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An Overview of Astronomy

A Concise Guide to the Universe


The Outer Jovian Planets

Jupiter

A. Jupiter is named after the Roman king of the gods. It is more than twice as massive as all the other planets in the solar system combined.

B. Jupiter is a giant ball of hydrogen-based gasses, whose density is only slightly greater than water. Its surface features many alternating bands of clouds that spin in opposite directions. White bands are known as zones, dark bands are known as belts. They are caused by material rising from, and failing to, the interior of the planet. The risings and failings are due to convection currents within the planet. The Coriolis effect on the surface causes the winds to spin in opposite directions. Organic chemicals cause the color of the belts.

C. The Great Red Spot on Jupiter is caused by winds spinning in opposite directions on the surface - essentially a permanent hurricane.

D. Jupiter has no surface per se. As one descends through the clouds, they become increasingly dense. At a certain point, they are so dense, they are essentially liquid.

E. Jupiter gives off twice as much energy as it receives from the sun. This energy is probably residual heat from the formation of the planet.

F. Jupiter has 16 named moons. The four largest can be seen easily from Earth, and are known as the Galilean satellites.

  1. Io is particularly interesting, as it is extremely volcanically active. The enormous gravitational field of Jupiter pulls and distorts Io to such an extent that internal friction heats up the moon. The internal heat manifests itself in volcanic activity, which produces sulfur, making the surface of the moon look reddish-orange.
  2. Europa, the next moon out, contains an icy crust covering a mantle of liquid water. Some planetary scientists think that Europa might be able to harbor life. Its surface has recently been found to be covered with a great deal of Epsom salts.
  3. Ganymede, the next moon, is the largest moon in the solar system, and is in fact larger than the planet Mercury.
  4. Callisto, the last Galilean moon, is similar in composition to Ganymede, but not quite as large. The moons of Jupiter formed in such a way as to mimic the formation of the solar system. The small rocky moons formed towards the interior of the planetary nebula and the larger, lighter moons formed further out.

G. Jupiter, as well as all of the Jovian planets, has rings. However, its rings are not as large or as visible as those of Saturn. The rings were not discovered until spacecraft were able to image the planet from behind so as to see the rings in relief.

H. Jupiter exhibits differential rotation, in which the equator of the planet spins faster than the north and south poles. Only a planet made essentially out of gas could do this. All of the Jovian planets exhibit this feature.

SATURN

A. Saturn is named after the Roman god of the harvest. It is known for its complex ring system, as well as for its moon, Titan. Saturn is almost as large as Jupiter, but has less than one-third the mass. In a large enough ocean, Saturn would float, as it has an average density of less than one.

B. Saturn lacks the striking banded cloud patterns of Jupiter. The reason is that it is just too cold. Saturn is nearly twice as far away from the sun as Jupiter. The exterior of Saturn is made up of frozen ammonia clouds. The interior is otherwise similar to Jupiter's.

C. Saturn has a large and complex system of rings. They may be from several different sources. First, the rings lie at about 2.44 the planetary radii of Saturn. At that distance, it as been calculated that any moon made of similar substance to the planet will be pulled apart by the gravitational attraction of the planet. This distance is known as the Roche Limit. Any satellite within this limit will be pulled apart. Additionally, the planet may have captured satellites, and broken them apart. There is a large gap in the rings of Saturn called the Cassini Division. The gaps in the rings are most probably due to shepherding satellites that orbit in resonance with the rings. The rings are made of myriad individual particles of rock and ice. They are 50,000 miles across and 200 yards deep.

D. Saturn has 22 moons, the most interesting of which is Titan. Titan is one of two moons in the solar system with its own atmosphere. Some planetary scientists believe that Titan is one of the most likely places in the solar system for life to be found.

URANUS

A. Uranus is named after the Roman god of' the sky. Uranus was discovered in 1781 by Sir William Herschel by careful observation of the heavens. He saw an object in his telescope which was disk-shaped and too large to be a star. After several months it became apparent to astronomers that this was a planet because it moved against the background stars but didn't grow a tail like a comet did.

B. Uranus is a relatively featureless, pale blue planet. It lacks the cloud bands that are distinctive on the surface of Saturn and Jupiter. Part of the reason for this is it has a rotation about its axis that is almost in the plane of its orbit. In other words, whereas most planets, including Earth, spin almost directly up and down with respect to their orbital path, Uranus has been pushed over on its side and is spinning with its north or south pole occasionally pointing directly at the sun. This rotational pattern keeps Uranus from having a dynamic weather pattern. Half of the planet has almost constant heating for half the year, whereas most planets have the entire planet covered with partial heating over very short periods of rotation. This odd rotation was probably caused by the impact of a large object early in the formation of the planet.

C. Most of the moons of Uranus are named for characters in Shakespearean plays. There are moons named after Juliet and Puck, as well as other famous characters. Two of the moons are named after characters in Pope's Rape of the Lock, Umbrial and Belinda.

NEPTUNE

A. Neptune is named after the Roman god of the sea. The two largest moons of Neptune are Triton and Nereid. Triton is the Roman name for mermen, and Nereid the name for mermaids, thus keeping with the oceanic theme. Neptune was discovered simultaneously by English and French astronomers.

B. Neptune is approximately the same size and composition of Uranus, but has a radically different atmosphere. Because it spins vertically on its axis, rather than on its side, it has a more dynamic surface, with the fastest winds in the solar system at well over a thousand miles per hour.

C. Neptune was once seen to have a large blue spot, discovered by the Viking Space Probe, much like the Great Red Spot on Jupiter. However, recent pictures from the Hubble Space Telescope show that this spot has disappeared.

D. Neptune's moon, Triton, joins Saturn's moon, Titan, as the only other moon in the solar system with its own atmosphere. It is the largest of Neptune's many moons.

PLUTO

A. Pluto, named after the Roman god of the underworld, is not a Jovian Planet. American astronomer Clyde Tombaugh discovered the planet in 1930. Pluto's large moon is named after Charon, the ferryman over Styx, the river of the dead. Charon is very large compared to Pluto, and in fact it could be said that Pluto and Charon orbit one another, rather than Charon orbiting Pluto. Charon was not discovered until 1977.

B. Not much is known about Pluto because of its small size and vast distance from the sun. It is presumed to be made of mostly frozen gasses and water, similar to most comets, and is thus probably a remnant left over from the formation of the solar system. In addition, because Pluto is very cold, the water on it would have a consistency similar to that of steel on Earth.

C. Pluto has a very elliptical orbit, which is inclined to the plane of orbit of the other planets by approximately 17 degrees. This trajectory takes it well away from the path of the other planets. Because of its highly elliptical orbit, Pluto is occasionally closer to the sun than Neptune.

 

Comparative Data on the Jovian Planets and Pluto

Quantity

Jupiter

Saturn

Uranus

Neptune

Pluto

Equatorial diameter (Km)

142796

120000

50800

48600

3300-2800(?)

Density (kg/cm(3))

1330

706

1270

1700

2000?

Mass (Earth=1)

1318.7

743.6

14.6

17.2

0.0025?

Surface gravity (Earth=1)

2.643

1.159

1.11

1.21

?

Escape velocity (km/s)

60.22

32.26

22.5

23.9

?

Mean distance from sun (AU)

5.2028039

9.5388437

19.181843

30.057984

39.4

Mean distance from sun (miles)

4.836x10(8)

8.8722x10(8)

1.7837x10(8)

2.7946x10(8)

3.6642x10(8)

Mean distance from sun (10(6)Km)

778.3

1427.0

2869.0

4497.1

5900

Orbital period (Earth years)

11.867

29,461

84,013

164,793

247.7

Orbital period (Earth days)

4334.3

10,760

30,685

60,189

90,465

Orbital velocity (Km/sec)

13.06

9.64

6.81

5.43

4.74

Ave. Surface Temperature (K)

125

95

60

60

 

Satellites of Planets in the Solar System

Planet

Name of Satellite

Average Diameter in Km

Earth

Moon

3476

Mars

Phobos
Deimos

24
14

Jupiter

Metis
Adrastea
Amalthea
Thebe
lo
Europa
Ganymede
Callisto
Leda
Himalia
Lysithea
Elara
Anake
Carme
Pasiphae
Sinope

40
40
170
100
3630
3140
5260
4800
15
185
35
75
30
40
50
35

Saturn

Pan
Atlas
Prometheus
Pandora
Janus
Epimetheus
Mimas
Enceladus
Tethys
Telesto
Calpyso
Dione
Helene
Rhea
Titan
Hyperion
Iapetus
Phoebe

20
30
100
90
190
120
380
500
1050
25
25
1120
30
1530
5150
255
1440
220

Uranus

Cordelia
Ophelia
Bianca
Cressida
Desdemona
Juliet
Portia
Rosaline
Belinda
Puck
Miranda
Ariel
Umbrial
Titania
Oberon

25
30
45
65
60
85
110
60
68
155
470
1160
1170
1580
1520

Neptune

Naiad
Thalassa
Despina
Galatea
Larissa
Proteus
Triton
Neried

60
80
150
160
190
420
2700
340

Pluto

Charon

1200


An Overview of Astronomy

A Concise Guide to the Universe

 


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