Europa (moon)

Europa is one of the four Galilean satellites that orbit Jupiter. Only slightly smaller than the Earth’s Moon, Europa is covered by a relatively smooth layer of highly reflective fractured ice. Tidal forces exerted by Jupiter cause internal heating on Europa that apparently results in the periodic resurfacing of watery flows, which have obliterated most impact craters and other blemishes over time. Heat flow may be sufficient to maintain a liquid water subsurface layer that could harbor simple life forms.

Overview

Europa is one of the four-largest satellites of the planet Jupiter, known as the Galilean satellites after their discoverer, Galileo Galilei. In order of their distance from Jupiter, they are Io, Europa, Ganymede, and Callisto. Jupiter has at least sixty-three satellites, but only the Galilean satellites are large enough to be observed from Earth through small telescopes. With a diameter of 3,138 kilometers, Europa, the smallest Galilean satellite, is slightly smaller than Earth’s Moon of 3,476 kilometers. By contrast, the largest Galilean satellite, Ganymede, measures 5,260 kilometers in diameter, larger than the planet Mercury (4,878 kilometers). Thus, if the Galilean satellites orbited the Sun instead of Jupiter, they would be considered full-fledged planets. Despite its relatively small size compared to its Galilean companions, Europa is nevertheless the sixth-largest planetary satellite in the solar system. It is about 780 million kilometers from the Sun, about 5.2 times the distance from the Earth to the Sun.

Europa orbits Jupiter at an average distance of 670,900 kilometers; its orbital period (time to complete one orbit) is 3.55 Earth days. Its rotational period around its axis is also 3.55 days, which means that Europa always shows the same face toward Jupiter. The other Galilean satellites and Earth’s own Moon also follow this 1:1 ratio of orbital to rotational period, termed a synchronous relationship.

Using a crude homemade telescope, Galileo discovered Europa and two of the other three large Jovian satellites (Io and Callisto) on January 7, 1610. At first, he believed the tiny points of light in line with Jupiter were small stars, but later, he realized that they, in fact, orbited Jupiter as if in a miniature solar system. Galileo originally called the moons the Medician planets, after the powerful Italian Medici family, and numbered each satellite with a Roman numeral, beginning with the one closest to Jupiter. Europa, in this scheme, was designated II. Another observer, Simon Marius (Simon Mayr), who claimed to have discovered the Jovian satellites before Galileo in November 1609 but was tardy in publishing his results, later named the bodies as we know them in the twenty-first century.

The name Europa comes from a Phoenician princess, one of many mortal consorts of the supreme Greek god Zeus, whose Roman name was given to the planet Jupiter. (The other Galilean satellites are similarly named for Zeus's conquests.) The most intriguing aspect of Europa is its unusual surface. Images beamed to Earth in 1979 by the Voyager1 and 2 spacecraft as they flew through the Jupiter system show a relatively smooth ice ball that some scientists compared in appearance to a fractured antique ivory billiard ball. The satellite is covered by a globally encompassing shell of water ice, frozen at 128 kelvins, that gives Europa an extremely high albedo (reflective power). Sixty-four percent of the light striking the surface is reflected in all directions, giving Europa an albedo of 0.64, while rocky surfaces like that of Earth’s Moon or Mercury reflect only about ten percent.

Europa’s density, 3.013 grams per cubic centimeter, suggests that most of the Moon, like Earth, is composed of rocky silicate material. The icy surface layer, therefore, must be relatively thin; most estimates lie in the range of 15 to 25 kilometers thick, with the water below potentially reaching depths of 100 or even 150 kilometers. The surface shows relatively little topographic relief, nothing higher than one kilometer, and displays only a few small, scattered impact craters. This is in dramatic contrast to its neighbors, the two outer satellites Ganymede and Callisto, which have numerous large craters on the order of 50 to 100 kilometers in diameter. Most craters on Europa do not exceed about twenty kilometers in diameter. This suggests that Europa’s icy surface is relatively young. This indicates that resurfacing by liquid ice flows or other processes has covered any large craters formed during early periods of heavy meteoroids impacting the Jovian system. Estimates of the surface age of Europa range from twenty to two hundred million years old, with the more likely age being between forty and 90 million years. This suggests a significant resurfacing of the planet in the later stages of its history.

In December 1995, the Galileo spacecraft entered orbit around Jupiter. For thirty-eight orbits, Galileo investigated the giant planet Jupiter and flew by its many satellites, paying particular attention to Europa, Ganymede, and Callisto. In 1997, Galileo produced images of a large, multi-ringed impact crater on Europa, probably buried beneath the ice crust. Evidence for the crater includes diffuse, dark, concentric, arc-like bands and associated fractures that define a structure more than five thousand kilometers in diameter. The presence of this buried crater shows that the rocky surface below the ice layer was subjected to significant impact early in Europa’s history. It further suggests that the ice crust formed time, probably after heavy meteoroid bombardment had greatly diminished.

The most striking aspects of Europa’s surface are its mottled, colored terrains and the linear fractures that crisscross most of its globe. Based on color and subtle topographic expressions, mottled terrains are of two varieties: brown and gray. Brown landscapes contain numerous pits and depressions from one to ten kilometers in diameter. Several large plateaus range from a few kilometers to tens of kilometers wide and nearly one hundred kilometers long. Some circular depressions without raised crater rims may represent degraded impact craters. Gray terrains are similar to brown but are generally smoother and less hummocky. The relationship between the two terrains is unknown, but their differences may result from different ages, degrees of surface development, or both. The ultimate origin of these mottled terrains also remains unknown. Still, a reasonable hypothesis is that they represent the effects of hydrothermal upwelling, causing heating and expansion of affected crustal areas. The nonmottled areas on Europa are very light in color and have very smooth topographies. These icy plains contain most of the observed linear surface fractures.

Linear features on Europa’s surface may extend for thousands of kilometers. They are classified into three categories: (1) dark triple bands, some containing dark outer bands with a white strip down the center, thought to represent icy geyser deposits erupted along the axis of the fractures; (2) older and brighter lineaments that are crosscut by the triple bands and resemble them in some cases; and (3) very young cracks that crosscut the other two fracture types. Detailed analysis of the orientation of these three fracture types indicates that each type shows a distinct orientation that can be correlated with the relative age of the fractures. The data show that the direction of tidal stresses in Europa’s crust has rotated clockwise over time. This observation has been used to suggest that Europa’s rotation is not perfectly synchronous. Over time, Europa may rotate faster than the synchronous rate, causing the surface to be progressively reoriented relative to tidal forces.

High-resolution images from the Galileo orbiter show places on Europa resembling ice flows in Earth’s polar regions. Large, angular pieces of ice have shifted away from one another, some rotating in the process, but reconstructions show that they fit together like puzzle pieces. This evidence for motion involving fluid flow and the possibility of geyser eruptions shows that the ice crust has been or is still being lubricated from below by warm ice or even liquid water. The heating source to produce this watery fluid is tidal forces by gravitational interaction with massive Jupiter, along with some escaping heat produced by radioactive minerals in the underlying silicate crust.

Galileo spacecraft’s mission was expanded to include the Galileo Europa Mission, during which it flew several close flybys to focus its instrumentation and cameras specifically on Europa. On one close encounter with Europa, Galileo came within two hundred kilometers of the satellite's icy surface. Ultimately, the Galileo spacecraft was purposely directed to plunge to its destruction in Jupiter’s atmosphere on September 21, 2003. This was to safeguard any possible life forms on Europa against the plutonium inside Galileo’s radioisotope generators if it crashed into the satellite.

Europa remains a high-priority location within the solar system for astrobiology studies. With the demise of the Galileo spacecraft, plans were proposed to send another spacecraft, this time to orbit Europa, for prolonged and repeated studies. A more ambitious plan arose, called Jupiter Icy Moons Orbiter or JIMO. JIMO would have been the flagship mission of a more extensive program called Project Prometheus to develop nuclear propulsion as a means to cut down the travel time between Earth and the rest of the solar system. However, after initial funding was granted, the National Aeronautics and Space Administration (NASA) was forced to cancel Project Prometheus in 2005 in favor of other expenditures arising from the Vision for Space Exploration program under the George W. Bush administration. Returning to Europa with a robotic spacecraft was, therefore, put on hold for the early portion of the twenty-first century. Data from the Cassini spacecraft in orbit about Saturn revealed information about the icy satellite Enceladus and Titan's thick atmosphere and organic compounds that diverted the attention of many astrobiologists away from Europa.

Methods of Study

Jupiter and its four largest satellites have been studied using telescopes since Galileo first trained his on the system in 1610. Before the advent of interplanetary space probes, telescopic observations resulted in a remarkable treasure trove of data on the Galilean satellites.

For example, in the 1920s, the astronomers Willem de Sitter and R. A. Sampson obtained reasonably accurate data on their masses. Calculations involved observing how each satellite disturbed the orbits of the others and noting the nature of the resonant orbits of the inner three, first described by Pierre-Simon Laplace in the late eighteenth century. These resonant orbits dictate that Europa revolves two times and Ganymede four for every rotation of Io around Jupiter. This orbital resonance scheme implies a specific ratio for the masses of the bodies, which assisted de Sitter and Sampson in their calculations.

The satellites' diameter was accurately known with the advent of stellar occultation studies in the 1970s and later, when spacecraft imaging produced precise values. Before that, Europa was described by a popular 1950s-era science text as having a diameter of 1,800 miles (2,880 kilometers), only a bit less than the accepted value of 3,138 kilometers.

Although the first Earth-launched space probes encountered the Jovian system in 1973 (Pioneer 10) and 1974 (Pioneer 11), they paid scant attention to the Galilean satellites. At the time, the community of planetary scientists viewed the satellites of the gas giants in the outer solar system to be nothing more than boring ice balls. Even on the Voyagers, few planetary science studies were devoted to any of the icy satellites. Only imaging studies of Io and Titan, and to a lesser extent Europa, were planned as major portions of Voyager flyby operations in the Jupiter or Saturn system.

In 1979, however, knowledge of these bodies dramatically expanded as images of all four satellites were beamed back to Earth by Voyagers1 and 2. The first pictures of Europa showed a previously unknown world with a smooth, highly reflective surface mottled by brown and tan patches and crisscrossed by a complicated network of curved and straight lines. Four months later, higher-resolution images from Voyager 2 confirmed the presence of even more linear structures, which were interpreted as fractures with virtually no associated relief. In addition to its imaging work, the Voyager probes also made precise measurements of the mass of the Galilean satellites by analyzing the gravitational effects of the planets on spacecraft trajectories; this, combined with improved size determinations, allowed for more accurate calculations of density, which in turn is used to assess planetary composition.

In 1995, the Hubble Space Telescope discovered a thin oxygen atmosphere on Europa; this was later confirmed by the Galileo orbiter during its extended Europa Mission. The official term for this rarefied atmosphere is a surface-bound exosphere. Hubble used its highly sensitive spectrometers to analyze the energy spectrum of light reflected from the Moon’s surface. Europa’s atmosphere is so tenuous that its surface pressure is only one hundred-billionth that of Earth. It is estimated that if all the oxygen on Europa were to be compressed to the surface pressure of Earth’s atmosphere, it would fill an area the size of a dozen Houston Astrodomes.

The Galileo spacecraft investigated the Jupiter system from late 1995 through 2003. After launching an atmospheric probe into Jupiter itself relatively early in its mission, the Galileo orbiter assumed an elliptical orbit, allowing it to make several close passes by all four Galilean satellites. The resolution of Voyager images of Europa made it possible to view surface features no smaller than about four kilometers across. In contrast, Galileo swooped down closer than either Voyager spacecraft. With its more sophisticated cameras, it achieved resolutions of around ten meters per pixel, allowing objects the size of earthly buildings to be discerned. From these high-resolution images, scientists have observed evidence of both tensional and compression ridges and have documented features like water-ice geysers, possible ice volcanoes, and jumbled ice flows that resemble puzzle pieces. These observations paint a picture of a dynamic planet in which tectonic faulting and flooding by liquid water occur periodically. The dark color of many surface fractures may result from the injection of water or warm ice mixed with darker silicates that well up into the fractures and freeze. Galileo's images generated renewed interest in the idea that a layer of liquid water exists below the ice or existed sometime in the recent past.

Galileo also carried a magnetometer to detect a planet’s magnetic field's existence and measure its strength. During a December 1996 pass of Europa, this magnetometer detected the first evidence of a magnetic field. Ganymede, the next moon from Europa, also has a magnetic field. Although it is about four times weaker than that of Ganymede, Europa’s field is still of substantial magnitude. Combined with gravity data suggesting a dense core, Europa's magnetic measurements indicate the probable existence of a sizable metallic core and a layered internal structure similar to that of Earth. The magnetic field data also provided constraints on the nature of the water on Europa. The magnetic field could not be explained by assuming pockets of salty water within a crust of ice; instead, it requires a spherical shell of liquid water.

In 2012, Hubble captured images of clouds of water vapor coming from the Moon's southern region, supporting the theory that a warm and active ocean is beneath the sheet of ice. In 2014, scientists learned that Europa may have a form of plate tectonics, which would make it the only known body in the solar system other than Earth to have a dynamic crust. The European Space Agency (ESA) launched its probe, the Jupiter Icy Moons Explorer (JUICE), on its eight-year voyage to Jupiter in April 2023.

In October of 2024, NASA launched the Europa Clipper. The Clipper is scheduled to travel 1.8 billion miles to Jupiter and conduct 49 close flybys of Europe. The mission is designed to determine if Europa currently has habitable conditions. Due to Europa's dangerous radiation zone, the Clipper can only spend less than a day there on each orbit.

Context

The Jovian system has long been of interest to scientists as a possible model analogous to the more extensive solar system of the Sun and planets. In this model, Jupiter is a substitute for the Sun, and the Galilean satellites represent the planets, particularly the rocky planets from Mercury to Mars. The Galilean satellites' considerable masses and stable orbits suggest that they originated along with Jupiter during its formation from the gaseous solar nebula. If so, do these satellites show evidence of having evolved in a manner that parallels that of the solar system's inner planets, including Earth?

In the early 1970s, planetary scientist John Lewis pointed out that the densities of the two outer satellites, Ganymede (1.93 grams per cubic centimeter) and Callisto (1.83 grams per cubic centimeter), are consistent with condensation of solar-composition gas (the solar nebula), where water ice is a stable compound. He predicted that these two bodies should be composed of about equal parts water ice and silicate rock, a generally accepted view. The two inner bodies, Io and Europa, however, have higher densities (3.53 and 3.01 grams per cubic centimeters, respectively) and thus would be expected to contain less in the way of low-density materials like ice, with a density of 1.0 grams per cubic centimeter. In fact, these bodies show evidence of being largely composed of rocky material, with no ice on Io and only a thin crust of ice over an ocean of liquid water on Europa. What processes could have produced such a density distribution?

In the early 1950s, astronomer Gerard P. Kuiper suggested that Jupiter had been hot during its early history. Building on Kuiper’s earlier work, hypotheses confirm that Jupiter was probably hot enough in its infancy to have forced low-mass, volatile gaseous materials to the outer fringes of the Jovian region, leaving heavier compounds to accrete as planetoids closer to Jupiter. Compared to denser silicate minerals, the lighter volatile gas would contain a high proportion of elements that would eventually freeze as ice. These materials would eventually accrete to produce Ganymede and Callisto, while the volatile-poor inner gas would eventually accrete as Io and Europa. Europa, farther from Jupiter than Io, has more volatiles such as ice than Io, which has virtually no ice on its surface. Io probably also lost much of its volatile components due to long-term volcanic activity due to tidal heating produced by Jupiter’s gravitational field.

Similarly, the solar system exhibits a composition distribution with high-density rocky (or terrestrial) planets near the Sun and more volatile-rich bodies in the outer regions. The inner planets show a similar density distribution. Thus, the Jovian satellite system shows that any evolving planetary system on a scale large enough to have a hot central star predictably develops a density distribution where low-density, high-volatile planets dominate the outer regions and high-density rocky bodies dominate the inner areas.

The study of Europa is essential in terms of its possible role as a site of extraterrestrial life-forms. Images from the Galileo space probe have confirmed ideas spawned after the Voyager flybys that Europa may have a globally encircling layer of liquid water beneath its surface ice layer. Where water exists in liquid form on a planet, life as we know it can evolve. Europa has joined the ranks of Mars and Saturn’s moon, Titan, as possible sites where primitive life could exist.

Robotic spacecraft could actively study comparative planetology inside the Jovian system. Voyager provided only tantalizing images and raised many questions. Galileo data strongly suggested that the four large Jovian satellites share essentially similar cores in terms of size. Io is volcanically active, taking material deep inside and turning itself inside out by resurfacing itself with sulfur and sulfur compounds; therefore, Io is devoid of an icy shell. Europa has internal heat that is insufficient to melt the ice cover in total but supplies warmth to a subsurface layer of liquid water. Ganymede and Callisto have thick, icy shells and retain more primordial character because they lack significant heating from their cores. Ganymede’s magnetic field is a paradox because its character resembles the dipole field produced by convection within a molten iron core.

Europa once held the primary attention of astrobiologists as the favored place within the solar system for finding some type of life beyond Earth. As a result of extensive robotic investigation of other portions of the outer solar system, it must share that hopeful spotlight with Titan and Enceladus.

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