Moon rot!

Recently on the arxiv: Long-term degradation of optical devices on the moon, Murphy et al. This paper talks about the retroreflectors left on the Moon by the Apollo and Lunokhod missions, and observes that they have dropped in effectiveness by a factor of ten since they were placed. So far from the moon being a hostile but static place, something has been steadily degrading these mirrors.

Among the things the Moon missions left behind were arrays of retroreflectors. Like street signs, bicycle reflectors, or those weird-looking radar octahedra, these take incoming light and beam it back to where it came from. These are useful scientific tools, because you can fire lasers at them and time how long it takes the pulses to come back, in the process measuring the Earth-Moon distance. The current best setup, APOLLO, measures the distance to the nearest millimeter, which lets us test theories of gravity, detect a liquid core to the Moon, and watch the Moon recede from us (well, at 38 mm/year).

The moon is very far away (unlike the International Space Station, which orbits at an altitude of only a hundred kilometers). So when you beam a laser at it, even if you use a telescope to collimate the beam, it's spread over 7 km when it reaches the Moon. Then any imperfection of the retroreflector, or simple diffraction, spreads the return beam over an even larger area (20 km); your telescope picks up as much of the returned light as it can, but APOLLO sends out pulses of 10¹⁷ photons and gets back only about one photon per pulse. These tremendous losses are a challenge, so the authors of this paper (who work on APOLLO) monitor the efficiency of the system.

What they noticed, spurring this paper, was that the efficiency dropped substantially — by a factor of fifteen — near the full Moon. Now, obviously the full Moon is very bright, so background photons make detection more challenging, but it is easy to measure the background and estimate how much harder it makes detection; this effect is far from enough to explain the dip. By itself, a dip at full Moon isn't that exciting; since the Apollo 15 retroreflector is pointed at the Earth, a full Moon is when the Sun is illuminating it nearly face-on, so thermal effects might explain it (and in fact since it works by total internal reflection, it only serves as a reflector out to about 17° from the vertical, while the dip is about that wide, at 30°; the authors don't mention this, so it may be coincidence).

To investigate this dip in efficiency, though, the authors of the paper went back and looked at older observations. In the initial years, lunar laser ranging was done with the 2.7m Macdonald Observatory Smith Telescope. But rather than compete for time on this large telescope, in 1985, the program switched to using smaller, dedicated telescopes. Unfortunately these smaller telescopes couldn't see the reflections near full Moon, so there's no data from 1985 to 2006, when APOLLO went online. But comparing the APOLLO data to the MST data, they find that the dip at full Moon was not present at first, and gradually grew as time went on. What's more, if they looked at the return rate away from full Moon, they found a uniform decay; right now the retroreflectors are returning about a tenth what they did initially.

So what's happening to the retroreflectors? How are they getting worse? It's obviously not rain or wind, the moon being devoid of either, let alone the kinds of organic decay you see here on Earth. As with almost all real science, the authors cannot offer a definitive answer, but they do discuss some possibilities.

First of all it's worth pointing out that the huge drop in efficiency doesn't mean the reflectors are absorbing all that extra energy; they are almost certainly reflecting it but in the wrong direction. They're cube corner reflectors, and if you distort the shape of a cube corner reflector it doesn't reflect light back in quite the same direction it came in. The authors find it would take about 4°C temperature difference across each cube to produce the full-Moon losses they see. So if for some reason the reflectors are now absorbing a small fraction of the blazing unfiltered sun at full Moon and being heated by it, that could explain the dip in effectiveness. But why are these mirrors absorbing more and more of the sunlight?

The authors' most plausible answer, to my eye, is dust. The surface of the Moon is covered with dust, made by thermal weathering of rocks and by micrometeorite impact. This dust does not of course blow around the way terrestrial dust does, and in a vacuum a tiny dust grain should fall as fast as a rock, so it initially seems difficult to explain how much dust could get on top of the reflector. There are micrometeorites, though, and there is an effect I hadn't heard of before: dust particles become electrostatically charged through irradiation and are either levitated or thrown upward in "fountains" by electrostatic repulsion. We think. What we do know is that observations from the lunar surface show the optical effect of dust above the ground. So however this dust gets up there, some of it can plausibly fall on optical equipment left there by astronauts.

A layer of dust on the surface would also naturally explain the general decay in effectiveness even when not being heated by the Sun. So it looks like perhaps things left exposed on the lunar surface get covered with dust fairly rapidly. This I find interesting in its own right, but there are also various plans to build telescopes on the lunar surface, since these would share the advantage of the Hubble space telescope of images undistorted by atmosphere, while being able to rely on gravity to hold things in place, and possibly even being able to be built out of lunar materials. If they rapidly become coated with dust, those plans will have to come up with some scheme for cleaning the optical elements.