Stronger Than Dirt
Lunar explorers will have to battle an insidious enemy—dust.
- By Trudy E. Bell
- Air & Space magazine, September 2006
(Page 2 of 4)
A mere two days in the Colorado dunes was enough to convince me of the insidious mechanical challenges that dust will present to future astronauts and their machines. The windborne grit scoured our skin raw, sifted into hair and zippers and shoes, messed up the transmissions of all four rovers, and jammed the buttons of cellular phones.
Even on the airless moon, the dirt doesn’t lie still. Astronauts in lunar orbit aboard the Apollo 8, 10, 15, and 17 spacecraft repeatedly observed and sketched what they variously called “bands,” “streamers,” or “twilight rays” for about 10 seconds before lunar sunrise or sunset. The drawings are reminiscent of the slanting rays that filter up through clouds during sunsets on Earth. Some scientists chalked the phenomenon up to light reflecting from dust suspended above the lunar surface, but others remained unconvinced: Without an atmosphere, how could dust be suspended above the moon? Even if the particles were kicked up by, say, a meteorite impact, they would quickly settle back to the surface.
It wasn’t until last year that Timothy Stubbs and his colleagues at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, came up with an explanation, which they called the “dynamic fountain” model. In a drinking fountain, the arc of water from the spout appears suspended in one position, but the water molecules are constantly in motion. Similarly, according to Stubbs, microscopic grains of lunar dust are constantly leaping from the surface and falling back again due to a weird phenomenon unknown on Earth: electrostatic lofting.
Just as rubbing a balloon against your shirt creates a static charge that can levitate the hair on your head, lunar dust particles with opposite charges will attract each other, and like-charged particles will repel. How does the dust become charged? On the moon’s sunlit side, solar ultraviolet and X-ray radiation beats down relentlessly, knocking electrons from atoms in the lunar soil. The electrons escape into space, and positive charges build up on the lunar surface. The tiniest motes of dust—just a few hundred-thousandths of an inch in size—are repelled from the rest and launched upward, some reaching miles above the surface. Lunar gravity eventually pulls them back down, but electrostatic repulsion kicks them off again. The process is repeated over and over to form a tenuous “atmosphere” of moving dust particles.
The same happens on the lunar far side, which is bombarded by solar wind particles flowing around the moon—except that the net charge is negative since the solar wind is mostly electrons. Data from the 1998 Lunar Prospector mission suggests that the electrical potential might amount to hundreds of volts on the night side, even higher than on the day side, possibly launching dust particles to higher velocities and altitudes.
Evidence supporting the dynamic fountain model may be buried in old data from the Lunar Ejecta and Meteorites experiment, left on the moon by Apollo 17 in 1972. LEAM had three sensors that could record the speed, energy, and direction of tiny particles. The experiment was designed to look for fallout from lunar meteorite impacts, as well as material raining down from comets or interstellar space. But in a classic case of serendipity, “LEAM recorded a high number of particles every lunar sunrise,” recounts Gary Olhoeft, professor of geophysics at the Colorado School of Mines, “mostly from east or west rather than from above, and mostly much slower than expected.” Even stranger, a few hours after every lunar sunrise, LEAM’s temperature rocketed up so high—near that of boiling water—that the instrument had to be turned off because it was overheating. Olhoeft and others now suspect that dust lofted from the moon covered the LEAM, darkening its surface so the experiment package absorbed sunlight rather than reflected it. But nobody knows for sure. LEAM operated only briefly before the Apollo program ended.
Then there are the puzzling rays seen by the Apollo astronauts. Because the specks of dust bouncing around on the moon would be too small to see with the naked eye, explorers on the surface wouldn’t likely notice them. But astronauts on the night side around sunrise might see the moving dust causing the sunlight to scatter, looking like “a weird, shifting glow extending along the horizon, almost like a dancing curtain of light,” according to Stubbs. And at certain times during the lunar cycle, when the moon passes through an active part of Earth’s magnetosphere, Stubbs speculates that “dust would start flying at high velocities”—not at densities that could be seen from Earth, but perhaps in large enough amounts to get into unprotected machinery on the moon.
Last year, in a 77-page report listing 20 risks that required further study before we should commit to a human Mars expedition, NASA’s Mars Exploration Program Analysis Group ranked dust number one. The report urged study of its mechanical properties, corrosiveness, grittiness, and effect on electrical systems. Most scientists think the only way to answer the questions definitively is by returning samples of Martian soil and rock to Earth well before launching any astronauts.
Many also believe a lunar sample return will be necessary. True, the Apollo astronauts brought back some 800 pounds of lunar rocks from six landing sites. But the dust played a dirty trick: The gritty particles deteriorated the knife-edge indium seals of the bottles that were intended to isolate the rocks in a lunar-like vacuum. Air has slowly leaked in over the past 35 years. “Every sample brought back from the moon has been contaminated by Earth’s air and humidity,” Olhoeft says. The dust has acquired a patina of rust, and, as a result of bonding with terrestrial water and oxygen molecules, its chemical reactivity is long gone. The chemical and electrostatic properties of the soil no longer match what future astronauts will encounter on the moon.