Oh, glorious Venus! How fragrant are your sulphuric skies! How your rainbow clouds do shimmer!

Actually the sulfuric acid-laden clouds of our neighboring planet would be anything but pleasant for humans, but ESA’s Venus Express orbiter did spot some iridescent hues as it flew over. The picture above, made from images acquired on July 24, 2011, show a circular “rainbow” effect known as a glory. It’s the backscattering of sunlight observed around the shadow point of an object when the Sun is directly behind it from the perspective of the observer (in this case, Venus Express itself.)

Glories are often seen here on Earth from aircraft (but sometimes even from the ground within banks of dense fog) and the mechanics of this one on Venus are pretty much the same — except that the composition of Venus’ clouds is very different, leading to a perfect opportunity for science! Read more…

From a news article from ESA:

The atmosphere of Venus is thought to contain droplets rich in sulphuric acid. By imaging the clouds with the Sun directly behind the Venus Express spacecraft, scientists hoped to spot a glory in order to determine important characteristics of the cloud droplets.

They were successful. The glory in the images here was seen at the Venus cloud tops, 70 km above the planet’s surface, on 24 July 2011. It is 1200 km wide as seen from the spacecraft, 6000 km away.

See a glory observed by a NASA Earth-orbiting satellite here.

From these observations, the cloud particles are estimated to be 1.2 micrometres across, roughly a fiftieth of the width of a human hair. The fact that the glory is 1200 km wide means that the particles at the cloud tops are uniform on this scale at least.

The variations of brightness of the rings of the observed glory is different than that expected from clouds of only sulphuric acid mixed with water, suggesting that other chemistry may be at play. One idea is that the cause is the “UV-absorber”, an unknown atmospheric component responsible for mysterious dark markings seen in the cloud tops of Venus at ultraviolet wavelengths. More investigation is needed to draw a firm conclusion.

Source: ESA