Snapped a casual 400 kilometers (250 miles) over Australia aboard the International Space Station on October 7, a hauntingly beautiful image shows Earth glowing a hearty orange hue. While the image looks to be something straight out of James Cameron’s Avatar, the good folks at NASA offered up a perfectly rational – even natural – explanation.
The aptly-named “airglow” phenomenon is really a layer of nighttime light produced through high-altitude chemical reactions stretching between 80 and 640 kilometers (50 and 400 miles) into Earth’s atmosphere. This colorful illumination happens all over the world (though light pollution and cloudy skies typically hide these glowing particles from eyesight) and at any time of the solar day for a total of three different types: during the day it’s called “dayglow,” at twilight it’s called “twilight glow,” and at night it’s called – wait for it – “nightglow”. Each has a different process, but the overall result is the same.
Dayglow happens when molecules in the atmosphere absorb sunlight giving them the extra energy that, when released, takes the form of light. Twilight glow is roughly the same process but takes place only in the upper atmosphere as it’s still receiving sunlight even though us earthlings are shrouded in darkness.
Nightglow uses a process called chemiluminescence and produces light in a similar way as a glow stick or glow-in-the-dark silly putty. Because there is no sunlight on the night atmosphere, this process is slightly different than day or twilight glow.
“The phenomenon typically occurs when molecules (mostly nitrogen and oxygen) are energized by ultraviolet (UV) radiation from sunlight. To release that energy, atoms in the lower atmosphere bump into each other and lose energy in the collision,” wrote the space agency in a statement.
During the day, sunlight deposits its energy into the atmosphere, some of which is stored in oxygen. As explained by The Conversation, this extra energy causes the oxygen molecules to rip apart into individual atoms for several hours. Eventually, this atomic oxygen comes back together to form molecular oxygen, which is then released in the form of light. Nightglow can take on several colors, including the famous green glow illuminating over the UK in 2016.
It’s not to be confused with aurora borealis, which is created from the collision of charged particles from solar flares penetrating through Earth’s magnetic shield with atoms and molecules in the atmosphere, producing bursts of light (photons).
NASA writes that these events help us understand how the upper reaches of our atmosphere works, the intersect of Earth and space particle movement, and the various connections between space and Earth weather systems.