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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 20 Nov 2013 12:54:47 -0800 (PST)
Message-ID: <201311202054.rAKKsldI008366_at_zagami.jpl.nasa.gov>

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
Received on Wed 20 Nov 2013 03:54:47 PM PST