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.
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.
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.
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).
A computer-generated model of the LADEE spacecraft.
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.