Chapter summary

The Earth is the planet that we can study in the most detail. In this chapter, we looked at the properties of the Earth and the Moon. We also looked at the physical processes responsible for their current properties.

So much heat was trapped when the Earth formed that it still has a molten core of iron and nickel. Radioactive materials provide heating near the surface, keeping the mantle soft, like plastic. The radioactive materials also provide us with a way of dating various rocks. The oldest rocks on the Earth are 3.7 billion years old. The plastic nature of the material below the surface allows the continental plates to drift slowly. These plate tectonics are responsible for most of the geological activity that we see - volcanoes, earthquakes, and mountain building.

The temperature of the Earth (or any planet) is determined by the balance between the radiation absorbed from the Sun and that given off by the Earth. The atmosphere plays an important role in modifying that balance. The atmosphere is a very thin layer (relative to the radius of the Earth itself). The pressure is determined by hydrostatic equilibrium, and drops very quickly with altitude, falling to half of its sea level value at an altitude of about 6 km. The temperature distribution depends on the sources of heating for different parts of the atmosphere. In the ozone layer, solar ultraviolet radiation is absorbed, and directly heats that part of the atmosphere. Near the ground, the source of heating is the ground, either by infrared radiation from the ground, or convection of the air heated just above the ground. The trapping of infrared radiation near the ground produces the greenhouse effect, and raises the temperature near the ground by about 25 K. The temperature is high enough in the upper atmosphere for most of the hydrogen to have escaped. The general flow in the atmosphere is strongly influenced by the Earth's rotation.

The molten iron-nickel core provides the Earth with a magnetic field, similar to that of a dipole magnet. The field is strongest at the north and south magnetic poles (which don't coincide with the rotation poles). The magnetic field has the important role of trapping charged particles that hit the Earth from outer space. These charged particles give off a lot of long wavelength radio emission. They are also responsible for the glowing aurorae.

The Moon produces tidal effects on the Earth. These depend on the fact that the Moon's gravitational force is weaker on the side of the Earth away from the Moon than on the side closer to the Moon. This produces the water tides. The Sun has a similar effect, but it is not as large as the Moon's. The tidal effects also cause dissipation of some of the Moon's orbital energy, and have resulted in the Moon keeping the same face towards the Earth. The Sun and Moon also cause the direction of the Earth's rotation axis to precess, like a top, with a period of 26 000 years.

The lunar surface is particularly interesting since it has been preserved over most of the history of the Moon. This is because the Moon has no atmospheric erosion. The lunar rocks show some differences between the maria and the highlands. An important difference is that the highland rocks are somewhat older (with the oldest being 4.48 billion years). The lunar interior is different from that of the Earth. In particular, the Moon does not have a molten core. The question of the lunar origin is still up in the air.

Questions

23.1. Why do we think that the Earth is more 23.4. If the Earth and Moon formed together, why than a billion years old? are the oldest rocks on the Moon older than

23.2. What does the fact that the Earth is differ- the oldest rocks on Earth?

entiated tell us about its history? 23.5. What does continental drift tell us about

23.3. Do you think that radioactive uranium would the Earth's interior? be useful for dating the bones of a cave person?

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