Earth Pulled a Heist on the Moon's Water

Artist’s impression of the collision that formed our moon.

While we’ve found extensive evidence of ice on the moon, it’s a very dry body compared to Earth. Some researchers say it is drier than the driest desert on our own planet. It’s an interesting dataset given that the moon was likely created after a Mars-sized body crashed into our planet several billion years ago. The shards collected into the moon, but something — some process astronomers aren’t sure about — kept most of the water on Earth.

Previous studies suggest that a bunch of water escaped from the debris of the impact before the moon was formed, but a new study has a different take. It suggests that water and other volatiles tended to deposit more on our planet because of the way the moon came together.

“We began with an existing computer simulation of the moon’s accumulation from a disk orbiting the Earth produced by a giant impact,” said Robin Canup, an astrophysicist at the Southwest Research Institute, in an e-mail interview with Discovery News. “This model was combined with models for how the temperature and chemical composition of the disk material evolve with time.”

Graphic showing all the water on Earth, compared to the size of the Earth itself.

The simulation showed that during this process, the moon picks up about half of its mass “from melt condensed in the inner portions of the disc”, Canup said, which resided between the Earth and the moon’s first orbit. As the protomoon grew larger, it continued to collide with this material, causing its orbit around the Earth to expand.

“When the Mmoon is distant enough, it can no longer efficiently accumulate inner disk melt, which is instead scattered inward and assimilated by the Earth,” Canup added. “We find that this transition occurs prior to the condensation of the depleted volatile elements, so that the final half of the Moon’s mass forms from volatile-poor melt.”

Coincidentally, Canup’s group’s work was published around the same time an article in Science suggested that volcanic rock may be responsible for at least some of the Earth’s water — as opposed to say, comets bringing water to the surface.

“We have not followed water explicitly yet in our models,” she said.  “But in order for the moon to retain water at depth, one would need to invoke a water-rich Earth and/or giant impactor before the impact, which I believe is consistent with this new paper.”

Read more at Discovery News