Download The Solar System

Introduction to
the Solar System
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Some Basics
Facts
The solar system consists of
The Sun
Nine planets
More than a hundred (100) satellites of
the planets
A large number of smaller bodies
Asteroids and comets
Cosmic dust
Countless grains of broken rock
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Planetary Orbits in the Solar System
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Inner Solar System
The Sun and the four planets closest to it belong to
what is called the inner solar system
Thus the inner planets are
Mercury
Venus
Earth
Mars
These 4 inner planets
are also called
terrestrial planets
The Earth and the
other three are
similar in that they
are composed mainly
of rocks and metals
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Outer Solar System
The part of the solar system outside the inner
solar system is called the outer solar system
Thus, the planets in the outer solar system are
Jupiter
Saturn
Uranus
Neptune
Pluto
Jupiter, Saturn,
Uranus, and
Neptune are
several times larger than the Earth and hence
are called the giant planets
They are also called the jovian planets
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Some More Basic Facts
Object
Sun
Jupiter
Comets
Percentage of solar system’s
total mass
99.80
0.10
0.05
All other planets
0.04
Satellites & rings
0.00005
Asteroids
0.000002
Cosmic dust
0.0000001
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The Nine Planets (1)
The planets’ sizes are not to scale
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The Nine Planets (2)
Outer planets
Inner planets
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The planets’ sizes are to scale
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Main Characteristics of the Planets
Name
Distance
from Sun
(AU)
Revolution
Period
(Years)
Diameter
(km)
Mass
(1023 kg)
Density
(g/cm3)
Mercury
0.39
0.24
4,878
3.3
5.4
Venus
Earth
Mars
0.72
1.00
1.52
0.62
1.00
1.88
12,102
12,756
6,787
48.7
59.8
6.4
5.3
5.5
3.9
Jupiter
5.20
11.86
142,984
18,991
1.3
Saturn
9.54
29.46
120,536
5,686
0.7
Uranus
19.18
84.07
51,118
866
1.2
Neptune
30.06
164.82
49,660
1,030
1.6
Pluto
39.44
248.60
2,200
0.01
2.1
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(Natural) Satellites
Only Mercury and Venus do not have a
moon or (natural) satellite
There are more than 100 satellites known
today
More are being discovered on a regular basis
The largest of the satellites are as big as a
small planet, including
our Moon
the four largest moons of Jupiter
called the Galilean satellites after their discoverer
the largest moons of Saturn and Neptune
respectively called Titan and Triton
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Rings (1)
Each of the jovian planets has rings made up of
countless small bodies
They range in size from grains of dust to
terrestrial mountains
Each ring system orbits its
planet at its equator
The rings of Saturn are
the best known, the
widest, and by far the
easiest to see
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Rings (2)
Recent missions near the jovian
planets showed that the rings
have complex shapes
are influenced by the pull of the planets'
satellites
Jupiter’s rings
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Uranus’ rings
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Neptune’s rings12
Asteroids
These are
rocky and metallic objects, quite modest in size
important members of the solar system
found in great number
between the orbits of
Mars and Jupiter
now believed to be
remnants of the initial
population of the solar
system
Asteroids Mathilde, Gaspra, and Ida
Some of the smallest
satellites of the planets
are believed to be asteroids captured by the
gravitational pull of the planets
The two moons of Mars are nowadays thought to have
such an origin
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Comets
These are another class of small bodies
They are composed in part of ice, made of
frozen gases such as water, carbon dioxide,
carbon monoxide, and methyl alcohol
Comets are believed to be remnants
from the formation of the solar system
With rare exceptions, comets orbit the
Sun in distant, cooler regions
Their orbits are large and very eccentric
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Cosmic Dust
Solar system contains countless grains of broken
rock which one refers to simply as cosmic dust
Comic-dust particles constantly collide with the
planets and become trapped by their gravitational
pull
Million of these hit Earth's atmosphere every day
When these particles enter the atmosphere, they
heat up quickly, burning and producing brief flashes
of light that we see as shooting stars or meteors
Occasionally some of the larger chunks survive
the passage through the atmosphere and land on
Earth to become meteorites
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Terrestrial or Rocky Planets
Mercury, Venus, Earth, and Mars are called
terrestrial or rocky planets
They
are composed primarily of rocks and metals
have relatively high densities
have relatively slow rotation
have solid surfaces
have no rings and few satellites
have oxidized chemistry
because they are largely composed of oxygen compounds
Venus has an unusual direction of rotation about
its axis, compared to the other planets
It spins on its axis backwards
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Jovian or Giant
Planets
Jupiter, Saturn,
Uranus, and Neptune
are called jovian or giant planets
They
are composed primarily of hydrogen and helium
generally have low densities
have relatively rapid rotation
have deep atmospheres
have extensive ring system and many satellites
have reduced chemistry
because they are largely composed of hydrogen and its
compounds
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Trends in Temperature
Planets and satellites
do not generate their own heat as the Sun does
are heated by the radiant energy of the Sun
Mathematically, the temperatures decrease
approximately in proportion to the square
root of the distance from the Sun
Thus, the farther a planet/satellite is from
the Sun, the cooler it is
Mercury has a surface temperature of 500 K
Pluto’s temperature is only about 50 K, which is colder
than water’s freezing point (273 K)
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Earth’s Uniqueness for
Life Support
Because of the strong dependence of
temperature on distance from the Sun,
Earth is the only planet where
the surface temperatures lie between the
freezing and boiling points of water
water can be liquid on the planet’s surface
Therefore, Earth is the only planet that
can support life — at least life as we
know it
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Physical Appearances
The external appearance of a planet is
determined by
its surface composition
external bombardments on it
geological activity on the planet
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Planets under Attack!!
Planets are bombarded with a slow
rain of projectiles from space
The impact of the projectiles on the
surface of the planet leaves craters of all
sizes
The amount of craters a planet has on its
surface may provide clues about its
history
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Shoemaker-Levy 9
A dramatic example:
impacts of large
pieces of Comet
Shoemaker-Levy 9
with Jupiter in
summer 1994
Sequence of images taken by Hubble Space Telescope
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Geological Activity (1)
The geological activity
on terrestrial planets
involves the internal
forces that constantly
reshape their surfaces by
buckling and twisting their crusts
building up mountain ranges
creating and erupting volcanoes
Geological activity results from a hot interior
Mountains/volcanoes arise from the buildup and
release of heat escaping from a planet’s core
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Geological Activity (2)
Each of the planets is believed to be heated at
the time of its birth
This heat initially powered extensive volcanic
activity
Small bodies, such as our Moon, soon cooled off
The larger the body, the more likely it is to
retain its internal heat
Thus, it is more likely to see surface evidence of
geological activity on the larger planets/satellites
The inner planets appear to conform to this
expectation
Mercury and our Moon are geologically dead
Earth and Venus are still geologically active
Mars represents an intermediate case
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The Dating Game
Two popular methods for estimating the age of a
planet or satellite or its surface
Crater counting
Radioactivity dating
The crater counting method is based on the
assumptions that
the rate at which asteroids or comets bombard a
planet’s surface is roughly constant for a long time
the planet’s surface has not been reshaped, and
the craters have never been erased, since it
experienced a major change
If these assumptions are satisfied, then the
number of craters will be proportional to the
length of time the surface has been exposed
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Counting Craters
The number of craters on
Moon
the surface of a planet
provides a clue about the length of time the
surface has undergone cosmic bombardment
since it solidified or geological activity
stopped
Mercury
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Radioactive Decays (1)
Radioactivity is a natural phenomenon that
was discovered at the beginning of the 20th
century
Some atomic nuclei are not stable, but can
spontaneously split apart, or decay, into smaller
nuclei
Such nuclei are said to be radioactive
For any one radioactive nucleus, the decay
process happens randomly — it is not possible to
predict when the decay will occur
Radioactive decays involve the emission of
particles, such as electrons, or of radiation in the
form of gamma rays
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Radioactive Decays (2)
The rate at which nuclei decay is most easily
expressed in terms of their half-life
The half-life is a specific time period during which the
chances are fifty-fifty that decay will occur for any one
of a large number of radioactive nuclei
For example, if you had 1 gram of pure radioactive
nuclei of one type whose half-life is 10 years, then
after 10 years you would have ½ gram, after 20 years
¼ gram, after 30 years 1/8 gram, and so on
After many half-lives, the original radioactive nuclei
do not disappear, but instead are replaced by their
decay products
Sometimes the original nuclei are called parents and
the decay products are called daughters
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Radioactive Decays (3)
The number of radioactive nuclei in a sample
decreases exponentially
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Radioactive Elements as Clocks
Radioactive elements can serve as natural
clocks if one can determine how many of the
initial radioactive parents have been
replaced with their daughters
By comparing how much of a radioactive parent
material is left in a rock with how much of its
daughter material has accumulated, one can
learn how long the decay process has been going
on and hence how long ago the rock formed
This method involves the assumption that the
rock being studied has been isolated since its
formation — none of the parents and daughters
in the rock have leaked out of it or been polluted
by outside contaminants
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Common Dating Elements
Radioactive Decay Reactions Used to Date Rocks
Parent Nucleus
Daughter Nucleus
Half-Life
Samarium (147Sa)
Neodymium (143Nd)
106
Rubidium (87Ru)
Strontium (87Sr)
48.8
Thorium (232Th)
Lead (208Pb)
14.0
Uranium (238U)
Lead (206Pb)
4.47
Potassium (40K)
Argon (40Ar)
1.31
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Ages of the Moon and Earth
Astronauts that visited the Moon brought back
lunar rock for radioactive age dating
Counting craters had given different ages for
different parts of the Moon’s surface
Samples brought back enabled an accurate
dating of the lunar surface which showed that
the Moon is an ancient, geologically dead body
Radioactive dating of rocks brought back by the
Apollo mission and of rocks here on Earth
indicated that the two bodies have similar ages
According to this dating method, both were
formed some 4.5 billion years ago
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Origin of the Solar System
An observation of the Sun and the nine
planets of reveals some patterns
The planets all revolve around the Sun in the
same direction
Also, the planets lie in nearly the same flat plane
The Sun spins in the same direction about its
own axis
Astronomers regard these facts as evidence
that the Sun and the planets formed
together as a spinning system of gas and
dust, referred to as the solar nebula
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Origin and Composition (1)
The composition of planets provides another
clue about the origin of the solar system
Spectroscopic analysis allows a determination of
what chemical elements are present in the Sun
and the planets
Such analysis shows that the Sun has the same
hydrogen-dominated composition as Jupiter and
Saturn
This suggests that these three bodies were formed
from the same reservoir of material
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Origin and Composition (2)
By contrast, spectroscopic analysis shows
that the terrestrial planets and satellites
are relatively deficient in the light gases and the
various ices composed of the common elements
oxygen, carbon, and nitrogen
contain mostly heavy elements like iron and
silicon — these are rare on the Sun and the giant
planets
This pattern suggests that the processes
which led to the formation of the inner
planets must have somehow rejected much
of the lighter materials
These appear to have escaped, leaving behind
just a residue of heavy stuff
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Other Planetary Systems (1)
An additional approach to understanding the
origin of the solar system is to look outside the
solar system for evidence
Many stars is space are much younger than the
Sun, and planet formation might be occuring in
some of these star systems that could be
accessible to direct observation
Astronomers have observed other “solar
nebulas” or circumstellar disks — flattened
spinning clouds of gas and dust surrounding
young stars
These are believed to be examples of what our
solar system may have looked like when it
started to form
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Other Planetary Systems (2)
Unfortunately, because of the enormous distance
involved, and the fact that planets forming are not
likely to reflect light very efficiently, astronomers
have yet to observe the actual formation of planets
in distant solar nebulas
Astronomers have now developed tools that enable
the observation of planets orbiting distant stars
The technique is still limited to the detection of giant
planets only
Observations nonetheless indicate clearly that other
solar systems have configurations that vastly differ
from the configuration of our solar system
Some have planets with very elliptical orbits
In other cases, giant planets are very near their stars
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