Most people would be surprised to learn that the Moon has an atmosphere, or that NASA scientists want to monitor how badly we’ve polluted it, but that’s exactly what the new LADEE mission is trying to find out. Launched in November last year, the Lunar Atmosphere and Dust Environment Explorer (LADEE) is currently in orbit around the moon studying its tenuous atmosphere to understand why there is so much dust levitating above the surface, and how rocket engine emissions are changing the composition of that environment.Everybody knows that the Moon is a harsh airless world, with wild extremes of temperature. During the day, the full energy of the sun beats down, heating rocks and dirt to above the boiling point of water, while at night the cold vacuum of space allows those same rocks to cool down to almost absolute zero. Without a thick Earth-like atmosphere to insulate the surface, there is nothing to stabilise the temperature to a relatively narrow range that allows life to thrive.
But this does not mean that the Moon is blanketed in pure vacuum. Rather, the Apollo astronauts found that it is surrounded by the thinnest of possible atmospheres, which planetary scientists call an exosphere. Exosphere’s are so thin that their atmospheric pressure is lower than what we can achieve in our hardest laboratory vacuums. Unlike the air in a dense atmosphere, the gas molecules almost never interact, so that exospheres can neither trap nor conduct heat, meaning that there is no weather or climate. It also means that aerosols like dust and smoke cannot float, but rather fall to the ground like rocks.
This leads to one of the strangest discoveries made by those early Lunar missions: Dust levitates on the Moon. When the Sun rises, individual microscopic particles of dust rise up from the surface and form a thin haze of material kilometers up from the surface. When night falls, so does the dust. The most logical explanation is that individual high-energy photons of sunlight strike the dust particles and knock off a few electrons, leaving a positive charge on the dust. Positive repels positive, and the dust is thrown up into the air until it is high enough that the electrostatic repulsion is weak enough to be balanced by gravity. But without proper close-up observations and data, this theory remains merely a hypothesis.
Enter LADEE, which was launched into space on 6 September by one of the US Air Force’s Minotaur V rockets, converted from an old ICBM. After a leisurely float through space, it entered lunar orbit on 20 November and turned on its science instruments. It currently orbits the Moon every two hours, at an altitude that varies from twelve to sixty kilometers above the surface. This orbit allows it to monitor the entire Moon at various times of day, and sample the exosphere at a wide range of altitudes.
“A thorough understanding of the characteristics of our lunar neighbor will help researchers understand other small bodies in the solar system, such as asteroids, Mercury, and the moons of outer planets,” said Sarah Noble, LADEE program scientist at NASA Headquarters in Washington. This is because we now know that nearly all so-called airless bodies in the Solar System do actually possess exospheres of their own, which have an important effect on the long-term behaviour of an object.
Since astronomers became aware of the Moon’s exosphere, some have begun to worry about the effect we’re having on the lunar environment. While there is no life on the Moon, and therefore no ecosystem to damage, the exosphere is still quite vulnerable. After all, while the emissions of a few rocket landings might not seem significant in the context of an entire world, there is so little gas in the exosphere that those few landings can significantly alter its composition – a fact confirmed when the Chinese Chang’e 3 mission landed in December last year and LADEE was able to detect the change in exosphere composition and dust volumes.
Because the exosphere is so thin, individual gas molecules hardly ever interact with each other, meaning that there is no atmospheric chemistry to speak of: chemical compounds do not break down, but last forever. This makes LADEE one of the most important active missions today, and the only one with an unmovable deadline: we need to study the exosphere right now, while we still can, before we change it forever.