Europa (moon)

Europa (ⁱ/jʊˈroʊpə/^[11] (Greek: Ευρώπη), or Jupiter II, is the sixth closest moon of the planet Jupiter, and the smallest of its four Galilean satellites, but still one of the largest bodies in the Solar system. Europa was discovered in 1610 by Galileo Galilei^[1] and possibly independently by Simon Marius around the same time. Progressively more in-depth observation of Europa has occurred over the centuries by Earth-bound telescopes, and by space probe flybys starting in the 1970s.

Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably 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 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.^[12] This hypothesis proposes that heat energy from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics.^[13]

Although only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The Galileo mission, launched in 1989, provided the bulk of current data on Europa. A new mission to Jupiter's icy moons, the Europa Jupiter System Mission (EJSM), was proposed for a launch in 2020.^[14] Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions.^[15][16]

Discovery and naming

Europa was discovered in January 1610 by Galileo Galilei,^[1] and possibly independently by Simon Marius. The moon is named after a Phoenician noblewoman in Greek mythology, Europa, who was courted by Zeus and became the queen of Crete.

Europa, along with Jupiter's three other largest moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei in January 1610. The first reported observation of Io was made by Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, January 8, 1610 (used as the discovery date for Europa by the IAU).^[1]

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 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.^[17][18]

The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century.^[19] 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". 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 Jupiter II.^[19]

Orbit and rotation

Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. 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 0.470°.^[20] Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is the line intersecting this point.^[21] 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.^[22]

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.^[23] Thus, the tidal flexing kneads Europa's interior and gives the moon a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes.^[13][23] 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.^[23][24]

Physical characteristics

Europa is slightly smaller than Earth's Moon. At just over 3,100 kilometres (1,900 mi) in diameter, it is the sixth-largest moon and fifteenth largest object in the Solar System. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined.^[25] Its bulk density suggests that it is similar in composition to the terrestrial planets, being primarily composed of silicate rock.^[26]

Internal structure

It is believed that Europa has an outer layer of water around 100 km (62 mi) 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.^[27] Europa probably contains a metallic iron core.^[28]

Surface features

Europa is one of the smoothest objects in the Solar System.^[29] 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.^[30][31] Europa's icy crust gives it an albedo (light reflectivity) of 0.64, one of the highest of all moons.^[20][31] 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.^[32] There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.^[33]

The radiation level at the surface of Europa is equivalent to a dose of about 540 rem (5400 mSv) per day,^[34] an amount of radiation that would cause illness or death in human beings.^[35]

Lineae

Europa's most striking surface features are a series of dark streaks crisscrossing the entire globe, called lineae (English: 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 more than 20 km (12 mi) across, often with dark, diffuse outer edges, regular striations, and a central band of lighter material.^[36] 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.^[37] 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.^[38] 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.^[39]

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.^[40]

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.^[40] The smooth, dark spots could be formed by meltwater released when the warm ice breaks through the surface. The rough, jumbled lenticulae (called regions of "chaos"; for example, Conamara Chaos) would then be formed from many small fragments of crust embedded in hummocky, dark material, appearing like icebergs in a frozen sea.^[41]

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. ^{[42] [43]}

In November 2011, a team of researchers from the University of Texas at Austin and elsewhere presented evidence in the journal Nature suggesting that many "chaos terrain" features on Europa sit atop vast lakes of liquid water.^[44][45] These lakes would be entirely encased in the moon's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. Full confirmation of the lakes' existence will require a space mission designed to probe the ice shell either physically or indirectly, for example using radar.

Subsurface ocean

Most planetary scientists believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat.^[46] 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.^[9] 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.^[46] 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.^[47] The different models for the estimation of the ice shell thickness give values between a few kilometers and tens of kilometers.^[48]

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 100 km (60 mi) deep.^[32] 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 200 metres (660 ft). 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.^[48]

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 7.3×10¹⁷ J of kinetic energy, which is two hundred times larger than that of the flow excited by the dominant tidal forces.^[49][50] 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.^[51] 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.^[28] 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.^[52] Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically.^[53] 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.^[54]

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).^[55][56] The surface pressure of Europa's atmosphere is 0.1 μPa, or 10⁻¹² times that of the Earth.^[10] 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,^[57][58] 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.^[59] 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.^[60] 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.^[61][62]

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.^[63] 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.^[64]

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. ^[65]

Potential for extraterrestrial life

Europa has emerged as one of the top Solar System locations in terms of potential habitability and possibly, hosting extraterrestrial life.^[66] 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 Vostok.^[67] Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.^[68][69] 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.^[70]

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 reach of sunlight, 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.^[71] An environment'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 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.^[71] 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.^[64] 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.^[72] However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface.^[73] Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit 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.^[74] 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.^[74] In September 2009, planetary scientist Richard Greenberg calculated that cosmic rays impacting on Europa's surface convert some water ice into free oxygen (O₂) 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.^[75]

In 2006, Robert T. Pappalardo, an assistant professor in the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder 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.^[16]

In November 2011, a team of researchers presented evidence in the journal Nature suggesting the existence of vast lakes of liquid water entirely encased in the moon's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell.^[44][45] If confirmed, the lakes could be yet another potential habitat for life.

Exploration

Most human knowledge of Europa has been derived from a series of flybys beginning in 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 compared to later missions.

The two Voyager probes traveled through the Jovian system in 1979 providing more detailed images of Europa's icy surface. The images caused many scientists to speculate about the possibility of a liquid ocean underneath.

Starting in 1995, Galileo began a Jupiter orbiting mission that lasted for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included, Galileo Europa Mission and Galileo Millennium Mission, with numerous close flybys of Europa.^[76]

New Horizons imaged Europa in 2007, as it flew by the Jovian system while on its way to Pluto.

Future missions

Various proposals have been made for future missions. The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its subsurface ocean.^[68][77] Any mission to Europa would need to be protected from Jupiter’s high radiation environment.^[15] Europa receives about 540 rem of radiation per day.^[78]

The Jupiter Europa Orbiter, as part of the Europa Jupiter System Mission (EJSM), is a joint NASA/ESA proposal for exploration of Jupiter's moons targeting a launch in 2020. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the other Outer Planet Flagship Mission, the Titan Saturn System Mission (TSSM).^[79] ESA's contribution will still face funding competition from other ESA projects.^[80] 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 effort.

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.^[81]

Prior to EJSM, the plan for the extremely ambitious Jupiter Icy Moons Orbiter (JIMO) was cancelled in 2005.^[15][81] Before that, the Europa Orbiter received a go-ahead in 1999 but was canceled in 2002. This Orbiter featured a special radar that would allow it to look below the surface.^[29]

Jovian Europa Orbiter was a ESA Cosmic Vision concept study from 2007. 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.^[82][83]

More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for biosignatures that might be frozen in the shallow subsurface.^[84][85]

Another proposal put forward in 2001 calls for a large nuclear-powered "melt probe" (cryobot) which would melt through the ice until it reached the ocean below.^[15][86] Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) which would gather information and send it back to Earth.^[87] 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.^[88] This proposed mission has not yet reached a serious planning stage.^[89]

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
• Snowball Earth hypothesis
• Terraforming of Europa (moon)

References

Further reading

• Bagenal, Fran; Dowling, Timothy Edward; and McKinnon, William B. (2004). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 0-521-81808-7. 
• Rothery, David A. (1999). Satellites of the Outer Planets: Worlds in Their Own Right. Oxford University Press US. ISBN 0-19-512555-X. 
• Harland, David M. (2000). Jupiter Odyssey: The Story of NASA's Galileo Mission. Springer. ISBN 1-85233-301-4. 
• Greenberg, Richard (2005). EUROPA The Ocean Moon. Springer. ISBN 3-540-22450-5. 

External links

• Europa, a Continuing Story of Discovery at NASA/JPL
• Europa Profile at NASA's Solar System Exploration site
• Astronomy Cast: Europa. Frasier Cain, 2010
• Europa page at The Nine8 Planets
• Europa page at Views of the Solar System
• The Calendars of Jupiter
• Are our nearest living neighbours on one of Jupiter's Moons?
• Preventing Forward Contamination of Europa - SSB Study of Planetary
• Protection policies for Europa
• Images of Europa at JPL's Planetary Photojournal
• Movie of Europa's rotation from the National Oceanic and Atmospheric Administration
• Europa map with feature names from Planetary Photojournal
• Europa nomenclature and Europa map with feature names from the USGS planetary nomenclature page
• Paul Schenk's 3D images and flyover videos of Europa and other outer solar system satellites; see also
• Large, high-resolution Galileo image mosaics of Europan terrain from Jason Perry's (normally Io-related) blog: 1, 2, 3, 4, 5, 6, 7
• Europa image montage from Galileo spacecraft, NASA APOD