Download Document

Overview of the Solar System
• The solar system formed about 4.5 billion years ago from
a giant cloud of gas and dust
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
1
Inventory
• Mercury, Venus, Earth, Mars, Jupiter, and
•
•
•
Saturn were known to the ancients
Uranus, Neptune, and Pluto were
discovered after the invention of the
telescope
The masses of the planets have been
known for centuries based on Newton’s
Law of Gravity
Each planet rotates on it own axis



Most rotate in the same direction as they orbit
Venus rotates slowly backward
Uranus and Pluto are nearly tipped on their
sides
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
2
Terrestrial Planets
• The four inner planets are called the terrestrial
planets



Mercury, Venus, Earth, Mars
Composed of rock and metal
The Moon is often classified as a terrestrial body
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
3
Composition of the Terrestrial Planets
Pla net Mass (Earth = 1 ) (%)
Mercury
5.50%
Ve nus
82 .00%
Earth
10 0.00 %
Moon
1.23%
Mars
10 .70%
• Venus, Earth and Mars have similar compositions


1/3 iron-nickel and iron-sulfur
2/3 silicates
• Mercury has a higher proportion of iron
• The Moon has a much lower proportion of iron
• All terrestrial planets show differentiation

While molten, dense materials sink toward the center
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
4
Positions of the Inner Planets
• Positions of the inner planets as of February 5, 2001
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
5
Gas Giants
• The next four planets are called the giant planets



Jupiter, Saturn, Uranus, Neptune
Composed of lighter ices, liquids, and gases
Jupiter is the largest planet comprising 0.1% of the
total mass of the solar system
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
6
Composition of the Gas Giants
Pla net Mass (Ju pite r = 1 ) %
Jupiter
10 0.00 %
Saturn
29 .67%
Uranu s
4.59%
Neptune
5.41%
• Bulk of Jupiter and Saturn is compressed, liquefied
hydrogen

Liquid giants!
• Jupiter and Saturn have 75% hydrogen and 25% helium,
much like the primordial distributions

Uranus and Neptune have much less hydrogen and helium
• The gas giants have a small rocky central core

Jupiter’s core is about 10 - 15 times the size of Earth
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
7
Positions of the Outer Planets
• Positions of all the planets as of February 5, 2001
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
8
Density of Planets
• Density is defined as
mass per unit volume
• By measuring the
density, one can infer
the composition of the
planet
• The terrestrial planets
are much denser than
the gas giants
• The density of gas
giants implies that they
must have a solid core
ISP 205 - Astronomy Gary D. Westfall
Pla net Dens ity
3
Mercury
Ve nus
Earth
Mars
Jupiter
Saturn
Uranu s
Neptune
Plu to
Lecture 8
g/cm
5.4
5.3
5.5
3.9
1.3
0.7
1.2
1.6
2.1
9
Pluto
• Pluto seems to be similar to the moons of the outer planets
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
10
Geological Activity
• The crusts of the terrestrial planets show evidence
of being hit by objects from space
• Earth and Venus show evidence of volcanoes and
built-up mountain ranges


Geological activity
Planets were once molten

Early crater formation
erased

Moon and Mercury retain
crater hits
• Surface of Venus mapped
by Magellan’s radar
shows “recent” lava flows
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
11
Moons
• All the planets except Mercury and Venus have moons
orbiting them


The four largest moons of Jupiter are called the Galilean
satellites
Several of the moons are as large as Mercury (4878 km)
Pla net Numb er
La rgest Dia. (km ) Rela tive to Moon
Earth
1 Moon
37 46
10 0.0%
Mars
2 Pho bos
23
0.6%
Jupiter
16 Ganyme de
52 62
14 0.5%
Saturn
19 Ti tan
51 50
13 7.5%
Uranu s
18 Ti tani a
16 10
43 .0%
Neptune
8 Tri ton
27 20
72 .6%
Plu to
1 Charo n
12 00
32 .0%
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
12
Rings
• The Gas Giants all have rings as well as moons
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
13
Asteroids
• There are small bodies that are much
smaller than the planets and their moons

Asteroids
• Located mainly in a band between Mars
and Jupiter

Astronomers speculate that there was once a
planet or planets that were destroyed by the
gravity of Jupiter
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
14
Comets
• Comets are remnants from the formation of the solar system
• They orbit farther from the Sun than the asteroids

Oort cloud, Kuiper belt
• Many are in very eccentric orbits that bring them close to the Sun

Warmth from Sun caused the frozen material to evaporate and stream out
into the distinctive tail
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
15
Radioactive Dating
• One can determine the age of a world by determining the age of its
•
•
rocks
The ages of rocks can be determined using radioactive dating
Radioactivity is the spontaneous splitting or decaying into of nuclei
into smaller or different nuclei

The decaying nucleus is changed into a different nucleus
• Radioactivity involves the emission of various kinds of particles



Alpha particles ()
Electrons and positrons ()
Gamma rays ()
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
16
Half Life
• We cannot know exactly when a nucleus might decay
• If we have a very large number of nuclei, we can define
the half-life


After one half life, half the nuclei will have decayed
A particular nucleus may survive longer or decay earlier
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
17
Decays used to Date Rocks
Pare nt
Daug hter
Z
Sam arium -147
Rubi dium -87
Th orium -232
Urani um-23 8
Potassi um-40
147
87
Rb
232
238
40
Sm
N
62
85 Neod yn ium-1 43
37
50 Stro ntiu m-87
Th
90
14 2 Le ad-20 8
U
92
14 6 Le ad-20 6
19
21 Arg on-40
K
Z
143
87
60
83
10 6
38
49
48 .8
Pb
82
12 6
14
Pb
82
12 4
4.47
18
22
1.31
Sr
208
206
40
Nd
Half Life
N (bi llio n yr)
Ar
• These decays can be used to date rocks
• The parent products are created in stars when the
material that makes up the rocks is created
• Radioactive decay begins at that time
• We measure the relative amount daughter and
parent and calculate back the age of the rock
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
18
Origin of the Solar System
• We saw that all the planets orbit in the same direction and almost in
•
•
the same plane
The planets and the Sun rotate on their axes in the same direction
(except Venus and Uranus)
Suggests that the solar system was formed from a spinning system
of gas and dust called the solar nebula
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
19
Formation of Other Solar Systems
• We can look for formation of other solar systems
in space
• Such planetary nurseries have been observed



Solar nebula
Circumstellar disks
These system are too far away to observe planet
formation directly
• Astronomers recently have been able to identify
large Jupiter class planets around distant stars but
as yet there is no evidence for terrestrial class
planets
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
20
Orion Nebula
• In the Orion Nebula
•
•
star formation is
visible
1500 light years away
Contains new stars
about 100,000 years
old

Young compared with
our 5 billion year old
Sun!
ISP 205 - Astronomy Gary D. Westfall
Lecture 8
21