The moon just keeps getting wetter and wetter. Trace amounts of water were found in rocks returned from manned Apollo and unmanned Soviet missions. Then in the late 1990s, during orbital missions to the moon to map and study its surface minerals, scientists detected the water ice in permanently shadowed craters in the moon’s polar regions. It was also found bound up in lunar minerals here and there across the entire lunar globe.
Now, researchers from NASA and the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, report that streams of meteoroids striking the Moon infuse the thin lunar atmosphere with a short-lived water vapor.
Finding water on the sun-baked orb with its vacuum-like atmosphere is incredible in itself. But there’s also a practical side. Water is a crucial resource for a long-term stay on the moon. If we find extract it from the rocks, astronauts might live there for months or years. The team made the recent discovery after sifting through the data collected by LADEE (Lunar Atmosphere and Dust Environment Explorer) and finding dozens of events. LADEE studied lunar dust and the moon’s scanty atmosphere during its 7-month mission in 2013-2014.
“We traced most of these events to known meteoroid streams, but the really surprising part is that we also found evidence of four meteoroid streams that were previously undiscovered,” said Mehdi Benna of NASA’s Goddard Space Flight Center and the lead author of the study. The new ones seen by LADEE occurred on Jan. 9, April 2, April 5 and April 9, 2014.
Meteoroids are bits of rock shed by comets and asteroids. When a meteoroid strikes Earth’s atmosphere and burns up it’s called a meteor. Meteoroid streams are responsible for our favorite meteor showers like the Perseids in August and Geminids in December. Meteors from those same showers also strike our lunar neighbor, but instead of burning up, they slam into and zap the surface.
Most of the time the moon has next to no water vapor in its skimpy atmosphere, but when it passed through one of the streams enough vapor was released for LADEE to detect. When the shower was over, the H2O — and its cousin, OH, called the hydroxyl radical — fizzled away.
To release water, the meteoroids had to penetrate at least 3 inches (8 cm) below the surface. That may sound challenging for a rock only a few millimeters across but consider that it’s traveling at tens of thousands of miles an hour. Second, there’s no atmosphere to slow them down. Third, the lunar soil, called regolith, is fluffy. Even a meteoroid one-fifth of an inch (5 mm) wide can burrow down deep enough to release a puff of water vapor.
With each impact, a small shock wave fans out and ejects water from the surrounding area. About two-thirds of that vapor escapes into space, but about one-third lands back on the surface of the Moon.
The top layer of lunar regolith is bone-dry but below that there’s a hydrated layer where water molecules likely stick to bits of soil and rock. Based on measures of the moon’s exosphere (its barely-there atmosphere), researchers calculated that the hydrated layer has a water concentration of about 200 to 500 parts per million, or about 0.02 to 0.05 percent by weight. That’s far drier than the driest desert soils on Earth. To squeeze half a quart of water out of the moon, you’d have to process more than a 1.1 tons (1 metric ton) of regolith.
These findings could help explain the deposits of ice in cold traps in the dark reaches of craters near the poles. Untouched by sunlight water can remain stable up to several billion years. Water vapor liberated by meteoroid impacts may have migrated to the poles where it remains in “safe keeping.”
Some meteoroids contain water and hydroxyls, but the team confirmed that most of the water detected had to be the moon’s because the meteoroids contain too little to account for what was detected. So where does the water come from? Sources include ancient bombardment by water-rich comets and asteroids, meteoroids and even the steady stream of protons (hydrogen atoms stripped of their electrons) blowing from the sun in the solar wind. Protons hit the regolith at high speed and combine with oxygen to make the precious stuff life finds so necessary.