Venus is made of the same stuff of Earth, but is bone-dry, hot enough to melt lead and has a chokingly thick atmosphere. It even spins backwards.
Astronomers have spent decades trying to explain Venus' mysterious properties.
Now one scientist thinks the planet's formation may explain all: Two huge, protoplanetary bodies collided head-on and merged to form our planetary neighbor, but obliterated nearly all water in the process.
"The probability that two protoplanets collided to form Venus is not at all implausible," said John Huw Davies, a geodynamicist at Cardiff University in Wales who developed the idea.
A majority of scientists think Earth's moon formed when a protoplanet about the size of Mars smacked into the planet at an angle. Davies thinks Venus was born of a far worse cosmic train wreck.
"What if the moon-Earth collision isn't that big in planetary terms?" Davies told SPACE.com. "A head-on blow between two similarly sized bodies would have been about twice as energetic."
Astronomers have had little time to react to Davies' proposition, which is detailed in recent issue of the journal Earth and Planetary Science Letters, but already some are wary.
Despite the cautionary responses from other scientists, Davies thinks his idea is worth exploring.
Earth harbors an enormous volume of water, even in its searing interior. The life-giving molecule emerges as a vapor with molten lava, carrying with it a radioactive gas known as argon-40.
The isotope is generated from radioactive potassium deposits inside of our planet, as well as in Venus.
Davies thinks the relatively low amount of that isotope of argon detected in Venus' atmosphere — about 400 times scarcer than on Earth — is a sign that water never really seeped out of the parched, volcano-covered planet.
"The only way water could have out-gassed is very early in Venus' history," Davies said. "The argon-40 gives us a timescale of water leaving the ground because it's produced over time, and only a little of it has been released."
A mega-collision between two bodies of roughly equal size could have provided the energy necessary to rip water, which is made of two hydrogen atoms and one oxygen atom, into pieces.
The hydrogen would escape into space, while oxygen would bond with iron and sink to the planet's core.
Although the Earth suffered a catastrophic impact that formed the moon, Davies explained that the process did not dry out the two bodies.
"It wasn't as energetic, limiting the reaction of iron and water," he said.
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 (447 degrees C), 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."
Evidence from Venus, he said, does not suggest that this is the case.
Another clue that Davies said gives his theory legs is the odd rotation of Venus.
The planet rotates in a clockwise or retrograde direction when viewed looking down on its north pole, which is the opposite spin of almost every other planet in the solar system.
(Uranus may also spin "backwards," but since it also spins on its side, possibly due to another planetary collision, its north and south poles are somewhat arbitrarily labeled.)
"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."
All the other seven major planets except for Mercury have moons.
Alan Boss, a scientist at Carnegie Mellon University in Pittsburgh, thinks massive, indirect collisions — not a head-on merger — 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, noting that moons like the Earth's are thought to frequently form from such impacts.
If so, then where is Venus' moon?
Boss explained a protoplanet impact could have formed a moon around Venus that has since drifted out of the solar system, into the Sun or even smacked back into the planet.
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.
New Venusian visitor?
Aside from planning to create a detailed computer model for the hypothesized mega-collision, as has been done for moon-formation theory, Davies said another way to test his idea is to send a new spacecraft to Venus.
Russia's space program successfully landed nearly 10 spacecraft on Venus' surface in the 1970s and 1980s.
But Davies said none of them scouted for water-containing minerals such as mica — evidence that would challenge his hypothesis.
"They made remote chemical measurements of the surface," Davies said, but none indicated hydrated rocks. "If a new spacecraft finds a lot of hydrated minerals, it would show there is still abundant water on Venus. Then my hypothesis would be out."
Spacecraft that have recently encountered Venus can't detect such minerals from space, he said, because of a layer of reflective hydrogen sulfide in the atmosphere.
"A rover of some sort could scout for such minerals before it fails from the intense heat, or maybe a satellite below the hydrogen sulfide [layer]," he said.
Boss, however, said even detecting such minerals might not rule out a collision.
"Water can always be added as a 'late veneer' by ... icy planetesimals that helped finish building the planet," Boss said, although Davies thinks comets and other such bodies could only deliver a small amount of water to the planet.
Even if hydrated rocks on Venus' surface could rule out a cataclysmic formation, other data could provide better clues to the planet's origins, Francis Nimmo of the University of California, Santa Cruz thinks.
"There a lot of things that would be very nice to do on Venus, like put a seismometer on the surface," said Nimmo, a planetary scientist. "The reason we know anything about Earth's interior is from such devices."
Whether or not someone launches a new spacecraft to scout out Venus' surface, and whatever its scientific mission is, Davies said it will have to investigate quickly.
"You have to take all of your measurements before the lander, or whatever it is, quite literally burns up," Davies said. "The longest any spacecraft has lasted is less than two hours."
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