Download An Overview of the Solar System

GEOLOGY 12
CHAPTERS 22 NOTES
COMPARATIVE PLANETOLOGY
Name __________________
An Overview of the Solar System
Basics
The solar system consists of the Sun; the nine eight planets, at least 2 dwarf planets, sixty four
satellites of the planets, a large number of small bodies (the comets and asteroids), and the
interplanetary medium. The inner solar system contains the Sun (Sol), Mercury, Venus, Earth and
Mars. The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and the lesser or
dwarf planets Pluto and Eris. The asteroid belt separates these two groups.
The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and
Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane
(called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7
degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the
ecliptic with an inclination of 17 degrees. The planets all orbit in the same direction (counterclockwise looking down from above the Sun's north pole); all but Venus and Uranus also rotate
counterclockwise.
There are numerous smaller bodies in the solar system: the satellites or moons of the planets; the
large number of asteroids (small rocky bodies) orbiting the Sun; and the comets (small icy
bodies) which come and go from the inner parts of the solar system. With a few exceptions, the
planetary satellites orbit in the same direction (counterclockwise) as the planets and approximately
in the plane of the ecliptic but this is not generally true for comets and asteroids.
Classification
The classification of these objects is a matter of minor controversy. Traditionally, the solar system
has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons,
variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and
comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been
found to be more complicated than this would suggest:
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there are several moons larger than Pluto and Eris (formerly known as "Xena" or 2003 UB313)
and two larger than Mercury;
there are several small moons that are probably captured asteroids;
comets sometimes fizzle out and become indistinguishable from asteroids;
the Kuiper Belt objects and others like Pluto’s moon, Charon, don't fit this scheme well (The
Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU
(astronomical unit = distance from sun to earth) from the Sun containing many small icy
bodies. It is now considered to be the source of the short-period comets.);
The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets".
Other classifications based on chemical composition and/or point of origin can be proposed which
attempt to be more physically valid. But they usually end up with either too many classes or too
many exceptions. The bottom line is that many of the bodies are unique; our present
understanding is insufficient to establish clear categories.
Conventional Categorizations
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The nine bodies conventionally referred to as planets are often further classified in several ways:
1. BY COMPOSITION
(a) terrestrial or rocky planets: Mercury, Venus, Earth, and Mars
• The terrestrial planets are composed primarily of rock and metal and have relatively
high densities, slow rotation, solid surfaces, no rings and few satellites.
(b) Jovian or gas giants: Jupiter, Saturn, Uranus, and Neptune
• The gas planets are composed primarily of hydrogen and helium and generally have
low densities, rapid rotation, deep atmospheres, rings and lots of satellites.
(c) under debate
• Pluto and Eris
2. BY SIZE
(a) small planets: Mercury, Venus, Earth, Mars and Pluto.
• The small planets have diameters less than 13000 km.
(b) giant planets or gas giants: Jupiter, Saturn, Uranus and Neptune.
• The giant planets have diameters greater than 48000 km.
(c) lesser or dwarf planets
• Mercury, Pluto and Eris are sometimes referred to as lesser planets (not to be
confused with minor planets which is the official term for asteroids)
3. BY POSITION RELATIVE TO THE SUN
(a) inner planets: Mercury, Venus, Earth and Mars
(b) outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto
(c) asteroid belt: between Mars and Jupiter; forms the boundary between the inner solar
system and the outer solar system
4. BY POSITION RELATIVE TO EARTH
inferior planets: Mercury and Venus
• closer to the Sun than Earth
• The inferior planets show phases like the Moon's when viewed from Earth.
superior planets: Mars thru Pluto
• farther from the Sun than Earth
• The superior planets always appear full or nearly so.
5. BY HISTORY
classical planets: Mercury, Venus, Mars, Jupiter, and Saturn
• known since prehistorical times
• visible to the unaided eye
modern planets: Uranus, Neptune, Pluto and Eris
• discovered in modern times
• visible only with telescopes
Chapter 22 Notes
Page 2
The Nebular Disk Theory and The Origin of the Solar System
Here is a brief outline of the current theory of the events in the early history of the solar system:
1. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its
own gravity. The disturbance could be, for example, the shock wave from a nearby supernova.
2. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust
to vaporize. The initial collapse is supposed to take less than 100,000 years.
3. The center compresses enough to become a protostar and the rest of the gas orbits around it.
Most of that gas flows inward and adds to the mass of the forming star, but since the gas is
rotating the centrifugal force prevents some of the gas from reaching the forming star. Instead,
it forms an "accretion disk" or “NEBULAR DISK” around the star. The disk radiates away its
energy and cools off.
4. The gas cools off enough for the metal, rock and ice to condense out into tiny particles. (i.e.
some of the gas turns back into dust). The metals condense almost as soon as the accretion
disk forms; the rock condenses a bit later. The lighter elements remain in gaseous form.
5. The dust particles collide with each other and form into larger particles. This goes on until the
particles get to the size of boulders or small asteroids. Once the larger of these particles get
big enough to have significant gravity, their growth accelerates and, very quickly, they form
protoplanets. (Much like the dust bunnies under your bed!)
6. About 1 million years after the nebula cooled the sun would have generated a very strong solar
wind which swept away all of the gas left in the protoplanetary nebula. If a protoplanet was
large enough its gravity would pull in the nebular gas and it would become a gas giant. If not,
it would remain a rocky or icy body.
7. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants.
The "planetesimals" slowly collided with each other and become more massive. Once the
solar system was mostly clear of debris, planet building ended.
8. The original atmosphere of the Earth, Venus and Mars consisted of Hydrogen and Helium.
Those light elements, however, were heated to escape velocity by solar radiation and thus
much of this original atmosphere escaped. The current atmospheres of the Earth, Venus and
Mars evolved from other processes. Early bombardment brought some of the materials from
which atmospheres and oceans formed. Outgassing, gasses blown out of volcanoes, is
another likely source for atmosphere's formation.
9. It is likely that there was NOT enough water released via outgassing to account for the
present day oceans. A significant portion of the ocean water may have been delivered to the
Earth after it formed by the impact of comets.
10. On Earth, oxygen was produced by photosynthetic organisms (cyanobacteria / blue-green
algae / stromatolites) breaking down CO2.
11. Rings around giant planets, such as Saturn, are probably the result of stray planetesimals
being torn apart by gravity (tidal forces) when they ventured too close to planet.
Chapter 22 Notes
Page 3
Planetary Characteristics
ROCKY PLANETS
Diverse atmospheres cover terrestrial planets. Shells of silicates overlay inner metallic cores.
1. Mercury - The surface of Mercury and the Earth's Moon have very similar, old crusts
displaying a nearly complete record of impacts over the last 4 billion years. Caloris Basin is a
ringed impact crater of an asteroid that hit 4 billion years ago. No satellites.
2. Venus - An atmosphere so thick that its surface can only be mapped by radar. The surface
contains abundant evidence for volcanoes such as Fula Mons which is 1.8 miles high.
3. Earth's Moon - Same age as the earth (4.6 billion years). The moon's crust, with craters,
fractures, and lava flows, is four times thicker than earth's. Large mare (“lunar seas”) on the
near side of the moon are actually composed of dark basalts. Highlands dominate the far side
of the moon. Interior heat from radioactivity caused partial melting and basaltic material rose
along fractures, filling basins layer by layer to form the mare.
4. Mars - Shows polar ice caps, large volcanoes (Olympus Mons sours 15 miles above its
surface, has a base 335 miles across, and a 45 mile wide caldera), rift canyons extending the
length of the United States (Valles Marineris is 150 miles wide and 4 miles deep), shifting sand
dunes and other indications of wind erosion, and river canyons that must have been cut by
torrential rains.
GIANT PLANETS
Emitting more energy than they receive from the sun, the gas giants are actually composed of
much the same material as the sun. Together they account for more than 90% of the mass of the
solar system.
1. Jupiter - Cloudy with high-pressure storms lasting years to centuries. Internal heat drives the
turbulence. Jupiter's cloud cover consists of three layers: ammonia crystals over ammonium
hydrosulfide over ice crystals. Below the atmosphere is a sea of liquid hydrogen and helium.
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Io - Youngest surface in the solar system. Erupts continually. Heated by gravitational tugs
(tidal forces) from Jupiter and Europa. Displays three types of plumes due of interactions
between its molten silicate interior, sulfurous mantle, and hard sulfur crust.
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Europa - Has one of the smoothest surfaces in the solar system - most likely a thin ice
crust over a global ocean of water. Streaks that paint the surface were probably fissures in
the ice that were filled by upwelling water or soft ice. Their patterns suggest that at one
time Europa's ice crust was expanding and cracking on a large scale. Only a few impact
craters are found the moon's surface, indicating it is relatively young.
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Ganymede - Largest moon of solar system. Fresh white ice was ejected by most recent
meteorite impacts. Strips of alternating parallel grooves and ridges indicate crustal
movement millions of years ago. Crustal faulting and movement has reworked the surface
to leave only the most recent impact craters.
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Callisto - Concentric ridges were heaved up by the collision of a huge meteorite. The
impact basin has been filled in by ice, and, though later battered by smaller meteorites, is
nearly level. Callisto shows a nearly complete record of impacts. (∴No tectonics.)
Chapter 22 Notes
Page 4
2. Saturn - Saturn's atmosphere is like Jupiter's but less dynamic because it is further from the
Sun and, hence, colder. The rings of Saturn are 100,000 km across and as few as ten meters
thick
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Mimas - The ammonia and water ice of Mimas is nearly as rigid as granite.
Enceladus - Ices oozed through fissures.
Titan - Main gases are nitrogen and methane with methane in three phases (s, l, g).
ICY PLANETS
Have cores equal to the closer-in giants but have much smaller accretions of gas. Huge mantles of
water, methane, and ammonia probably underlay their dense atmospheres.
1. Uranus - Hides its atmospheric features beneath a deep layer of hydrogen. Very bland
surface. Unlike Jupiter, Saturn, and Neptune, it has no severe turbulence beyond local
thunderheads.
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Miranda - A complex terrain which may have been repeatedly shattered by collisions and
reassembled by gravity. Ammonia and water ices, erupting as lava does on Earth, could
have partly smoothed the surface. Has ice cliffs higher than the Earth's Grand Canyon.
2. Neptune - Orbiting in a deep freeze within 60°C of absolute zero, Neptune unleashes winds
that may be the fastest in the solar system. Storms form as convection shoots hydrocarbon
gases up into colder regions where they condense into bright ices.
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Triton - Largest moon
3. Pluto – Formerly the farthest planet from the Sun and by far the smallest. Pluto is smaller than
seven of the solar system's moons. Pluto's orbit is highly eccentric. At times it is closer to the
Sun than Neptune (1979 until 1999). Pluto rotates in the opposite direction from most of the
other planets. Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it
is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the
surface seem to be covered with ices of nitrogen with smaller amounts of solid methane and
carbon monoxide.
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Charon is named for the mythological figure who ferried the dead across the River Styx
into Hades (the underworld). Prior to 1978 it was thought that Pluto was much larger since
the images of Charon and Pluto were blurred together. Charon's composition is unknown,
but its surface seems to be covered with water ice. Charon was formed by a giant impact
similar to the one that formed Earth's Moon.
4. Eris – (formerly Xena or 2003 UB313) In Greek mythology, Eris (known as the goddess of
discord, strife, and chaos) caused a quarrel among the goddesses that sparked the Trojan
War, while her daughter Dysnomia was known as the goddess or spirit of lawlessness. Not
surprisingly, the discovery of Eris and the finding that it is larger than Pluto have also caused
strife in the astronomical community, forcing some astronomers to produce a strict definition of
the term "planet" which eventually led to Pluto losing its status as the "ninth planet" that it had
held since its discovery in 1930.
Initially nicknamed Xena, the object is currently located at around 98 AU, a distance that is up
to more than three times farther out than that of Pluto or Neptune. However, Eris will
eventually move in as close as Pluto and Neptune in a 557-year orbit around the Sun. It has
an elliptical orbit that is more eccentric than Pluto's. The dwarf planet is also tilted almost 44.2
Chapter 22 Notes
Page 5
degrees from the ecliptic so many astronomers assume that encounters with a more massive
object moved it into its current, highly inclined orbit.
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Dysnomia - On September 10, 2005, astronomers and engineers at the Keck Observatory
on Mauna Kea in Hawaii discovered that Eris has a satellite or "moon."
The Ecliptic - where most of the planets orbit
The Eccentric Orbits of Pluto and Eris
Chapter 22 Notes
Page 6
Geological Processes
One reason surface processes are emphasised both in planetary and conventional Earth based
geology is that they are the easiest to observe. Also, they give evidence of the evolution of that
planet. You should be aware of what drives these processes, both internal and external. There
are various forms of energy that do work on different materials.
Source of Energy
Related Processes
Gravitational Energy
accretion of planets
impact cratering
internal differentiation
isostatic adjustment
atmospheric circulation
hydrospheric circulation
tidal heating and orbit evolution
Radioactive Decay
convection in planets
volcanism
tectonism
planetary differentiation
Solar Energy
atmospheric circulation
hydrospheric circulation
solar wind
biologic activity
Chemical Energy
atmospheric evolution
rock alteration and weathering
condensation of nebula
biological activity
NOTE:
No simple scheme fits all circumstances, but the list should be helpful as a way to understand
some of the driving forces.
For example, some very complex processes are likely
going on surrounding Io, the inner moon of Jupiter. Not
only is this satellite thought to be heated by tidal
interactions (Jupiter's strong magnetic field and intense
radiation belts interact to control the way the sulphur fumes
from its volcanoes are removed from the surface) but a
huge electric field and strong electrical currents are also
likely involved.
Be sure to check our website or Google images for color
photos of Io. It has appeared on previous exams.
Chapter 22 Notes
Page 7
Geological Processes (cont’d)
Volcanism
Volcanic activity occurs on a planet or moon when sufficient heat inside the solid body causes
partial melting of the interior rock. Eruptions of the molten rock, called magma, and gases onto the
surface produce a variety of volcanic landforms. Surfaces of the terrestrial planets show volcanic
mountains of different shapes and sizes, fields of lava flows, lava channels, and ash and cinder
deposits. The forms of volcanoes and other volcanic products is controlled by such factors as the
composition and fluidity of the magma, nature of the vent, eruption rate and duration, planetary
atmosphere, and gravity.
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Impact Cratering
(see text p.500)
Impact cratering involves the collision of solid bodies, such as a meteorite into a planet, resulting
in roughly circular, excavated holes. Rock material excavated from the crater is called ejecta. This
ejecta is distributed radially from the crater onto the surface of the planet as fragmental debris.
The size and shape of the crater, and the form and extent of the ejecta depend upon a number of
factors: impact energy, strength of the target material, rock compositions, presence of volatiles
(such as water), and gravity.
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Tectonics
Tectonics is a general term referring to the large-scale deformation (or change) of rock in
response to forces causing faulting and folding. The forces acting upon a rock mass are generally
termed tensional (pulling apart), compressional (squeezing together), or shear (parallel sliding).
The magnitude and direction of the force (stress), the temperature and confining pressure on the
rock, the composition of the rock, and the rate at which the rock is deformed determine how the
rock changes in length, shape or volume. Common landforms resulting from tectonic processes
are mountain ranges, rift zones, faults, fractured rock, and folded rock masses.
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Weathering and Erosion or Gradation
Gradation is the set of processes that work on surface rocks to break, loosen, move, and finally
deposit the smaller pieces. The processes are: weathering (the chemical decay or mechanical
break-up of rocks), erosion, transportation (the removal of weathered rocks by moving water, wind,
ice, or gravity), and deposition. The presence of water and atmosphere drive the chemical decay
of rock. Gravity moves landslides. Moving water, wind, and ice create landforms such as river
channels, deltas, beaches, sand dunes, glacial deposits and valleys. On airless, waterless planets,
gradation appears to be limited to impact cratering and the pull of gravity.
Chapter 22 Notes
Page 8
Comparison Charts of Geological Processes
TERRESTRIAL PLANETS
Erosion and
Transportation
Weathering
Planet
chemical
decay
mechanical
break-up
Mercury
"solar wind"
chemical
changes
meteorite
impacts
Venus
heat, acid
yes
Earth
pressure of ice,
water, acid,
roots; heating &
oxidation,
cooling; saltation
organics
in rivers
Moon
meteorite
impacts, heating
& cooling
Mars
water and
carbon dioxide
ice
oxidation
Deposition
water wind ice gravity
yes
yes
yes
yes
yes
yes
yes
yes
landslides on innercrater rims, crater
ejecta
yes
landslides, wind
streaks
yes
landslides, deltas,
flood plains, beaches,
sand dunes, glacial
deposits, soil, dust
yes
landslides on innercrater rims, crater
ejecta, regolith
yes
landslides, crater
ejecta, wind streaks,
sand dunes, channels,
regolith, dust
Volcanism
Planet
lava
flows
Impact Cratering
Tectonics
yes
yes
pyroclastics volcanoes
Mercury
yes
probably
Venus
yes
probably not
yes
yes
yes
Earth
yes
yes
yes
yes
yes, global plate
tectonics
Moon
yes
yes
yes
yes
Mars
yes
yes
yes
yes
yes
In addition to the textbook, be sure to check our website or Google images for photos of the
various geological processes. They have appeared on previous exams.
Chapter 22 Notes
Page 9
Model of the Earth’s Interior
By Composition
V
Crust
U
W
Mantle
X
Y
Core
Z
- 7 km to 70 km thick
- silicate rocks
- 2900 km thick
- ultramafic,
ferromagnesian
silicates
- 3400 km thick
- nickel and iron
V
U
W
Crust
Lithosphere
Asthenosphere
7 km to 70 km thick
100 km thick
250 km thick
X
Mantle
2900 km thick
Y
Outer Core
2000 km thick
Z
Inner Core
1400 km thick
By Physical properties
Oceanic Crust
Continental Crust
100 km thick
Cool, rigid rock
Asthenosphere
250 km thick
Hot, plastic rock
Mesosphere
2500 km thick
Hot, rigid and brittle rock
Outer Core
2000 km thick
Liquid iron, nickel
Inner Core
1400 km thick
Solid iron, nickel
Lithosphere
Compare the cross-section of the Earth with that of the moon and Jupiter.
How do we know these models are
correct?
Although we don’t have “solid proof”
scientists study gravitational and
magnetic measurements to suggest
models for the interior structure of the
planets. For the Earth and the moon
we have also have the advantage of
being able to study seismic waves.
(i.e. earthquakes and moonquakes).
Chapter 22 Notes
Page 10
The following diagrams show a cross-section of the Earth, and a graph of earthquake wave
velocities against depth in the Earth.
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The Earth’s overall density is 5.5 g/cm3, yet the density
of the crust averages only 2.8 g/cm3. This fact implies
that the densities of the mantle and core must be
greater than that of the crust
•
Both P and S wave velocities increase as they pass
down through layer X or the mantle. This increase in
velocity is because with increasing depth, layer X
becomes denser.
•
The S waves have no velocity or stop below a depth of
about 2900 km. This is because the S waves are
entering a liquid layer.
Chapter 22 Notes
Page 11
Earth’s Composition and Structure
Chapter 22 Notes
Page 12