively, it could exist clinging to the lower surface of the moon's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean. However, if Europa's ocean were too cold, biological processes similar to those known on Earth could not take place. Similarly, if it were too salty, only extreme halophiles could survive in its environment.

In September, 2009, planetary scientist Richard Greenberg calculated that cosmic rays impacting on Europa's surface convert the ice into oxidizers, which could then be absorbed into the ocean below as water wells up to fill cracks. Via this process, Greenberg estimates that Europa's ocean could eventually achieve an oxygen concentration greater than that of Earth's oceans within just a few million years. This would enable Europa to support not merely anaerobic microbial life but potentially larger, aerobic organisms such as fish.

In 2006, Robert T. Pappalardo, an assistant professor in the Laboratory for Atmospheric and Space Physicsmarker at the University of Colorado in Bouldermarker said,


Most human knowledge of Europa has been derived from a series of flybys since the 1970s. The sister crafts Pioneer 10 and Pioneer 11 were the first to visit Jupiter, in 1973 and 1974, respectively; the first photos of Jupiter's largest moons produced by the Pioneers were fuzzy and dim. The Voyager flybys followed in 1979, while the Galileo mission orbited Jupiter for eight years beginning in 1995 and provided the most detailed examination of the Galilean moons that is expected until the end of the 2020s.

Various proposals have been made for future missions. Any mission to Europa would need to be protected from the high radiation levels sustained by Jupiter. The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its subsurface ocean. Europa receives about 540 rem of radiation per day.

Spacecraft proposals and cancellations

Plans to send a probe to study Europa for signs of liquid water and possible life have been plagued by false starts and budget cuts. Proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) is a joint NASAmarker/ESAmarker proposal for exploration of Jupiter's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. ESA's contribution will still face funding competition from other ESA projects. EJSM consists of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter. Russia has expressed interest in sending a lander to Europa as part of an international flotilla.

Prior to EJSM, the plan for the extremely ambitious Jupiter Icy Moons Orbiter was cancelled in 2005.. Before that, the Europa Orbiter received a go-ahead in 1999 but was canceled in 2002. Another possible mission, known as the Ice Clipper mission, would have used an impactor similar to the Deep Impact mission—it would make a controlled crash into the surface of Europa, generating a plume of debris which would then be collected by a small spacecraft flying through the plume.

More ambitious ideas have been put forward including a capable lander to test for evidence of life that might be frozen in the shallow subsurface, or even to directly explore the possible ocean beneath Europa's ice. One proposal calls for a large nuclear-powered "melt probe" (cryobot) which would melt through the ice until it hit the ocean below. Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) which would gather information and send it back to Earth. Both the cryobot and the hydrobot would have to undergo some form of extreme sterilization to prevent detection of Earth organisms instead of native life and to prevent contamination of the subsurface ocean. This proposed mission has not yet reached a serious planning stage.

See also


Further reading

External links

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