Europa: Moon Of Jupiter, Internal Structure, Discovery, Atmosphere, Exploration

Europa(Moon of Jupiter)

(Europa: Moon Of Jupiter, Internal Structure, Discovery, Atmosphere, Exploration)

Europa /jʊˈroʊpə/ (  listen) (Jupiter II), is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the legendary mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter).

Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron–nickelcore. It has a tenuous atmosphere composed primarily of oxygen. Its surface is striated by cracks and streaks, whereas craters are relatively rare. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s.

Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably harbor extraterrestrial life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below. Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers.

The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agency's Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede that is due to launch in 2022, and will include two flybys of Europa. NASA's planned Europa Clipper will be launched in the mid-2020s.

Discovery and naming

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

Europa is named after Europa, daughter of the king of Tyre, a Phoenician noblewoman in Greek mythology. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter. Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who discovered the four satellites independently.Marius attributed the proposal to Johannes Kepler.

The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II (a system also 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.

Internal structure

It is estimated that Europa has an outer layer of water around 100 km (62 mi) thick; a part frozen as its crust, and a part as a 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. This layer is likely a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.

Surface features 

Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. However; according to one theory, Europa's equator may be covered in icy spikes called penitentes, which may be up to ten meters high, due to direct overhead sunlight on the equator, causing the ice to sublime forming vertical cracks. The prominent markings crisscrossing Europa appear to mainly be albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young. Europa's icy crust has an albedo (light reflectivity) of 0.64, one of the highest of all moons. This indicates a young and active surface, based on estimates of the frequency of cometary bombardment that Europa likely experiences, the surface is about 20 to 180 million years old. There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.

The radiation level at the surface of Europa is equivalent to a dose of about 5400 mSv (540 rem) per day, an amount of radiation that would cause severe illness or death in human beings exposed for a single day.

Atmosphere

Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope, first described in 1995, revealed that Europa has a thin atmosphere composed mostly of molecular oxygen (O2). The surface pressure of Europa's atmosphere is 0.1 μPa, or 10−12times that of the Earth. 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, providing evidence of an atmosphere.

Magnetic field around Europa. The red line shows a trajectory of the Galileospacecraft during a typical flyby (E4 or E14).

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. 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 adsorbedand "sputtered" into the atmosphere. The same radiation also creates collisional ejections of these products from the surface, and the balance of these two processes forms an atmosphere. Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it 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.

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. 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.

The molecular hydrogen that escapes Europa's gravity, along with atomic and molecular oxygen, forms a gas torus 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.

Exploration 

Exploration of Europa began with the Jupiter flybys of Pioneer 10 and 11 in 1973 and 1974 respectively. The first closeup photos were of low resolution 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, the Galileo spaceprobe orbited Jupiter for eight years, until 2003, and provided the most detailed examination of the Galilean moons to date. It included the "Galileo Europa Mission" and "Galileo Millennium Mission", with numerous close flybys of Europa. In 2007, New Horizonsimaged Europa, as it flew by the Jovian system while on its way to Pluto.

Future missions

Conjectures regarding extraterrestrial life have ensured a high-profile for Europa and have led to steady lobbying for future missions.The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans. Robotic missions to Europa need to endure the high-radiation environment around itself and Jupiter. Europa receives about 5.40 Sv of radiation per day.

In 2011, a Europa mission was recommended by the U.S. Planetary Science Decadal Survey. In response, NASA commissioned Europa lander concept studies in 2011, along with concepts for a Europa flyby (Europa Clipper), and a Europa orbiter. The orbiter element option concentrates on the "ocean" science, while the multiple-flyby element (Clipper) concentrates on the chemistry and energy science. On 13 January 2014, the House Appropriations Committee announced a new bipartisan bill that includes $80 million funding to continue the Europa mission concept studies.

        Europa Multiple-Flyby Mission — In July 2013 an updated concept for a flyby Europa mission called Europa Clipper was presented by the Jet Propulsion Laboratory (JPL) and the Applied Physics Laboratory (APL). In May 2015, NASA announced that it had accepted development of the Europa Clipper mission, and revealed the instruments it will use. The aim of Europa Clipper is to explore Europa in order to investigate its habitability, and to aid selecting sites for a future lander. The Europa Clipper would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude flybys of Europa during its envisioned mission. The probe would carry an ice-penetrating radar, short-wave infrared spectrometer, topographical imager, and an ion- and neutral-mass spectrometer.

          In 2012, Jupiter Icy Moon Explorer (JUICE) was selected by the European Space Agency (ESA) as a planned mission. That mission includes 2 flybys of Europa, but is more focused on Ganymede.

   

Habitability potential

    A black smoker in the Atlantic Ocean. Driven by geothermal energy, this and other types of hydrothermal vents create chemical disequilibria that can provide energy sources for life.

             Europa has emerged as one of the most likely locations in the Solar System for potential habitability. Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean hydrothermal vents. Even if Europa lacks volcanic hydrothermal activity, a 2016 NASA study found that Earth-like levels of hydrogen and oxygen could be produced through processes related to serpentinization and ice-derived oxidants, which do not directly involve volcanism. In 2015, scientists announced that salt from a subsurface ocean may likely be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water in contact with Europa's rocky mantlehas spurred calls to send a probe there.

             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. Although 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. The energy from tidal flexing could never support an ecosystem in Europa's ocean as large and diverse as the photosynthesis-based ecosystem on Earth's surface. Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean. If Europa's ocean is too cold, biological processes similar to those known on Earth could not take place. If it is too salty, only extreme halophiles could survive in that environment.

           Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell. If confirmed, the lakes could be yet another potential habitat for life.

           Evidence suggests that hydrogen peroxide is abundant across much of the surface of Europa. Because hydrogen peroxide decays into oxygen and water when combined with liquid water, the authors argue that it could be an important energy supply for simple life forms.

         Clay-like minerals (specifically, phyllosilicates), often associated with organic matter on Earth, have been detected on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet.

          Life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called lithopanspermia.