Terrestrial v Jovian Planets
Paul Fisher
June 2010
Terrestrial versus Jovian Planets
Contents
The project brief
4
Abstract
4
1
5
2
3
4
5
Introduction
1.1
This project
5
1.2
The Solar System
5
1.3
The terrestrial planets
5
1.4
The Jovian planets
6
Atmospheric chemistry
7
2.1
The terrestrial planets
7
2.2
The Jovian planets
8
2.3
Summary
9
Density and internal structure
10
3.1
Overview
10
3.2
Terrestrial planets
10
3.3
Jovian planets
11
3.4
Summary
12
Magnetic field
13
4.1
The terrestrial planets
13
4.2
The Jovian planets
14
4.3
Summary
15
Moons and rings
16
5.1
The terrestrial planets
16
5.2
The Jovian planets
16
5.3
Summary
18
References
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Paul Fisher
19
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Table of Figures
Figure 1: The terrestrial planets, to approximate scale
Figure 2: The Jovian planets to approximate scale
Figure 3: Jupiter and Earth to approximate scale
Figure 4: Jupiter's Great Red Spot Source: NAA/JPL
Figure 5: Proportions of atmospheric gases
Figure 6: Density of the planets
Figure 7: Interior structure of Mercury
Source: Wikipedia
Figure 8: Internal structure of Earth Source: NASA
Figure 9: Jupiter's magnetic field Source: NASA
Figure 10: Magnetic field of Uranus Source: Wikimedia Commons
Figure 11: Jupiter and the Galilean moons
Figure 12: Saturn from spacecraft Cassini
Terrestrial versus Jovian Planets
Paul Fisher
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Terrestrial versus Jovian Planets
The project brief
In this project you will compare and contrast the terrestrial and jovian planets. Choose at least
five characteristics (such as atmosphere and volcanism for example) to compare between the
two groups. Make note of any planets in these two categories that are exceptions to the rule and
mention why you think this might be. To achieve high marks in this project you will need to tie in
the formation mechanisms of the planets with your arguments.
Abstract
This project compares and contrasts the terrestrial and Jovian planets. It describes the Solar
System in general terms and identify the two groups of planets – terrestrial and Jovian. We
compare the two groups using five characteristics and describe the how the planetary
characteristics are shaped by their formation mechanisms.
This paper reviews each of the planets individually, and in their groups, in terms of five
characteristics: atmospheric chemistry, density, temperature, magnetic field, moons & rings.
The paper identifies which characteristics are consistent across each group, and which are
independent. It relates each of the characteristics to each other, and to the theory of the
formation of the Solar System.
Terrestrial versus Jovian Planets
Paul Fisher
Page 4
1 Introduction
1.1
This project
This project will compare and contrast the terrestrial and Jovian planets. It will describe the
Solar System in general terms and identify the two groups of planets – terrestrial and Jovian. We
will compare the two groups using five characteristics and describe the how the planetary
characteristics are shaped by their formation mechanisms.
1.2
The Solar System
The Solar System is dominated by the Sun and planets. The Sun is our local star, the centre of
the system and the largest and most massive member. There are eight planets orbiting the sun,
four termed the terrestrial (Earthlike) and four termed Jovian (like Jupiter). The two groups have
very different characteristics, which will be explored in this project.
In addition to the sun and planets, there are very many other objects within the Solar System,
including the dwarf planets, asteroids, comets, Kuiper Belt objects and Oort Cloud objects.
Except to the extent these minor players are important to the story of the planets, we will not
deal with them in this project.
1.3
The terrestrial planets
The four planets closest to the Sun – Mercury, Venus, Earth and Mars – share certain
characteristics which are quite distinct from the Jovian planets. These planets are smaller that
the Jovians, and are of a rocky rather than gaseous nature. Because of their general similarity to
Earth, the inner four planets are referred to as the terrestrial planets.
Mercury
Venus
Earth
Mars
Figure 1: The terrestrial planets, to approximate scale
Source: NASA
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Paul Fisher
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1.4
The Jovian planets
The outer four planets – Jupiter, Saturn, Uranus and Neptune – are gas giants. They are much
larger than the terrestrial planets and have a very different composition. Because of their
general similarity to Jupiter, they are called Jovian planets.
Jupiter
Saturn
Uranus
Neptune
Figure 2: The Jovian planets to approximate scale
Source: NASA
Figure 3: Jupiter and Earth to approximate scale
Source: NASA
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2 Atmospheric chemistry
2.1
The terrestrial planets
Mercury
The atmosphere of Mercury is extremely tenuous, and is essentially a vacuum, with a surface
pressure less than 10-15 bars. Its composition is: 42% O2, 29% Na, 22% H2, 6% He and 0.5% K.
(Casewestern1Web)
The Messenger spacecraft has detected quantities of magnesium, as well as sodium and
calcium. Because it is so tenuous, the atmosphere of Mercury is unstable and subject to solar
wind effects. (MessengerWeb)
Venus
The atmosphere of Venus is composed primarily of carbon dioxide. The surface pressure is
approximately 92 bars at the surface. The composition is 96.5% CO2, 3.5% N2 and traces of other
gases. (Casewestern2Web)
The density and concentration of carbon dioxide has led to a runaway greenhouse effect, with
surface temperatures of 450°C. The upper atmosphere (45 to 70km above the surface)
comprises dense clouds of sulphuric acid (H2SO4). At these altitudes, winds of up to 370km/h
blow constantly, circling the planet in about 4 days. It is not understood how such high speed
winds are generated on a planet with an extremely slow rotation. (UniverseTodayWeb,
BasqueWeb)
Earth
Earth’s atmosphere is 78% N2, 21% O2, nearly 1% Ar and traces of other gases, including
0.035% CO2. The surface pressure is 1 bar. Water vapour is also present in varying amounts,
depending on location and the current weather situation. (Casewestern3Web)
Mars
The Martian atmosphere is substantially thinner than that of Earth or Venus, with a surface
pressure of only 6.36*10-4 bar. It is composed of 95.3% CO2, 2.7% N2, 1.6% Ar and traces of
other gases. (Casewestern4Web)
Overview
It is thought that the terrestrial planets all formed in that part of the solar nebula disk closest to
the sun. This region of higher temperatures and stronger solar wind was stripped of the lighter
elements which were unable to condense out of the disk. These planets are therefore rocky in
nature. Other than Mercury, the atmospheres of the terrestrials are deficient in the light gases
hydrogen and helium, and have a preponderance of heavier gases such as nitrogen, oxygen and
CO2.
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Paul Fisher
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2.2
The Jovian planets
Jupiter
As a gas giant, Jupiter’s atmosphere is thought to blend into an
ocean of liquid hydrogen with no clear interface. The gaseous
atmosphere is about 1000 km thick. The atmospheric
composition is 89.8% H2, 10.2% He, 0.3% CH4, and traces of
other gases. Polysulphur compounds and phosphine (PH3) may
be present, and contribute to the distinctive colours of the
planet’s clouds. (Casewestern5Web)
The high speed winds and enormous storms (including the
Great Red Spot) are distinctive features of Jupiter’s
atmosphere.
Figure 4: Jupiter's Great Red Spot
Source: NAA/JPL
Saturn
Saturn is similar to Jupiter in structure, with a thick gaseous / cloudy atmosphere overlying a
liquid hydrogen ocean, with no defined interface. The atmospheric composition is 96.3% H2,
3.25% He, 0.45% CH4, and traces of other gases including water ice aerosol. (Casewestern6Web)
The lower proportion of He seen in the atmosphere may be due to the “rain out” effect.
Similarly to Jupiter, Saturn has very high wind speeds, up to 450m/sec (1620km/hr) (Showman
2009). However, recent measurements by the Cassini probe indicate winds of “only”
1100km/hr.
Uranus
Uranus has a different internal structure from Jupiter and Saturn. It has a thick gaseous
atmosphere overlying an ocean of hydrogen, helium, and water, with small amounts of
ammonia and methane. The composition of the atmosphere is 82.5% H2, 15.2% He, and 2.3%
CH4, with traces of other gases. It is the methane which gives Uranus its distinctive blue colour.
In addition, aerosols of ammonia ice, water ice, and ammonia hydrosulfide have been detected.
(Casewestern7Web)
Neptune
Neptune has a very similar structure to Uranus. Its atmospheric composition is 80% H2, 19% He
and 1.5% CH4. (Casewestern8Web) As with Uranus, there are aerosols of ammonia ice, water
ice, and ammonia hydrosulfide in the atmosphere.
Overview
The Jovian planets all have atmospheres composed mainly of hydrogen and helium. Saturn’s
atmosphere is nearly all hydrogen, while the outer planets Uranus and Neptune have lower
levels of hydrogen, but larger traces of methane.
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Paul Fisher
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2.3
Summary
Figure 5 shows the proportions of the major constituents in each planet’s atmosphere. It is seen
that the Jovian atmospheres are very largely composed of hydrogen, with varying amounts of
helium. These gases are almost totally absent from Venus, Earth and Mars.
The inner planets show more variation in their makeup. Mercury’s ultra-thin atmosphere is
quite anomalous, containing nearly 20% hydrogen and a large proportion of sodium. Both Venus
and Mars have atmospheres almost entirely composed of carbon dioxide, while Earth is the only
planet with a nitrogen – oxygen atmosphere.
The high percentages of hydrogen and helium in the Jovian planets is consistent with their
distance from the sun. The lower temperatures and weaker solar wind at these distances
allowed the lighter gases to condense from the protoplanetary disk. The higher temperatures of
the terrestrial planets did not allow hydrogen and helium to condense out.
O2
1
Na
0.8
N2
0.6
He
0.4
H2
CO2
0.2
CH4
0
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus Neptune
Figure 5: Proportions of atmospheric gases
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Paul Fisher
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3 Density and internal structure
3.1
Overview
The densities of each of the planets are set out in Table 1 and shown graphically in Figure 6.
(CasewesternWeb1)
6
5
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
5.427
5.243
5.515
3.933
1.326
0.687
1.270
1.638
Table 1
4
3
2
1
0
Figure 6: Density of the planets
Density of the planets (Water = 1)
It is clearly seen that the terrestrial planets have much higher densities than the Jovians. This is
expected, considering the terrestrials are generally rocky, while the Jovians are thought to be
largely gaseous with liquid oceans. We will briefly consider the current theories concerning the
make-up of each of the planets, and see the effect on their density.
3.2
Terrestrial planets
Mercury
Mercury has a density slightly lower than Earth. However, it is noted
that because of its much smaller mass, the effect of gravity
compressing materials within Mercury would be much less than on
Earth. It is therefore supposed that Mercury must have a much larger
core in proportion to the size of the planet. Current estimates
indicate a core of 1,800 km radius, or about 42% of the volume of the
planet, compared to 17% for Earth’s core. (Strom & Sprague 2003)
A number of theories have been advanced as to why Mercury has
such a proportionately large core. These include:
1. Crust—100–300 km thick
2. Mantle—600 km thick
3. Core—1,800 km radius
Figure 7: Interior structure of
Mercury
Source: Wikipedia
 The orbits of lower density (eg silicate) planetisimals in the solar
nebula decayed due to gas drag at a higher rate than denser (iron)
planetisimals. Consequently a higher proportion of dense material accreted into the planet.
(Weidenschilling 1978)
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Paul Fisher
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 Mercury may have been impacted by a large planetesimal which blasted away much of its
outer layers. (USGSWeb)
 Mercury may have formed as a much larger planet than it is at present. Subsequent increases
in the temperature of the solar nebula may have vaporised the outer layers, which were
carried away by the solar wind. (Cameron 1985)
Venus and Earth
These planets have very similar mass, radius and density. This
suggests that their internal structures are probably similar.
(RankWeb)
The internal structure of Earth is composed of several layers:




Rocky crust: 8 to 40 km thick
Rocky mantle: 2900 km thick
Liquid iron/nickel outer core: 2250 km thick
Solid iron/nickel inner core: 2600 km diameter
The mantle is primarily peridotite, which is composed of
silicon, oxygen, iron, and magnesium.
Mars
Figure 8: Internal structure of Earth
Source: NASA
The density of Mars is somewhat lower than the other terrestrial planets, indicating a lower
percentage of iron and a smaller core. (NASAMarsWeb)
The internal structure is thought to be:
 Rocky (basalt and andesite) crust: 50 km thick
 Peridotite mantle 1350 - 1850 km thick
 Solid iron / nickel & sulphur core 1500 – 2000km radius
3.3
Jovian planets
The high percentage of hydrogen in the makeup of the Jovian planets will dictate much lower
average densities than the terrestrials. From Figure 6 it is seen that the Jovians are about 12 –
20% of the density of the terrestrial planets.
Jupiter
The structure of Jupiter is thought to be as follows (CaseWestern5Web, NASAJupiterWeb):




Gaseous atmosphere: 1,000 km thick
Liquid hydrogen: 39,000 km thick
Metallic hydrogen: 18,000 km thick
Liquid iron / rocky core: 14,000 km radius
Saturn
Saturn has a similar makeup to that of Jupiter. However is the least dense of all the planets. Its
density is less than water. (OregonWeb)
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Paul Fisher
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



Gaseous atmosphere: 1,000 km thick
Liquid hydrogen: 30,000 km thick
Metallic hydrogen: 15,000 km thick
Liquid iron / rocky core: 15,000 km radius
Uranus & Neptune
Uranus and Neptune are often referred to as the “ice giants” rather than gas giants. In each
case, there is probably a core of liquid rock, surrounded by an ocean of hydrogen, helium,
water, ammonia and methane. The ocean graduates into a gaseous atmosphere.
(CaseWestern7Web, CaseWestern8Web)
3.4
Summary
In all cases, the planets are thought to have a core of iron or molten rock. In the case of the
terrestrial planets, the core is surmounted by a rocky mantle and crust. The larger Jovian planets
(Jupiter and Saturn) have “mantles” of metallic hydrogen. All the Jovians have an ocean of liquid
hydrogen and helium (plus other components), which graduates into a gaseous atmosphere.
By reason of their relatively large cores and rocky mantles, the terrestrial planets all have a
higher density than the Jovians. This is consistent with the solar nebula theory of the formation
of the Solar System, in which higher temperatures and stronger solar wind nearer the Sun do
not allow hydrogen and helium to condense out of the protoplanetary disk. Only the heavier
materials condense out and accrete at these high temperatures. Further out, the temperatures
are much lower, enabling rocks and ices to accrete into planetesimals whose gravity attracted
the lighter gases. The Jovian planets swept up the hydrogen and helium within their orbits and
grew to become very large, low density planets.
Note that the “Nice model” (Tsiganis et al 2005; Gomes et al 2005) suggests that the Jovian
planets originally formed in orbits much closer to each other, with Neptune inferior to Uranus.
Interaction with the remnant planetissimal field distorted the orbits of Jupiter and Saturn. When
these planets achieved a 1:2 orbital resonance the gravitational effect pulled Uranus closer to
the Sun and forced Neptune out to the Kuiper belt, thus triggering the late heavy bombardment.
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4 Magnetic field
4.1
The terrestrial planets
Overview
Mercury
With a dipole field strength of 0.0033 gauss-Rh3 tilted at 169°, (NASAMercuryFactWeb), the
magnetic field of Mercury is only 4 x 10-4 of that of the Earth. Luhmann & Russell (1997,1)
conjecture that because Mercury is much smaller than Earth, it would have cooled faster with its
solid core forming a larger proportion of the total. The liquid component of the core (which
generates the magnetic field) may be constrained to a thin shell. This, together with the slow
rotation of the planet, account for the weakness of its magnetic field.
Venus
Measurements by orbiting probes place the Venusian magnetic field strength at about 10-5 that
of Earth. Luhmann & Russell (1997,2) explain that the dynamo generating a planetary field in
the terrestrial planets is the result of interactions between the solid inner core “stirring” the
liquid outer core. This process apparently does not exist on Venus either: a) because the entire
core has solidified, or b) because the core has remained entirely (or nearly so) molten. Luhmann
& Russell also discount the slow rotation of Venus as a cause of the weak field.
Nimmo (2002) contends that the dynamo effect relies on convection within the inner core,
driven by the extraction of heat from the core into the mantle. On Earth, the mantle is cooled
by plate tectonics, which does not occur on Venus. Therefore the mantle and core have reached
an equilibrium temperature and the dynamo effect has ceased.
Earth
Earth’s dipole field moment is 0.3 Gauss-Re3 (NASAEarthWeb). The field is generated by the
dynamo effect of Earth’s solid inner core and liquid outer core.
Mars
Mars does not have a global magnetic field, although very strong fields exist in areas of the
crust. It is thought the global dynamo ceased operating 3.5 billion years ago.
(NASAMarsSurveyorWeb)
By examining patterns of magnetism in the various impact basins, some scientists believe that
the dynamo was disrupted by catastrophic asteroid collisions during the late heavy
bombardment. Others, however, believe that the dynamo ceased operating of its own accord.
(ScienceNowWeb)
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Paul Fisher
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4.2
The Jovian planets
Jupiter
Jupiter has the strongest magnetic field
in the Solar System, other than
localised areas of the Sun.
The dipole field strength is 4.28 gaussRj3 and the magnetic field tilt is 9.6°
(NASAJupiterFactWeb)
Jupiter’s field is caused by a dynamo
effect in the metallic hydrogen layer of
the planet’s interior. (UTennesseeWeb)
The field is complex, with the planet’s
rapid rotation distorting the radiation
belts into a “plasma sheet”. Several of Figure 9: Jupiter's magnetic field Source: NASA
Jupiter’s satellites orbit within the
magnetic field and discharge ions into the field. In particular the volcanically active moon Io
orbits in a “plasma torus” of ionised particles.
Saturn
Saturn’s magnetic field is much weaker than may be expected, with its dipole moment being
only 1/30 of Jupiter’s, despite the relatively small difference in radius. In addition, the field’s axis
aligns almost exactly with the planet’s axis of rotation. (Luhmann & Russell 1997,3)
The dipole field is strength 0.210 gauss-Rs3 and the tilt is <1° ( NASASaturnFactWeb)
The relative weakness of the field would be consistent with the smaller quantity of liquid
metallic hydrogen compared with Jupiter.
Uranus
The axial of Uranus is about 98°, meaning that the planet
rotates on its side, when compared with the ecliptic.
However, the magnetic field is tilted some 59° to the
planet’s axis. The magnetic field is asymmetric, with the
dipole axis being offset some 0.3 times the planetary
radius. The field’s maximum strength is approximately
10 times the minimum. (Ness et al, 1986)
As Uranus does not have a metallic hydrogen layer, it is
most likely that the dynamo operates within a thin shell
of the ice layer, composed of H2O, CH4, NH3 and H2S Figure 10: Magnetic field of Uranus Source: Wikimedia
(Stanley & Bloxam 2004). This would explain the non- Commons
axis symmetric nature of the field (Arnou 2004).
However it has also been conjectured that the offset could have been caused by the same event
that caused the planet’s rotation axis to tilt over (Freedman & Kaufmann 2008).
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Paul Fisher
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Neptune
Neptune has a magnetic dipole field strength of 0.142 gauss-Rn3, tilted at 46.9°.
(NASANeptuneFactWeb)
Like Uranus, Neptune’s field is both tilted and offset. (Stanley & Bloxam 2004; Arnou 2004) As
with Uranus, the non-axis symmetric nature of the field is thought to be caused by the dynamo
only acting within a thin shell of fluid ices.
4.3
Summary
The magnetic fields of the planets show great variety. They range in strength from effectively
zero at Mars and Venus, to enormously powerful on Jupiter. Mars has no planetary field today,
but shows evidence of a strong field which disappeared 3.5*109 years ago.
In so far as the terrestrials have a field, it is thought to arise from convection currents in the
liquid nickel/iron component of the planetary core, perhaps “stirred” by the solid inner core. In
the gas giants Jupiter and Saturn, the field dynamo is in the liquid metallic hydrogen layer. On
Uranus and Neptune, it is thought to lie in a thin fluid shell within the ice layers.
The planetary fields are more-or-less aligned with the axis of rotation, except Uranus and
Neptune, where they are offset and tilted up to 60°.
The peculiarities of the fields are consistent with the standard model of solar system formation:
they are generated in the iron cores of terrestrial planets and in the hydrogen or ice layers of
the Jovians. This reflects the distribution of material in the solar nebular due to temperature
gradients and solar wind effects as the planets were formed.
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Paul Fisher
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5 Moons and rings
5.1
The terrestrial planets
Mercury and Venus
Mercury and Venus have no known natural satellites or ring systems.
Earth
Earth has one natural satellite – the Moon. The Moon is large in proportion to the size of its
planet - about 27% of Earth’s diameter. Current theory is that the Moon was formed by a
collision between the Earth and a body about the size of Mars. The body may have formed in
Earth’s L4 or L5 Lagrangian points then gradually had its orbit perturbed by interactions with the
gravity of other planets and planetesimals, eventually colliding with Earth in a tangential
trajectory. The collision debris would have formed a disk around Earth, which condensed into
the Moon. (Hartman & Davis 1975; Bellbruno & Gott 2005)
Earth has no ring system.
Mars
Mars has two very small satellites, Phobos and Deimos. Both are thought to be captured
carbonaceous rock asteroids. (UTexasWeb).
Phobos is irregularly shaped, with a mass of 1.1*1016 kg. Its average distance from the planet is
9378 km, which places it close to the Riche limit. Its orbital period is only 7h39m. Phobos is
being pulled closer to Mars by tidal forces, and will eventually either crash into the planet, or
disintegrate and form a ring around the planet. (UTexasWeb). The density of Phobos is very
low, leading to the conclusion that it is not a solid body but a loosely bound conglomeration – a
“rubble ball”.
Deimos is even smaller than Phobos, with a mass of 1.8*1015 kg. It orbits at 23,460 km, and is
gradually spiralling away from the planet. (UTexasWeb)
5.2
The Jovian planets
Jupiter
Jupiter has at least 63 moons. (NASAJupterMoonsWeb) The four
largest moons Ganymede, Callisto, Europa and Io were discovered by
Galileo and are referred to as the Gallilean moons. At 5268 km
diameter, Ganymede is the largest moon in the Solar System, and is
actually larger than Mercury.
Each of the Galilean moons is a distinct world with its own
characteristics. Io is volcanically active, with internal heat being
generated by tidal effects. Europa has a water ice surface, while
Callisto and Ganymede are both thought to contain large quantities
of water.
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Paul Fisher
Figure 11: Jupiter and the
Galilean moons
Source: NASA
Page 16
Most of the other satellites are thought to be captured asteroids.
Jupiter's diameter is approximately 143,000 km. The ring system begins about 92,000 km from
Jupiter's centre and extends out to 250,000 km. (NASAJupiterRingsWeb) The Galileo spacecraft
has provided evidence that the rings are formed by dust being shed from the inner moons
Amalthea, Thebe, Adrastea and Metis, under bombardment from asteroids and other particles.
The Jovian rings are far less spectacular than Saturn’s and were not seen until the Voyager
spacecraft undertook a flyby.
Saturn
Saturn is noted for its spectacular system of rings which are easily visible through a small
telescope on Earth. The rings are a complex
system of multiple strands, associated in many
cases with small “shepherd moons” which orbit
along the periphery of each ring set and
maintain their structure by gravitation. The
main rings span some 282,000 km, but are less
than a kilometre thick. (NASASaturnRingsWeb).
Figure 12: Saturn from spacecraft Cassini
Source: NASA
The rings are composed of water ice and rocky
particles.
The Spitzer space telescope has discovered new,
extremely tenuous, rings extending far beyond the classical system to a distance of 207 Saturn
radii (1.2x107 km). (Verbiscer, Skrutskie & Hamilton 2009)
Saturn has some 63 natural satellites. Many of these are intimately involved with the ring
system, as shepherd moons and as a source of dust and ice to replenish the rings.
(NASASaturnMoonsWeb)
The largest moon is Titan, with a diameter of 5150 km. Titan has a thick atmosphere of about
95% nitrogen – similar to that of Earth’s early history. The small ice moon Enceladus has a series
of enormous water ice geysers near its south pole. (JPLWeb)
Uranus and Neptune
The ice giants have 27 and 13 known satellites respectively. (NASAUranusMoonsWeb)
The larger moons are a mixture of water ice and rock, while the others are thought to be
captured asteroids. Neptune’s moon Triton has a retrograde orbit and is thought to be a
captured Kuiper belt object. It is slowly being pulled toward the planet by tidal forces and will
eventually break up to form a large ring. (NASANeptuneMoonsWeb)
Both planets have narrow rings. Those of Neptune contain arcs which are thicker than the rest
of the rings – it is thought that gravity of the moon Galatea shepherds the ring particles into
these arcs. (Namouni & Porco 2002) Uranus’ ring system comprises 13 rings, one of which has
an embedded satellite, Mab. (HubbleWeb)
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Paul Fisher
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5.3
Summary
Each of the Jovian planets has a multitude of moons, and a system of rings. Many of the moons
are captured asteroids. Satellites seem to play a role in the formation and maintenance of the
ring systems, either as shepherds or as a source of replenishing particles. The ring systems
appear to rely at least in part on having a large number of satellites to maintain the rings’
structural integrity.
The terrestrial planets have no ring systems. Earth has a single large moon, likely caused by a
collision with a Mars size object. Mars has two tiny satellites, most likely captured asteroids.
It is likely that the inner regions of the solar nebula were largely cleared of accreting particles
very early in the history of the solar system. (Gomes et al 2005) There was insufficient material
to form satellites of the planets. Further, the giant planets would likely have formed in their
own protoplanetary disks, from which their larger moons were also formed. The terrestrial
planets are so relatively small that such disks would not have had sufficient material to form
satellites.
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Paul Fisher
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th
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