Europa
From Palaeos.org
Europa (Template:PronEng Template:Respell Template:Audio; or as Greek Ευρώπη) is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei (and possibly independently by Simon Marius. It is the smallest of the four Galilean moons. At just over 3000 km in diameter, Europa is slightly smaller than Earth's Moon and is the sixth-largest moon in the Solar System. 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.<ref>Mass of Europa: 48 Yg. Mass of Triton plus all smaller moons: 39.5 Yg (see note g here)</ref> It is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Does life generate the oxygen? 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.<ref>Template:Cite web</ref> Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.<ref name=geology>Template:Cite web</ref>
Although 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. Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions.<ref name="PlanetarySocEuropa">Template:Cite web</ref><ref name=Europabudget>Template:Cite web</ref>
Subsurface ocean
Many astronomers believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat.<ref name=greenberg /> 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 at the equator and only 50 K at the poles, keeping Europa's icy crust as hard as granite.<ref name=cyclo /> 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 (spacecraft) imaging team members argue for the existence of a subsurface ocean from analysis of Voyager program and Galileo images.<ref name=greenberg>Greenberg, R.; Europa: The Ocean Moon: Search for an Alien Biosphere, Springer Praxis Books, 2005</ref> 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 surface.<ref name=greeley>Greeley, R. et al.; Chapter 15: Geology of Europa, in Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, 2004</ref> The different models for the estimation of the ice shell thickness give values between a few hundred meters and tens of kilometers.<ref name="Billings">Template:Cite journal</ref>
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 Template:Convert deep.<ref name="Schenk"/> 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 Template:Convert. The "thin ice" model allows regular contact of the liquid interior with the surface through open ridges.<ref name="Billings"/>
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.<ref name="Zimmer">Template:Cite journal</ref> 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.<ref name="Kivelson"/> 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.<ref>Template:Cite web</ref> Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically.<ref>Template:Cite web</ref> 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.<ref name=Calvin>Template:Cite journal</ref>
Possible extraterrestrial life
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.<ref>Exotic Microbes Discovered near Lake Vostok, Science@NASA (December 10, 1999)</ref> Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.<ref name="EuropaLife"/><ref>Jones, N.; Bacterial explanation for Europa's rosy glow, NewScientist.com (11 December 2001)</ref> 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.<ref>Phillips, C.; Time for Europa, Space.com (28 September 2006)</ref>
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 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.<ref name=smoker>Template:Cite web</ref> 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.<ref name=smoker /> 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. Europa's unlit interior is now considered to be the most likely location for extant extraterrestrial life in the Solar System.<ref>Template:Cite web</ref>
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. 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.<ref>Template:Cite web</ref> However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface.<ref>Template:Cite web</ref> 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. 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.<ref name=limit>Template:Cite web</ref> 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.<ref name= limit />
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."<ref name="Europasbudget">David, L.; Europa Mission: Lost In NASA Budget, Space.com (7 February 2006)</ref>
