Astronomy Answers: Universe Family Tree: Moon

# Astronomy AnswersUniverse Family Tree: Moon

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 Type :  Planetary System ───  Moon Size :  0.1 AE ───  3476 km Earth :  the Earth System ───  the Moon

There is no fundamental physical difference between a moon and a planet. If two or more celestial bodies that could each be a planet or a moon orbit around each other, then the bigger one is called the planet, and the smaller ones are the moons.

There are at least 156 known moons in the Solar System. A number of (usually very small) moons still waits for an official name. The distribution of the known moons over the planets is shown in the following table, which also displays for each planet the last year of the discovery of a moon (per 2006-05-25). It may take a year or two before the existence and orbit of a newly discovered moon are sufficiently clear that it can get an official name.

Table 1: Satellite Discoveries in the Solar System

planet moons latest discovery
Mercury 0
Venus 0
Earth 1 prehistory
Mars 2 1877
Jupiter 63 2003
Saturn 47 2005
Uranus 27 2003
Neptune 13 2003
Pluto 3 2005

Below, a number of tables are presented that compare different characteristics of the moons from our Solar System. For each characteristic, information is given for at least the ten moons that have the most of that characteristic. If a certain planet has none of its moons in the top ten, then the moon from that planet that has the characteristic the most is also mentioned in the table. These are the characteristics for which the moons are compared:

## 1. Distance from the Planet

This table shows the moons that are on average the furthest from or the closest to the center of their planet, measured in units of 1000 km.

Moon Planet Distance
156 1 S/2002 N4 Neptune 48387
155 2 S/2003 N1 Neptune 46695
154 3 S/2003 J2 Jupiter 29541
153 4 S/2004 S8 Saturn 25108
152 5 Cyllene Jupiter 24349
151 6 Callirrhoe Jupiter 24103
150 7 Autonoe Jupiter 24046
149 8 Kallichore Jupiter 24043
148 9 Aoede Jupiter 23981
147 10 Hegemone Jupiter 23947
107 50 Ferdinand Uranus 20901
50 107 Moon Earth 384
28 129 Pan Saturn 134
25 132 Metis Jupiter 128
10 147 Bianca Uranus 59
9 148 Ophelia Uranus 59
8 149 Despina Neptune 53
7 150 Thalassa Neptune 50
6 151 S/2005 P2 Pluto 50
5 152 Cordelia Uranus 50
3 154 Deimos Mars 23
2 155 Charon Pluto 20
1 156 Phobos Mars 9

The largest moon orbit (of S/2002 N4) is about 5000 times greater in diameter than the smallest moon orbit (of Phobos).

## 2. Diameter

Seventeen moons and one asteroid have a diameter of at least 1000 km. Those celestial bodies and the largest moon of Mars are listed in the following table, with their diameters measured in kilometers.

Moon Planet Diameter
1 Ganymede Jupiter 5262
2 Titan Saturn 5151
Mercury 4880
3 Callisto Jupiter 4821
4 Io Jupiter 3643
5 Moon Earth 3474
6 Europa Jupiter 3122
7 Triton Neptune 2707
Pluto 2274
8 Titania Uranus 1578
9 Rhea Saturn 1529
10 Oberon Uranus 1523
11 Iapetus Saturn 1469
12 Charon Pluto 1186
13 Umbriel Uranus 1169
14 Ariel Uranus 1158
15 Dione Saturn 1125
16 Tethys Saturn 1060
17 1 Ceres asteroid 1003
71 Phobos Mars 22

Two moons are larger than the planet Mercury, and seven moons are larger than the planet Pluto.

## 3. Relative Diameter

The next table shows the moons with the largest diameter relative to their planets, in general and for each planet.

Moon Planet Relative Diameter
1 Charon Pluto 0.52
2 Moon Earth 0.27
3 Triton Neptune 0.055
4 Titan Saturn 0.043
5 Ganymede Jupiter 0.037
6 Callisto Jupiter 0.034
7 Titania Uranus 0.031
8 Oberon Uranus 0.030
9 Io Jupiter 0.025
10 Umbriel Uranus 0.023
25 Phobos Mars 0.003

## 4. Mass

This table lists the moons with the greatest mass, compared to the mass of the Moon, in general and for each planet. I do not have information about the masses of all of the moons. The missing masses are most likely smaller than the mass of Charon, but may in some cases be greater than the mass of Phobos.

Mass Planet Moon
1 Ganymede Jupiter 2.02
2 Titan Saturn 1.83
3 Callisto Jupiter 1.46
4 Io Jupiter 1.22
5 Moon Earth 1.00
6 Europa Jupiter 0.65
7 Triton Neptune 0.29
8 Titania Uranus 0.048
9 Oberon Uranus 0.041
10 Rhea Saturn 0.031
12 Charon Pluto 0.022
66 Phobos Mars 0.00000015

## 5. Relative Mass

The next table displays the greatest masses of moons relative to the mass of their planets. Only moons of which I know the mass have been included.

Moon Planet Relative Mass
1 Charon Pluto 0.13
2 Moon Earth 0.012
3 Titan Saturn 0.00024
4 Triton Neptune 0.00021
5 Ganymede Jupiter 0.000078
6 Callisto Jupiter 0.000057
7 Io Jupiter 0.000047
8 Titania Uranus 0.000041
9 Oberon Uranus 0.000035
10 Europa Jupiter 0.000025
28 Phobos Mars 0.000000017

## 6. Gravity

This table shows the moons with the greatest gravity at their surface (compared to the gravity at the surface of the Earth):

Table 2: Moons with the Greatest Gravity at their Surface

Moon Planet Gravity
1 Io Jupiter 0.183
2 Moon Earth 0.166
3 Ganymede Jupiter 0.146
4 Titan Saturn 0.138
5 Europa Jupiter 0.134
6 Callisto Jupiter 0.126
7 Triton Neptune 0.080
Pluto 0.067
8 Titania Uranus 0.038
9 Oberon Uranus 0.035
10 Charon Pluto 0.031
62 Phobos Mars 0.000000017

The gravity on seven moons is greater than the gravity on the planet Pluto.

## 7. Tidal Forces from the Sun

The orbits of moons that are far enough away from their planet are influenced by the gravity of the Sun. We can measure this influence through the kmoon of the tidal boundary. The moons with the greatest influence of the Sun on their orbits are:

Moon Planet kMoon
1 S/2003 J2 Jupiter 0.39
2 Cyllene Jupiter 0.32
3 Callirrhoe Jupiter 0.31
4 Autonoe Jupiter 0.31
5 Kallichore Jupiter 0.31
6 Aoede Jupiter 0.31
7 Hegemone Jupiter 0.31
8 Sinope Jupiter 0.31
9 S/2003 J4 Jupiter 0.31
33 S/2002 N4 Neptune 0.28
48 S/2004 S8 Saturn 0.27
62 Ferdinand Uranus 0.21
73 Moon Earth 0.18
101 Deimos Mars 0.015
106 S/2005 P1 Pluto 0.0059

## 8. Density

Some moons are made of heavier elements, and others of lighter elements. We can calculate the mass density of a moon if we know its mass and its size. For many of the smallest moons, we do not know the mass and size very accurately, so for such moons we cannot calculate the mass density accurately, either. The below table includes moons with a diameter of 100 km or more, and also smaller moons for which we know the shape (i.e, not just an estimate of the radius) and for which we have an estimate of the mass. The table lists the moons with the greatest density (compared to water), in general and for each planet, and the moons with the smallest known density.

Moon Planet Density
1 Io Jupiter 3.5
2 Moon Earth 3.5
3 Europa Jupiter 3.0
4 Himalia Jupiter 2.6
5 Triton Neptune 2.0
6 Ganymede Jupiter 1.9
7 Titan Saturn 1.9
8 Phobos Mars 1.9
9 Charon Pluto 1.9
10 Callisto Jupiter 1.8
11 Titania Uranus 1.7

The moons with high densities consist mainly of rocky material. Moons with ever lower densities contain ever more ice.

## 9. Orbital Speed

The next table shows the moons with the greatest and smallest speed in their orbit around their planet, measured in kilometers per second. 1 kilometer per second is the same as 3600 kilometers per hour, or 2237 miles per hour.

Moon Planet Orbital Speed
Mercury 47.9
Venus 35.0
140 1 Metis Jupiter 31.6
Earth 29.8
138 3 Amalthea Jupiter 26.4
Mars 24.1
137 4 Thebe Jupiter 23.9
136 5 Io Jupiter 17.3
135 6 Pan Saturn 16.9
134 7 Atlas Saturn 16.6
133 8 Prometheus Saturn 16.5
132 9 Pandora Saturn 16.4
131 10 Janus Saturn 15.9
121 20 Ophelia Uranus 11.4
Pluto 4.7
34 107 Phobos Mars 2.1
33 108 S/2003 J14 Jupiter 2.1
19 122 Deimos Mars 1.35
18 123 Ymir Saturn 1.28
16 125 Moon Earth 1.02
10 131 S/2002 N1 Neptune 0.61
9 132 Prospero Uranus 0.59
8 133 Setebos Uranus 0.58
7 134 S/2002 N3 Neptune 0.58
6 135 S/2002 N2 Neptune 0.56
5 136 Ferdinand Uranus 0.54
4 137 S/2002 N4 Neptune 0.40
3 138 Francisco Uranus 0.39
2 139 S/2003 N1 Neptune 0.37
1 140 Charon Pluto 0.22

The fastest moons are very fast, but not as fast as the planets Mercury and Venus. Over ninety moons have a smaller average orbital speed than the planet Pluto.

## 10. Orbital Period

The next table shows the moons with the longest and shortest sidereal orbital periods ("months"). For each moon, the period is shown in hours, days, and/or years, but it is the same period each time. So: the orbital period of Setebos is 2246 days, which is about 6.1 years.

Moon Planet Hours Days Years
156 1 S/2002 N4 Neptune 25.6
155 2 S/2003 N1 Neptune 24.3
154 3 S/2002 N3 Neptune 3183 8.7
153 4 S/2002 N2 Neptune 2953 8.1
152 5 Ferdinand Uranus 2887 7.9
151 6 Setebos Uranus 2196 6.0
150 7 Prospero Uranus 1980 5.4
149 8 S/2002 N1 Neptune 1735 4.7
148 9 Margaret Uranus 1642 4.5
147 10 S/2004 S8 Saturn 1486 4.1
136 21 S/2003 J2 Jupiter 1037 2.8
Mars 687 1.9
Earth 365 1.0
59 98 S/2005 P1 Pluto 41.4
57 100 Moon Earth 27.3
35 122 Deimos Mars 30.3 1.3
18 139 Pan Saturn 13.8
10 147 Bianca Uranus 10.4
9 148 Ophelia Uranus 10.3
8 149 Galatea Neptune 10.3
7 150 Cordelia Uranus 8.1
6 151 Despina Neptune 8.0
5 152 Phobos Mars 7.7
4 153 Thalassa Neptune 7.5
2 155 Metis Jupiter 7.1

There are 88 moons that take more than a year to complete one orbit, and 31 moons that take less than a day. The moon S/2002 N4, which takes longest of all moons to orbit its planet, takes about 31,000 times longer to do that than the moon Naiad which does it in the shortest time.

## 11. Temperature

The next table shows the moons with the highest and lowest equilibrium temperature (average surface temperature), based on the visual geometric albedo of the moons, and measured in kelvin (K) and degrees Celsius (). Many of the smaller moons of a given planet have the same estimated albedo, so they have the same estimated temperature.

Moon Planet K
℃
1 Moon Earth 270 −3
2 Phobos Mars 222 −51
3 Deimos Mars 222 −51
4 Elara Jupiter 121 −152
5 Himalia Jupiter 121 −152
6 Callirrhoe Jupiter 121 −152
7 Chaldene Jupiter 121 −152
8 Erinome Jupiter 121 −152
9 Harpalyke Jupiter 121 −152
10 Iocaste Jupiter 121 −152
66 Europa Jupiter 93 −180
67 Methone Saturn 90 −183
111 Francisco Uranus 63 −210
138 Ariel Uranus 56 −217
133 Tethys Saturn 60 −213