When Meteorites Strike the Moon, They Launch Water Vapor Into Space

We tend to think of the moon as an airless wasteland, and that’s mostly true. The moon actually has a tenuous envelope of water and hydroxyl (OH) molecules. NASA launched the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission in 2013 to study the wispy gas around the moon. Now, NASA reports LADEE spotted what scientists had long expected. Meteorite impacts on seemingly dry patches of the moon can throw up a mist of water vapor that regenerates the “exosphere” and moves water around the surface.

LADEE was a short-term lunar exploration mission, spending less than a year orbiting the moon from September 2013 to April 2014. The probe contained spectrometers for quantifying the molecules floating around in the moon’s exosphere. A separate instrument also aimed to identify any lunar dust particles that might be ejected from the surface.

Models have predicted that meteoroid impacts could have a correlation with water and hydroxyl molecules in the lunar exosphere, and now we have confirmation from the LADEE data. The team has identified dozens of strikes on the lunar surface that caused an increase in water vapor concentration in space around the moon.

According to NASA, most small objects that hit the moon make barely a dent. However, a slightly more massive space rock around 0.2-inches (5 millimeters) across can penetrate the 3-inch (8-centimeter) bone-dry surface layer of regolith. Below that, the moon has a layer of hydrated soil. The water stuck to these bits of dust and rock get launched into space by these impacts where they remain in the exosphere for a brief time. The moon doesn’t have enough gravity to retain an atmosphere, so they eventually drift off into space, but some ends up falling back to the surface.

The team says this finding helps explain how ice ends up in craters on the moon. These so-called “cold traps” can keep water ice stable for billions of years, and impacts on the moon might be responsible for transporting it to and from those areas.

Future human missions to the moon might rely on the water available on (and under) the surface. LADEE’s measurements of water in the lunar exosphere helped scientists calculate how much water lies beneath the surface. Unfortunately, it’s not an embarrassment of riches. The water concentration of the hydrated layer is about 200 to 500 parts per million, or about 0.02 to 0.05 percent by weight. Thus, it would take more than a metric ton (2,204 pounds) of hydrated regolith to collect 16 ounces of pure water.

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