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Earth Pulled a Heist on the Moon's Water

When the moon was formed from a massive planetary collision billions of years ago, somehow very little of Earth's water ended up on the moon -- and planetary scientists aren't sure why. Continue reading →

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.

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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. | Jack Cook, Woods Hole Oceanographic Institution

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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."

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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."

Canup acknowledged a few limitations in the model - such as how the moon ended up with so much heavy zinc under this scenario - but said that they are planning further research to probe our neighbor's origins. You can read the published research in Nature.

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

Apollo has never looked better. More than 40 years after the astronauts explored the surface, two photo projects are showing us the moon as only a handful of people has seen before.

The Project Apollo Archive

uploaded thousands of scanned NASA images to Flickr. Also, a new crowdfunded book called "

Apollo: The Panoramas

" easily exceeded its initial goal and will begin shipping hardcover books next year. This is some of the science these photos helped NASA perform.

MORE: Moonwalkers: Stunning Photos from Apollo 11

Some rumors circulated in the late 1960s that the first Apollo astronauts to land on the moon may very well sink into the surface. That wasn't really the worry for NASA during Apollo 11; after all, the Surveyor 3 (U.S.) and Luna 9 (Soviet) spacecraft had arrived safely, among others, with little evidence of subsidence. But NASA was interested in how well the lunar module performed when astronauts arrived on the surface. In pictures, NASA surveyed information such as how far the feet penetrated into the surface, and how much of a divot the engine exhaust left behind.

MORE: Fragile Apollo Artifacts in Need of Some Love: Photos

NASA was also interested in knowing how the investigators' experiments worked on the surface. Some of the experiments were used multiple times in missions, such as the foil solar wind composition collector seen here from Apollo 12. By asking the astronauts and looking at pictures of the deployed experiments, the investigators could make improvements from mission to mission to better data collection or other aspects of the mission.

MORE: Distant Earth: A History of 'Pale Blue Dots'

Without the lunar rover, the Apollo missions would have had severely limited surface operations. The rovers could carry equipment, samples and astronauts for many miles across the surface, allowing for more intensive investigations. NASA took care to make sure the astronauts observed a "walkback limit", just in case the rover broke down and the crew had to hike back to the lunar lander.

MORE: Moon Landing: Rarely Seen Photos Inside Apollo 11

As the Apollo program matured, the astronauts received advanced training in geology so they could better make choices about their work on the surface. This allowed them to select rocks representative of the environment, and to give detailed descriptions to Mission Control about their surroundings that could be recorded for the geology team. As a part of that, Apollo 15 commander Dave Scott did a brief survey of the landscape before even setting foot, perched inside the lunar lander and sticking his torso outside to take pictures and relay information to NASA.

MORE: Return to Earth: Rarely Seen Photos Inside Apollo 11

Where were the astronauts on the surface of the moon? That was no trivial question after astronauts began using rovers to get miles away from their landing site -- this shot from Apollo 16 gives you a taste. NASA closely monitored the astronauts' discussions, looking at television cameras and trying to plot their best estimates of the crew's location on orbital maps. Later on, when the pictures were developed, NASA and others looked at them to pin down where the astronauts physically were.

MORE: When (Part of) Apollo 13 Reached the Moon

While the astronauts described their surroundings as best as possible, NASA had a special color tool available to "calibrate" the images on the moon to their true color. An identical copy of this scale was on Earth, making it possible to do comparisons from afar as to what color the moon's regolith (soil) really was. That turned out to be very important on Apollo 17, when the astronauts found what they thought was orange regolith in an otherwise greytone landscape. They were right; it was the tint of volcanic glass.

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