The "blanket" around the Earth that is called the atmosphere is made up of several layers, which perform different tasks in protecting the surface from harmful radiation. The troposphere is the layer nearest to Earth, where weather occurs. Beyond the troposphere lies the stratosphere, which also contains the ozone layer. Above the stratosphere, the mesosphere is a thinner layer that slows the descent of meteorites and causes them to burn. The far upper reaches of the atmosphere consist of the thermosphere with its layer of ionized gas particles called the ionosphere, which bounces radio signals transmitted from Earth, and the exosphere, the last thin layer between Earth and deep space.
Like other planets, the Earth is constantly bombarded by cosmic rays: intense, high-energy radiation emitted by the sun and other bodies such as black holes and supernovae. When these rays, which can damage human DNA and trigger a variety of cancers, approach Earth, they are blocked by oxygen and nitrogen molecules in the atmosphere, which prevent them from reaching the Earth's surface.
Gamma rays are produced by violent cosmic events such as the explosion of a supernova, as well as by the decay of radioactive material in space. Other sources of gamma rays and X-rays include neutron stars and protons. Gamma-ray bursts can release more energy in 10 minutes than the sun in its entire lifetime. Although some theories propose that gamma-ray bombardment may be responsible for mass extinctions in the Earth's distant past, the complex layered structure of the atmosphere stops them from reaching the planet's surface.
Ultraviolet radiation comes to Earth primarily from the sun. UV radiation exists in three forms: UV-A, B and C. Of these, UV-A is the weakest and least harmful to Earth life, while C is most harmful. UV-C rays are stopped in the stratosphere by the ozone layer and never reach Earth. However, some UV-B rays and nearly all UV-A radiation are capable of passing through the atmosphere and reaching the surface, so health professionals advise people to protect themselves against these types of cosmic radiation by reducing sun exposure and using sunscreen.