The first data from the planetary probe Venus Express is helping scientists shape the story of how a young planet with everything needed to support life became instead a sulphurous, baking hell.
The authors of nine papers, to be published in the journal Nature on Thursday, conclude the data showsVenus is more Earth-like than scientists had previously believed.
It's a "sister planet gone wrong," according to Andrew Ingersoll of the California Institute of Technology who in 1969 coined the phrase "runaway greenhouse" to describe the effect of carbon dioxide in Venus's atmosphere.
Venus Express, which is the European Space Agency's first Venus probe, has been circling the planet since April 2006, returning a torrent of data telling us how Venus's atmospheric circulation, chemistry, energy balance and the greenhouse effect act together to produce a climate the authors of the overview paper call "defiantly different" from Earth's.
When they were formed and cooled, the young planets had remarkably similar masses, radii, distances from the sun and geologies. Both had water and probably vast oceans. But Venus is closer to the sun than Earth, and in its first billion years, the water boiled away. Now, there is a tiny amount of water in the atmosphere and none in the furnace of the planet's surface.
"Venus once had an ocean's worth of water but lost it," Ingersoll, who wrote an analysis of the nine papers for Nature, told CBC News. The process is continuing, he said. "It is just a trickle at the end of a flood."
The evidence is found in the ratio of deuterium to hydrogen in the atmosphere: about 150 times that of Earth — which has kept its water — and double that amount high above the clouds. That cannot happen, say the scientists, unless a massive amount of water had evaporated from the surface over time.
Venus had the same amount of carbon in its atmosphere as the early Earth, but whereas on Earth the carbon became locked into the planet's crust, in Venus it became trapped in the atmosphere.
Ingersoll said if you imagine "cranking up the sun," you would see how a runaway greenhouse effect occurs as carbon builds up in the atmosphere.
"It would get warmer and that would evaporate some water and water is a very effective greenhouse gas and that traps the heat, and that makes the ocean even warmer and that evaporates more water and that traps more heat, and it just cycles away."
This extreme climate, driven by an atmosphere that is 96.5 per cent carbon dioxide, "reminds us of pressing problems caused by similar physics on Earth," according to the authors of the overview paper.
Lightning provides puzzler
In addition to the lost water, another set of data from Venus Express hints at the planet's lost potential.
Against all logic, there appears to be lightning in the atmosphere. Author Christopher Russell of the University of California, Los Angeles told the CBC the evidence comes from the discovery of radio waves called whistler-mode waves. On earth, whistlers mean lightning.
And that's a significant finding, he said, because on Earth, lightning creates chemical reactions and molecules that some believe could be precursors to life.
"Now Venus is an unpleasant sulphuric place and we don't expect it has the atmosphere for life but who knows at the beginning?"
According to Ingersoll, our understanding of terrestrial lightning means it shouldn't happen on Venus. He points out the atmosphere of Venus is similar to "19th century London on a smoggy day with sulphuric acid in the air and a general haze." And you don't get lightning in that kind of atmosphere, nor has anyone ever spotted its telltale flash.
Large temperature differences also puzzling
Ingersoll was also surprised by the large temperature differences between day and night on Venus. Logic suggests the planet's massive atmosphere should smooth out those differences to perhaps 4 to 5 C.But Venus Express found differences of 30 to 40 degrees.
These are "puzzlers," he said. "And puzzles are always interesting because they shake up your prejudices and open your eyes to things."
Just as Earth could have shared the fate of Venus had it been closer to the sun, so might Venus have shared Earth's had it been further away.
Russell said, "If I were designing the planets, I would exchange Mars and Venus's position."
Venus is warmer and has a thick atmosphere, while Mars doesn't have enough atmosphere for life.
"If you put Mars where Venus is and Venus where Mars is, that would be perfect" for life to evolve, Russell said.