Space Physics Project The Moons of the Planets Ria Becker October 27, 2008 Contents 1 Contents 1 Introduction 2 2 Definitions 2 3 The Earth's Moon 3 4 The Moons of Mars 3 4.1 Phobos . 3 4.2 Deimos . 4 5 The Moons of Jupiter 4 5.1 Io........................................... 5 5.2 Europa . 5 5.3 Ganymede . 6 5.4 Callisto . 6 6 The Moons of Saturn 7 6.1 Titan . 7 6.2 Enceladus . 8 6.3 Mimas . 8 6.4 Hyperion . 9 6.5 Epimetheus and Janus . 9 7 The Moons of Uranus 9 8 The Moons of Neptun 10 8.1 Triton . 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 solar system. The longest and best known is naturally the Earth's moon. In 1610 discovered Galileo Galilei four moons of Jupiter: Io, Europa, Ganymede and Callisto. Those where the first celestial objects confirmed to orbit 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 planet 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 Venus 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 natural satellite is a body, which orbits a planet or smaller body (e.g. dwarf planet), 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 momentum is this distance increasing each year 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 years 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 Moons of Mars Mars has two tiny satellites, Phobos and Deimos, which are presumably captured asteroids 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 orbital period 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-Galilean moons 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 tidal heating. Since Io is in orbital resonance 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 orbital resonance 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].