The Secrets behind Earth’s Multi-colored Glow

What does our planet look like from space? Most are familiar with the beloved images of the blue marble or pale blue dot — Earth from 18,000 and 3.7 billion miles away, respectively. But closer to home, within the nearest region of space, you might encounter an unfamiliar sight. If you peer down on Earth from just 300 miles above the surface, near the orbit of the International Space Station, you can see vibrant swaths of red and green or purple and yellow light emanating from the upper atmosphere. This is airglow. Airglow occurs when atoms and molecules in the upper atmosphere, excited by sunlight, emit light in order to shed their excess energy. Or, it can happen when atoms and molecules that have been ionized by sunlight collide with and capture a free electron. In both cases, they eject a particle of light — called a photon — in order to relax again. The phenomenon is similar to auroras, but where auroras are driven by high-energy particles originating from the solar wind, airglow is energized by day-to-day solar radiation.