Earth's UV Shield Gas Explained

The question asks to identify the specific gas responsible for protecting Earth's surface from the Sun's ultraviolet (UV) radiation.

Identifying the Ultraviolet Shield

The gas that shields the Earth's surface from harmful ultraviolet radiation originating from the Sun is Ozone.

• Ozone, with the chemical formula $O_3$, exists primarily in the stratosphere, forming the ozone layer.
• This layer acts as a natural filter, absorbing the majority of the Sun's dangerous UV-B and UV-C radiation.
• By absorbing this radiation, ozone prevents it from reaching the Earth's surface, thus protecting living organisms from its damaging effects, such as skin cancer and genetic mutations.

Why Other Gases Are Not the Primary UV Shield

Let's consider why the other options are incorrect:

• Nitrogen ($N_2$): The most abundant gas in the atmosphere, it does not effectively absorb UV radiation.
• Carbon dioxide ($CO_2$): This gas primarily absorbs infrared radiation (heat) and plays a role in the greenhouse effect, not UV shielding.
• Oxygen ($O_2$): While oxygen molecules absorb very high-energy UV radiation (UV-C) to form ozone in the upper atmosphere, molecular oxygen itself is not the primary shield against the UV radiation that reaches the lower atmosphere and surface.

Therefore, Ozone is the correct answer as it is the principal gas responsible for absorbing harmful UV radiation.