The Venusian “glory” in false colours showing ultraviolet, visible, and near-infrared wavelengths. The centre of the concentric circles is the pale yellow patch at left, and the glory extends over at least 1200 kilometres (Image: ESA/MPS/DLR/IDA) Simulated views of the glory phenomena on Venus (left) and Earth (right) (Image: C. Wilson/P. Laven/NASA)

A rare type of rainbow called a “glory” has been spotted in Venus’s atmosphere, the first time such a ring of colour has been imaged on another planet. The glory’s shape and size could help solve a decades-old mystery about the hazy planet’s makeup.


“A full glory has never been seen before outside of the terrestrial environment,” says Wojciech Markiewicz at the Max Planck Institute for Solar System Research in Gottingen, Germany, who was part of the team that spotted the glory on Venus.

Ordinary rainbows, and glories, are both caused by sunlight filtering through cloud droplets, which on Earth consist of water particles. But where ordinary rainbows are typically seen from the ground, glories are only seen from above and look like smaller concentric circles, like a saint’s halo.

Scientists think glories occur when sunlight bounces about inside the droplets and ends up leaving in the same direction it came from, making a ring of light that can only be seen if you’re precisely between the sun and the centre of the glory. The particles must also be nearly spherical and almost all the same size.

Bacterial culprit?

Ever since the first spectra of Venus was taken in the 1920s, scientists have known that the hazy planet’s atmosphere contains sulphuric acid “clouds”, which could create glories. But there was another, unidentified component in the atmosphere that absorbed light in the ultraviolet range. Scientists have come up with a variety of possible culprits, but none has been confirmed. “Even such bizarre things as bacteria were proposed, but no one really knows what it is,” says Markiewicz.

In 2011, he and his colleagues manoeuvred the European Space Agency’s Venus Express spacecraft into position to hunt for Venusian glories, and on 24 July they were successful, as they reported last month in the journal Icarus.

The glory they spotted is 1200 kilometres wide, meaning the particles at the cloud tops must be all of the same size for a swatch of sky at least that big. The team found that sulphuric acid droplets alone cannot explain the glory, but droplets coated with elemental sulphur or mixed in with ferric chloride fit the data well.

“This could be the so-called unknown absorber that people had been trying to identify for years,” Markiewicz says. “We cannot say for sure, but we can say that this is one more piece of the puzzle for the whole thing.”

Journal reference: Icarus, DOI: 10.1016/j.icarus.2014.01.030