If all goes as planned, at 11:27 p.m. tonight, NASA will launch LADEE – the Lunar Atmosphere and Dust Environment Explorer, pronounced "laddie" – from its Wallops Island facility in Virginia. Most of the Eastern Seaboard will be able to watch the beginning of what sounds like an impossible mission: to study the moon's atmosphere and water cycle.
Most of us learned in school that the moon doesn't have either one of those. It turns out the story is a little more complicated, and a lot more interesting.
Past missions have looked down at the moon's surface (Apollo, LCROSS) and interior (GRAIL), or even up to the lunar skies (Artemis), but LADEE is doing something different: "We'll be looking sideways, through the atmosphere and dust, to understand what's happening just above the surface," says Sarah Noble, LADEE's program scientist at NASA's Washington, DC headquarters.
Finding air on the airless moon
LADEE was created to answer "two mysteries," says Rick Elphic, LADEE's project scientist, "the first being the thin, tenuous, very exotic lunar atmosphere, utterly unlike our own; the second being this mysterious 'lofted substance,' reportedly dust, that the Apollo astronauts saw above the surface."
As the astronauts flew over the moon, they saw what looked like "dust particles or aerosols," says Dr. Elphic, who is based at NASA Ames Research Center in Menlo Park, Calif. "But since the moon is an awfully good vacuum, and has only a thin trace of an atmosphere, we know that dust particles couldn't be suspended ... so it's a real mystery."
The moon is indeed airless, but it has an "exosphere" – that's the word for an atmosphere with so few particles in it that they drift around like ships at sea, never interacting with each other. Earth's exosphere is far, far out at the raggedy edge of our thick, gassy atmosphere, out past the International Space Station. On the moon, the exosphere is right at the surface, making it a "surface boundary exosphere." Surface boundary exospheres are actually more common than gaseous atmospheres, says Dr. Noble: Mercury has one, as do Pluto, most of the moons of the gas giant planets, and even some of the bigger asteroids. "It turns out to be the most common class of atmosphere" in our solar system, she says, "yet it is one we don't know much about."
LADEE's Ultraviolet and Visible Light Spectrometer (UVS) will scan sideways to analyze the exosphere and dust particles, while the Neutral Mass Spectrometer (NMS) and Lunar Dust Experiment (LDEX) will vacuum up whatever particles or dust the orbiter passes through. The UVS's telescope will pay special attention to the terminator, the line between day and night.
LADEE won't follow a circular orbit, because of the moon's "lumpy" gravitational field, but NASA scientists are taking advantage of that to time the closest approaches – when LADEE will skim just 12 miles above the moon's surface – to match up with passes by the terminator. "That's what our orbit is designed to optimize for: so we can go over the terminator as many times as we can, as close as we can, to see what's really going on," says Noble. LADEE will follow its uneven orbit for 100 Earth days, orbiting three times per day, so it will have a chance to observe the rising or setting sun some 600 times in its three-month mission.
As the terminator sweeps around the moon, bringing the rising sun across the lunar surface, scientists expect something very interesting to happen. Along with golden sunlight, sunbeams include photons strong enough to kick off the negatively-charged electrons, leaving the moon's sun-drenched side with a net positive electrical charge. The night side, the side of the moon facing away from the sun, has a net negative charge. At the terminator, breaking dawn brings with it an abrupt electrical change from negative to positive. The resulting current could zap dust grains into leaping sideways, some scientists have theorized.
In addition, since like repels like, a positively-charged dust grain near the surface will be repelled away from the positively-charged surface immediately below it. "If that (repelling) force can overcome gravity, then it's possible for that dust to levitate around on the surface – to be transported around by electric fields," says Timothy Stubbs, a lunar scientist with NASA's DREAM center. He adds that this isn't limited to the terminator, but can happen "anywhere on the moon, because the whole of the moon charges up ... becomes electrified."
The moon's 'water cycle'
Sunlight doesn't just get the dirt excited; it also perturbs exospheric particles. For one thing, it's carrying hydrogen atoms, that can connect with stray oxygen to form water or hydroxyl ions (water's kid sister). And then there's all that energy, heating the surface to a balmy 273 degrees Fahrenheit.
"The same solar wind that comes in and unites with oxygen and possibly creates water and hydroxyls comes in with a kilovolt (kev) of energy. That's enough energy to really rattle the lattice of mineral grains," says Elphic. "You can imagine a high-energy cueball going into a bunch of billiard balls: Some of those billiard balls rattle around and collide with other billiard balls and come back out. Others get sent deeper into the mineral grain. But the few that make it back out then, basically, constitute this exosphere." So instead of nitrogen or carbon dioxide, like Earth's atmosphere, he says, "You're going to see things representing the silicate mineralogy of the lunar surface – silicon, oxygen, calcium, argon – so we'll be looking for those." And for the very, very faint traces of water vapor.
A recent mission into a crater near the moon's north pole – a dark hole that hasn't seen sunlight in millions or perhaps billions of years – revealed ice, riddled with other impurities. "So we know there's ice there," says Elphic. "But how did it get there? That's the question. You can imagine a number of scenarios that deliver ice to the cold regions of the moon. One might be cometary impact. But there's another possibility," he says.
"Slowly, over the millenia, molecule by molecule, water molecules have vibrated," getting knocked around like billiard balls and occasionally getting kicked upward, "leaping off the surface, falling in this nice parabolic arc to some other place on the surface, leapfrogging around," he explains.
"Imagine thousands and billions of these things, doing the same thing" over millions and millions of years, he says. It can only happen during the day, while sunlight pours in. When the sun sets, the moon's surface temperature plunges almost 500 degrees Fahrenheit and the particles in the exosphere "collapse" to the ground, says Elphic, waiting for the next pass of the terminator to bring the rising sun. "On the day side, it's warm enough that these guys are hopping around ... like bunnies across the surface."
Moving water one molecule at a time makes for a slow transport process, to be sure, and it only goes one way. Once a molecule hops into one of those permanently shadowed polar regions, they get "cold-trapped" and end up stuck, perhaps forever.
"LADEE's in perfect position to look at the possibility of water and hydroxl in the lunar atmosphere, so we'll be able to check that pathway, that water cycle, if you will, that takes the possible water from the midlatitudes to the polar regions."
And it all starts tonight. Go outside early enough to let your eyes adjust some before the 11:27 p.m. launch. Wallops is located in the middle of the Eastern Seaboard, so "theoretically, if we have really clear skies, some 60 percent of the US population ought to be able to see it," says Noble.