406 PART 4^ |^ THE SOLAR SYSTEM
■ Figure 19-6
A meteor is a sudden streak of glowing gases produced by a bit of material
falling into Earth’s atmosphere. Friction with the air vaporizes the material
about 80 km (50 mi) above Earth’s surface. (Daniel Good)
■ Figure 19-7
(a) The radioactive atoms (red) in a mineral sample decay into daughter atoms
(blue). Half the radioactive atoms are left after one half-life, a fourth after
two half-lives, an eighth after three half-lives, and so on. (b) Radioactive
dating shows that this fragment of the Allende meteorite is 4.56 billion years
old. It contains a few even older interstellar grains, which formed long before
our solar system. (R. Kempton, New England Meteoritical Services)
years to form lead-206 and other isotopes. Rubidium-87 decays
into strontium-87 with a half-life of 47 billion years. Any of
these substances can be used as a radioactive clock to fi nd the age
of mineral samples.
Of course, to fi nd a radioactive age, you need to get a
sample into the laboratory, and the only celestial bodies of
which scientists have samples are Earth, the moon, Mars, and
meteorites. Th e oldest Earth rocks so far discovered and dated
are tiny zircon crystals from Australia that are 4.4 billion years
old. Th at does not mean that Earth formed 4.4 billion years ago.
As you will see in the next chapter, the surface of Earth is active,
and the crust is continually destroyed and reformed with mate-
rial welling up from beneath the crust. Th ose types of processes
tend to dilute the daughter atoms and spread them away from
the parent atoms, eff ectively causing the radioactive clocks to
reset to zero. Th e radioactive age of a rock is actually the length
of time since the material in that rock was last melted.
Consequently, the dates of these oldest rocks tells you only a
lower limit to the age of Earth, in other words, that Earth is at
least 4.4 billion years old.
One of the most exciting goals of the Apollo lunar land-
ings was to bring lunar rocks back to Earth’s laboratories
where their ages could be measured. Because the moon’s
Visual-wavelength image
100
100
A mineral sample
containing radioactive
atoms , which decay
into daughter atoms
Percentage of
radioactive and
daughter atoms
in the mineral
Percentage of
radioactive atoms
remaining
1/2 remain1/4 remain1/8 remain
50
50
Percentage remaining
0
0123
Age in half-lives
456
