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Europa ( ; Greek ''Ευρώπη'') is the sixth nearest and fourth largest Natural Satellite of the Planet Jupiter . Europa was discovered in 1610 by Galileo Galilei (and independently by Simon Marius shortly thereafter) and is the smallest of the four Galilean Moons named in Galileo's honor. Europa is primarily composed of Silicate rock, has an outer layer of Water , and likely has an Iron core. At just over 3000 kilometers in Diameter , it is slightly smaller than the Earth's moon and the Sixth Largest Moon in the Solar System . The satellite has a very tenuous Oxygen atmosphere and one of the smoothest surfaces in the solar system. The young surface of the moon is striated by cracks and streaks, while craters are relatively infrequent. Due to an hypothesized water ocean beneath its icy surface, and an energy source provided by Tidal Heating , Europa has been cited as a possible host of Extraterrestrial Life .2 The heat energy ensures the ocean remains liquid and also drives geological activity. The intriguing character of Europa has led to a number of ambitious exploration proposals; to date, only flyby missions have visited the moon. The '' Galileo '' mission provided the bulk of current data on the satellite, while the abortive Jupiter Icy Moons Orbiter , cancelled in 2005, was the most ambitious planned spacecraft. Conjecture on extraterrestrial life has ensured a high profile for the moon and led to continued lobbying for future missions.34 DISCOVERY AND NAMING Europa was discovered in January 1610 by Galileo along with the three other large satellites of the planet Jupiter, Europa is named after the mythological Europa , daughter of Agenor , king of the Phoenicia n city of Tyre (now in Lebanon ), and sister of Cadmus , founder of Thebes, Greece . The name fell out of favor for a considerable time (as did those of the other Galilean moons), and was not revived in common use until the mid-20th century.Marazzini, C.; (2005); ''The names of the satellites of Jupiter: from Galileo to Simon Marius'', Lettere Italiana, Vol. 57, No. 3, pp. 391-407 In much of the earlier Astronomical literature, it is simply referred to by its Roman Numeral designation as (a system introduced by Galileo) or as the "second satellite of Jupiter". The discovery of Amalthea in 1892, closer than any of the Other Known Moons Of Jupiter , pushed Europa to 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 . ORBIT Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 Km (416,900 Mi ). The satellite follows a very nearly circular orbit, with an eccentricity of only 0.009. Europa's Orbital Inclination relative to the Jovian equatorial plane is also slight, at 0.470°.7 Like all the Galilean satellites, Europa is Tidally Locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. Research suggests the tidal locking may not be full, as a Non-synchronous Rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that "the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice," in keeping with the proposed subsurface ocean.8 Because of its orbit's slight eccentricity, maintained by the gravitational disturbances from the other Galilean satellites of the planet, the sub-jovian point oscillates about a mean position. Europa strives to assume a slightly elongated shape pointing towards Jupiter in response to the Tidal Force of the giant planet; because different parts of Europa end up being on different points of this departure from sphericity at varying times, the crust flexes up and down. This motion dissipates energy from Jupiter's rotation into Europa ( Tidal Heating ), giving the moon a source of heat and energy, allowing the subsurface ocean to stay liquefied and driving subsurface geological processes. PHYSICAL CHARACTERISTICS Internal structure Europa is somewhat similar in bulk composition to the , Vol. 289, No. 5483 ( 25 August 2000 ), pp. 1340-1343 (accessed 15 April 2006 ) Surface features The Europan surface is relatively smooth; few features more than a few hundred metres high have been observed, but topographic relief in places approaches a kilometre (0.62 mi). Europa is one of the smoothest objects in the solar system.9 The prominent markings crisscrossing the moon seem to be mainly , 1996 )10 Europa's Albedo (light reflectivity) of 0.64 is one of the highest of all moons because of its icy surface. This would seem to indicate a young and active surface; based on estimates of the frequency of Comet ary bombardment that Europa probably endures, the surface is about 20 to 180 million years oldSchenk, P. M.; Chapman, C. R.; Zahnle, K.; Moore, J. M.; ''Chapter 18: Ages and Interiors: the Cratering Record of the Galilean Satellites'', in ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, 2004 (the geological features of the surface clearly show a variety of ages). Cynthia Phillips, a member of SETI and an expert on Europa states there is currently no consensus among the often contradictory explanations for the surface features of Europa. {Link without Title} Lineae See Also: List of lineae on Europa Europa's most striking surface feature is a series of dark streaks criss-crossing the entire globe. 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 roughly 20 km (12 mi) across commonly with dark diffuse outer edges, regular striations, and a central band of lighter material. Among the controversial hypotheses put forward to explain these features, one states that they may have been produced by a series of Volcanic Water Eruptions or geysers 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 Ridge s. 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 have occurred 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 to 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 of no faster than once every 10,000 years for the surface relative to its interior. Other geological features See Also: List of geological features on Europa region]] Another type of feature present on Europa are circular and elliptical ''lenticulae'', Latin for " Freckle s". 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. Among the controversial hypotheses put forward to explain these features, one states that these lenticulae were formed by Diapir s of warm ice rising up through the colder ice of the outer crust, much like Magma Chamber s in the Earth's crust. The smooth dark spots could be formed by melt water 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, perhaps like Iceberg s in a frozen sea. Subsurface ocean It is thought that under the surface there is a layer of liquid water kept warm by ," a common feature on Europa's surface that some interpret as a region where the subsurface ocean 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 surface.Greeley, R.; ''et al.''; ''Chapter 15: Geology of Europa'', in ''Jupiter: The Planet, Satellites and Magnetosphere'', Cambridge University Press, 2004 The different models for the estimation of the ice shell thickness give values between a few hundred meters and tens of kilometers.11 The best evidence for the so called "thick ice" model is a study of Europa's large craters. The largest craters 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 kilometers (5–20 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km (60–65 mi) deep. This leads to a volume of Europa's oceans of 3m³, slightly more than two times the volume of Earth's oceans. The so called "thin ice" model is a conclusion of the formation of ridges and the flexure of the ice shell due to loading at the surface. With these models the crust of solid ice could be as thin as 200 meters. The "thin ice" model allows regular contact of the liquid interior with the surface through open ridges. The '' Galileo '' orbiter has also found that Europa has a weak Magnetic Field (about one quarter the strength of Ganymede's field and similar to Callisto's) which varies periodically as Europa passes through Jupiter's massive magnetic field. A likely explanation of this is that there is a large, subsurface ocean of liquid salt water. 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. Sulfur compounds are suspected. Atmosphere |
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