The Glory of Venus
Tricks of the light on other planets
On 24 July 2011, something rather magical appeared in the cloud-tops of our nearest planetary neighbour, Venus.
The Venusian atmosphere is thick and choking. A runaway greenhouse effect means that the air is ninety-six percent carbon dioxide, and pressure at the surface is more than ninety-two times that of Earth. It’s by far the hottest planet in the solar system.
But at the top of the dense blanket of clouds, seventy kilometres above Venus’s fiery surface, things are rather calmer. Temperatures are close to those on Earth and there’s even talk of setting up balloon colonies.
If we did send floating colonies to Venus, their shadows below would be ringed by a concentric rainbow circle known as a glory. These occur on Earth when sunlight shines on a cloud comprised of uniformly-sized, spherical droplets. We often see them from airplanes, or during foggy dawns and dusks, as they require the Sun to be positioned behind you.
Now, however, we’ve observed them on other planets too. The European Space Agency’s Venus Express orbiter has snapped a photo of a 1200 kilometre wide glory on top of the Venusian clouds. Here it is in all its… er… glory:
Rather than water, the Venusian clouds are comprised of droplets of sulphuric acid, and by studying the glory we can examine some of their characteristics.
From the observations made by the Venus Express orbiter, we can tell that the droplets at 1.2 micrometres across — about a fiftieth of the width of a human hair. The extremely wide glory also tells us that the droplets are the same size across a very large area.
There’s just one mystery. The variation in the brightness of the rings doesn’t match what we’d expect from clouds of sulphuric acid mixed in with a little water, which suggests that there’s something going on that we’re not aware of.
It’s likely that there’s a chemical of some sort in there that we haven’t identified — a theory lent credibility by the fact that we see dark markings in the tops of the clouds of Venus at ultraviolet wavelengths. Something is absorbing the ultraviolet light, and we’re not totally sure what, but iron, chlorine and sulphur have been implicated.
Paper author W.J. Markiewicz writes: “We investigated several possibilities and argue that either small ferric chloride (FeCl3) cores inside sulfuric acid particles or elemental sulfur coating their surface are good explanations of the observation.”
The details of the Venusian glory were published in Icarus.
Hat tip to Frank Swain of Futures Exchange.
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