Peeling Away the Moon's Mysteries

Sitting on the couch in the family room the other night, my eye caught a bright light shining in through the window: the moon, almost full, rising over the hills east of our house.

Remember the moon? There’s been so much news coming from more distant reaches of our solar system lately: from spacecraft like the Mars rovers Curiosity and Opportunity on their geologic expeditions on the red planet; Cassini wheeling about the Saturn system; New Horizons getting ready for its historic flyby of Pluto next year; and even 30+ year veteran Voyager 2, recently announced to have, at long last, crossed the boundary of the solar system, the plasma “membrane” imposed by the sun’s solar wind as it pushes against the gases of interstellar space.

But the moon often seems like an ancient relic of space exploration, that dusty, dry, airless ball of rock and soil that we visited decades ago and have since left alone—possibly because we found nothing there but dust, rock, and soil?

Not so fast. Lately, the moon has begun to reveal a new face, fresh facets of fascination that are making us reconsider it not as Earth’s closest, but dead, partner in space, but as a world of subtle character with mysteries that still surprises us.

One of those surprising finds is water. An experiment performed in 2009 by NASA’s LCROSS (Lunar Crater Observation and Sensing Satellite) mission revealed the presence of significant amounts of water ice deposits in the perpetually shaded floors of Cabeus Crater at the Moon’s South Pole. LCROSS bombarded the crater floor with a large impacting spacecraft—the upper stage of the rocket that took it to the Moon—and in the plume of dust that was kicked up water vapor was detected.

One possible source for these polar crater ice deposits is the solar wind, the constant flow of material (mostly hydrogen ions) that blows outward from the sun throughout the solar system. Over time, it is thought, the interaction of solar wind particles with lunar surface material may form water molecules, which can build up as layers in the shady recesses that never receive direct rays of sunlight.

Around the same time, India’s Chandrayaan-1 lunar orbiter made another watery discovery in Bullialdus Crater near the moon’s equator: rock contained in the crater’s central peak—material that was once buried deep under the moon’s surface but exposed by the asteroid impact that formed the crater—contains a high concentration of hydroxyl (OH) molecules, indicating the presence, long ago, of water beneath the moon’s surface, water that originated from within the moon itself.

NASA launched another probe to the moon earlier this month, further asserting that our scientific curiosity about the moon is alive and well. LADEE, the Lunar Atmosphere and Dust Environment Explorer, will make a study of another thing that we don’t associate with the moon: atmosphere.

Yes, contrary to what you learned in the classroom, the moon does possess an atmosphere, though we must use that term loosely. The density of this lunar atmosphere is somewhere around a million particles per cubic centimeter—particles like sodium and potassium atoms. That may sound like a lot, but when we compare it to air density on Earth’s surface (10 billion billion molecules per cc), we find that the lunar atmosphere is actually a purer vacuum than we can create in most laboratories. LADEE is still being maneuvered into what will eventually be a tight nearly circular orbit that will carry it around the moon once every 24 hours, so the fascinating facts of the moon’s “air”, as well as the dust environment closer to the surface, is yet to come.

Long ago, before we got so good at exploring places in outer space, people imagined all sorts of things on the moon: vast seas of water, strange life forms and civilizations, deities and springs bubbling forth the elixir of life. Then our scientific scrutiny of that sphere reduced it to a dead, dry, and long unchanged cinder.

But now that we’re getting even better at exploring, some “life” is returning to our musings of the moon. No life forms or vestiges of civilizations (so far), no black obelisks, and no wide liquid planes with waves lapping against shorelines (that would be Saturn’s moon Titan).

But, yes, polar ice, possible igneous sources of subsurface water, and a light whiff of air dancing around the skies. Sound like the makings of a hot vacation spot.