Europa (moon) Totally Explained

Everything about Europa Moon totally explained

Europa (yew-ROE-pə ; or as Greek Ευρώπη) is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei (and, some say, independently by Simon Marius), and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete. 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. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.
Although by 2007 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, while the Jupiter Icy Moons Orbiter, cancelled in 2005, would have targeted Europa, Ganymede and Callisto. 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. His discovery helped buttress Nicolaus Copernicus's heliocentric cosmology by proving that not all objects in the universe orbit the Earth. Like all the Galilean satellites, Europa is named after a lover of Zeus (the Greek Jupiter), in this case Europa, daughter of the king of Tyre. 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's still sometimes referred to as . Like its fellow 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 a layer of subsurface liquid separates the icy crust from the rocky interior.
The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate about a mean position. As Europa comes slightly nearer to Jupiter, the planet's gravitational attraction increases, causing the moon to elongate towards it. As Europa moves slightly away from Jupiter, the planet's gravitational force decreases, causing the moon to relax back into a more spherical shape. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives the moon a source of heat, allowing its ocean to stay liquid and driving subsurface geological processes.

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. Cynthia Phillips, an expert on Europa, states that there's currently no consensus among the often contradictory explanations for the surface features of Europa.

Lineae

Europa's most striking surface feature is a series of dark streaks crisscrossing the entire globe, called lineae (Latin: "lines"). 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 across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.
One hypothesis states that these lineae 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 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.

Subsurface ocean

Many astronomers believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat. 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 surface. The different models for the estimation of the ice shell thickness give values between a few hundred meters and tens of kilometers. 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's 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. The existence of the induced moment requires a layer of a highly electrically conductive material in the moon's interior. A likely candidate for this role is a large subsurface ocean of liquid saltwater. 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

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 1 μPa, or 10⁻¹¹ that of the Earth. At equivalent pressure to Earth's atmosphere at sea level, Europa's oxygen would "fill only about a dozen Houston Astrodomes (under normal conditions—pressure 100 kPa, temperature 300 K)". providing evidence of an atmosphere.
   Unlike the oxygen in Earth's atmosphere, Europa's isn't of biological origin. As first predicted by R. E. Johnson and colleagues, 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 doesn't 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's light enough to escape Europa's surface gravity.
   Observations of the surface have revealed that some of the molecular oxygen produced by radiolysis isn't ejected from the surface. Since the surface may interact with the subsurface ocean (based on the geological discussion above), this molecular oxygen may make its way to the ocean, where it could aid in biological processes.
   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.

Possible extraterrestrial life

It has been suggested that life may exist in Europa's under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean. So far, there's 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 ocean depths, where sunlight can't reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. A world's ability to support life was thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Rift 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.
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. 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 couldn't take place. Similarly, if it were too salty, only extreme halophiles could survive in its environment.

Exploration

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.

Spacecraft proposals and cancellations

[[Image:JEO at Europa.jpg|thumb|JEO is a spaceprobe to be sent by the ESA to Europa The only mission currently scheduled to go to Europa is the European Space Agency's Jovian Europa Orbiter (JEO), but a launch date hasn't yet been determined. The plan for the extremely ambitious Jupiter Icy Moons Orbiter was cancelled in 2005.

Carry a small lander to determine the surface chemistry directly, and to measure seismic waves, from which the level of activity and ice thickness could be determined. However, it's far from certain that NASA will actually fund this mission, as funding for it isn't included in NASA's 2008 budget plan, and no "flagship" planetary expeditions are likely before 2017. Planetary scientist Ronald Greeley said about the Europa mission: Without the need for an insertion and relaunch of the spacecraft(s) from an orbit around Jupiter or Europa, this would be one of the least expensive missions since the necessary amount of fuel would be decreased.
   More ambitious ideas have been put forward for 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. The Planetary Society says that drilling a hole below the surface would be a main goal. 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 hasn't yet reached a serious planning stage.
   Even though the U.S. Congress, the National Academy of Sciences, and the NASA Advisory Committee have all supported a mission to Europa, funding has still been halted. The Planetary Society plans to create an "International Europa Task Force" to convince NASA and other space agencies to fund a Europa mission. Other people, such as Congressman John Culberson, have even tried to go against NASA's budget cuts.
   A "Solar System Exploration Roadmap" published for NASA by the Universities Space Research Association in 2006 placed exploration of Europa high on its list, and suggested that plans for a "flagship-class" mission to Europa begin by 2008 with hopes to launch by 2015. Another mission, the Europa Orbiter, was also cancelled.

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