FEATURE
The Dust Busters
Scientists Hope to Gather Measurements of the Moon’s Dusty Environment
Scientists investigate instrument to Search for Signs of Cosmological Inflation
It won’t be easy living and working on the Moon. On certain days each month, a veritable “dusty sleet” made up of irregularly shaped, razor-sharp dust grains traveling at hurricane-like speeds could pelt the astronauts, possibly damaging their spacesuits and the robotic machinery they will use to establish their permanent outposts.
These ultra-tiny dust grins — formed by millions of years of meteorite impacts that repeatedly melted rocks into glass and then broke the glassy rocks into powder — are highly electrostatic. Because of these issues, NASA has ranked lunar dust as among the top hazards to mitigate before sending human astronauts to the Moon for extended stays. However, before engineers can design a detailed dust-mitigation strategy, NASA needs to better understand the physics that drive the phenomenon, many dust experts believe.
Goddard scientist Bill Farrell is one of them.
He and his colleagues recently won research and development funding to carry out feasibility studies on hardware requirements for a follow-on instrument to the Lunar Ejecta and Meteorites (LEAM) instrument, which Goddard dust pioneer Otto Berg developed for the Apollo 17 mission. With this funding, Farrell is investigating the development of a higher-precision instrument that would help identify the processes that create the dusty environment and accelerate the movement of dust grains on and above the surface.
“It’s not about just dust,” he says. “It’s an environmental system question. If you understand the system, you can do predictions and develop ways to better alleviate, mitigate, or simply avoid the dust hazard.”
| Electrostatic Clinging
Scientists now know that lunar dust clings because of its varying shapes and jagged edges that hook into objects like microscopic burrs. However, another reason for the tenacious clinging is the dust’s electrostatic charge.
On the day side of the Moon, harsh, unshielded ultraviolet radiation from the Sun kick electrons out of the upper layers of the lunar regolith or soil, giving the surface of each dust particle a net positive charge. This positive charge builds up until a small fraction of the looser grains, some measuring one micrometer or smaller, are repelled and lofted from meters to kilometers high — a phenomenon observed by the Apollo astronauts and the Lunar Surveyor missions. They eventually fall back, but the process repeats itself, creating an atmosphere of vertically moving dust particles.
|
Dust covered astronaut Harrison Schmitt’s spacesuit as he retrieved lunar samples during the Apollo 17 mission.
|
On the dark side of the Moon, the situation is a little different. Plasma currents from the Sun also charge the lunar surface, but negatively. The situation gets interesting where the two sides meet at the terminator — the moving line between lunar day and night. The transition between the two could create more complex and stronger electric fields, further accelerating the grains.
Strong Electric Fields
And, indeed, that’s what Berg’s LEAM found in the mid 1970s. Although designed to measure hypervelocity micrometeorite impacts to the Moon, the instrument mostly detected charged dust traveling many hundreds of miles per hour primarily at the terminator. While scientists suspect that strong electric fields form at the terminator and are believed to accelerate the dust, they have not yet obtained critical correlated measurements in the active regions.
Knowing these measurements are even more relevant now that NASA is considering Shackleton Crater as a site for a lunar base. The crater is aligned with the terminator and is potentially exposed to this high-energy, highly variable dust for extended periods of time.
For astronauts, the situation will be made worse by the fact that they carry their own charge and will attract dust as they rove about the Moon. And because the grit is so adhesive, it doesn’t simply brush off like commonplace house dust. It embeds itself into material, which means that it’s easily tracked into living habitats.
In some sense, Farrell and his colleagues are acting as lunar “weather men,” trying to understand, model, and predict the character of the dusty lunar exosphere. “Our knowledge of the lunar electrostatic environment is still limited,” Farrell says. “An updated LEAM to answer some of these questions is what we need before we return astronauts on the Moon for long-duration stays.”
|
