Airglow

The night sky is never truly dark.

High in the air, gases glow with a soft light. This light comes from the sun. During the day, the sun splits parts of the air. At night, those parts join back together. This joining makes the sky glow.

This glow can look blue to people on Earth. It is not the same as the lights called auroras. It is a special light from the air itself.

Scientists use space tools to see it better.

The night sky is never completely dark.

Even without starlight, a faint light stays in the air. This is called airglow. It is a soft light from gases in our atmosphere.

Airglow happens in two ways. During the day, the sun's energy splits air molecules apart. This is called dayglow. It is too faint to see in the bright sun. At night, those split parts join back together. This is called nightglow. When they join, they let out light. One way this happens is when nitrogen and oxygen join to make nitric oxide. This process gives off a photon, which is a tiny bit of light.

Airglow is not the same as an aurora. Auroras are caused by different things. Airglow is also found on other planets. Scientists saw it on Venus. They also found green oxygen airglow on Mars. On Earth, airglow can make it hard for telescopes to see faint stars. This is why space telescopes like Hubble work so well. They can look past the glow from our air.

The night sky is never truly dark. Even when you remove the light from stars, a faint glow remains in our atmosphere. This is called airglow. It is a soft light made by gases in the upper atmosphere.

While it looks a bit like an aurora, airglow is actually different. Auroras are caused by Earth's magnetism and sunspots. Airglow comes from the gases themselves. It is a constant part of our planet's sky.

Airglow works through two main ways. During the day, the sun's energy hits molecules in the air. This energy splits the molecules apart. This daytime version is called dayglow. It is too faint to see because the sun is so bright.

At night, the split pieces begin to recombine. When these atoms join back together, they release light. One way this happens is when nitrogen and oxygen join. They form a molecule called nitric oxide. This process releases a photon, which is a tiny bit of light. Other chemicals like sodium and lithium also create this glow.

People have known about this light for a long time. Ancient Greeks like Aristotle and Pliny described it. They called these bright nights "Chasmata." Later, a Swedish physicist named Anders Ångström identified it in 1868. In 1901, Simon Newcomb was the first to study it scientifically.

Since then, scientists have used laboratories to watch these chemical reactions. They have even used small satellites to study it from space. One small satellite called SwissCube-1 took its first airglow image in March 2011.

Airglow can be a hard job for astronomers to deal with. The glow makes it difficult for telescopes on the ground to see faint stars. For example, a large 10-meter telescope might see 35 photons from a star. However, it might see 3,500 photons from the airglow at the same time. This makes the star hard to spot.

Because of this, space telescopes like Hubble are much better. Hubble is above the air, so it does not have to look through the glow. It can see much fainter objects than telescopes on Earth.

We can find airglow on other worlds, too. The Venus Express spacecraft found emissions from Venus. These emissions come from oxygen and nitric oxide. On Mars, scientists found green oxygen airglow in 2020. They used an instrument called NOMAD on the TGO spacecraft.

This shows that airglow is a common thing in our solar system. It helps us understand how different atmospheres work. Just like Earth, other planets have gases that glow in the dark.

Airglow is a faint emission of light from a planetary atmosphere. This optical phenomenon means the night sky is never completely dark. Even after you remove starlight and diffused sunlight, a soft glow remains. This light comes from self-illuminated gases in the upper atmosphere. It is important to distinguish airglow from aurorae. Aurorae are caused by Earth's magnetism and sunspot activity. Airglow has no relationship to those magnetic forces. Instead, it is a constant chemical process in our atmosphere.

This phenomenon occurs through two interlinked processes. During the day, the sun's energy causes dayglow. Solar energy hits atmospheric molecules and splits them apart. This process is called photoionization. Dayglow is too faint to see because of sunlight glare. At night, the process reverses to create nightglow. The split atoms and molecules begin to recombine. As they join back together, they release energy as light. This light is released in the form of a photon.

Many different chemical reactions contribute to this glow. One specific mechanism involves nitrogen and oxygen atoms. Solar energy dissociates nitrogen (N2) and oxygen (O2) molecules. These free atoms then encounter each other in the upper atmosphere. When a nitrogen atom combines with an oxygen atom, they form nitric oxide (NO). This recombination process emits a photon. This photon can have several different wavelengths. Other chemicals also create airglow. These include hydroxyl (OH), atomic oxygen (O), sodium (Na), and lithium (Li).

Humans have observed this light for many centuries. Ancient Greeks like Aristotle and Pliny described the phenomenon. They referred to bright airglow nights as "Chasmata." The scientific identification of airglow began much later. Swedish physicist Anders Ångström first identified it in 1868. In 1901, Simon Newcomb became the first person to scientifically study and describe it. Since then, scientists have used laboratories to observe these specific chemical reactions. They have even used small satellites like SwissCube-1 to capture images from space.

Airglow creates significant challenges for ground-based astronomy. The glow limits the photosensitivity of optical telescopes. It acts as a source of noise that hides faint objects. For example, consider a 10-meter diameter ideal ground-based telescope. In one second, it might receive 35 photons from a star. However, it might receive 3,500 photons from the airglow over the same area. This creates a low signal-to-noise ratio. Because of this, space telescopes like Hubble are much more effective. Hubble sits above the atmosphere and is not restricted by airglow.

Scientists can also induce airglow through human activity. Experiments have used high-power radio emissions to target the ionosphere. These radiowaves interact with the ionosphere to create visible light. This is known as induced airglow. This effect can be observed at specific wavelengths under certain conditions. This shows how energy can be used to trigger atmospheric chemical responses.

Airglow is not unique to Earth. We can observe similar emissions on other planets in our solar system. The Venus Express spacecraft detected near-infrared emissions from Venus. These emissions come from molecular oxygen and nitric oxide. This might explain the "ashen light" seen from Venus since the 17th century. In 2020, scientists detected green oxygen airglow on Mars. They used the NOMAD instrument on the TGO spacecraft to see it. This proves that atmospheric gases can glow on many different worlds.