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 34.1 Phobos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.2 Deimos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5 The Moons of Jupiter 45.1 Io . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.2 Europa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.3 Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.4 Callisto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

6 The Moons of Saturn 76.1 Titan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76.2 Enceladus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86.3 Mimas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86.4 Hyperion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96.5 Epimetheus and Janus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7 The Moons of Uranus 9

8 The Moons of Neptun 108.1 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 oursolar system. The longest and best known is naturally the Earth’s moon. In 1610 discoveredGalileo Galilei four moons of Jupiter: Io, Europa, Ganymede and Callisto. Those where thefirst celestial objects confirmed to orbit an body other than the Earth. Until today there aremore than 165 confirmed moons in the solar system discovered and the number of moonsbelonging 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 solarsystem, 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. dwarfplanet), 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 alwaysfacing the same side towards its primary. Mostly it is believed that regular satellites are oftenformed at the same time and place like the planet. Irregular satellites often orbit inclinedand are too far away from the planet to be tidally locked. It is presumed that they have beencaptured by their primary.

3 The Earth’s Moon 3

3 The Earth’s Moon

Figure 2: The moon as seen by an observer onthe Earth [1].

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 onefourth of the Earth’s diameter. The distancebetween Moon and Earth is approximatelysixty times the radius of the Earth, whichis 384, 400 km. Due to tidal effects of theMoon on the Earth and conservation of an-gular momentum is this distance increasingeach year by 3.8 cm. This slows also the ro-tation of the Earth down by 0.002 secondsper day per century.With a mass density of3, 341 g/cm3 the Moon is about 81 timeslighter than the Earth. The Moon orbits theEarth in 27.3 days and since it is a regu-lar satellite, it faces always the same side tothe Earth. The atmosphere is negligible andthe 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 aredifferent 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 amountof 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 onlyabout 7% of incident light. A dry ashen layer (called Regolith) covers the surface, on whichall-around impact craters can be found. This shows the absence of strong erosion and anygeological 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 asteroidsand were discovered in 1877. Until now there is no satisfactory theory about the reason whythose two bodies came into the orbit of Mars. Both, Phobos and Deimos, are tidally lockedto its primary. They have a lower albedo than the Earth’s Moon and their surface is thereforevery low-reflective.

4.1 Phobos

Phobos (Fig. 3a) is irregularly shaped, its size is 27 × 21.6 × 18.8 km. The distance to theMars is only 9377 km and therefore has Phobos the smallest orbit of all known planetarymoons. The orbital period is only 7.66 hours. Since the rotational period of Mars is 24.6hours, Phobos rises twice each day in the west and sets in the east. The fast orbital velocityleads also to a declining of the orbit from which follows that in a few ten millions of yearsPhobos 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-biter.

(b) Deimos, image taken by the Viking 1 orbiter.

Figure 3: Moons of Mars [1].

be solid rock and therefore there is a suggestion that Phobos might contain a substantialreservoir 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 beseen in Figure 3b, it is also highly non-spherical. The orbital period of Deimos is 30.35 hourswhich means it orbits slower than Mars rotates. This leads to an increasing orbit and thefact, 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 untilnow 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 istiny and irregular, that means their orbits have high inclinations and eccentricities. Those aremostly 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-Galileanmoons of Jupiter combined. The physical and orbital characteristics of the satellites differgreatly, from nearly circular to highly eccentrical orbits as well as diameters of barely 1 kmto 5262 km, which is the diameter of Ganymede, the largest satellite of the solar system. Theorbital 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 arepresented.

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 systemand slightly greater than the Moon of the Earth. The orbit of Io has a distance of 421700 kmfrom the center of Jupiter and a period of 42.5 hours. It is tidally locked to its primary andthus is Io a regular satellite.Io is the most geological active object in the solar system. It has more than 400 active volcanoson its surface. There can be found almost no impact craters due to large surface changescaused 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. SinceIo is in orbital resonance with Europa and Ganymede which maintains Io in its orbit, thetidal dissipation leads to a significant heating in the interior. On Io more than 100 mountainscan be found whereas some are taller than the Mount Everest on the Earth are. Large partsof 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 significantlyhigher than the density of the other Galilean moons. In contrary to them Io has almost nowater.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 slightlysmaller than Earth’s Moon and the sixth largest in the solar system. It is a regular satelliteand has an orbital period of 3.55 days. The orbit is slightly eccentric due to gravitationaldisturbances from the other Galilean moons. Since Europa is in orbital resonance with Ioand Ganymede the tidal flexing produces also an internal heating.Europa has a tenuous atmosphere consisting of oxygen. The interior of Europa is mainlysilicate 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 theequator to −220◦ C at the poles. The surface of Europa is one of the smoothest of bodies inthe solar system. Dark streaks as well as infrequent craters can be found on it. The streaksare called lineae, they are supposable cracks in the ice crust whereas the crust on each sideof the streaks has moved relative to each other. Due to its icy crust, Europa has one of thehighest 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 kmis it larger than the planet Mercury. Ganymede is the seventh moon out from Jupiter. It istidally 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. Thedensity is 1.94 g/cm3. The regions of the surface can be distinguished into two differenttypes: the dark ones, which have a highly rate of impact craters and can be dated back tofour billion years ago. The other lighter regions are younger and crosscut by grooves andridges, which are probably caused by tectonic activity due to tidal heating. This heatingresults also from the orbital resonance of Ganymede with Io and Europa. The mean surfacetemperature is −160◦ C.Ganymede is the only satellite with a magnetosphere. This is most likely caused by convectionwithin the liquid iron core.

5.4 Callisto

Callisto (Fig. 5b) is with a diameter of 4821 km the second largest satellite of Jupiter andthe third largest of the solar system. It is also the outermost of the Galilean moons. Theorbital radius is about 1880000 km, which is 26.3 times the radius of Jupiter. Since this issignificantly larger than the orbit of Ganymede, Callisto is not in orbital resonance with theother 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 Moons of Saturn 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 subsurfaceprocesses like earthquakes, volcanos or plate tectonics. Hence the surface is extremely old andheavily 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 waterice. Callisto has like the other Galilean satellites an extremely thin atmosphere consisting ofcarbon dioxide and possibly molecular oxygen.The most favorable theory of the origin of Callisto is a slow accretion in the Jovian subnebulaafter Jupiters formation.

6 The Moons of Saturn

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

6.1 Titan

Figure 6: The moon seen from theCassini–Huygens spacecraft. [1].

Titan was discovered in the year 1655 byChristian Huygens and is therefore the firstknown satellite of Saturn. With a diameterof 5150 km it is the second largest moon inthe solar system and the largest of Saturn.Titan is a regular satellites, is tidally lockedto its primary and has an orbital period of15.9 days.Titan is composed half of water ice and halfof rocky material. This is ordered in differentlayers. With its diameter, mass and density(1.9 g/cm3) is Titan comparable to Jupiter’smoons Ganymede and Callisto.Titan is the only satellite with a dense atmo-sphere, which consists similar to the Earth’satmosphere to 98.8% of nitrogen. It has aheight of about 200 to 880 km and methaneand ethane clouds can be found. The climate has seasonal weather patterns and produces asurface on Titan which is similar to the Earth. There are streaky features like large plainscovered with sand dunes caused by windblown particles and liquid hydrocarbon lakes in thepolar 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 methaneclouds punctuate the haze so that the surface temperature is about −179◦ C. Since waterdoes not sublimate or evaporate at this temperature there is almost no water vapor in theatmosphere of Titan.Titan is seen analogous to the early earth except at a much lower temperature. It is assumedthat 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. Thediameter is 505 km. Enceladus is a regular moon with an orbital radius of 238000 km and aperiod of 32.9 hours. It is orbiting in the densest part of the E-ring, which is the outermostone of Saturn. There is the hypothesis that Enceladus is the source for the material in theE-ring due to cryovolcanic plume from the south polar region. Another factor is the meteoricbombardment, it causes the raising of dust particles from the surface. Enceladus is thesmallest 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 reflectedby the surface. Hence Enceladus has perhaps the most reflective surface of any body in thesolar system.

6.3 Mimas

Mimas was discovered in the year 1789 and has a diameter of 397 km. With that Mimas isthe smallest known astronomical body in the solar system with an almost spherical shapecaused by self gravitation. Mimas is tidally locked to Saturn and has an orbital period of22.5 hours. While orbiting, Mimas is clearing material from the Cassini division, which is thegap between the A and B ring of Saturn. One remarkable feature of the surface of Mimas isa large impact crater with a diameter of 130 km, which is one third of the size of the satelliteitself.

Figure 7: Enceladus as seen by Voyager 2,August 26, 1981 [1].

Figure 8: Mimas, image taken by Cassini-Huygens on August 2, 2005 [1].

7 The Moons of Uranus 9

(a) Hyperion, September 26, 2005. (b) Epimetheus (lower right) andJanus, 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 highlyirregular bodies in the solar system. The size of Hyperion is 360 × 280 × 225 and it has avery low density of 0.57 g/cm3. Therefore it consists mainly of water ice and only a smallamount of rock. Peculiar is the surface of Hyperion. It is covered with deep, sharp-edgedcrates and gives Hyperion the appearance of a sponge. Almost 40% of the moons interior isempty space. Hyperion has not only an unusual appearance, it also is the only known moonin the solar system with such a chaotic rotation. This rotation makes the orientation of thesatellite in space unpredictable.

6.5 Epimetheus and Janus

Epimetheus and Janus (Fig. 9b) is a unique arrangement in the solar system. Those twosatellites 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 onespeeds up, therefore its inner momentum boosts, the orbit increases. The outer satellite losesmomentum likewise and drops into a lower orbit. Consequently the two moons have switchedtheir orbits. Therefore their nearest approach to each other are about 10000 km. The tradingof 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 ahydrostatic equilibrium, 13 orbit within the ring system of Uranus. There are also nine outerirregular moons. In the region between the main rings are the small moons crowded andtherefore 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 orbitamong 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 anyknown moon in the solar system. In the following only the satellite Triton, which is by farthe largest one of Neptun, is presented closer.

8.1 Triton

Figure 10: Triton, picture taken in 1989 byVoyager 2 [1].

Triton has a diameter of 2707 km, an orbitalradius of 354760 km and a period of 5.9 days.It is in synchronized rotation with Neptunbut it is the only large satellite in the solarsystem with a retrograde orbit. This meansTriton orbits in the opposite direction thanNeptun rotates. The shape of the orbit isalmost perfectly circular, but still the radiusis decaying so that in about 3.6 billion yearsit is believed that Triton will either collidewith the atmosphere of Neptun or break upand create a ring system. The rotational axisof Triton is close to the plane of Neptun’sorbit so that during Neptun’s orbit aroundthe sun once on each poles is about 40 yearssummer and winter on the other one.The density of Triton is 2.1 g/cm3, the coreis composed of substantial rock and metal, the mantel is icy and frozen nitrogen forms thecrust. This is geologically active due to geysers erupting nitrogen. Therefore the surface ofTriton is relatively young. On the “winter-side” the temperature of the surface can cool downto 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 methanecloser 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 threekilometers 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 sameregion of the solar nebula as Neptun. Therefore Triton is presumably captured from theKuiper belt. It also composed identical like Pluto which leads to the opinion that they havethe 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