Europa - Just the facts

Europa - Just the facts (A article written in 2007)

Europa is a moon of the planet Jupiter. It is the sixth nearest moon to Jupiter, & the fourth largest of Jupiter's moons. It was discovered in 1610 by Galileo Galilei (and independently by Simon Marius shortly thereafter) & is the smallest of the four Galilean moons named in Galileo's honor. Due to the hypothesized ocean beneath its icy surface, Europa is one of the most likely places in the solar system to host primitive extraterrestrial life.

Europa is named after Europa, daughter of Agenor, king of the Phoenician city of Tyre, now in Lebanon, & sister of Cadmus, founder of Thebes, Greece.

Although the name "Europa" was suggested by Simon Marius soon after the moon's discovery, the name fell out of favor for a considerable time (as did those of the other Galilean moons), & was not revived in common use until the mid-20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation as Jupiter II or as the "second satellite of Jupiter". The discovery of Amalthea in 1892, closer than any of the other known moons of Jupiter, pushed Europa to 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.

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

Like all the Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. Europa is also being gravitationally pulled in different directions by Jupiter & by other satellites of the planet (tidal flexing). This gives the body a source of heat & energy, allowing the subsurface ocean to stay liquified, & driving subsurface geological processes.

Physical characteristics

Interior of Europa
Internal structure
Europa is somewhat similar in bulk composition to the terrestrial planets, being primarily composed of silicate rock. It has an outer layer of water thought to be around 100 km thick (some, as frozen ice upper crust; some, as liquid ocean underneath the ice), & recent magnetic field data from the Galileo orbiter probe, which orbited Jupiter & studied Europa between 1995 & 2003, shows that Europa generates an induced magnetic field by interacting with Jupiter's field, which suggests the presence of a subsurface conductive layer which is likely a salty liquid-water ocean. Europa probably also contains a metallic iron core.

Surface features
The Europan surface is relatively smooth; few features more than a few hundred meters high have been observed, but topographic relief in places approaches a kilometer (0.62 miles)[citation needed]. Europa is one of the smoothest objects in the solar system. The prominent markings crisscrossing the moon seem to be mainly albedo features, which emphasize low topography. There are very few craters on Europa because its surface is active & young. Europa's albedo (light reflectivity) of 0.64 is one of the highest of all moons because of its icy surface. This would seem to indicate a young & 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 (the geological features of the surface clearly show a variety of ages).

Approximately natural color image of Europa by the Galileo spacecraftEuropa's most striking surface feature is a series of dark streaks criss-crossing the entire globe. Close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are roughly 20 km (12 miles) across commonly with dark diffuse outer edges, regular striations, & a central band of lighter material. These may have been produced by a series of volcanic water eruptions or geysers as the Europan crust spread open to expose warmer layers beneath. The effect is similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter; Since Europa is tidally locked to Jupiter, & therefore always maintains the same orientation towards the planet, the stress patterns should form a distinctive & predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to have occurred at increasingly different orientations the older they are. This can be explained if Europa's surface rotates slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon's surface from its rocky mantle & to the effects of Jupiter's gravity tugging on the moon's outer ice crust. Comparisons of Voyager & Galileo spacecraft photos suggest that Europa's crust rotates no faster than once every 10,000 years relative to its interior.

Craggy mountains & smooth plates jumbled together in the Conamara Chaos regionAnother type of feature present on Europa are circular & elliptical lenticulae, Latin for "freckles". Many are domes, some are pits & some are smooth dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below. It is thought that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in the Earth's crust. The smooth dark spots could be formed by meltwater released when the warm ice breaks the surface, & the rough, jumbled lenticulae (called regions of "chaos", for example the Conamara Chaos) appear to be formed from many small fragments of crust embedded in hummocky dark material, perhaps like icebergs in a frozen sea.

Subsurface ocean

Europa's subsurface oceanIt is thought that under the surface there is a layer of liquid water kept warm by tidally generated heat. The temperature on the surface of Europa averages about 110 K (-163 °C) at the equator & only 50 K (-223 °C) at the poles, & so the surface water ice is permanently frozen. The first hints of a subsurface ocean came from theoretical considerations of the tidal heating (a consequence of Europa's slightly eccentric orbit & orbital resonance with the other Galilean moons). Galileo imaging team members have analyzed Voyager & Galileo images of Europa to argue that Europa's geological features also demonstrate the existence of a subsurface ocean. The most dramatic example is "chaos terrain," a common feature on Europa's surface that some interpret as a region where the subsurface ocean melted through the icy crust. This interpretation is extremely controversial. Most geologists who have studied Europa favor what is commonly called the "thick ice" model, in which the ocean has rarely, if ever, directly interacted with the surface. The different models for the estimation of the ice shell thickness give values between a few kilometers & tens of kilometers.

The best evidence for the so called "thick ice" model is a study of Europa's large craters. The largest craters are surrounded by concentric rings & appear to be filled with relatively flat, fresh ice; based on this & on the calculated amount of heat generated by Europan tides, it is predicted that the outer crust of solid ice is approximately 10-30 kilometers (5-20 miles) thick, which could mean that the liquid ocean underneath may be about 100 km (60-65 miles) deep.

The Galileo orbiter has also found that Europa has a weak magnetic field (about one quarter the strength of Ganymede's field & similar to Callisto's) which varies periodically as Europa passes through Jupiter's massive magnetic field. A likely explanation of this is that there is a large, subsurface ocean of liquid salt water. Spectrographic evidence suggests that the dark reddish streaks & features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, since these materials are colorless or white when pure, some other material must also be present to account for the reddish color. Sulfur compounds are suspected.

Atmosphere
In 1994, observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope revealed that Europa has a very tenuous atmosphere (1 micropascal surface pressure) composed of oxygen.[17] Of all the moons in the solar system only six others (Io, Callisto, Enceladus, Ganymede, Titan & Triton) are known to have atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. It is most likely generated by ultraviolet sunlight & charged particles hitting Europa's icy surface, splitting water into hydrogen & oxygen. The hydrogen escapes Europa's gravity due to its low atomic mass, leaving the oxygen behind.

Exploration of Europa
Most of our knowledge of Europa comes from the flybys by the Voyager & Galileo missions. 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. The aims of these missions have ranged from examining Europa's chemical composition, to searching for extraterrestrial life in its sub-surface ocean.

Possible extraterrestrial life

Life in such an ocean could possibly be similar to life on Earth's deep oceanIt has been suggested that life may exist in this under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok. Life in such an ocean could possibly be similar to life on earth in the deep ocean. So far, there is no evidence that life exists on Europa but due to the likely presence of liquid water there are proposals to send a probe there. Robert Pappalardo, an assistant professor within the University of Colorado's space department, said "We’ve spent quite a bit of time & effort trying to understand was Mars 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". However, recent budget cuts have prevented proposed missions to search for life.

Spacecraft proposals & cancellations

Artist's concept of the cryobot & hydrobotThe plan for the extremely ambitious Jupiter Icy Moons Orbiter was canceled in 2005.

The 2006 NASA budget includes Congressional language imploring NASA to fund a mission that would orbit Europa. Such a mission would be able to confirm a subsurface ocean using gravity & altimetry measurements, elucidate the origin of surface features by imaging much of the surface at high resolution, constrain the chemistry of surface materials using spectroscopy, & probe for subsurface liquid water using ice-penetrating radar. The mission might even carry a small lander to determine the surface chemistry directly, & to measure seismic waves, from which the level of activity & ice thickness could be determined. However, at present it is far from certain that NASA will actually fund this mission, as funding for it is not included in NASA's 2007 budget plan. Planetary scientist Ronald Greeley said about the Europa mission:

"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, & 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."

Another possible mission, known as the "Ice Clipper" mission, would use an impactor similar to the Deep Impact DI 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. Without the need for an insertion & relaunch of the spacecraft(s) from an orbit around Jupiter or Europa, this would be one of the least expensive missions since the necessary amount of fuel would be decreased.

More ambitious ideas have been put forward for a capable lander to test for evidence of life that might be frozen in the shallow subsurface, or even to directly explore the possible ocean beneath Europa's ice. One proposal calls for a large nuclear powered "Melt Probe" (cryobot) which would melt through the ice until it hit the ocean below. The Planetary Society says that drilling a hole below the surface would be a main goal, & provide protection from radiation. Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot), which would gather information & send it back to Earth. Both the cryobot & the hydrobot would have to undergo some form of extreme sterilization to prevent it from detecting earth organisms instead of native life & to prevent contamination of the subsurface ocean. This proposed mission has not yet reached a serious planning stage.

Even though Congress, the National Acadamy of Sciences, & the NASA advisory committee have all supported a mission to Europa, funding has still been halted. The Planetary Society plans on creating the "International Europa Task Force" to convince NASA & other space agencies to fund a Europa mission. Other people, such as congressman John Culberson, have also tried to go against NASA's budget cuts.

A "Solar System Exploration Roadmap" published for NASA by the Universities Space Research Association in 2006 placed exploration of Europa high on its list, & suggested that plans for a "flagship-class" mission to Europa begin by 2008 with hopes to launch by 2015.

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