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.
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