Venus mysteries blamed on colossal collision

Diabolical deuterium
Tobias Owens, a planetary scientist at the University of Hawaii, thinks Davies has "swept deuterium under the rug." This form of hydrogen gas, Owens explained, can form high in a planet's atmosphere when ultraviolet sunlight breaks apart a water molecule.

"When a Venus probe sent back readings of deuterium on the planet, everybody was astonished," Owens said of a Russian Venus lander mission. "There was a huge fraction of deuterium 150 times greater than you see on Earth. You have to explain that."

Owens and other scientists argue that at 836 degrees F, Venus' surface would have instantly baked water into vapor and pushed it into the upper atmosphere, where sunlight is two times more intense than at Earth. Over time, he said, the water would degrade.

Davies, however, said a lack of molecular oxygen — the same type we breathe — produced by the photo-degradation process does not support such an origin of deuterium.

"Venus has virtually no oxygen, whereas Earth's atmosphere is about 20 percent oxygen," Davies said. "If not trapped in the atmosphere, then rocks would have to absorb it." And evidence from Venus, he said, does not suggest that this is the case.

Spin factor
Another clue that Davies said gives his theory legs is the odd rotation of Venus. The planet rotates in a clockwise or retrograde direction, which is the opposite spin of every planet in the inner solar system. "Another peculiarity is that it has no moon," Davies said. "If the head-on impact I've hypothesized was a little off of the mark, it could explain Venus' retrograde rotation without making a moon."

ESA Venus' southern hemisphere, as seen in ultraviolet.

Alan Boss, a scientist at the Carnegie Institution in Washington, D.C., thinks massive collisions — including head-on mergers — were the norm for terrestrial plants early in their histories and could explain our sinister twin's backwards habits.

"Venus must have suffered a giant impact during its formation, as did all the terrestrial planets. That is how the final phase of terrestrial planet formation occurs," Boss said in an e-mail. "This could have been a head-on impact, which might not have produced a moon, or it could have been an off-center impact, like the impact that led to Earth's moon."

If the latter was the case, then where is Venus' moon? Boss explained that if a Venusian moon formed via a giant impact, its orbit could have decayed and spiraled the body into the planet's surface.

Davies thinks the simpler explanation is his own.

"Of course it is possible, but it is unclear whether it is probable," Davies said. Whatever the case, Davies, Boss and most other scientists think big collision events were common in the solar system's formative years.

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