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Soundings: Chemistry of Venusian atmosphere

After the sun goes down, even before the sky grows dark, the “evening star” shines bright in the west: Venus. (Look for the crescent moon close to and above the planet today).

Venus reflects sunlight better than any other planet.

Earth and Venus are twins – virtually the same size, likely composed of the same materials.



But the surface of Venus is bone-dry. What happened to its oceans?

Venus is one-and-a-third times closer to the sun than Earth; Venusians (were there any) would see the sun, not one-and-a-third times larger, but one-and-a-third squared (1 and 1Ú3 times 1 and 1Ú3) times larger … almost twice as large as the disk we see from Earth.




With its disk nearly twice as large as the disk we see on Earth, the sun’s light on Venus is almost twice as strong.

Spacecraft that have entered the Venusian atmosphere have radioed back evidence that Venus did, indeed, once have lots of water. But under such scorching sunlight, the water could not remain liquid; it evaporated.

Water is an excellent “greenhouse gas” – better, even, than carbon dioxide. If the air stays moist after a winter storm (the sky remaining cloudy), water molecules absorb infrared (heat) beaming off the ground, radiate it back down at us, and we enjoy a (relatively) balmy night. But should the clouds disappear, infrared energy beaming off the surface continues out into space, and the clear, starlit night gets very, very cold.

(A large part of the global warming debate ultimately focuses, not so much on the warming caused by carbon dioxide, but on the extra water that will evaporate into the air because of that warmth.)

With an ocean of water vapor in its atmosphere, Venus suffered a “runaway-greenhouse” – its surface temperature soared to hundreds of degrees Fahrenheit.

Drifting upward, water molecules were zapped by solar ultraviolet, which shattered the molecules into atoms of hydrogen and oxygen. Lightweight hydrogen atoms, flying faster than the planet’s escape velocity, sped off into space.

And took with them any chance of recombining with oxygen atoms to re-make Venus’s water.

As they do on Earth, volcanoes on Venus belch out carbon dioxide. Carbon dioxide on Earth – over millions of years – combines with minerals in the rocks and is washed to the sea, where it is buried in the muck. But the process requires water, and, with hardly any water remaining, Venus’s carbon dioxide has accumulated; it now makes up roughly 95 per cent of the atmosphere … and, through the “greenhouse effect,” keeps the surface of Venus hotter than a kitchen oven broiler.

If not water, what are the Venusian clouds made of?

Venusian volcanoes, again like Earth’s, also exhale fire and brimstone … better known as sulfur. Sulfur combines with oxygen to make SOx Ð sulfur oxides. And SOx combines with whatever water it can find to form sulfuric acid.

Place a beaker of concentrated sulfuric acid, full to the brim, on a lab bench on a humid day, and the beaker will overflow. Sulfuric acid “loves” water. As it pulls water from the air (much as salt in a shaker pulls water from the air and becomes hard to pour), its volume increases.

What’s good for the goose is good for the gander.

Water “loves” to dissolve sulfuric acid, and most of Earth’s sulfurous emissions are quickly rained out of the atmosphere.

But because Venus lacks the water to rain out the acid, it accumulates. The clouds of Venus – the clouds that reflect sunlight to make the evening star so bright and beautiful – are composed of droplets of sulfuric acid.

ooo

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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