On March 11, the European Space Agency reported that its Venus Express orbiter observed a rainbow-like effect known as a glory. The phenomenon is frequently observed on Earth by airplanes flying at high altitude, but this is the first time that the effect has been seen on another planet.
Rainbows and glories both appear when sunlight shines on cloud droplets. A rainbow is seen when the droplets are between the Sun and the observer, while a glory is seen when the observer is directly between the Sun and the droplets. While rainbows appear as giant arcs across the sky, glories appear as a series of small colored concentric rings surrounding a bright core. These will differ in appearance based on the composition of the clouds, which are composed of water droplets on Earth versus droplets composed mostly of sulfuric acid mixed with water vapor on Venus. A simulation of this can be found in the accompanying slideshow.
For glories to appear, there must be spherical cloud droplets of similar sizes. The observation of a glory therefore tells scientists that the particles at the cloud tops are roughly uniform in size.
The glory was observed on July 24, 2011 at a height of 70 kilometers above the planet's surface. The spacecraft was 6,000 kilometers from Venus, where the glory appeared to be 1,200 kilometers across. From this information, scientists calculated that the cloud particles are about 1.2 micrometers across, or about the size of a bacterium.
The simulation of sulfuric acid and water does not match the observation of the glory on Venus, which leads scientists to believe that other chemistry is at work. One possibility is that the same unknown ultraviolet-absorbing chemical which is responsible for dark markings observed in Venusian cloud tops at ultraviolet wavelengths could also be responsible for the variation from expected glory brightness, but more observations are needed to know for sure.