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Home , Lineae, Orbital resonance, Regular moon

Space Physics Project

The of the

Ria Becker

October 27, 2008 Contents 1

Contents

1 Introduction 2

2 Definitions 2

3 The ’s 3

4 The Moons of 3 4.1 ...... 3 4.2 ...... 4

5 The Moons of 4 5.1 ...... 5 5.2 ...... 5 5.3 ...... 6 5.4 ...... 6

6 The Moons of 7 6.1 ...... 7 6.2 ...... 8 6.3 ...... 8 6.4 ...... 9 6.5 and ...... 9

7 The Moons of 9

8 The Moons of Neptun 10 8.1 ...... 10 1 Introduction 2

1 Introduction

The aim of this project is to give an overview over the different moons of the planets in our . The longest and best known is naturally the Earth’s moon. In 1610 discovered Galileo Galilei four : Io, Europa, Ganymede and Callisto. Those where the first celestial objects confirmed to an body other than the Earth. Until today there are more than 165 confirmed moons in the solar system discovered and the number of moons belonging to a differs greatly. The Earth has one large one, Mars has two small ones, Jupiter and Saturn have about 60, Uranus has 27 and Neptun 13. In contrary to that Mer- cury and possess no natural satellites. Due to this large number of moons in the solar system, this report covers only the most interesting ones.

Figure 1: Selected moons of the solar system [1].

2 Definitions

A moon or is a body, which a planet or smaller body (e.g. ), called primary. Satellites can be classified in regular and irregular ones. Former are those which orbit rela- tively circular and close to the planet. They are tidally locked, that means they are always facing the same side towards its primary. Mostly it is believed that regular satellites are often formed at the same time and place like the planet. Irregular satellites often orbit inclined and are too far away from the planet to be tidally locked. It is presumed that they have been captured by their primary. 3 The Earth’s Moon 3

3 The Earth’s Moon

The Earth’s Moon is the fifth largest satel- lite in the solar system. It has a diame- ter of 3, 500 km, which is approximately one fourth of the Earth’s diameter. The distance between Moon and Earth is approximately sixty times the radius of the Earth, which is 384, 400 km. Due to tidal effects of the Moon on the Earth and conservation of an- gular is this distance increasing each by 3.8 cm. This slows also the ro- tation of the Earth down by 0.002 seconds per day per century.With a mass density of 3, 341 g/cm3 the Moon is about 81 times lighter than the Earth. The Moon orbits the Earth in 27.3 days and since it is a regu- Figure 2: The moon as seen by an observer on lar satellite, it faces always the same side to the Earth [1]. the Earth. The atmosphere is negligible and the temperature of the surface differs between lunar day (about 107◦C) and night (about −153◦C). It is believed that the formation of the Moon was about 4.5 billion ago. There are different speculations about the process whereas today the giant impact hypothesis is pre- vailed. Therefore a Mars-sized body collided with the proto-Earth, whereby a huge amount of material was thrown into the orbit and accreted to the Moon. The surface of the Moon is one of the poorest reflector in the solar system, it reflects only about 7% of incident light. A dry ashen layer (called Regolith) covers the surface, on which all-around impact craters can be found. This shows the absence of strong erosion and any geological activity. The Moon is the only celestial body which humans have traveled to or even landet upon. There where about six Moon landings between 1969 and 1972.

4 The

Mars has two tiny satellites, Phobos and Deimos, which are presumably captured and were discovered in 1877. Until now there is no satisfactory theory about the reason why those two bodies came into the orbit of Mars. Both, Phobos and Deimos, are tidally locked to its primary. They have a lower albedo than the Earth’s Moon and their surface is therefore very low-reflective.

4.1 Phobos Phobos (Fig. 3a) is irregularly shaped, its size is 27 × 21.6 × 18.8 km. The distance to the Mars is only 9377 km and therefore has Phobos the smallest orbit of all known planetary moons. The is only 7.66 hours. Since the rotational period of Mars is 24.6 hours, Phobos rises twice each day in the west and sets in the east. The fast orbital velocity leads also to a declining of the orbit from which follows that in a few ten millions of years Phobos will crash into Mars. The density of Phobos is about 1.9 g/cm3. It is too low to 5 The Moons of Jupiter 4

(a) Phobos, obtained by Mars Reconnaissance Or- (b) Deimos, image taken by the Viking 1 orbiter. biter.

Figure 3: Moons of Mars [1]. be solid rock and therefore there is a suggestion that Phobos might contain a substantial reservoir of ice. Yet this presumption has not been ruled out by spectral observations.

4.2 Deimos Deimos (Fig. 3b) is smaller than Phobos, with a size of only 10 × 12 × 16 km. As it can be seen in Figure 3b, it is also highly non-spherical. The orbital period of Deimos is 30.35 hours which means it orbits slower than Mars rotates. This leads to an increasing orbit and the fact, that Deimos rises in the east and sets in west in contrary to Phobos.

5 The Moons of Jupiter

Jupiter, which is the largest planet of the solar system with a radius of 71398 km has until now 62 confirmed moons. This is the highest number of satellites belonging to one planet. Not all of them have yet been named. There are eight regular moons and the rest of them is tiny and irregular, that means their orbits have high inclinations and eccentricities. Those are mostly captured from solar orbits. The most famous are the four regular Galilean satellites: Io, Europa, Ganymede and Callisto, listed in the order of increasing distance from Jupiter. Europa, the smallest of them, is about 5000 times more massive than all of the non- of Jupiter combined. The physical and orbital characteristics of the satellites differ greatly, from nearly circular to highly eccentrical orbits as well as diameters of barely 1 km to 5262 km, which is the diameter of Ganymede, the largest satellite of the solar system. The orbital periods vary likewise: from a period of seven hours to almost three earth years. Due to the large number of moons in the following only the four Galilean satellites are presented. 5 The Moons of Jupiter 5

5.1 Io Io (Fig. 4a) is the innermost of the Galilean satellites and the fifth moon out from Jupiter. It has a diameter of 3642 km and is therefore the fourth largest moon in the solar system and slightly greater than the Moon of the Earth. The orbit of Io has a distance of 421700 km from the center of Jupiter and a period of 42.5 hours. It is tidally locked to its primary and thus is Io a regular satellite. Io is the most geological active object in the solar system. It has more than 400 active volcanos on its surface. There can be found almost no impact craters due to large surface changes caused by volcanic plumes an lava flows. Those colored the surface in different shades of red, yellow, white, black and green. This geological activity results from the . Since Io is in orbital with Europa and Ganymede which maintains Io in its orbit, the tidal dissipation leads to a significant heating in the interior. On Io more than 100 mountains can be found whereas some are taller than the Mount Everest on the Earth are. Large parts of the surface are coated with sulfur and sulfur dioxide frost. The interior of Io consists mainly of silicate rock an iron. This causes a density of 3.53 g/cm3, which is higher than that of any other satellite in the solar system. It is also significantly higher than the density of the other Galilean moons. In contrary to them Io has almost no water. There is an extremely thin atmosphere, which consists mainly of sulfur dioxide.

5.2 Europa Europa (Fig. 4b) is the sixth moon out from Jupiter. With a diameter of 3122 km slightly smaller than Earth’s Moon and the sixth largest in the solar system. It is a regular satellite and has an orbital period of 3.55 days. The orbit is slightly eccentric due to gravitational disturbances from the other Galilean moons. Since Europa is in with Io and Ganymede the tidal flexing produces also an internal heating. Europa has a tenuous atmosphere consisting of oxygen. The interior of Europa is mainly silicate rock and there is an iron core. The outer layer of about 100 km consists of water

(a) Io. (b) Europa.

Figure 4: Two inner Galilean Satellites, images taken by NASA’s Galileo spacecraft [1]. 5 The Moons of Jupiter 6 and the surface is composed of an ice crust. The temperature varies from −160◦ C at the equator to −220◦ C at the poles. The surface of Europa is one of the smoothest of bodies in the solar system. Dark streaks as well as infrequent craters can be found on it. The streaks are called lineae, they are supposable cracks in the ice crust whereas the crust on each side of the streaks has moved relative to each other. Due to its icy crust, Europa has one of the highest albedo of all moons, almost 64% of incident light is reflected.

5.3 Ganymede Ganymede (Fig. 5a) is the greatest satellite in the solar system. With a diameter of 5262 km is it larger than the planet . Ganymede is the seventh moon out from Jupiter. It is tidally locked to its primary and has an orbital period of 7.2 days. The interior is composed of silicate rock, a layer of water and and ice crust as surface. The density is 1.94 g/cm3. The regions of the surface can be distinguished into two different types: the dark ones, which have a highly rate of impact craters and can be dated back to four billion years ago. The other lighter regions are younger and crosscut by grooves and ridges, which are probably caused by tectonic activity due to tidal heating. This heating results also from the orbital resonance of Ganymede with Io and Europa. The mean surface temperature is −160◦ C. Ganymede is the only satellite with a magnetosphere. This is most likely caused by convection within the liquid iron core.

5.4 Callisto Callisto (Fig. 5b) is with a diameter of 4821 km the second largest satellite of Jupiter and the third largest of the solar system. It is also the outermost of the Galilean moons. The orbital radius is about 1880000 km, which is 26.3 times the radius of Jupiter. Since this is significantly larger than the orbit of Ganymede, Callisto is not in orbital resonance with the other Galilean satellites and therefore there is no tidal heating. But still, Callisto is tidally

(a) Ganymede. (b) Callisto.

Figure 5: Two outer Galilean Satellites, images taken by NASA’s Galileo spacecraft [1]. 6 The 7 locked to Jupiter. The orbit is slightly eccentric and its period is 16.3 days. Callisto is composed of rock and ice and its density is 1.83 g/cm3. There are no subsurface processes like earthquakes, volcanos or plate tectonics. Hence the surface is extremely old and heavily cratered. Some craters show multiring structures, whereas two of them are enormous: one has a bright central region with a diameter of 600 km and rings with a radius of 1800 km, the other one has a diameter of 1600 km. The surface of Callisto has a low albedo of 0.2, but there are small highly reflecting patches (about 80% of incident light) of pure water ice. Callisto has like the other Galilean satellites an extremely thin atmosphere consisting of carbon dioxide and possibly molecular oxygen. The most favorable theory of the origin of Callisto is a slow in the Jovian subnebula after formation.

6 The Moons of Saturn

Since the consists of icy objects with dimensions spreading from one centimeter to hundreds of meters, each with an own orbit, there is now sharp boundary between objects of the and satellites. Hence there is no precis number of moons of Saturn. Until now there are 60 confirmed satellites, whereas eight of them have not been named yet. Saturn has 22 regular satellites, two orbit within gaps of the rings and the remaining are small and irregular. There are also four small trojans, these are moons which share an orbit with a larger one. Due to the large amount of satellites of Saturn in the following only a few most interesting moons are presented.

6.1 Titan Titan was discovered in the year 1655 by Christian Huygens and is therefore the first known satellite of Saturn. With a diameter of 5150 km it is the second largest moon in the solar system and the largest of Saturn. Titan is a regular satellites, is tidally locked to its primary and has an orbital period of 15.9 days. Titan is composed half of water ice and half of rocky material. This is ordered in different layers. With its diameter, mass and density (1.9 g/cm3) is Titan comparable to Jupiter’s moons Ganymede and Callisto. Titan is the only satellite with a dense atmo- sphere, which consists similar to the Earth’s Figure 6: The moon seen from the atmosphere to 98.8% of nitrogen. It has a Cassini–Huygens spacecraft. [1]. height of about 200 to 880 km and methane and ethane clouds can be found. The climate has seasonal weather patterns and produces a surface on Titan which is similar to the Earth. There are streaky features like large plains covered with sand dunes caused by windblown particles and liquid hydrocarbon lakes in the polar regions. Craters are filled due to raining hydrocarbons or volcanoes. The surface of Ti- tan is geologically young, relatively smooth, with several mountains and few impact craters. Titan is among the Earth the only known astronomical body in the solar system with liquid 6 The Moons of Saturn 8 patches on the surface. The haze in the atmosphere causes the surface to be colder than the upper atmosphere be- cause it is reflecting the sunlight. This is also called anti-greenhouse effect. But the methane clouds punctuate the haze so that the surface temperature is about −179◦ C. Since water does not sublimate or evaporate at this temperature there is almost no water vapor in the atmosphere of Titan. Titan is seen analogous to the early earth except at a much lower temperature. It is assumed that there is perhaps microbial extraterrestrial life or at least a prebiotic environment.

6.2 Enceladus Enceladus is the sixth largest, but also one of the smallest spherical satellite of Saturn. The diameter is 505 km. Enceladus is a with an orbital radius of 238000 km and a period of 32.9 hours. It is orbiting in the densest part of the E-ring, which is the outermost one of Saturn. There is the hypothesis that Enceladus is the source for the material in the E-ring due to cryovolcanic plume from the south polar region. Another factor is the meteoric bombardment, it causes the raising of dust particles from the surface. Enceladus is the smallest known body in the solar system with such a recent geologically activity. Another interesting fact about Enceladus is its albedo, almost 99% of incident light is reflected by the surface. Hence Enceladus has perhaps the most reflective surface of any body in the solar system.

6.3 Mimas Mimas was discovered in the year 1789 and has a diameter of 397 km. With that Mimas is the smallest known astronomical body in the solar system with an almost spherical shape caused by self gravitation. Mimas is tidally locked to Saturn and has an orbital period of 22.5 hours. While orbiting, Mimas is clearing material from the Cassini division, which is the gap between the A and B ring of Saturn. One remarkable feature of the surface of Mimas is a large impact crater with a diameter of 130 km, which is one third of the size of the satellite itself.

Figure 7: Enceladus as seen by Voyager 2, Figure 8: Mimas, image taken by Cassini- August 26, 1981 [1]. Huygens on August 2, 2005 [1]. 7 The 9

(a) Hyperion, September 26, 2005. (b) Epimetheus (lower right) and Janus, January 21, 2006.

Figure 9: Two moons of Saturn, images taken by Cassini [1].

6.4 Hyperion Hyperion (Fig. 9a) is one of Saturn’s irregular satellites. It is one of the largest highly irregular bodies in the solar system. The size of Hyperion is 360 × 280 × 225 and it has a very low density of 0.57 g/cm3. Therefore it consists mainly of water ice and only a small amount of rock. Peculiar is the surface of Hyperion. It is covered with deep, sharp-edged crates and gives Hyperion the appearance of a sponge. Almost 40% of the moons interior is empty space. Hyperion has not only an unusual appearance, it also is the only known moon in the solar system with such a . This rotation makes the orientation of the satellite in space unpredictable.

6.5 Epimetheus and Janus Epimetheus and Janus (Fig. 9b) is a unique arrangement in the solar system. Those two satellites of Saturn are co-orbital. Their orbital radii differ only about 50 km from each other. In fact, due to those close orbits, Epimetheus and Janus should approach each other and even- tually collide. But something else happens: When the two moons approach, the inner one speeds up, therefore its inner momentum boosts, the orbit increases. The outer satellite loses momentum likewise and drops into a lower orbit. Consequently the two moons have switched their orbits. Therefore their nearest approach to each other are about 10000 km. The trading of the orbits happens every four years and last time it was in January 2006.

7 The Moons of Uranus

Uranus has until today 27 named satellites. Five of them are massive enough to be in a hydrostatic equilibrium, 13 orbit within the ring system of Uranus. There are also nine outer irregular moons. In the region between the main rings are the small moons crowded and therefore perturbed by each other. This causes a chaotic and unstable system which leads 8 The Moons of Neptun 10 also to collisions between the satellites caused by crossing orbits.

8 The Moons of Neptun

Neptun has 13 confirmed satellites. Six of them are regular, whereas some of them orbit among the rings of Neptun. The remaining mons are irregular, their orbits have high incli- nations. Two of them have with a orbital period of about 25 years the largest orbits of any known moon in the solar system. In the following only the satellite Triton, which is by far the largest one of Neptun, is presented closer.

8.1 Triton Triton has a diameter of 2707 km, an orbital radius of 354760 km and a period of 5.9 days. It is in synchronized rotation with Neptun but it is the only large satellite in the solar system with a retrograde orbit. This means Triton orbits in the opposite direction than Neptun rotates. The shape of the orbit is almost perfectly circular, but still the radius is decaying so that in about 3.6 billion years it is believed that Triton will either collide with the atmosphere of Neptun or break up and create a ring system. The rotational axis of Triton is close to the plane of Neptun’s orbit so that during Neptun’s orbit around the sun once on each poles is about 40 years Figure 10: Triton, picture taken in 1989 by summer and winter on the other one. Voyager 2 [1]. The density of Triton is 2.1 g/cm3, the core is composed of substantial rock and metal, the mantel is icy and frozen nitrogen forms the crust. This is geologically active due to erupting nitrogen. Therefore the surface of Triton is relatively young. On the “winter-side” the temperature of the surface can cool down to at least −237◦ C. The surface has also a rather high albedo of 0.7. Triton has an extremely thin atmosphere consisting of nitrogen and small amounts of methane closer to the surface. About 8 km above the surface is a weather region caused by turbulences. The seasonal winds can move particles larger than one micrometer. Circa one to three kilometers above the surface there are also nitrogen clouds. Because of the retrograde orbit of Triton it is not believed that it is formed in the same region of the solar nebula as Neptun. Therefore Triton is presumably captured from the . It also composed identical like which leads to the opinion that they have the same origin. References 11

References

[1] Wikipedia, 2008-10-17, http://www.wikipedia.com

[2] T. Encrenaz, J.-P. Bibring, M. Blanc, The Solar System, Springer Verlag, Berlin, 1990

[3] Bill Arnett, The Eight Planets, 2008-10-17, http://www.nineplanets.org