Characteristics
and Origins of the Solar System
Lecture 7
September 13, 2000
Radioisotopes and
Ages of Rocks (Rocks of Ages?)
Today I
will start talking about one of the most important concepts in solar system
astronomy. How do we determine how long
ago solar system objects came into existence?
More generally, how do we put
“time stamps” on events in the
remote past?
This
material is on p138, Chapter 6 of your textbook, but I will be putting a lot
more info than is there.
The
technique used is radioisotope dating, and the basic idea is used in
archaeology as well as planetary science.
Here goes.
- Elements
are determined by the number of protons in tne nucleus, i.e. 6 for
carbon, 7 for nitrogen, and 37 for Rubidium. It is this atomic number that determines its chemical
characteristics.
- In
nature, elements have a number of isotopes, in which the atomic nuclei have the
same number of protons, but different numbers of neutrons. Neutrons are subatomic particles which
have the almost the same mass as a proton, but no electric charge. So the atomic weight of the
isotopes differs.
- Examples:
Seawater on Earth consists
of H2O. The vast majority of the hydrogen atoms
are plain old hydrogen, 1H, but about 150 out of a million
atoms is Deuterium, 2H or D, in which the nucleus has a
proton and a neutron.
Another famous example is carbon, in which the most common isotope
is C12, but C14 is present as well.
- Some
isotopes are stable, meaning that once formed, they will last
forever. However, some of them are
radioactive, meaning that
they transform themselves to a different type of nucleus, and emit nuclear
radiation in the process.
An example of a radioactive decay
process is
37Rb87
→ 38Sr87 + β + ν