November 2007 Archives

Venus is about the same size and mass as Earth, and of roughly the same composition. And before the space age, planetary scientists imagined an Earth-like environment, perhaps even tropical jungles, obscured by Venus’s perpetual cloud cover. But in 1958, when astronomers measured intense microwaves emanating from the planet, they first got a hint that it was not as lush as they had imagined.

Subsequent visits by spacecraft confirmed that the surface temperatures exceed 800 degrees Fahrenheit, hot enough to melt tin and lead. Although Venus is closer to the Sun than is Earth, the clouds reflect much of the sunlight, and the high temperatures largely result from the heat-trapping effects of an atmosphere that is almost pure carbon dioxide and about 100 times as dense as Earth’s.

Scientists imagine that Venus formed with much liquid water, just like Earth, but that because it is closer to the Sun, with sunlight twice as intense as on Earth, the water began to evaporate. Water vapor, also a greenhouse gas, trapped heat.

“That heats up the surface and leads to more evaporation,” said David Grinspoon of the Denver Museum of Nature & Science. “It’s a powerful feedback.”

The evaporation accelerated until all the liquid water had turned into a thick atmosphere of water vapor. As the water molecules floated in the air, scientists hypothesize, ultraviolet rays from the Sun broke them apart into hydrogen and oxygen atoms. Chemical reactions with minerals in the rocks transformed the oxygen into carbon dioxide. The hydrogen, the lightest of atoms, escaped into outer space.

Measurements from Venus Express, which arrived at the planet last year, support that hypothesis, looking at amounts of hydrogen remaining in the atmosphere compared with concentrations of deuterium, a heavier version of hydrogen. The heavier deuterium would escape more slowly into space, and Venus Express detected a deuterium-to-hydrogen ratio 150 times as high as on Earth, a finding that agreed with earlier measurements. What was surprising, though, was that the deuterium concentration turned out to be 2.5 times as high in the upper atmosphere as near the ground.

“We haven’t completely figured out what it means yet,” Dr. Grinspoon said. “Once we crack this mystery, this will be an important clue to this overall question of the history of water.”