A Cold Day on Mercury

Global climate change caused by the greenhouse effect is one of the greatest challenges facing humanity today. To survive, we'll need to maintain the delicate energy balance that makes Earth a place for life to thrive.

The hot daytime temperature on Mercury probably wouldn’t surprise you it’s well over  but the nighttime temperature might. Without the sun’s direct illumination, the surface of Mercury drops to a frigid  overnight.

Sunlight is absorbed by Mercury, which heats it up. But that heat energy doesn’t just stay there—it gets re-emitted as infrared light, the slice of the electromagnetic spectrum that we use for night or thermal vision. When Mercury emits light, that energy is lost forever, causing Mercury to cool down.

It’s constantly being cooled by emission, but on the day side the sun is enough to keep it hot. At night, with no sunlight temperatures drop to a frigid 

On Earth, sunlight energy and infrared emission are in balance. During the day, sunlight warms Earth even as some heat energy is emitted as infrared light. At night, Earth cools as it emits stored heat energy as infrared light.

But Earth has an atmosphere—a blanket of air that is largely transparent to visible light but opaque to infrared. Very little energy emitted by Earth as infrared is lost forever. Most gets stuck in Earth’s atmosphere, absorbed mostly by water vapor and carbon dioxide. Our atmosphere provides an oasis in the cold expanse of space, which gives this effect its name: the greenhouse effect.

Thanks to our atmosphere, the average temperature on Earth is about , with relatively mild temperature swings between day and night. This makes Earth unique in the solar system as the only planet with a climate which allows water to occupy all three of its phases: vapor, liquid, and ice.

The cycle of water through these phases has changed the face of our planet, and is responsible for sustaining life and creating the ecosystems around us. If the temperature tips too far in one direction, we get an ice age. If it tips in the other direction, we overheat the oceans and flood the atmosphere with water vapor.