False-colour composite of the glory, composed of three images in the ultraviolet, visible, and near-infrared wavelengths.
The first "glory" rainbow ever seen on another planet has been spotted on Venus, revealing new clues about the planet's atmospheric composition.
For the first time, a type of rainbow known as a "glory" has been seen and photographed on a planet other than Earth. The glory — a type of rainbow caused when light is reflected directly back towards its source — occurred on Venus, our closest neighbouring planet.
Glories are similar to rainbows in that they are caused by refracted light off droplets of liquid from a source behind the observer; however, glories are smaller in size and, rather than a sweeping arc, they consist of a number of concentric coloured rings around a bright core. They are usually only seen from above, such as from an aeroplane, and require two compositional conditions: first, that the cloud particles are spherical (which indicates that they are liquid), and that they are all the same size.
The glory in question was photographed by the European Space Agency's Venus Express. Astronomers have long thought that Venus' atmosphere was rich in liquid sulfuric acid, as well as some other mysterious atmospheric component that absorbs UV light, which accounts for dark markings seen across the Venusian cloud tops at ultraviolet wavelengths. The ESA arranged for the orbiter to take a photo of Venus' clouds when it was directly between the sun and the planet.
The resultant glory is the first of its kind seen by human scientists. "A full glory has never been seen before outside of the terrestrial environment," Wojciech Markiewicz of the Max Planck Institute for Solar System Research in Gottingen, Germany, told New Scientist.
What the glory — a full 1200 kilometres wide — revealed is that the cloud particles must be of a uniform size on a large scale, around 1.2 micrometers in diameter — about a fiftieth of the size of a human hair. However, the brightness variations seen in the glory are not consistent with what would be expected from sulfuric acid mixed only with water — which leads the team to believe that some other chemical is present.
"This could be the so-called unknown absorber that people had been trying to identify for years," Markiewicz said. "We cannot say for sure, but we can say that this is one more piece of the puzzle for the whole thing."
The team's research was published last month in the journal Icarus.