Why Does Venus Glow in the Dark? Just Say NO

Right now people in the Northern Hemisphere are enjoying the last few weeks when Venus will shine bright in the night.

Around the end of March the “evening star” becomes the “morning star,” and the planet won’t grace the dusk skies again until next year.

(Read more at EarthSky to find out why Venus makes this transition.)


Morning or night, Venus appears so bright from Earth not only because it’s so close to us (a mere 26,027,649 miles, or 41,887,440 kilometers), but because its thick atmosphere is highly reflective.

Venus takes on a blueish hue in a filtered image from the Galileo orbiter

—Image courtesy NASA/JPL

It’s no surprise that sunlight bouncing off Venus’s atmosphere would make the planet glow. But what’s perhaps even more exciting is that Venus glows on its night side too.

No, conspiracy theorists, this is not because colonies of aliens are living on the Venusian surface. Sunlight interacting with the atmosphere is still the key player.

Planets with atmospheres—including Earth, Mars, and Venus—can glow in the dark thanks to a process called chemiluminescence, a fancy way of saying “light produced by chemical reactions.”

As ultraviolet light from the sun enters the atmosphere, it breaks apart some air molecules. The freed atoms can then recombine into new molecules, and in some cases the process causes the atoms to lose energy, which we see as light.

Today scientists working with ESA’s Venus Express orbiter announced they have seen the first infrared signatures of nitric oxide (NO) in the nighttime atmosphere of Venus, a sure sign that chemiluminescence is under way.


According to the ESA press release, Venus’s infrared nightglow has been seen before, revealing oxygen and hydroxyl. But this is the first time we’ve ever seen NO in the infrared on any planet.

It’s a subtle distinction, but a helpful one if you’re a planetary scientist.

The oxygen and hydroxyl were seen at around 56 to 62 miles (90 to 100 kilometers) above the planet’s surface, while the NO is glowing at about 68 to 74 miles (110 to 120 kilometers) high.

—Image courtesy ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

Knowing what’s where, chemically speaking, can reveal new information about Venus’s atmospheric circulation, which in turn could help us refine models of atmospheric conditions on Earth.

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