1 Ch. 23: “Touring Our Solar System” 23.1: “The Solar System”

2 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.

3 Planetary Data

4 The Planets: An Overview 23.1 The Solar System  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.

5 Scale of the Planets

6 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. The Age of the Solar System

7 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.

8 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.

9 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.

10 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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12 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.