Europa

Europa, as seen by the Galileo spacecraft
Discovery
Discovered by	G. Galilei S. Marius
Discovery date	January 7, 1610
Orbital characteristics[1]
Epoch January 8, 2004
Periapsis	664,862 km[2]
Apoapsis	676,938 km[2]
Mean orbit radius	670,900 km[3]
Eccentricity	0. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German Events 1325 - Alfonso IV becomes King of Portugal. 1558 - France takes Calais, the last continental In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. Events 871 - Battle of Ashdown - Ethelred of Wessex defeats a Danish invasion army "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 009[3]
Orbital period	3. The orbital period is the time taken for a given object to make one complete Orbit about another object 551181 d[3]
Average orbital speed	13. The orbital speed of a body generally a Planet, a Natural satellite, an artificial satellite, or a Multiple star, is the speed at which it 740 km/s[3]
Inclination	0. Inclination in general is the Angle between a Reference plane and another plane or axis of direction 470° (to Jupiter's equator)[3]
Satellite of	Jupiter
Physical characteristics
Mean radius	1,569 km (0. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. 245 Earths)[3]
Surface area	3. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Equation A spheroid centered at the origin and rotated about the z axis is defined by the implicit equation \left(\frac{x}{a}\right^2+\left(\frac{y}{a}\right^2+\left(\frac{z}{b}\right^2 09×107 km2 (0. Square Kilometre ( US spelling square kilometer) symbol km2, is a decimal multiple of the SI unit of 061 Earths)[4]
Volume	1. The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically 593×1010 km3 (0. CM3 redirects here If you were looking for the 3rd game in the Cooking Mama series abbreviated as CM3 see here. 015 Earths)[4]
Mass	4. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object 80×1022 kg (0. 008 Earths)[3]
Mean density	3. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different 01 g/cm3[3]
Equatorial surface gravity	1. For other uses of the words gram or gramme see Gram (disambiguation. A cubic centimetre or cubic centimeter (symbol cm3 —the abbreviation cc, though widely used is deprecated is a commonly used unit of Volume The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface 314 m/s2 (0. 134 g)[2]
Escape velocity	2. In Physics, escape velocity is the speed where the Kinetic energy of an object is equal to the magnitude of its Gravitational potential energy 025 km/s[2]
Rotation period	Synchronous[5]
Axial tilt	0. The rotation period of an astronomical object is the time it takes to complete one revolution around its Axis of rotation relative to the background stars In Astronomy, synchronous rotation is a planetological term describing a body orbiting another where the orbiting body takes as long to rotate In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. 1°[6]
Albedo	0. The albedo of an object is the extent to which it diffusely reflects light from the sun 67 ± 0. 03[7]
Surface temp.    Surface	min mean max ~50 K[8] 103 K 125 K
Apparent magnitude	5. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 29 (opposition)[7]
Adjectives	Europan
Atmosphere
Surface pressure	1 µPa[9]

Europa (pronounced /jʊˈroʊpə/ yew-ROE-pə listen (help·info); or as Greek Ευρώπη) is the sixth moon of the planet Jupiter. Opposition is a term used in Positional astronomy and Astrology to indicate when one celestial body is on the opposite side of the sky when viewed from Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is 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. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German Phoenicia ( Phoenician: Phoenician nunsvg|12px|נ]]Phoenician nun Europa ( Greek Εὐρώπη was a Phoenician woman of high lineage in Greek mythology, from whom the name of the Continent Europe Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology Crete ( Greek: Κρήτη transliteration: Krētē, modern transliteration Kriti) is the largest of the Greek islands and the It is the smallest of the four Galilean moons. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei.

At just over 3,000 kilometres (1,900 mi) in diameter, Europa is slightly smaller than Earth's Moon and is the sixth-largest moon in the Solar System. Geometry, a diameter of a Circle is any straight Line segment that passes through the center of the circle and whose Endpoints are on the EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The tables below of Natural satellites in the Solar System are ordered from largest to smallest by average diameter The Solar System consists of the Sun and those celestial objects bound to it by Gravity. 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. ^[10] It is primarily made of silicate rock and likely has an iron core. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 It has a tenuous atmosphere composed primarily of oxygen. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the 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. Extraterrestrial life is Life originating outside of the Earth. ^[11] Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i ^[12]

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. Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer The Jupiter Icy Moons Orbiter ( JIMO) was a proposed Spacecraft designed to explore the icy moons of Jupiter. TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek TemplateInfobox Planet.--> Callisto (kəˈlɪstoʊ, or as Greek Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions. ^[13][14]

Discovery and naming

Europa, along with Jupiter's three other largest satellites, Io, Ganymede, and Callisto, was discovered by Galileo Galilei in 1610. TemplateInfobox Planet.--> Io (ˈaɪoʊ, or as Greek TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek TemplateInfobox Planet.--> Callisto (kəˈlɪstoʊ, or as Greek His discovery helped buttress Nicolaus Copernicus's heliocentric cosmology by proving that not all objects in the universe orbit the Earth. In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. Cosmology (from Greek grc κοσμολογία - grc κόσμος kosmos, "universe" and grc -λογία -logia) is study ^[15] 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. Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology In Roman mythology, Jupiter was the king of the gods and the god of Sky and Thunder. Europa ( Greek Εὐρώπη was a Phoenician woman of high lineage in Greek mythology, from whom the name of the Continent Europe Tyre ( Arabic صور Ṣūr, Phoenician Phoenician wawsvg|12px|ו]] Ṣur, Hebrew The naming scheme was suggested by Simon Marius, who apparently discovered the four satellites independently, though Galileo alleged that Marius had plagiarized him. Simon Marius ( Latinized from German Simon Mayr) ( January 10, 1573 &ndash December 26, 1624) was a German Marius attributed the proposal to Johannes Kepler. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer ^[16][17]

The names fell out of favor for a considerable time, and were not revived in general use until the mid-20th century. ^[18] In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II (a system introduced by Galileo) or as the "second satellite of Jupiter". Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Roman numerals are a Numeral system originating in ancient Rome, adapted from Etruscan numerals. In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. TemplateInfobox Planet.--> Amalthea (, or as in Greek Αμάλθεια 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 Jupiter II. See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and ^[18]

Orbit

Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand With an eccentricity of only 0. 009, the orbit itself is nearly circular, and the orbital inclination relative to the Jovian equatorial plane is small, at 0. Inclination in general is the Angle between a Reference plane and another plane or axis of direction In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " 470°. ^[19] Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. A separate article treats the phenomenon of Tidal resonance in Oceanography. 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. In Astronomy, synchronous rotation is a planetological term describing a body orbiting another where the orbiting body takes as long to rotate This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior. ^[20]

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. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their TemplateInfobox Planet.--> Io (ˈaɪoʊ, or as Greek ^[21] 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. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i ^[12][21] The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance. ^[22][21]

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. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere It has an outer layer of water thought to be around 100 km (62 mi) thick; some as frozen-ice upper crust, some as liquid ocean underneath the ice. Water is a common Chemical substance that is essential for the survival of all known forms of Life. 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. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer 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. True Polar wander is a phenomenon in which a planet or moon changes its orientation so that different points become the North and South Poles This can happen [3] Europa probably contains a metallic iron core. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 ^[23]

Surface features

Mosaic of Galileo images showing features indicative of internal geologic activity: lineae, domes, pits and Conamara Chaos. Conamara Chaos is a region of chaotic terrain on Jupiter 's moon Europa.

Europa is one of the smoothest objects in the Solar System. ^[24] The prominent markings crisscrossing the moon seem to be mainly albedo features, which emphasize low topography. An albedo feature is a large area on the surface of a Planet (or other Solar system body which shows a contrast in brightness or darkness ( Albedo) with There are few craters on Europa because its surface is tectonically active and young. In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body ^[25][26] Europa's icy crust gives it an albedo (light reflectivity) of 0. The albedo of an object is the extent to which it diffusely reflects light from the sun 64, one of the highest of all moons. ^[19][26] 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. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[27] Cynthia Phillips, an expert on Europa, states that there is currently no consensus among the often contradictory explanations for the surface features of Europa. ^[28]

Lineae

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

See also: List of lineae on Europa

Europa's most striking surface feature is a series of dark streaks crisscrossing the entire globe, called lineae (Latin: "lines"). This is a list of Lineae on Europa. Most Europan lineae are named after characters and places in the legends of Cadmus and Europa; others are named after Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. 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, often with dark, diffuse outer edges, regular striations, and a central band of lighter material. ^[29]

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. A cryovolcano is literally an Icy Volcano. Cryovolcanoes form on Icy moons and possibly on other low- Temperature Astronomical objects ^[30] The effect would have been similar to that seen in the Earth's oceanic ridges. A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by 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. ^[31] Comparisons of Voyager and Galileo spacecraft photos serve to put an upper limit on this hypothetical slippage. See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and The full revolution of the outer rigid shell relative to the interior of Europa occurs over a minimum of 12,000 years. ^[32]

Other geological features

See also: List of geological features on Europa

Enhanced-color view of part of Conamara Chaos, showing ice rafts up to 10 km (6 mi) across. This is a list of named geological features on Europa, a moon of the planet Jupiter. Conamara Chaos is a region of chaotic terrain on Jupiter 's moon Europa. White areas are ejecta rays from the crater Pwyll. The Impact crater Pwyll (named after the Pwyll of Celtic mythology) is thought to represent one of the youngest features on the surface of Jupiter

Craggy, 250 m high "mountains" and smooth plates are jumbled together in a close-up of Conamara Chaos. Conamara Chaos is a region of chaotic terrain on Jupiter 's moon Europa.

Other features present on Europa are circular and elliptical lenticulae (Latin for "freckles"). Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. 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. ^[33]

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. A diapir (ˈdaɪəpɪər) ( French, from Greek diapeirein, to pierce through is an Intrusion caused by Buoyancy and Pressure A magma chamber is a large underground pool of molten rock lying under the surface of the earth's crust ^[33] 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, perhaps like icebergs in a frozen sea. Meltwater is the water released by the Melting of Snow or Ice, including glacial ice and Ice shelfs over oceans Conamara Chaos is a region of chaotic terrain on Jupiter 's moon Europa. An iceberg is a large piece of freshwater Ice that has broken off from a snow-formed Glacier or Ice shelf and is floating in open water ^[34]

Subsurface ocean

Many astronomers believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced ^[35] 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 110 K (−160 °C/−260 °F) at the equator and only 50 K (−220 °C/−370 °F) at the poles, keeping Europa's icy crust as hard as granite. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The Celsius Temperature scale was previously known as the centigrade scale. Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 ^[8] 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). In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and ^[35] 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. Chaos terrain (or chaotic terrain) is an astrogeological term used to denote planetary surface areas where features such as ridges cracks and plains appear jumbled 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. ^[36] The different models for the estimation of the ice shell thickness give values between a few hundred meters and tens of kilometers. ^[37]

Two possible models of Europa

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 km (6–19 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km (60 mi) deep. ^[27] 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 so-called "thin ice" 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 ice crust could be as thin as 200 metres (660 ft). The "thin ice" model allows regular contact of the liquid interior with the surface through open ridges. ^[37]

The Galileo orbiter found that Europa has a weak magnetic moment, which is induced by the varying part of the Jovian magnetic field. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current 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 tesla (symbol T) is the SI derived unit of Magnetic field B (which is also known as "magnetic flux density" and "magnetic ^[38] 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. ^[23] 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. Magnesium sulfate is a Chemical compound containing Magnesium and Sulfate, with the formula MgSO4 ^[39] Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. ^[40] 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. Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 ^[41]

Atmosphere

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 Goddard High Resolution Spectrograph (GHRS or HRS was a Spectrograph installed on the Hubble Space Telescope. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the ^[42][9] 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)". For the aeronautical use see Astrodome (aviation Reliant Astrodome, also known as the Houston Astrodome or simply the Astrodome ^[9] 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,^[43][44] providing evidence of an atmosphere. Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object.

Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five As first predicted by R. E. Johnson and colleagues,^[45] the "surface-bounded atmosphere" forms through radiolysis, the dissociation of molecules through radiation. Radiolysis is the dissociation of Molecules by Radiation. It is the cleavage of one or several Chemical bonds resulting from exposure to high-energy Dissociation in Chemistry and Biochemistry is a general process in which ionic compounds ( complexes, Molecules, or Salts) separate 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. Adsorption is a process that occurs when a gas or liquid Solute accumulates on the surface of a solid or a liquid (adsorbent forming a film of molecules or atoms (the Sputtering is a process whereby Atoms are Ejected from a solid target material due to bombardment of the target by energetic Ions It is commonly used for The same radiation also creates collisional ejections of these products from the surface, and the balance of these two processes forms an atmosphere. ^[46] 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. Hydrogen peroxide (H2O2 is a very pale blue liquid which appears colorless in a dilute solution slightly more Viscous than water Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity. ^[47][48]

Observations of the surface have revealed that some of the molecular oxygen produced by radiolysis is not 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. ^[49]

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. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its Models predict that almost every atom or molecule in Europa's torus is eventually ionized, thus providing a source to Jupiter's magnetospheric plasma. ^[50]

Possible extraterrestrial life

A black smoker in the Atlantic Ocean. A black smoker or sea vent is a type of Hydrothermal vent found on the Ocean floor. Driven by geothermal energy, such hydrothermal vents create chemical disequilibria that can provide energy sources for life. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change

This giant tube worm colony dwells beside a Pacific Ocean vent. Giant tube worms, Riftia pachyptila, are marine Invertebrates in the phylum Annelida ref> (formerly grouped in phylum Pogonophora The Pacific Ocean is the largest of the Earth 's Oceanic divisions While the worms require oxygen, some microbes in the vent communities do not. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the An anaerobic organism is any Organism that does not require Oxygen for growth and may even die in its presence

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. Extraterrestrial life is Life originating outside of the Earth. A hydrothermal vent is a Fissure in a planet's surface from which geothermally heated Water issues Lake Vostok (восток "east" is the largest of more than 140 subglacial Lakes found under the surface of Earth's southern-most ^[51] Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean. A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually The deep sea, or deep layer, is the lowest layer in the Ocean, existing below the Thermocline, at a depth of 1000 fathoms or more ^[52][53] 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. ^[54]

Until the 1970s, life, at least as the concept is generally understood, was believed to be entirely dependent on energy from the Sun. Life is a state that distinguishes Organisms from non-living objects such as non-life and dead organisms being manifested by growth through Metabolism 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. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. Food chains, also called food networks and/or trophic networks, describe the feeding relationships between species within an Ecosystem. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. ^[55] 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. Alvin ( DSV-2) is a 16-ton manned deep-ocean research Submersible owned by the United States Navy and operated by the Woods Hole Oceanographic Giant tube worms, Riftia pachyptila, are marine Invertebrates in the phylum Annelida ref> (formerly grouped in phylum Pogonophora Clam is a word which can be used for all some or only a few Species of Bivalve Mollusks the word is a Common name which has Structure of crustaceans As Arthropods crustaceans have a stiff Exoskeleton, which must be shed to allow the animal to grow ( Ecdysis or molting The common name mussel is used for members of several different families of Clams or Bivalve Molluscs, from both saltwater and freshwater habitats A black smoker or sea vent is a type of Hydrothermal vent found on the Ocean floor. ^[55] 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. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Hydrogen sulfide (or hydrogen sulphide) is the Chemical compound with the formula H 2 S. 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. Chemosynthesis is the biological conversion of one or more carbon molecules (usually Carbon dioxide or Methane) and nutrients into organic matter using the Oxidation It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. Astrobiology (from Greek grc ἄστρον astron, "constellation star" grc βίος bios, "life" and grc -λογία Europa's unlit interior is now considered to be the most likely location for (currently living) extraterrestrial life in the Solar System. ^[56]

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. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Cellular respiration is the set of the metabolic reactions and processes that take place in Organisms cells to convert biochemical energy from Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the An anaerobic organism is any Organism that does not require Oxygen for growth and may even die in its presence 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. ^[57] However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface. ^[58] 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. An endolith or cryptoendolith is an Organism ( Archaeum, Bacterium, Fungus, Lichen, Alga or Amoeba 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. ^[59] 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. Halophiles are Extremophiles that thrive in environments with very high concentrations of Salt. ^[59]

In 2006, Robert Pappalardo, an assistant professor within the University of Colorado's space department, said,

"We’ve spent quite a bit of time and effort trying to understand if Mars was once a habitable environment. Europa today, probably, is a habitable environment. We need to confirm this … but Europa, potentially, has all the ingredients for life … and not just four billion years ago … but today. "^[60]

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. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet ^[24] 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 as of 2008. See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei.

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. ^[13] The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its subsurface ocean. Extraterrestrial life is Life originating outside of the Earth. ^[52][61]

Spacecraft proposals and cancellations

Artist's concept of the cryobot and hydrobot

JEO is a spaceprobe to be sent by the ESA to Europa ^[62]

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. A cryobot or Philberth -probe is a Phallic Robot designed to operate in or around water ice. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member ^[63] The only mission currently scheduled to go to Europa is the European Space Agency's Jovian Europa Orbiter (JEO), but a launch date has not yet been determined. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The Jovian Europa Orbiter (JEO is a future mission by the European Space Agency to Jupiter's moon Europa. ^[62] The plan for the extremely ambitious Jupiter Icy Moons Orbiter was cancelled in 2005. The Jupiter Icy Moons Orbiter ( JIMO) was a proposed Spacecraft designed to explore the icy moons of Jupiter. ^[63][13] The 2006 NASA budget included Congressional language imploring NASA to fund a mission that would orbit Europa. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Such a mission would be able to do the following:

1. Confirm a subsurface ocean using gravity and altimetry measurements.
2. Elucidate the origin of surface features by imaging much of the surface at high resolution.
3. Constrain the chemistry of surface materials using spectroscopy.
4. Probe for subsurface liquid water using ice-penetrating radar. ^[64]
5. 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 is far from certain that NASA will actually fund this mission, as funding for it is not included in NASA's 2008 budget plan, and no "flagship" planetary expeditions are likely before 2017. ^{[13][60][65][66]} Planetary scientist Ronald Greeley said about the Europa mission:^[60]

I am disappointed that after so many false starts over the last decade, it looks like a mission to Europa is slipping once again. The planetary community remains essentially unanimous in setting Europa as the highest priority large mission to the outer solar system.

Additionally, NASA Chief Mike Griffin said the following about the Jupiter Icy Moons Orbiter:

It was not a mission, in my judgment, that was well-formulated. [A scientific mission to Europa] is extremely interesting on a scientific basis. It remains a very high priority, and you may look forward, in the next year or so, or maybe even sooner, to a proposal for a Europa mission as part of our science line. But we would not—we would, again, not—favor linking that to a nuclear propulsion system. ^[60]

Another possible mission, known as the Ice Clipper mission, would use 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. Deep Impact is an ongoing NASA Space probe launched on 12 January 2005 that was designed to study the composition of the interior of the Comet ^[67] 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. ^[68]

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. Nuclear power is any Nuclear technology designed to extract usable Energy from atomic nuclei via controlled Nuclear reactions A cryobot or Philberth -probe is a Phallic Robot designed to operate in or around water ice. ^[69] The Planetary Society says that drilling a hole below the surface would be a main goal. The Planetary Society is a large publicly supported non-government and not-for-profit organization that has many research projects related to Astronomy. ^[13] Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) which would gather information and send it back to Earth. ^[70] 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. Forward-contamination is the contamination of other worlds with Earth microbes ^[71] This proposed mission has not yet reached a serious planning stage. ^[72]

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 United States of America —commonly referred to as the ^[73][74] The Planetary Society plans to create an "International Europa Task Force" to convince NASA and other space agencies to fund a Europa mission. ^[75][76] Other people, such as Congressman John Culberson, have even tried to go against NASA's budget cuts. John Abney Culberson (born August 24, 1956) American Politician, is a Republican congressman from Texas, representing ^[77][78]

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. Universities Space Research Association was incorporated on March 12, 1969 in the District of Columbia as a private nonprofit corporation under the auspices ^[79] Another mission, the Europa Orbiter, was also cancelled. The Europa Orbiter was a cancelled NASA mission to Jupiter 's Moon Europa. ^[80]

See also

• Colonization of Europa
• Jupiter's moons in fiction
• List of craters on Europa
• List of geological features on Europa
• List of lineae on Europa
• Moons of Jupiter

References

Further reading

• Greenberg, Richard (2005). Europa - The Ocean Moon: Search For An Alien Biosphere. Springer. ISBN 3540224505.  
• Bagenal, Fran; Dowling, Timothy Edward; McKinnon, William B (2004). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 0521818087.  
• Rothery, David A. (1999). Satellites of the Outer Planets: Worlds in Their Own Right. Oxford University Press US. ISBN 019512555X.  
• Harland, David M. (2000). Jupiter Odyssey: The Story of Nasa's Galileo Mission. Springer. ISBN 1852333014.  

External links

• Europa, a Continuing Story of Discovery at NASA/JPL
• Europa Profile by NASA's Solar System Exploration
• The Calendars of Jupiter
• Are our nearest living neighbours on one of Jupiter's Moons?