[meteorite-list] Rediscovered Apollo Data Gives First Measure of How Fast Moon Dust Piles Up

[Ron Baalke]

http://news.agu.org/press-release/rediscovered-apollo-data-gives-first-measure-of-how-fast-moon-dust-piles-up/

Rediscovered Apollo data gives first measure of how fast Moon dust piles up
American Geophysical Union
November 20, 2013

Washington, D.C. - When Neil Armstrong took humanity's first otherworldly 
steps in 1969, he didn't know what a nuisance the lunar soil beneath his 
feet would prove to be. The scratchy dust clung to everything it touched, 
causing scientific instruments to overheat and, for Apollo 17 astronaut 
Harrison Schmitt, a sort of lunar dust hay fever. The annoying particles 
even prompted a scientific experiment to figure out how fast they collect, 
but NASA's data got lost.

Or, so NASA thought. Now, more than 40 years later, scientists have used 
the rediscovered data to make the first determination of how fast lunar 
dust accumulates. It builds up unbelievably slowly by the standards of 
any Earth-bound housekeeper, their calculations show - just fast enough 
to form a layer about a millimeter (0.04 inches) thick every 1,000 years. 
Yet, that rate is 10 times previous estimates. It's also more than speedy 
enough to pose a serious problem for the solar cells that serve as critical 
power sources for space exploration missions.

"You wouldn't see it; it's very thin indeed," said University of Western 
Australia Professor Brian O'Brien, a physicist who developed the experiment 
while working on the Apollo missions in the 1960s and now has led the 
new analysis. "But, as the Apollo astronauts learned, you can have a devil 
of a time overcoming even a small amount of dust."

That faster-than-expected pile-up also implies that lunar dust could have 
more ways to move around than previously thought, O'Brien added.

In his experiment, dust collected on small solar cells attached to a matchbox-sized 
case over the course of six years, throughout three Apollo missions. As 
the granules blocked light from coming in, the voltage the solar cells 
produced dropped. The electrical measurements indicated that each year 
100 micrograms of lunar dust collected per square centimeter. At that 
rate, a basketball court on the Moon would collect roughly 450 grams (1 
pound) of lunar dust annually.

Comparing the effects on cells from dust and from damaging high-energy 
radiation from the sun, O'Brien found that long-term dust accretion could 
diminish the output from shielded power supplies of a lunar outpost more 
than even the most intense solar outbursts.

Because the threat posed by radiation damage was recognized early on, 
solar-cell makers fortified their devices against that sort of harm. Yet, 
"while solar cells have become hardier to radiation, nothing really has 
been done to make them more resistant to dust," said O'Brien's colleague 
on the project Monique Hollick, who is also a researcher at the University 
of Western Australia, in Crawley. "That's going to be a problem for future 
lunar missions."

The work is detailed this week in Space Weather, a publication of the 
American Geophysical Union.

Answers from Apollo

Before Apollo 11 blasted off to the Moon in 1969, NASA scientists realized 
the Lunar Module would likely kick up a large amount of lunar soil on 
takeoff, potentially coating nearby science experiments with dust. Detachable 
covers would require either a small explosive or a physical mechanism 
to remove after the astronauts left, creating more engineering challenges 
and room for failure.

"Then I asked what I thought was a pretty common sense question," recalled 
O'Brien. "If we've got to guard ourselves against damage from the lunar 
module taking off, who's measuring whether any damage actually took place; 
who's measuring the dust?"

O'Brien proceeded to quickly invent the Lunar Dust Detector experiment 
as a small add-on device to the larger experiments. Requiring little power 
and weighing only 270 grams (0.6 pounds), the dust detector reported back 
to Earth alongside the non-scientific housekeeping data.

"It really got a free ride," O'Brien said.

The detectors flown on Apollo 12, 14 and 15 operated until NASA shut them 
off in September 1977 due to budgetary concerns. While the detectors worked 
properly, NASA did not preserve the archival tapes of the data they collected. 
For three decades NASA assumed the dust detector data had been lost forever, 
until 2006 when O'Brien heard about NASA's mistake and told them he still 
had a set of backup copies.

Lunar levitation

Each detector in the experiment had three solar cells, each covered with 
a different amount of shielding against incoming radiation. By comparing 
damage to the unshielded and shielded solar cells, O'Brien made his determination 
that dust, rather than radiation, caused the most degradation to the protected 
cells.

Previous model-based estimates of lunar dust accumulation assumed the 
dust came entirely from meteor impacts and falling cosmic dust. "But that's 
not enough to account for what we measured," O'Brien said.

With no atmosphere for wind, the Moon's soil should be stagnant. However, 
O'Brien said a popular idea of a "dust atmosphere" on the Moon could explain 
the difference. The concept goes that, during each lunar day, solar radiation 
is strong enough to knock a few electrons out of atoms in dust particles, 
building up a slight positive charge. On the nighttime side of the Moon, 
electrons from the flow of energetic particles, called the solar wind, 
which comes off the Sun strike dust particles and give them a small negative 
charge. Where the illuminated and dark regions of the moon meet, electric 
forces could levitate this charged dust, potentially lofting grains high 
into the lunar sky.

"Something similar was reported by Apollo astronauts orbiting the Moon 
who looked out and saw dust glowing on the horizon," said Hollick.

The idea of levitating lunar dust could soon be confirmed by NASA's Lunar 
Atmosphere and Dust Environment Explorer (LADEE), launched in September. 
The spacecraft orbits 250 kilometers (155 miles) above the surface of 
the Moon, searching for dust in the lunar atmosphere.

While LADEE scours the Moon's atmosphere, O'Brien looks back on a decades-long 
science experiment that finally has a result.

"It's been a long haul," said O'Brien. "I invented [the detector] in 1966, 
long before Monique was even born. At the age of 79, I'm working with 
a 23-year old working on 46-year-old data and we discovered something 
exciting - it's delightful."

Notes for Journalists
Journalists and public information officers (PIOs) of educational and 
scientific institutions who have registered with AGU can download a PDF 
copy of this early view article by clicking on this link:
http://onlinelibrary.wiley.com/doi/10.1002/2013SW000978/full

Or, you may order a copy of the final paper by emailing your request to 
Thomas Sumner at tsumner at agu.org. Please provide your name, the name of 
your publication, and your phone number.

Neither the paper nor this press release is under embargo.

Title
"Lunar weather measurements at three Apollo sites 1969-1976"

Monique Hollick and Brian O'Brien
School of Physics, University of Western Australia, Crawley, WA, Australia

Contact information for the authors
Brian O'Brien: Phone: +011 (+61) 04-0839-2401, Email: brianjobrien at ozemail.com.au

Monique Hollick: Phone: +011 (+61) 08-9387-3827, Email: monique.m.hollick at gmail.com

Note: Both authors are located in Western Australia (UTC+8:00)

AGU Contact:
Thomas Sumner
+1 (202) 777-7516
tsumner at agu.org