The moons of the planets

Space Physics Project

By Tobias Lind

Definition of a moonA moon is a natural satellite. That means it is a natural celestial body that orbits a primary object. For a satellite this primary object can be a galaxy, a planet, a dwarf planet or a asteroid but moons just orbit planets, dwarf planets and asteroids. The moon follows the primary object on its orbit .

ClassificationThere are at least two different classes of moons we have to distinguish.

Regular moons: These moons have circular orbits and most of them orbit their planets almost in equatorial plane. Regular planets are spherical and originated in different ways. Some of these moons originated together with their planets out of the suns protoplanetary disk . Some other moons have originated by collisions of two large objects.

Irregular moons:Irregular moons have orbits far away from the planets and these orbits are not circular and in equatorial plane. These moons are often captured asteroids and so it is possible that these moons have retrograde orbits. That means that the moon orbits the planet in the opposite direction of the planets orbits the sun.

List of moonsIn 2008, 335 bodies where

formally classified as moons in the solar system. 167 moons orbiting planets, six orbiting dwarf planets, 104 orbiting asteroids and 58 orbiting trans neptunian objects. The moons of the planets are distributed in the following way:

Mercury: 0

Venus: 0

Earth: 1

Mars: 2

Jupiter: 63

Saturn: 61

Uranus: 27

Neptune: 13

Selection of moonsIn the following section a selected number of moons will be described.

Luna (earth)Luna is the only moon of the earth and caused

of the relative small distanxce to the earth it is the only moon a human ever entered. The average distance between moon and earth is 384.405 km and its diameter is 3467 km, what is a quarter of the earth’s diameter. The time of circulation is 27 days, 7 hours and 44 minutes and the orbit is almost circular. Luna is tidally locked, so it shows always the same part of its surface to the earth. Because of a bombardment of asteroids the surface of this moon is cratered.

The latest theory of the origination of Luna is the “giant impact hypothesis”. This hypothesis says that a large object collided with the earth and a part of the surface was blowing up into space. Out of this dusk Luna originated.

Io (Jupiter)Io is one of the four “Galilean moons” (Io, Europa, Ganymed, Callisto). These moons were discovered by Galileo in 1610 and where the first objects which not orbit the sun or the earth.

The average distance between Io and Jupiter is421.600 km and the circulation time is one day 18 hours and 28 minutes. The diameter is 3642 km, so Io is the fourth largest moon in the solar system. Io is tidally locked so it has the same rotation velocity like the circulation time.

With over 400 active volcanoes Io is the most geologically active object in the solar system and the surface of the moon changes continuous. The energy of this eruptions is caused by the tidal forces between Io and the near objects like Jupiter, Europa and Ganymed. With its volcanoes it has a big influence of the magnetosphere of Jupiter because it loses one ton of charged particles to the magnetosphere of Jupiter every second

Europa (Jupiter)Europa is the smallest of the Galilean moons. It

has a diameter of 3100 km and an average distance of 670.900 km. The circulation time is three days 13 hours and 15 minutes. Like all other Galilean moons Europa is tidally locked.

Europa is primary composed of silicate rock and has a small iron core. The surface of the moon has an outer layer of water. There is a 10 to 15 km thick layer of ice and below this layer a 90 km deep ocean of liquid water which is heated by tidal forces. Because of this outer layer the moon has a smooth surface and a very high light reflectivity of 64%.

Ganymed (Jupiter)

Ganymed is another Galilean moon and with a diameter of 5264 km it is the biggest moon in the solar system. It is even bigger than Mercury but it has just half of Mercuries mass. The average distance is 1.070.400 km and the circulation time is seven days and tree hours. The moon is composed of rocky material and water (ice) and its surface is split in two different terrains. A very old dark region with many craters and a younger lighter one with riffs and faults. There are two independent tectonic plates too but the tectonic activities will end.

Ganymed is the only moon with its own magnetosphere.

Ganymed, Europa and Io are in orbital resonace, what means that in the time of on circulation of Ganymed Europa circulates two times and IO circulates four times.

Callisto (Jupiter)

Callisto is the third biggest moon in the solar system with a diameter of 4821 km. The average distance is 1.833.00 km so it is far away from Jupiter in relation to the other Galilean moons. Cause of this large distance the circulation time is 16 days, 16 hours and 32 minutes. It iscomposed of rocks and ice and eventually there is liquid water in a subsurface ocean. The moon has a heavily cratered an very dark surface. The light reflectivity is only 20 %.

Prometheus and Pandora(Saturn)Prometheus and Pandora are two moons of Saturn.

They were discovered in 1980 from photos taken by Voyager 1. The dimensions of the moons are 119*87*61 km (Prometheus) and 110*88*62 km (Pandora). The average distances are 139.400 and 141.700 km and the circulation times are 14 hours and 42 minutes for Prometheus and 15 hours and

5 minutes for Pandora. Both moons have a low density and high albedo ( light reflectivity), so they are maybe porous ice bodies.

Prometheus and Pandora orbit Saturn on the equatorial plane and act as “shepherd moons”. They have their orbits near the outer edges of a planetary ring of Saturn ( F- ring) and they maintain a sharply defined edge of this ring. If material is leaving a ring in front of a moon it will be decelerated

by the gravity and attracted to the inner ring. If the material is behind the moon it will be accelerated and tossed to the outer ring. So these moons clean up the place between different rings.

Phobos (Mars)

This moon was discovered in 1877 by Asphall Hall. With dimensions of 27*22*19 km it is a very small moon. The average duistance between the center of mass of Mars and Phobos is only 9377 km and so the circulation time is 7 hours, 39 minutes and 12 seconds.

Phobos is a captured asteroid from the main asteroid belt and so it is a irregular moon. It loses height and will impact in 11 million years if it not break up before. The surface of the moon is cratered and the biggest one is named Stickney after Asphal Halls wife.

Deimos Mars

Like Phobos, Deimos was discovered by Hall in 1877. Its dimension are 15*12,2*1 km, the average distance 23.453 km and the circulation time is one day, six hours and 18 minutes.

Like Phobos this moon is probably a captured asteroid.

Charon (Pluto)Charon was discovered in 1978 by James W. Christy. It is the largest satellite of Pluto and with a diamtere of 1207 km it has more than half the dimension of Pluto. The circulation time is six days and nine hours and the average distance is 19570 km.

The center of mass of the Pluto- Charon- system lies outside either bodies what is unique for an planet- moon system. So the relation between these both bodies could be defined as a dual dwarf system and not as a planet- moon system, but there isn’t a new official definition yet.

References

www.wikipedia.or

www.kufner-sternwarte.at

www.monde.de

I, Tobias Lind, hereby allow to publish this document