Why does Pluto’s atmosphere change as its distance from the Sun changes?

Pluto’s atmosphere changes in response to its distance from the Sun because it consists of vapors in equilibrium with their surface ices. Because these gases are in a delicate balance, even small fluctuations in temperature significantly impact the amount of gas present in the atmosphere.1

When Pluto is closer to the Sun—near its perihelion—it receives more sunlight, causing more of its frozen gases to vaporize and thicken the atmosphere. Conversely, at aphelion, when the dwarf planet is at its farthest point from the Sun and receives the least amount of solar energy, the surface temperatures drop, and the atmosphere may become undetectable as the gases freeze back onto the surface.1