Asteroid impacts may explain Venus’ missing oxygen

Asteroid impacts might be to blame for sweeping oxygen out of the atmosphere of Venus, a new study suggests — oxygen that was left behind after the planet lost most of its water.  

Venus was once a much wetter world, researchers suspect, but harsh ultraviolet radiation from the young, energetic sun probably zapped water molecules, splitting oxygen from hydrogen. The light, zippy hydrogen atoms fled into space. What became of the oxygen, however, isn’t as clear, says Kosuke Kurosawa, a planetary scientist at the Chiba Institute of Technology in Narashino, Japan.

The answer might lie in an asteroid blitz that pummeled the inner planets several hundred million years after the formation of the solar system, Kurosawa suggests in the Nov. 1 Earth and Planetary Science Letters. The flurry of impacts excavated and pulverized rocks, polluting the Venusian skies with subterranean dust. Orphaned oxygen could then chemically react with the iron-rich soot, aided by sweltering temperatures and a dense atmosphere that kept the particles afloat. Using computer simulations, Kurosawa found that these tiny sponges could soak up as much oxygen as is found in over 60 percent of Earth’s oceans.

It’s an interesting idea, says Ramses Ramirez, a planetary scientist at Cornell University. Another leading explanation for the mystery of the missing oxygen is absorption by a magma ocean that formed after the planet was repeatedly whacked by space rocks. The lofted debris, he says, could work in parallel with a molten surface to effectively hide the planet’s oxygen.

“All of this is about trying to understand the divergent evolution of Earth and Venus,” Ramirez says. The planets are near twins, roughly the same size and mass and made of pretty much the same material. “What is it about Venus that makes it so different compared to Earth?” he asks.

Understanding the contrasting histories of these neighboring worlds might also provide insight into what makes habitable planets elsewhere in the galaxy. “The simple answer is distance from the sun,” Ramirez says, “but it’s more complicated than that.”