Radioactive Dating and the Age of the Solar System
Even though radiation is generally something negative, sometimes it can be used for something positive as well. If
you know the half-life of a given radioactive element, you can in principle use the decay of that element as a clock
to measure a time interval. This clock, however, doesn't go 1, 2, 3, ... in fact it goes more like 1, 2, 4, 8, ... The decay
of very long-lived radioactive elements, for example, can be used to measure the age of rocks, that is, the time that
has elapsed since these rocks were formed.
How does radioactive decay work? If an isotope (forms of chemical elements that differ in the number of neutrons
in their atomic nuclei) is radioactive, it will break down naturally into a lighter element called a decay product. This
process occurs at a predictable rate and can be used to determine how old an object is.
What is a Half-life? A half-life is the time required for half of an element's atoms in a sample to change to the
decay product. In each half-life only half of the remaining radioactive atoms decay, no matter how large the sample
is. Look at the diagram below which represents the radioactive decay of uranium-238. The shaded area represents
the decay product which is lead-206. The half-life of uranuim-238 is 4.5 billion years. Since this object has gone
through two half-lives, it is 9 billion years old.
How can we determine the number of half-lives that have occurred ? We look at the decay-product ratio, which
is the ratio between the mass of a radioactive element and its decay product. If an object is made up of 50 % decay
product then the object has been through 1 half-life. 75% decay product equals 2 half-lives, 87.5% decay product
equals 3 half-lives, 93.76% decay product equals 4 half-lives, and so on. If we know how long is one half-life, we
can estimate the age.
In practice, the radioactive dating techniques used today measure the time since a rock was last molten. The age
of the Earth is 4.6 billion years. Nevertheless, according to radioactive dating techniques, the ``ages" of rocks
found on its surface are far younger. The material making up these rocks has been part of the Earth for all 4.6
billion years, but most of these rock were formed when molten material on or beneath Earth's surface solidified
only recently.
The oldest rocks in the Solar System -- the ones which have not been melted since they were first formed -- are
the meteorites which fall to Earth from interplanetary space. The meteorites are pieces of the asteroids -leftover planetesimals from the era of the formation of the planets. They solidified early in the history of the
Solar System and have not melted since. The asteroids are located between Mars and Jupiter. The large planet
Jupiter stirred up the motions of the nearby planetesimals and prevented them from accumulating into a planet.
These stirred planetesimals collided at high speed and fragmented. They were unable to stick together and
accumulate to form a large planet. These leftover planetesimals now form the asteroid belt. When asteroids
collide and fragment, pieces can be sent in our direction. Some of them reach the Earth - these are the
meteorites. When radioactive dating techniques are applied to meteorites they are uniformly found to be 4.6
billion years old.
The Young Earth
Asteroids