QUESTION: 
Is carbon dating used to determine the age of the soil on Mars, or is some
other method used?

ANSWER from Jack Farmer on November 20, 1996:
To date young materials like soils, we usually use the isotopes of carbon.
Specifically, the radioactive isotope called C-14. It has a half-life (the
time it takes for half of the C-14 in the sample to break down) of about
5000 years. We can now measure small enough quantities of C-14 in samples
to go back 60,000 years or so. For really old materials we use other
isotopes that have much longer half lives.