Another minor constituent in Earth’s atmosphere is
ozone (O3). This important molecule is formed when UV
light from the Sun breaks molecular oxygen (O2) into its individual
atoms (O) in a process called photodissociation.
These oxygen atoms can then recombine with other oxygen
molecules to form ozone (O2 + O → O3). Most of Earth’s
natural ozone is concentrated in the upper atmosphere
at altitudes between 20
and 50 km. There it acts
as a very strong absorber
of UV sunlight. Without
the ozone layer, this radiation would reach all the way to
Earth’s surface, where it would be lethal to nearly all forms
of life.