Europa (moon)

Europa (ew-roe'-pə, IPA: [juˈroʊpə] listen^ⓘ; Greek Ευρώπη) is a moon of the planet Jupiter. It is the sixth moon in position by length of orbital period and the fourth largest by diameter and mass of Jupiter's satellites. It was discovered in 1610 by Galileo Galilei and is the smallest of the four Galilean moons named in his honour.

Etymology

It is named after Europa, daughter of Agenor, king of Tyre, the Phoenician city, now in Lebanon, and sister of Cadmus, founder of Thebes, Greece. Europa, for whom the continent of Europe is named, was one of Zeus's many love interests in Greek mythology. Zeus, disguised as a white bull, abducted her and took her to the island of Crete where she bore him three sons, King Minos of Crete, King Rhadamanthus of the Cyclades, and Sarpedon.

Although the name "Europa" was suggested by Simon Marius soon after its discovery, the name fell out of favor for a considerable time (as did those of the other Galilean satellites), and was not revived in common use until the mid-20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation as Jupiter II or as the "second satellite of Jupiter". The discovery of Amalthea in 1892, inside the orbits of the Galilean satellites, pushed Europa to third position. The Voyager probes discovered three more inner satellites in 1979 and Europa is now considered Jupiter's sixth satellite.

Orbital characteristics

Europa has a mean distance from Jupiter of 670,900 km (416,900 miles) and orbits the gas giant in just three and a half days. Its orbit is very nearly circular with an eccentricity of 0.009.^[1]

Like all the Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. Europa is also being gravitationally pulled in different directions by Jupiter and by other satellites of the planet (tidal flexing). This heats the body, and allows for geology.^[2]

Physical characteristics

Europa is somewhat 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 100 km thick (some as frozen ice upper crust, some as liquid ocean underneath the ice), and recent magnetic field data from the Galileo orbiter shows that Europa generates an induced magnetic field by interacting with Jupiter's field, which suggests the presence of a subsurface conductive layer which is likely a salty liquid-water ocean. Europa probably also contains a metallic iron core.

Surface

The Europan surface is relatively smooth; few features more than a few hundred meters high have been observed, but topographic relief in places approaches a kilometer (1/2 mile). Europa is the smoothest object in the solar system. The prominent markings crisscrossing the moon seem to be mainly albedo features, which emphasize low topography. There are very few craters on Europa (there are about 20 larger than 10 km (6.2 miles) in diameter across the globe), and its albedo is 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 60 million years old (plus or minus a factor of 3). It is thought that under the surface there is a layer of liquid water kept warm by tidally generated heat. The temperature on the surface of Europa averages about 110 K (-163.15 °C) at the equator and only 50 K (-223.15 °C) at the poles, and so the surface water ice is permanently frozen. 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 kilometres (5-20 miles) thick, which could mean that the liquid ocean underneath may be about 100 km (60-65 miles) deep.

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 miles) across commonly with dark diffuse outer edges, regular striations, and a central band of lighter material. These 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 is 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; Europa's surface is thought to rise and fall up to 30 meters (nearly 100 feet) between high and low tides. Since Europa is tidally locked to Jupiter, and therefore always maintains the same 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 can 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 suggest that Europa's crust rotates no faster than once every 10,000 years relative to its interior.

Another type of feature 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. It is thought 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 the surface, and the rough, jumbled lenticulae (called regions of "chaos", for example the Conamara Chaos) appear to be formed from many small fragments of crust embedded in hummocky dark material, perhaps like icebergs in a frozen sea.

Atmosphere

In 1994, observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope revealed that Europa has a very tenuous atmosphere (1 micropascal surface pressure) composed of oxygen.^[3] Of all the moons in the solar system only six others (Io, Callisto, Enceladus, Ganymede, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes Europa's gravity due to its low atomic mass, leaving the oxygen behind.

Subsurface ocean

The Galileo orbiter has found that Europa has a weak magnetic field (about one quarter the strength of the Ganymedean 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.^[4] Spectrographic evidence suggests that the dark reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate (Epsom salt), 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.

It has been suggested that life may exist in this under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok. There is currently no observational evidence for this hypothesis, but efforts have nevertheless been made to avoid any possibility of contamination. The Galileo mission was concluded in September 2003 by crashing the spacecraft into Jupiter — if simply abandoned, the unsterilized craft might have eventually crashed into Europa and contaminated it with terrestrial microorganisms. The introduction of such microorganisms could make it impossible to determine if Europa ever had its own native life, or could even destroy such life if it exists.

Exploration of Europa

Artist's concept of the cryobot and hydrobot

Most of our knowledge of Europa comes from the flybys by the Voyager and Galileo missions. Various proposals have been made for future missions. The extremely ambitious Jupiter Icy Moons Orbiter was canceled in 2005.^[5]

The 2006 NASA budget includes Congressional language imploring NASA to fund a mission that would orbit Europa. Such a mission would be able to confirm a subsurface ocean using gravity and altimetry measurements, elucidate the origin of surface features by imaging much of the surface at high resolution, constrain the chemistry of surface materials using spectroscopy, and probe for subsurface liquid water using ice-penetrating radar. The mission might even 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, at present it is far from certain that NASA will actually fund this mission, as funding for it is not included in NASA's 2007 budget plan.

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. Once it reached the water, it would deploy an autononomous underwater vehicle (hydrobot), which would gather information and send it back to Earth. Both the cryobot and the hydrobot would undergo some form of extreme sterilization to negate the chance of contamination to any possible ecosystem on Europa. This proposed mission has not yet reached a serious planning stage.

Europa in popular culture

Europa plays a very important role in the film and book of Arthur C. Clarke's 2010: Odyssey Two and its sequels. Super-advanced aliens (who we learn in 2001 aided the development of humanity as a grand experiment) take an interest in the primitive life forms living under Europa's ice and transform Jupiter into a star called Lucifer to kick-start their evolution. In 2061: Odyssey Three, Europa has become a tropical ocean world.
In Greg Bear's novel The Forge of God (1987), Europa is destroyed by aliens who use chunks of its ice to terraform planets.
Creatures from Europa feature prominently in the Dan Simmons science fiction novel Ilium (2003).
In Schismatrix, Europa is inhabited by genetically re-engineered posthumans as a philosophical/political statement by Abelard Lindsay's Lifesider's clique.
In the video game Infantry, large cities lie underneath the ice sheets of Europa.
In the video game Battlezone, Europa is featured as a cold, ice covered world, where battles take place in cracks between the ice.
The computer game Abyss: Incident at Europa involves an underwater base in Europa's ocean.
The computer game Descent 3 has a level on Europa in which the player must destroy an energy refinery. The game depicts Europa as a snowy landscape.
The online script serial Banana Chan features Europa as a destination for Space Tourists in its 12th episode.
In Europa Strike by Ian Douglas, a massive ancient alien spacecraft lies in an ocean beneath the moon's surface, the discovery of which leads to a battle between Chinese forces and US Marines in 2067.
In the anime Cowboy Bebop, the crew of the Bebop is heading towards Europa when, after being involved in a hit and run accident, end up stranded on Io.
In the James Cameron IMAX film Aliens of the Deep, the idea of an underwater alien culture living beneath the surface of Europa is explored.
The Transhuman Space roleplaying game features Europa as the setting for a conflict between a faction dedicated to preserving a native microbial fauna and one attempting to introduce Earth-based life.

References

^ "Overview of Europa Facts" NASA webpage. URL accessed 15 April 2006
^ "Tidal Heating"
^ Hall, D. T. et al, "Detection of an oxygen atmosphere on Jupiter's moon Europa" (Abstract only) Nature 373, 677 - 679, 23 February 1995. URL accessed 15 April 2006.
^ Kivelson, M. G. et al, "Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa" Science 25 August 2000: Vol. 289. no. 5483, pp. 1340 - 1343. URL accessed 15 April 2006.
^ "NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer" 7 February 2005 Space.com article. URL accessed 15 April 2006.

External links

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[1] "Overview of Europa Facts" NASA webpage. URL accessed 15 April 2006

[2] "Tidal Heating"

[3] Hall, D. T. et al, "Detection of an oxygen atmosphere on Jupiter's moon Europa" (Abstract only) Nature 373, 677 - 679, 23 February 1995. URL accessed 15 April 2006.

[ocean-4] Kivelson, M. G. et al, "Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa" Science 25 August 2000: Vol. 289. no. 5483, pp. 1340 - 1343. URL accessed 15 April 2006.

[budget-5] "NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer" 7 February 2005 Space.com article. URL accessed 15 April 2006.

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