The Solar System in Close-Up

by John Wilkinson

  Io is thought to be volcanically active because of huge tidal forces created by Jupiter. Jupiter’s gravity pulls the surface of Io so much that the surface flexes, or bends back and forth. This movement generates enough heat to melt the interior and produce Io’s hot spot volcanism. Io’s thin atmosphere is mostly sulfur dioxide gas produced by the volcanoes. At night, some of this gas freezes, and produces the white areas seen on the surface. Io has little or no water (see Fig. 9.8).

  Fig. 9.8A huge 100 km-high volcanic plume (eruption) is seen rising above the surface of Jupiter’s moon Io. Lower down in the image, close to the shadow line, is a second, ring-shaped plume—about 75 km high (Credit: NASA).

  Europa is the second of the Galilean moons and the fourth largest moon of Jupiter. It is slightly smaller than Earth’s moon and orbits Jupiter once every 3.5 days. Europa is locked by gravity to its planet so that the same side always faces toward Jupiter. Io and Europa have a similar composition, consisting of mainly silicate rock. Unlike Io, Europa has a thin outer layer of ice and a layered internal structure, probably with a metallic core. Its surface is relatively smooth with no mountains and very few craters. Some astronomers think topographical features and a layer of liquid water may exist below the ice-covered surface. If Europa’s ocean is proven to exist, it would possess more than twice as much water as Earth. Fractures in the surface of this moon may be due to tidal forces caused by Jupiter.

  In 1995, astronomers discovered the atmosphere of Europa is very thin and contains molecular oxygen. This oxygen is thought to be generated by sunlight and charged particles hitting Europa’s icy surface producing water vapour that is split into hydrogen and oxygen. The hydrogen escapes, leaving behind the oxygen (see Fig. 9.9).

  Fig. 9.9Europa’s icy surface is covered by numerous streaks and cracks thought to be caused by tidal forces from Jupiter (Credit: NASA).

  In 2014 researchers announced they have clear visual evidence of Europa’s icy crust expanding.

  The surface of Europa is riddled with cracks and ridges. Surface blocks are known to have shifted in the same way blocks of Earth’s outer ground layer on either side of the San Andreas fault move past each in California. Many parts of Europa’s surface show evidence of extension, where miles-wide bands formed as the surface ripped apart and fresh icy material from the underlying shell moved into the newly created gap—a process similar to seafloor spreading on Earth.

  Ganymede is the third of the Galilean moons and is the largest moon in the Solar System. Its diameter is larger than the planet Mercury, although it’s mass and density is much less. Ganymede orbits Jupiter in synchronous rotation once every 7.16 days at a distance of about 1 million km.

  Ganymede has both dark and light areas on its surface. The dark areas are old and heavily cratered. The lighter regions are young, have few impact craters but do contain many grooves and ridges. The largest feature on Ganymede is Galileo Regio, a dark circular area of ancient crust 4000 km in diameter, which contains an abundance of craters.

  The crust is thought to be about 75 km thick and contain an outer layer of ice. Beneath lies a mantle of either water or ice, and a rocky or silicate-rich core. The Hubble Space Telescope has recently found evidence of oxygen in Ganymede’s atmosphere, very similar to Europa’s atmosphere.

  The Galileo space probe found that Ganymede has its own magnetic field embedded inside Jupiter’s huge field. This is probably generated in a similar fashion to the Earth’s, as a result of moving, conducting material in the interior. The magnetic field of Ganymede causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking back and forth”. By watching the rocking motion of the aurorae, scientists working with the Hubble Space Telescope announced in 2015 that they believe a large amount of saltwater exists beneath Ganymedes’s crust. The subterranean ocean is thought to have more water than all the water on Earth’s surface. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.

  Callisto is the fourth of the Galilean moons and the second largest moon orbiting Jupiter. It is only slightly smaller than the planet Mercury but has only one-third its mass. Callisto orbits Jupiter once every 16.69 days in synchronous rotation at a distance of about 2 million km.

  Callisto’s surface is a dark, ancient and icy crust, covered with many old impact craters. In fact it is the most heavily cratered object in the solar system. The craters and impact basins are relatively flat because of the nature of the surface. The largest impact basin, Valhalla, is about 3000 km in diameter and is surrounded by bright concentric rings of fractured ice. Valhalla may have formed as a result of a huge asteroid impact. Numerous smaller craters cover this feature, which suggests an age of about 4 billion years.

  Unlike Ganymede, Callisto has no grooved terrain, suggesting little if any tectonic activity occurred. It probably cooled very rapidly. Voyager instruments measured a temperature range of −118 °C during daytime and −193 °C at night.

  Data from the Galileo probe suggests Callisto has little internal structure, and is composed of about 40 % ice and 60 % rocky iron. Its atmosphere is very tenuous and contains mainly carbon dioxide. There is no evidence of a magnetic field.

  Other Moons of Jupiter

  In addition to the four Galilean moons, Jupiter has at least 59 other moons. The non-Galilean moons tend to be irregular in shape and smaller than 185 km in diameter. Four of these moons are closer to Jupiter than Io, the rest orbit in regions beyond Callisto. The outer bodies are probably captured asteroids, while the inner ones are probably pieces broken off a larger body.

  The four inner moons are Metis, Adrastea, Amalthea and Thebe. The largest of these is Amalthea, which was discovered in 1892 by the American astronomer Edward Barnard, and is only 167 km across. Amalthea was originally thought to be innermost moon, but the Voyager probes found Metis and Adrastea to be closer to Jupiter. Amalthea is a dark, heavily cratered irregularly shaped body with a reddish colour. The moons Metis, Adrastea and Thebe were discovered by the Voyager 1 probe in 1979 (Table 9.4).Table 9.4Details of the inner moons of Jupiter

  Name

  Distance from Jupiter (km)

  Period (days)

  Diameter (km)

  Discovered (year)

  Metis

  127,690

  0.29

  43

  1979

  Adrastea

  128,690

  0.30

  16

  1979

  Amalthea

  181,400

  0.50

  167

  1892

  Thebe

  221,900

  0.68

  98

  1979

  The four inner moons and the Galilean moons, all orbit Jupiter in an equatorial plane in near-circular orbits. The outer moon’s have more elliptical orbits and are more inclined to Jupiter’s orbital plane. More surprisingly, all the moons after Carpo orbit in the opposite direction to that of the other satellites, and opposite to the direction of Jupiter’s spin. These characteristics together with their small size suggest these outer satellites are captured asteroids, not originally part of the Jupiter system.

  As of 2007, Jupiter has four small inner moons, four large Galilean moons, and 55 other tiny satellites of sizes ranging from 1 km to 170 km. The tiny ‘asteroid-like’ satellites orbit between 7,284,000 km and 30,290,000 km distance from the planet and may be better called ‘moonlets’.

  Further Information

  For fact sheets on any of the planets including Jupiter check out

  http://​nssdc.​gsfc.​nasa.​gov/​planetary/​planetfact.​html

  www.​space.​com/​jupiter/​

  www.​nasm.​si.​edu/​etp/​

  www.​nasa.​gov/​juno

  © Springer International Publishing Switzerland 2016

  John Wilkinso
nThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_10

  10. Saturn: The Ringed Planet

  John Wilkinson1

  (1)Castlemaine, Victoria, Australia

  Highlights

  Space probes have revealed that Saturn’s rings are actually composed of hundreds of narrow, closely spaced ‘ringlets’. Some moons actually orbit inside the rings.

  The moons of Saturn are numerous and diverse ranging from tiny moonlets less than 1 km across to the enormous Titan which is larger than the planet Mercury.

  Radar images taken by the Cassini spacecraft of Saturn’s largest moon, Titan, have shown large lakes of hydrocarbons.

  In June 2005, the Cassini spacecraft detected auroral emissions associated with the solar wind around Saturn’s poles.

  The Cassini spacecraft has found evidence of water spewing from geysers on the moon Enceladus.

  Titan, Saturn’s largest moon, is the only moon in the solar system to have clouds and a dense atmosphere.

  In 2014 astronomers reported that Titan has a subsurface ocean made of water mixed with ammonia, and that the ocean may be as salty as the Dead Sea on Earth.

  Saturn is the second of the gas giants and the sixth planet from the Sun. This planet is the second largest in the Solar System with a diameter of 120,536 km. It travels around the Sun once every 29.46 years at an average distance of 1430 million km. At its closest approach to the earth, Saturn is about 1,278,000 km away.

  Saturn travels around the Sun in an elliptical orbit. Its distance from the Sun varies from about 1509 million km at its farthest point to about 1350 million km at its closest point.

  Saturn is about 85 % the size of Jupiter but twice as far from Earth. It is over 95 times as massive as the Earth and, with the exception of Jupiter, has more mass than all the other planets combined. However, Saturn has the lowest density of all the planets, only 0.69 g/cm3, which is less than the density of water and roughly half the density of Jupiter. This low density means Saturn must be composed of light elements.

  Saturn formed from the same swirling mass of gas and dust as the Sun and other planets. Like Jupiter, and unlike the inner planets, Saturn was far enough away from the Sun to retain its envelope of lighter gases, mainly hydrogen and helium. As it orbits the Sun, Saturn spins on its axis, at a rapid rate. Its axis is titled at an angle of about 27° from the perpendicular.

  Astronomers have studied Saturn for many years, as it is easily visible in the night sky of Earth. Ancient observers knew it, and its movement across the night sky has been accurately plotted against the background of stars for centuries. To the unaided eye, Saturn appears as a brilliant yellow-orange star like object in the night sky. Saturn’s main feature is its spectacular ring system, which can be seen through a telescope from Earth. The only other planets to have rings are Jupiter, Neptune and Uranus, but their rings aren’t as prominent as Saturn’s rings and they can’t be easily seen through a telescope from Earth (Table 10.1).Table 10.1Details of Saturn

  Distance from Sun

  1,430,000,000 km (9.54 AU)

  Diameter

  120,536 km

  Mass

  5.68 × 1026 kg (95.18 times Earth’s mass)

  Density

  0.69 g/cm3 or 690 kg/m3

  Orbital eccentricity

  0.056

  Period of revolution

  10,768 Earth days or 29.46 Earth years

  Rotation period

  10 h 40 min

  Length of year

  29.5 Earth years

  Orbital velocity

  34,704 km/h

  Tilt of axis

  26.73°

  Average temperature

  −185 °C

  Number of moons

  62

  Atmosphere

  Hydrogen, helium

  Strength of gravity

  10.4 N/kg at surface

  Early Views About Saturn

  Saturn has been observed in the night sky since prehistoric times. Mesopotamian astronomers called Saturn the ‘the old sheep’ or ‘the eldest old sheep’, while the Assyrians described the planet as a sparkle in the night sky and named it ‘Star of Ninib’. To the ancient Romans, Saturn was the god of agriculture, while the Greeks called it Cronus, after Zeus’s father, the overthrown ruler of the universe. Cronus was the also son of Uranus and Gaia. Saturn is the root of the English word ‘Saturday’.

  The modern era for Saturn began in 1610 when Galileo first observed it through his telescope, and described it as a triple-bodied object. Other observers thought Saturn had ‘handles’ or ‘ears’. In 1659, Christiaan Huygens reported that Saturn was circled by a broad, flat ring and had a moon; this was to be called Titan. In 1676, Italian astronomer, Giovanni Cassini, discovered a gap in Saturn’s ring system. With modern telescopes, Earth based astronomers have found Saturn has two prominent rings (A and B) and two faint inner rings (D and C). The gap between the A and B rings is now known as the Cassini division. A much fainter gap dividing the A ring and F ring is known as the Encke division after German astronomer Johann Franz Encke who allegedly saw it in 1838. Pictures taken by the Voyager probes show two additional faint outer rings G and E.

  Fig. 10.1Image of Saturn and its rings. Credit: NASA.

  Probing Saturn

  People on Earth have observed Saturn through telescopes based on Earth and in space since 1610 when Galileo first observed it, and more recently through space based telescopes.

  The first space probe to visit Saturn was Pioneer 11, on 1 September 1979. The probe passed within 21,000 km of the planet and within 3500 km of its outer ring. It travelled under the ring system and sent back many useful pictures of the rings. However, the images showed little new information about Saturn’s clouds and atmosphere.

  In November 1980, Voyager 1 passed within 124,123 km of Saturn before moving out of the solar system. Voyager 2 also encountered Saturn on August 26, 1981, getting to within 101,335 km of the planet before proceeding on to Uranus and Neptune. The Voyager probes provided many pictures and data about Saturn. They found three new moons around Saturn, four additional faint rings and provided pictures of atmospheric circulation. Before the Voyager probes, information about Saturn’s atmosphere was limited because astronomers could see only the tops of the clouds from Earth. The Voyager probes identified long-lived oval shaped structures inside the clouds and revealed three layers of clouds with slightly different compositions.

  In 1994, the Hubble Space Telescope, while orbiting Earth at 28,000 km/h, captured the first images of aurora in Saturn’s atmosphere. It also captured images of topographical features on Saturn’s largest moon, Titan, which suggest a continent once existed on this moon.

  The Cassini mission to Saturn (named after Giovanni Cassini) was one of the most ambitious ever attempted. It was joint venture of NASA, the European Space Agency (ESA) and the Italian Space Agency (known as ASI for its acronym in Italian), and was designed to explore the whole Saturnian system, the planet itself, its atmosphere, rings and magnetosphere and some of its moons. Launched in 1997, the Cassini space probe reached Saturn in 2004, and went into orbit around the planet. Cassini plunged between Saturn’s two outer rings at 80,000 km/h before it slowed down enough to be captured by Saturn’s gravity and begin its 4-year orbit of the planet. Instruments on board Cassini detected an eruption of atomic oxygen in Saturn’s E-ring.

  Cassini has also taken pictures of Saturn’s largest moon, Titan. In July 2004, Titan was found to be surrounded by a thick atmosphere, with areas of water ice on its surface. Cassini also released a smaller probe called Huygens on 24 December 2004. Twenty days later, the probe entered Titan’s atmosphere at about 6 km/s, and landed via parachute on Titan’s surface, on 14 January 2005. The probe landed in mud like wet clay covered by a thin crust. The first images showed a pale orange, rock-strewn, eroded landscape with drain channels. The ground temperature was a chilling −180 °C. Huygen’s was the first successful attempt by humans to land a
probe on another world in the outer Solar System.

  By the end of 2007, Cassini had flown by Titan 40 times and mapped over 60 % of Titan’s ‘Lake District’ north of latitude 60°. Dark areas on the surface are believed to be filled with a mixture of liquid ethane, methane and dissolved nitrogen. Some of the lakes appear to be fed by rivers that flow down from the surrounding hill country to shorelines of bays, peninsulas and islands. The rivers have many tributaries among the uplands. A few lakebeds appear dry (Fig. 10.4).

  Fig. 10.2Structure of Saturn’s rings.

  Fig. 10.3Image of Saturn and its largest moon Titan as seen by the Cassinispacecraft. The shadow of Saturn’s rings can be seen on the surface (Credit: NASA, ESA).

  From 2004 to November 2, 2009, the Cassini probe discovered and confirmed eight new moons of Saturn. Its primary mission ended in 2008 when the spacecraft had completed 74 orbits around the planet. The probe’s mission was extended to September 2010 and then extended again to 2017, to study a full period of Saturn’s seasons. In April 2013 Cassini sent back images of a hurricane at the planet’s north pole 20 times larger than those found on Earth, with winds faster than 530 km/h (Table 10.2).Table 10.2Significant space probes to Saturn