The Solar System in Close-Up

by John Wilkinson

  Other Moons of Saturn

  Excluding Titan, the six largest moons range in diameter from 390 km to 1530 km. In order of increasing distance from Saturn, these moons are Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus. These moons have average densities around 1.2 g/cm3, which suggests that they are made mostly of water ice with some rock. Having formed in a cold environment, these bodies retained water, methane, ammonia and nitrogen that condensed from the solar nebula. Some astronomers believe these mid-sized moons condensed from gas rings that surrounded Saturn about 4500 million ago.

  Mimas has a diameter of 392 km and is 185,520 km from Saturn. It takes only 23 h to orbit Saturn and is difficult to observe from Earth because it is so close to Saturn. Mimas has the distinction of resembling the Death Star from Star Wars Episode IV. The Voyager probe showed the surface of Mimas is dominated by a large impact crater, called Herschel, which is 130 km across (one third the diameter of Mimas). The walls of this crater are about 5 km high and parts of its floor measure 10 km deep. It has a central peak that rises 6 km above the crater floor. Fractures can be seen on the opposite side of Mimas that may be due to the large impact. Mimas has one of the heaviest cratered surfaces in the solar system, but they are all much smaller than Herschel (see Fig. 10.8).

  Fig. 10.8Mimas, one of Saturn’s moons about 400 km in diameter. The large crater at the top is Herschel crater, with walls about 10 km high (Credit: NASA).

  Enceladus has a diameter of 500 km and is 238,020 km from Saturn. The surface of this moon is covered with a smooth layer of water ice that makes it the most reflective of any known planetary body. There are many craters in one hemisphere and very few in the other hemisphere. The young surface of this moon was seen by Voyager 2 to contain a number of different types of formations, including ice flows, faults and striations. The crust is probably thin and lying on top of a molten interior (see Fig. 10.9).

  Fig. 10.9Surface of the moon Enceladus taken by Cassini. The surface is mostly cratered and crossed by numerous fault lines. The Cassini spacecraft has found evidence of icy water particles spewing from geysers on Enceladus (Credit: NASA).

  During 2006, the Cassini spacecraft had found evidence of icy water particles spewing from geysers on Enceladus. As a result some scientists have now placed this moon on the short list of places most likely to have extraterrestrial life. High-resolution images snapped by the orbiting Cassini probe confirmed the eruption of icy jets and giant water plumes from geysers at Enceladus’ south pole. If any life exists on the moon, it probably would be in the form of microbes or other primitive organisms. Icy particles ejected from Enceledus are thought to have created Saturn’s outermost E ring.

  Tethys has a diameter of 1060 km and is 294,660 km from Saturn. The Italian astronomer Giovanni Cassini discovered it in 1684. Like Mimas, the surface of this moon is heavily cratered in places, but there are also some smooth areas. The largest impact crater is Odysseus with a diameter of 400 km (making it larger than the moon Mimas). This crater is very old with a flat floor that corresponds to the curvature of the moon. A valley called Ithaca Chasma stretches three quarters the way around the moon and in places is up to 100 km wide. The Chasma may have been formed by the Odysseus impact or by tectonic activity.

  Dione is 377,400 km from Saturn with a diameter of 1120 km. It too is heavily cratered with a number of large impact craters scattered over its surface. There are also some smooth areas that may be due to coverings of water ice. Recent images show the surface contains features such as faults, valleys, and depressions caused by tectonic movement. The largest craters are about 100 km across, and bright streaks that are seen radiating from some craters are the result of material being ejected after impact.

  Rhea is Saturn’s second largest moon, being 1530 km in diameter. This moon is 527,040 km from Saturn. The surface is very old and shows little change by geological activity. Like Dione, parts of its surface are heavily cratered and parts are smooth. There are also some white streaks across the surface that may be ice-filled cracks. Rhea also has two very large impact basins on its anti-Saturnian hemisphere, which are about 400 and 500 km across. Rhea’s density of 1.23 times that of liquid water suggests that Rhea is three quarters ice and one-quarter rock (with some ammonia). In November 2010, NASA announced the discovery of a tenuous atmosphere on Rhea. It consists of oxygen and carbon dioxide in proportion of roughly 5–2. The main source of oxygen is thought to be radiolysis of water ice at the surface by ions supplied by the magnetosphere of Saturn. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon’s interior.

  Iapetus is the third largest moon orbiting Saturn but it is one of the most unusual. It has a diameter of 1440 km and orbits Saturn at a distance of 3,561,300 km from Saturn. One side of this moon is cratered and bright (probably due to water ice), while the other side is coated with a dark (probably carbon-based) material whose origin is not known. The dividing line between the two regions is relatively sharp. The Cassini probe revealed Iapetus to be heavily cratered with a bizarre patchwork of pitch-dark and snowy-white regions. The pole regions of Iapetus are as bright as its trailing hemisphere. Cassini discovered a 20 km tall equatorial ridge, which spans nearly the moon’s entire equator (see Fig. 10.10).

  Fig. 10.10Close up view of the cratered surface of the moon Iapetus as seen by Cassini. The large impact crater near the bottom is Engelier (Credit: NASA/ESA).

  Most of the remaining moons of Saturn range in diameter from 3 km to 255 km. Many of these moons are irregular in shape and have unusual orbits, suggesting they are fragments of larger bodies or captured asteroids.

  Hyperion is the largest of Saturn’s minor moons; it has an extremely irregular shape, and a very odd, tan-colored icy surface resembling a sponge. The surface of Hyperion is covered with numerous impact craters, most with diameters only 2–10 km. It is the only moon known to have a chaotic rotation, which means Hyperion has no well-defined poles or equator.

  The moon Phoebe orbits in a direction opposite to the orbits of the other moons and opposite to the direction of Saturn’s rotation. Phoebe is roughly spherical and has a diameter of 213 km. Phoebe rotates on its axis every 9 h and completes a full orbit around Saturn in about 18 months. Its surface temperature is −198 °C. Phoebe also has a highly inclined orbit and its surface is covered with a dark material. Spectroscopic measurement showed that the surface is made of water ice, carbon dioxide, phyllosilicates, organics and possibly iron bearing minerals. Phoebe is believed to be a captured object that originated from the Kuiper belt. It also serves as a source of material for the largest known ring of Saturn (see Fig. 10.12).

  Fig. 10.11The Cassini space probe took this photo of Epimetheus in December 2007. The moon is 116 km in diameter and irregular in shape. Heavy cratering on its surface indicates it may be several billion years old (Credit: NASA).

  Fig. 10.12The moon Phoebe has a diameter of 220 km but orbits in a direction opposite to that of the other moons (Credit: NASA).

  Pan is the innermost of Saturn’s known moons at a distance of 134,000 km. With a diameter of only 20 km, it was discovered on Voyager photographs in 1990. Pan orbits within the Encke Division (in Saturn’s A ring). Small moons near the rings produce wave patterns in the rings.

  Janus and Epimetheus are two irregularly shaped moons that are co-orbital (orbit together). These two bodies are separated by less than 100 km and their velocities are nearly equal. Their gravitational interaction causes them to exchange orbits every 4 years. Astronomers believe they are probably fragments of a single-body, now destroyed. Epimetheus and Janus are the fifth and sixth moons in distance from Saturn. Both are phase locked with their parent so that one side always faces toward Saturn. Being so close, they orbit in less than 17 h.

  A faint dust ring is present around the region occupied by the orbits of Epimetheus and Janus, as revealed by images taken by the Cassini spacecraft in 2006. The ring has a radial extent of a
bout 5000 km. Its source is thought to be particles blasted off their surfaces by meteoroid impacts, which have formed a diffuse ring around their orbital paths.

  All of Saturn’s regular moons except Iapetus orbit very nearly in the plane of Saturn’s equator. The outer irregular satellites follow moderately to highly eccentric orbits, and none is expected to rotate synchronously as all the inner moons of Saturn do (except for Hyperion). The exact number of Saturnian moons cannot be given, because there is no distinction between the countless small anonymous objects that form Saturn’s ring system and the larger objects that have been named as moons. Over 150 moonlets embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects.

  Chariklo

  Chariklo is an asteroid that orbits the Sun between the orbits of Saturn and Uranus at an average distance from the Sun of 15.87 AU. James Scotti of the Spacewatch program discovered the object on 15 February 1997. It takes 63.17 years to orbit the Sun and has a diameter of 250 km. A stellar occultation in 2013 revealed that Chariklo has two rings, one about 7 km wide and the other about 3 km wide, at 396 km and 405 km from Chariklo. This makes it the smallest known object to have rings. The existence of a ring system around an asteroid was unexpected because it had been thought that rings could only be stable around much more massive bodies. The origin of these rings remains a mystery, but they may be the result of a collision that created a disk of debris.

  Further Information

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

  www.​space.​com/​saturn/​

  https://​solarsystem.​nasa.​gov/​planets/​profile.​cfm (check on Saturn)

  © Springer International Publishing Switzerland 2016

  John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_11

  11. Uranus: The Coldest Planet

  John Wilkinson1

  (1)Castlemaine, Victoria, Australia

  Highlights

  Uranus is the coldest planet with a faint ring system and an extensive family of moons. The rings are composed of particles ranging in size from fine dust to several metres in diameter.

  Uranus has an odd magnetic field that is tilted at 59° to its axis of rotation. The magnetic field does not even pass through the centre of the planet.

  In August 2014 scientists using the Keck telescope in Hawaii photographed several huge bright storms on Uranus.

  Most of the moons of Uranus are quite dark possibly due to radiation darkening of methane on their surfaces.

  Uranus is the third of the gas giants and the seventh planet from the Sun. The planet is the third largest in the solar system with a diameter of 51,118 km. Uranus is one-third the diameter of Jupiter but four times the diameter of Earth. It is large enough to hold about 64 Earths. Its distance from the Sun varies from about 3010 million km at its farthest point to about 2739 million km at its closest point. Because of this large distance, it takes Uranus 84 years to orbit the Sun once. Since its discovery in March 1781 it has only gone around the Sun just over two and half times. Light from the Sun takes just over 8 min to reach Earth, but it takes about 2 h 40 min to reach Uranus.

  Uranus travels around the Sun in a slightly elliptical orbit and it spins on its axis once every 17 h 14 min. It is the furthest planet that can be seen with the unaided eye from Earth, however, it is faint and difficult to detect. Uranus was the first planet to be discovered by telescope.

  Uranus is pale blue in colour with few surface features. It is the coldest planet and has a faint ring system and an extensive family of moons. Little was known about the planet until the Voyager 2 probe flew by it in January 1986 (Fig. 11.1).

  Fig. 11.1The planet Uranus as seen by the Hubble Space telescope (Credit: NASA).

  It is possible to see Uranus without a telescope from Earth. At magnitude 5.3, Uranus is just within the brightness scale that a human eye can perceive. Of course, you’ve got to have extremely dark skies and know exactly where to look to see Uranus.

  Early Views About Uranus

  Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods). Uranus was not seen in the night sky by ancient observers because it is so far away from the Sun and therefore faint. The first recorded sighting of this object was made in 1690 by English astronomer, Joh Flamsteed, who incorrectly cataloged it as 34 Tauri. Another observer, Pierre Lemonnier, recorded Uranus as a star a total of 12 times during 1769.

  William Herschel officially discovered Uranus in 1781 while observing the night sky through one of his telescopes. Herschel noted that the object moved against the background of stars over several nights and had a bluish-green disc, unlike the stars, which were point sources of light. Herschel concluded that the faint object was a planet and he called it “the Georgium Sidus” (the George’s star) in honour of his patron, King George III of England; other people called the planet ‘Herschel’. At that time Saturn was the furthest know planet in the solar system. Uranus was the first planet to be discovered with a telescope and its discovery effectively doubled the size of the known Solar System. Herschel also discovered the two larger moons of Uranus, Oberon and Titania.

  Johann Bode, a German astronomer, named the planet Uranus after the Greco-Roman god who personified the universe and was the father of Saturn. The planet was officially named Uranus in 1850 (Table 11.1).Table 11.1Details of Uranus

  Distance from Sun

  2,870,990,000 km (19.2 AU)

  Diameter

  51,118 km

  Mass

  8.68 × 1025 kg (14.53 Earth’s mass)

  Density

  1.29 g/cm3 or 1290 kg/m3

  Orbital eccentricity

  0.046

  Period of revolution

  30,685 Earth days or 84.01 Earth years

  Rotation period

  17 h 14 min

  Orbital velocity

  24,516 km/h

  Tilt of axis

  97.86°

  Average temperature

  −200 °C

  Number of Moons

  At least 27

  Atmosphere

  Hydrogen, helium, methane

  Strength of gravity

  8.2 N/kg at surface

  Probing Uranus

  We know little about Uranus because it is so far from Earth. Most of what we do know came from the Voyager 2 probe, which flew by Uranus in early 1986. The probe passed within 82,000 km of the planet’s cloud tops. It took Voyager 2 five years to travel from Saturn to Uranus.

  Photographs taken by Voyager 2 revealed Uranus was a blue coloured planet with a few faint bands of clouds moving parallel to its equator. There were no signs of belts or storm spots. Ten additional moons were discovered around the planet and most have at least one shattering impact crater. Voyager 2 also found that the planet’s magnetic field was 50 times stronger than Earth’s field. The magnetic field is tilted at 59° to its axis of rotation, and does not even pass through the centre of the planet. Because of this strange arrangement, the magnetic field wobbles considerably as the planet rotates (Table 11.2).Table 11.2Significant space probes to Uranus

  Probe

  Country of origin

  Launched

  Notes

  Voyager 2

  USA

  1977

  Fly by in Jan 1986

  The possibility of sending the Cassini spacecraft to Uranus was evaluated during a mission extension planning phase in 2009. However, it would have taken about 20 years to get to the Uranian system after departing Saturn. A Uranus orbiter and probe was recommended by the 2013–2022 Planetary Science Decadal Survey published in 2011; the proposal envisages launch during 2020–2023 with a 13-year trip to Uranus. The ESA also looked at a “medium-class” mission
called Uranus Pathfinder.

  Position and Orbit

  Uranus is the third largest planetary member of the solar system and is the seventh planet from the Sun. Its orbit is slightly elliptical and lies between Saturn and Neptune. Uranus has a mean distance from the Sun of about 2871 million km, placing it about 19.2 times farther from the Sun than Earth. It travels around the Sun once every 84.01 years and rotates on its axis with a period of 17 h 14 min. The planet’s atmosphere rotates faster than its interior. The fastest winds on Uranus, measured about two-thirds of the way from the equator to the south pole, blow at about 720 km/h.

  Most planets spin on their axis nearly perpendicular to its orbital plane, but Uranus’ axis is almost parallel to this plane (nearly 98° to the vertical). This means the rotational axis is almost 8° below the orbital plane, so the planet appears to be tipped on its side. It is not known why Uranus has such a high axial tilt, but it may have been hit by another large body sometime in its past. At the time of Voyager 2’s flyby, the planet’s south pole was pointed almost directly at the Sun. As a result of this, Uranus’ polar regions receive more energy from the Sun than its equatorial regions. However, the temperature is still hotter at the equator than at its poles, for unknown reasons.