Download Ch. 23: “Touring Our Solar System”

Ch. 23: “Touring Our
Solar System”
23.1: “The Solar System”
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Useful information comes from the chemical composition of
the Solar System
 Terrestrial planets (inner):
primarily made of metallic compounds, rocks (silicates) and other heavy elements.
Their density is about 4 times that of water.
Have a slower rotation.
e.g. Mercury, Venus, Earth, Mars
Jovian planets (outer):
primarily made of gas, e.g. Jupiter is about 77% hydrogen, 22% helium and only trace
amounts of other materials. This composition is very similar to that of the Sun.
Average density is slightly greater than water.
Have a faster rotation.
e.g. Jupiter, Saturn, Uranus, Neptune
Saturn is so light that it would float in a bucket of
water, should you be able to find one big enough.
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Planetary Data
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23.1 The Solar System
The Planets: An Overview
 The Interiors of the Planets
• The substances that make up the planets are
divided into three groups: gases, rocks, and ices.
 The Atmosphere of the Planets
• The Jovian planets have very thick atmospheres
of hydrogen, helium, methane, and ammonia.
• By contrast, the terrestrial planets, including
Earth, have meager atmospheres at best.
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Scale of the Planets
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The Age of the Solar System
From radioactive dating we have a good knowledge
of the necessary timescales.
a) There are rocks on the surface of the Earth
which are at least 3.8 billion years old.
b) There are lunar rock samples 4.4 billion
years old.
c) Primitive meteorites
have ages of 4.5 billion years.
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What questions should we ask?
1. When did Solar System objects form?
A. The planets formed first and the Sun later.
B. The Sun formed first and the planets later.
C. Sun and planets formed at the same time.
2. Where did Solar System objects form?
A. In the same place.
B. In different places and then assembled later.
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The Nebular Hypothesis
The best chance of explaining these observations is to
assume that everything formed in the same place at
the same time, about 4.5 billion years ago.
It is now widely believed that the solar system was
formed in the gravitational collapse of a giant cloud
of gas and dust, known as the solar nebula.
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Initial stages
The Solar System is thought to
have started out as a giant gas
cloud, composed mainly of
hydrogen and helium, but also
containing some heavier
elements including ice and
metals.
Under its own gravitational
attraction, it collapses.
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Accretion Disk formation
As a gas cloud collapses, it begins to spin more
quickly.
The centrifugal force of this rotation flattens it out into
a rotating disk with a protostar at the center.
The nebula is hot because of gravitational energy
released by the collapse, and when the protostar
ignites at the centre it produces more heat. So there is
a strong temperature gradient across the disk: the
temperature gets hotter towards the center.
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Planetesimals
The temperature determines whether materials are solid or
gaseous. Towards the center of the disk only the metals are
solid, while further out ice can also condense.
These form chunks of oddly shaped bodies called
`planetesimals’ (itty bitty planets), perhaps ten kilometers
across.
The planetesimals merge together quickly to form objects (i.e.
planets) of perhaps the size of the moon in the inner solar
system, and 10 times more massive in the outer solar system
where ice is available.
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