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The Dirtiest Lunar Mystery Of All

The views expressed are those of the author and are not necessarily those of Scientific American.

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It's filthy work, but somebody has to do it...(NASA/Apollo)












There may be something funny going on with the stuff covering the Moon, and a new NASA mission launching next month is aiming to solve the mystery.

Gaze up at a brilliant Moon in the night sky and it’s hard to imagine that our companion world, Earth’s last high wilderness, is actually a rather dark and grimy place. The lunar albedo (fractional reflectivity) is only about 0.12 – in other words, over the visible spectrum, it reflects a mere 12% of the light hitting it, absorbing the rest.

By comparison the Earth has an average albedo of about 0.33, Venus – with its high and reflective clouds – is 0.76, and icy smooth Enceladus reflects almost all visible light with an astonishing near 1.0 albedo. In fact, out of all the major bodies in our solar system, only Mercury beats the Moon in terms of darkness, with a 0.11 reflectivity.

Despite its light absorbancy the Moon looks so bright to Earthlings because of its proximity to us, and because of our overall proximity to the Sun. But why is it so non-reflective? As with many phenomena, the answer is not entirely straightforward. The low average lunar albedo seems to be due to a variety of things, from the specifics of rock and ‘soil’ chemistry in a fairly intense radiation environment, to the physical texture of the surface.

Apollo 17 astronaut Gene Cernan looking a little smudgy (NASA/Apollo)

One of the key characteristics of the Moon is that it’s covered in dust. This isn’t household fluff either, it’s extremely abrasive, smells of gunpowder (probably from being implanted with solar wind ions), and sticks like crazy – as the Apollo astronauts discovered.

Without a thick protective atmosphere, the lunar surface has been pummeled by meteorites and micrometeorites for over 4 billion years, breaking rocks into finer and finer particles. These are raggedy things, with no wet weathering to smooth them, and their abrasive forms stick like crazy to spacesuits, humans, human nasal passages, and can even dig through Kevlar.

What exactly is this mysterious glow? Image taken by Clementine mission (BMDO/NASA)


But on a global scale lunar dust may exhibit some even more peculiar characteristics.

The Apollo astronauts were the first to witness a strange twilight phenomenon, a mysterious spread of light above and across the horizon. These remarkable displays were in stark contrast to the otherwise black skies, and a subject of considerable speculation.



The twilight horizon seen by Apollo 17 astronauts, from T-6 minutes to T-5 seconds before sunrise (NASA/Apollo)

They weren’t simple in structure, as seen in this page of sketches from Apollo 17 made during sunrise. In the span of a few minutes the glow above the horizon went from a centrally peaked luminosity to a set of ‘linear structures’ radiating outwards.

But what could be reflecting and scattering sunlight in the lunar vacuum? It could be something like the tenuous glow of irradiated sodium ions, but it could also be the glinting of lunar dust – levitated from the surface by powerful electrostatic charges generated by interplanetary radiation swirling across the landscape.

In fact, electrical charges might even produce dust ‘fountains’. As the rising Sun’s light and radiation sweeps across the lunar surface it could generate large positive charges, enough to kick dust particles a mile high, until they drop back, only to get kicked up again like a pulsing fountain.

Except we still don’t know whether this is really what’s happening, and the whole subject of the so-called lunar ‘exosphere’ (an incredibly tenuous atmosphere, a mere 1/100,000th the density of the Earth’s at sea level) is still relatively little understood.

This may all change after September 6th 2013, when NASA launches the Lunar Atmospheric and Dust Environment Explorer, or LADEE (Scotty would be proud, no doubt).

After a 30 day approach, and 30 days of checkout, LADEE will spend about 100 days orbiting the Moon and examining any dust that makes it to high altitude (it will carry a dust collector and analyzer) and the chemical contents of the atmosphere. It’s a modest enough set of goals, but they could help solve this long-standing mystery and help us better understand our rather filthy, but seemingly brilliant, nearest cosmic neighbor – an object that is very much a part of our own planetary history.

LADEE (NASA/Ames/GSFC) - also noteworthy for its Wallops Island, Virginia launch, which should be visible as far away as New York.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. David Cummings 10:59 am 08/19/2013

    That dust sounds nasty, but it also has its potential uses:

    Link to this
  2. 2. David Cummings 11:01 am 08/19/2013

    Here’s a great slideshow of the 3D dust-printing concept:

    Link to this
  3. 3. Kathy K. 11:31 am 08/19/2013

    Very interesting article. Do you know what time of day the launch will be? I’d like to be able to look up and try and spot it in the sky (from my vantage point in N.VA).
    Hopefully it will be a clear day.

    Link to this
  4. 4. Caleb A. Scharf in reply to Caleb A. Scharf 4:13 am 08/20/2013

    The nominal LADEE launch is set for 11.27pm EDT on Sept 6th (Friday), but that’s just the first launch window, the 7th-10th are also possible launch dates…

    Link to this
  5. 5. sjl4evr 9:50 am 08/20/2013

    OK, I am totally a layperson here, but why couldn’t it just be simple glass? We know that meteorite impacts can melt rock and presumably, in the right circumstances, form glass particles. And we all know from our TV sets that glass can be electrostatically charged. Does anyone know if “glass” always has to be silica-based or can it be made from other materials/elements in the right conditions?

    Link to this
  6. 6. Caleb A. Scharf in reply to Caleb A. Scharf 12:05 pm 08/20/2013

    …and that is an excellent insight. Yes, while I didn’t delve into this in the piece, in fact the dust particles tend to have a glassy coating due to the impact melt, and this certainly helps determine both their ‘abrasive’ nature (under the microscope some of them can look almost like sea urchins, spiky, nasty) and probably their electrical characteristics.

    Link to this
  7. 7. David Cummings 9:28 pm 08/20/2013

    Glass is good, binding into a 3d-printed material is better. The theory is heating the dust with microwave power no stronger than your kitchen microwave so that it will bind into a 3d-printing material, not fuse into glass.

    It would be nice to build a glass factory on the moon, but it’s more practical to use Giant Nasa spider robots to 3D print a lunar base using microwaves.

    Link to this
  8. 8. abemordent 9:38 pm 08/22/2013

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  9. 9. delspace 1:54 am 08/23/2013

    The lunar dust is also extremely hazardous to mechanical devices. Nearly all the moving joints on space suits, rovers, and tools froze up with the dust and were unusable after their relatively short exposure to lunar dust during the Apollo program. Much better seals against dust intrusion will be required on future lunar missions. As if extreme temperatures wasn’t enough of a challenge for lubricants already!

    Link to this

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