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Rather, each isotope has its own “half-life,” the time in which a nucleus has a 50% chance of decaying.

Half-lives range from less than a millisecond (highly unstable isotopes) to billions of years (nearly stable).

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Uranium-238

Thorium-234

Helium-4

Slide 11

Nuclear decay of a large sample

Slide 12

Radioactive Age Dating

A small percentage of all natural potassium is the radioactive isotope, potassium-40.

As a rock ages, its potassium-40 slowly disintegrates, leaving argon-40 atoms behind.

Argon is never incorporated into igneous crystals as they form, because it is a noble gas.

Therefore the ratio of argon-40 to potassium-40 is a direct measure of a rock’s age.

Possible problem: Heating a rock can allow trapped argon atoms to escape. If a rock has been heated, it might be older than we think it is.

Example: Potassium-40 decays to Argon-40 with a half-life of 1.4 billion years.

Slide 13

Radioactive Age Dates

Farmington Canyon Complex: 1.8 billion years

Oldest earth rocks: about 4 billion years

Oldest moon rocks: 4.6 billion years

Most meteorites: 4.6 billion years

Slide 14

Another interesting pattern . . .

Uranium 238 (half-life 4.5 billion years) is 140 times more common than uranium-235 (half-life 0.7 billion years). Other isotopes of uranium are not found on earth, although some have half-lives in the millions of years.

Elements heavier than uranium do not occur naturally at all on earth. The longest-lived example is an isotope of plutonium with a half-life of 80 million years.

Of the hundreds of isotopes with half-lives under 100 million years, only a few are found naturally on earth. These are being formed continuously by decay of heavier isotopes or cosmic ray bombardment.

Explanation: The earth is made of stuff that’s billions of years old, so short-lived isotopes are long gone.

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Solar System Patterns

The solar system is very flat. Why?

Nearly all the planets orbit and spin in the same direction. Why?

Inner planets are small; outer planets are big. Why?

Inner planets are mostly solid; outer planets are mostly gas and liquid. Why?

Inner planets have little hydrogen and helium; outer planets have lots. Why?

Partial answers are not hard to guess…

Detailed answers require an account of how the solar system formed.

Slide 16

Formation of the Solar System