Which of the following is not a unique function of the atmosphere?

The function of the atmosphere in relation to shielding Earth from asteroids and comets is not considered a unique function of the atmosphere. While the atmosphere does provide some level of protection against smaller meteoroids, larger asteroids and comets can travel through the atmosphere and impact the surface, causing significant damage. This protective role is often attributed more to the gravitational field surrounding the planet and its position in the solar system rather than the atmosphere itself.

In contrast, the other options clearly highlight unique aspects of what the atmosphere does for the planet. The presence of oxygen is essential for supporting animal life, and this is a direct result of the atmospheric composition. The atmosphere also protects living organisms by absorbing and scattering harmful ultraviolet radiation from the sun, preventing it from reaching the Earth's surface in damaging amounts. Additionally, the atmosphere plays a crucial role in the water cycle, enabling precipitation to occur, which is vital for many ecosystems. These functions collectively underscore the importance of the atmosphere in supporting life directly and indirectly, while shielding against asteroids and comets is not a primary function of the atmosphere alone.