H Radioactive decay.The vertical axis is the number of remaining nuclei N, divided by the original number N0.
taken place. We believe that the Earth's crust is billions of years old, so the alpha decay of 238U (uranium-238), with a half-life of 4.6 billion years, is ideal for studying its age.
A practical problem is that we often don't know how much of a given radioactive material the Earth started out with. This means that we must employ indirect methods, involving knowledge of the end products of the decays. For example, the alpha decay of 238U is followed by a series of alpha decays, eventually leading to the stable isotope 206Pb (lead-206). If we can assume that all of the 206Pb on the Earth came from such decays, we could use the amount of 206Pb as an indicator of the original amount of 238U.
However, the Earth may have been formed with some 206Pb, so we have to use even more involved techniques to correct for that effect. These techniques often involve measuring the relative abundances of certain isotopes, such as 204Pb and 206Pb. Such measurements require a means of separating the isotopes. This separation is much easier for gases than for solids. For this reason, we actually use a different age tracer. For example, 40K (potassium-40) beta decays with a half-life of 1.3 billion years. The decay product is 40Ar (argon-40). Since argon is a noble gas, it doesn't stay bound to the rocks. We can collect the argon and study the relative abundances of various isotopes.
In considering the history of the Earth, we must remember that the atmosphere has eroded the surface, altering its characteristics. (As we will see below, the motions of the continents also alter the surface.) The oldest rocks that we see are dated at 3.7 billion years. (The oldest fossil cells are dated at 3.4 billion years.) We think that the surface underwent significant alteration 2.2 to 2.8 billion years ago. This was a period of volcanic mountain building with the Earth having a very thin crust. The crust was probably broken into smaller platelets.
The age of the Earth, even as an approximation, is an important reference. It gives us an idea of what the appropriate time scales are for understanding the Solar System. For example, if the Earth formed as a by-product of the formation of the Sun, then the Sun must be at least as old as the Earth. This tells us that in understanding the Sun and other stars, we must be able to explain how they can shine for billions of years. On the other end of the time scale, we can see that, as far back as ancient history seems to us, some 5000 years, it is but a blink on the time scales of the Earth's history.
At that time, the atmosphere was mostly water, carbon dioxide, carbon monoxide and nitrogen. It is thought that any ammonia and methane could not have lasted very long. There was little oxygen, since oxygen is a product of plant life. Since there was no oxygen, there was no ozone (O3) layer to shield the Earth from the solar ultraviolet radiation. We think that this ultraviolet radiation must have stimulated the chemical reactions to make the simplest organic compounds. It has been shown in the laboratory that such reactions are greatly enhanced in the presence of ultraviolet radiation. We will talk more about the early evolution of the Earth's atmosphere in Chapter 27.
The layer below the thin crust is kept heated by radioactive decay. The amount of heat is not sufficient to melt the material completely, but it keeps it from being completely solid. It has the
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