Ultraviolet (UV) radiation is a form of invisible, high-energy electromagnetic wave emitted by the sun. If this energy reached the Earth’s surface unimpeded, it would sterilize the planet, making life impossible. Earth’s atmosphere acts as a natural shield, absorbing the vast majority of this solar UV radiation before it reaches the ground. This protective function is performed primarily by two specific gases: ozone and diatomic oxygen.
The Primary Role of Ozone
The most well-known gas responsible for blocking solar ultraviolet energy is ozone (\(\text{O}_3\)), a molecule made up of three oxygen atoms. Ozone is highly concentrated in a region of the stratosphere, commonly known as the ozone layer, spanning from roughly 15 to 35 kilometers above the Earth’s surface. This rarified layer is the atmosphere’s primary filter for medium-to-short wavelength UV light.
Ozone absorbs UV radiation through a process called photodissociation, where the energy from a UV photon causes the \(\text{O}_3\) molecule to break apart. Specifically, ozone absorbs nearly all of the sun’s UVC radiation and most of the highly energetic UVB radiation. The molecule splits into a diatomic oxygen molecule (\(\text{O}_2\)) and a single, highly reactive oxygen atom (\(\text{O}\)).
This absorption converts the sun’s high-energy radiation into thermal energy, effectively warming the stratosphere. The process is cyclical, as the freed oxygen atom quickly recombines with another oxygen molecule to reform ozone. This continuous cycle of destruction and reformation allows the ozone layer to maintain its protective density and shield the planet from these harmful wavelengths.
Diatomic Oxygen and High-Energy Shielding
While ozone handles most of the mid-range UV, diatomic oxygen (\(\text{O}_2\)) is responsible for absorbing the most energetic, shortest-wavelength radiation. This absorption occurs much higher up in the atmosphere, predominantly in the mesosphere and thermosphere, well above the main ozone layer. Molecular oxygen is an extremely efficient absorber of the shortest UVC wavelengths, specifically those below 200 nanometers.
When a high-energy UVC photon strikes an \(\text{O}_2\) molecule, the radiation contains enough energy to split the molecule into two separate oxygen atoms. This process is necessary to stop the highest-energy UV radiation before it can penetrate to lower altitudes. This initial splitting of \(\text{O}_2\) is also the first step in creating the ozone layer, as these freed oxygen atoms eventually migrate down and combine with other \(\text{O}_2\) molecules to form \(\text{O}_3\).
The absorption of this extreme UV light by \(\text{O}_2\) acts as the first line of defense, intercepting radiation that would be too energetic even for ozone to manage effectively. This two-part system, with \(\text{O}_2\) at the highest altitudes and \(\text{O}_3\) in the stratosphere, ensures that the most damaging solar radiation is dealt with far from the planet’s surface.
Distinguishing Between UV Wavelengths and Atmospheric Penetration
The effectiveness of atmospheric absorption is directly linked to the wavelength of the UV radiation, which is categorized into three types: UVC, UVB, and UVA. UVC radiation, with the shortest wavelengths (100–280 nanometers), carries the highest energy and is the most damaging to biological systems. UVC is completely blocked by the atmosphere, first by diatomic oxygen and then by ozone, meaning none of it reaches the Earth’s surface.
UVB radiation (280–315 nanometers) is less energetic but still poses a significant biological threat. Most UVB is absorbed by the stratospheric ozone layer, but a small percentage does penetrate to the ground. The amount of UVB that reaches the surface is variable, changing with latitude, season, and atmospheric conditions, and is the primary cause of sunburn.
UVA radiation (315–400 nanometers) has the longest wavelengths and the lowest energy of the three types. Ozone is a poor absorber of UVA, so this radiation passes through the atmosphere almost unimpeded. Consequently, nearly all of the UV radiation that reaches the Earth’s surface is UVA, accounting for approximately 95% of the total solar UV exposure.
The Essential Barrier for Life
The atmosphere’s ability to absorb UV radiation is a fundamental requirement for surface life on Earth. High-energy UV radiation, particularly UVC and UVB, is mutagenic because it is readily absorbed by DNA and proteins in living cells. This absorption leads to structural changes in these macromolecules, which can disrupt cellular function and cause mutations.
By filtering out all UVC and the majority of UVB, the atmospheric gases prevent widespread destruction of genetic material in surface organisms. The small amount of UVB that reaches the ground, along with the abundant UVA, still carries enough energy to cause biological damage, such as skin aging and increased cancer risk. The complete removal of the highest-energy wavelengths by oxygen and ozone is the protective mechanism that allowed complex life to evolve and thrive outside of the oceans.