The thin, glowing band hugging the curve of the planet in this photograph is airglow: faint light given off by Earth’s upper atmosphere itself. French ESA astronaut Thomas Pesquet captured it from the International Space Station on 30 July 2021, looking out through the station’s Cupola at the night side of Earth, with city lights below and stars above.
The short answer to “what is that glow?” is this: during the day, sunlight energises and splits atoms and molecules high in the atmosphere, and at night they release that energy again as a dim light. The orange-yellow layer comes mainly from sodium about 90 kilometres up; the greenish layer comes mainly from oxygen. Here is how it works, why it is so hard to photograph, and how you can even see it from the ground.
The photograph and when it was taken
Pesquet took the picture during his second long stay in orbit, ESA’s Alpha mission. He launched on SpaceX Crew Dragon Endeavour on 23 April 2021 with NASA astronauts Shane Kimbrough and Megan McArthur and JAXA’s Akihiko Hoshide, and spent 199 days in space before splashing down on 9 November 2021. In October that year he became the first French astronaut to command the station.
He shared a set of night images on his Flickr account in early September 2021 under the title “Earth and stars”. In his caption he joked about the contest playing out in front of him:
“Sometimes star lights battle it out with city lights for who’s the brightest and more beautiful. I’m just lucky to get to be the judge.” — Thomas Pesquet
Science sites picked the photo up quickly, partly because of its strange tilt. Shot looking sideways along the horizon, Earth appears as a dark wall with the luminous band running across it.
What airglow is
Even with no Moon, no city lights and no sunlight, Earth’s night sky would never be perfectly black. The atmosphere itself glows very faintly. Scientists split the effect into dayglow, which happens in sunlight and is drowned out by it, and nightglow, which is what astronauts and astronomers see after dark.
Several processes feed the glow:
- Recombination. Ultraviolet sunlight knocks electrons off atoms and molecules during the day (photoionisation). At night they recombine, and the spare energy comes out as light.
- Chemiluminescence. Chemical reactions between oxygen, nitrogen, hydroxyl and other species leave molecules in excited states that relax by emitting photons. One example: a nitrogen atom and an oxygen atom combine into nitric oxide and give off light.
- Cosmic rays. High-energy particles striking the upper atmosphere add a small contribution.
A common mix-up is to call this an aurora. It is not. Auroras are driven by charged particles from the Sun funnelled in by Earth’s magnetic field, they cluster near the poles, and they flare up during solar storms. Airglow is present all over the planet, every night, and has nothing to do with magnetism.
It was first identified by the Swedish physicist Anders Ångström in 1868, and the American astronomer Simon Newcomb described it scientifically in 1901.
Why the glow comes in colours
Each kind of atom or molecule emits light at its own characteristic wavelengths, so the colours in Pesquet’s picture are a map of the chemistry overhead.
- Green: excited atomic oxygen at about 90–100 km emits at 557.7 nanometres. This is the brightest airglow line and produces the classic green band seen from orbit.
- Yellow-orange: sodium atoms at around 90 km glow strongly near 589 nanometres. ESO astronomer Juan Carlos Muñoz told Live Science that the orange band in Pesquet’s photo comes from this sodium layer.
- Red: oxygen much higher up, roughly 150–300 km, plus hydroxyl molecules, add a deep red that long exposures pick up.
- Faint blue: excited oxygen molecules around 95 km contribute a weak bluish light.
Where does sodium come from at that height? From space rock. Meteors burn up (ablate) in the mesosphere, scattering metal atoms that collect in a layer roughly 80–105 km up and only about 5 km thick. Astronomers put that layer to work: large telescopes fire lasers at it to create artificial “guide stars” that help correct for atmospheric blurring.
One small correction to how this photo is often described. The glow does not come from molecules “breaking apart” at night; the splitting happens in daytime sunlight, and the light comes later as things recombine and react. The upper green layer is also mostly oxygen rather than a mix of oxygen and nitrogen.
Another of Pesquet’s night views from the Cupola: the airglow layer runs along the limb of Earth above the lights of cities. Photo: ESA/NASA–T. Pesquet.
Why it is so hard to photograph from orbit
Airglow is extremely dim. On a moonless night at a dark mountain observatory it is roughly as bright as a magnitude-22 star spread over every square arcsecond of sky, far too faint for the colour-sensing cells in our eyes. To record it, a camera needs a long exposure, keeping the shutter open long enough to collect what little light there is.
That is a challenge on a spacecraft moving at around 28,000 km/h. Pesquet admitted in his caption that he had missed more than his share of shots: the photographer has to hold perfectly still, yet the station’s own speed guarantees some motion in every frame. Astronauts often brace cameras against the window frame or use mounts, raise the sensor sensitivity, and shoot many frames hoping one is sharp.
The Cupola helps. Built for ESA by Thales Alenia Space and installed in February 2010, it has six side windows and one large round top window about 80 cm across, and it gives crews the widest view of Earth anywhere on the station. It doubles as a control post for the station’s Canadarm2 robotic arm.
You can see airglow from the ground too
You do not need to go to space. From a truly dark site, on a moonless night, airglow can be seen as pale grey or greenish streaks and bands that are easy to mistake for thin cloud. Practical tips from amateur observers:
- Give your eyes at least 20–30 minutes to adapt fully to the dark.
- Look about 10–15 degrees above the horizon, where the glow is brightest; lower than that, haze absorbs it.
- Move your head slowly to pick out the faint contrast between glowing patches and darker sky.
- Use a camera on a tripod with a wide lens and exposures of 15–30 seconds. The sensor will often show green and red that your eye sees only as grey.
- Activity is usually stronger around solar maximum, when the Sun is at its busiest.
Why scientists care about a faint glow
Airglow is more than a pretty effect. For ground-based astronomers it sets a floor on how faint an object they can detect, which is one reason space telescopes like Hubble can see fainter targets than much larger telescopes on Earth. For atmospheric scientists, it is a natural tracer: ripples and waves in the green and red layers reveal gravity waves, winds and temperature changes in a region that is too high for balloons and too low for most satellites.
Astronauts’ photographs like Pesquet’s add something the instruments cannot: a human-scale sense of how thin that glowing shell is. The whole layer sits only about a hundred kilometres above the ground, while the station that photographed it orbits roughly 300 km higher still.
Airglow is not unique to Earth either. ESA’s Trace Gas Orbiter has detected green oxygen airglow on Mars, using the same trick of looking at the planet’s edge that makes the glow visible from the ISS.
Sources: Thomas Pesquet on Flickr: Earth and stars (ESA/NASA–T. Pesquet); Live Science, 9 September 2021; Wikipedia: Airglow; Wikipedia: Sodium layer; Universe Today: How to see airglow; Wikipedia: Thomas Pesquet; Wikipedia: Cupola (ISS module); ScienceAlert. Photos: ESA/NASA–Thomas Pesquet.
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