By Thomas (guest blogger)
On the 6th of September NASA launched their LADEE mission (Lunar Atmosphere and Dust Environment Explorer) into space (Fig. 1). You might have heard of the launch as a frog photo-bombed a picture of the start (Fig. 2). So far the mission is fully on track. The mission will take about 30 days to travel to the Moon, 30 days for checkout and then around 100 days for science operations (Fig. 3).
So what`s hidden in that featureless term “science operations”?
The LADEE spacecraft contains three science instruments: The Ultraviolet and Visible Light Spectrometer (UVS), the Neutral Mass Spectrometer (NMS) and the Lunar Dust Experiment (LDEX). The instruments will analyse the light signatures of atmospheric materials, the variations in the composition of the lunar atmosphere in different heights over the Moon and dust particles in the atmosphere.
Furthermore LADEE carries the Lunar Laser Communications Demonstration (LLCD) which will not be used to investigate the lunar atmosphere. The purpose of the LLCD is to demonstrate the possibility to use lasers for communication with satellites and spacecrafts instead of the conventionally used radio transmitters. This will allow broadband speed in the communications between future satellites/spacecrafts and Earth. LADEE therefore does not only have research goals but also aims to make a major improvement in space flights from the engineering point of view.
But back to the science part: Why study the lunar atmosphere?
Asked about the lunar atmosphere most people would probably answer that there is none. That is understandable from an Earth perspective but, strictly speaking, is not true. A given volume of lunar atmosphere contains about 14 orders of magnitude fewer molecules than the terrestrial atmosphere. That means where you find 100,000,000,000,000 molecules on Earth you find 1 (sic!) molecule on the Moon. As a result the whole lunar atmosphere weights only about 10 tons.
The most abundant elements in the lunar atmosphere are Neon, Helium, Hydrogen and Argon. Those elements are mostly derived from the solar wind (→ Ne, He and H) or from radioactive decay of lunar potassium (→ Ar). At the same rate as the lunar atmosphere is gaining elements by the described processes it loses elements, e.g. through thermally induced escape, thanks to the low lunar escape velocity. Another possibility is that elements get ionized, which allows them to catch a lift on the solar wind due to its magnetic field. As a result of these processes an atom stays in the lunar atmosphere only for about 4 months.
As if that wasn’t enough the elemental abundance is highly variable during the Moon’s day and night cycle (which lasts roughly 29.5 Earth days). The abundance of Neon, Helium and Hydrogen decreases during the day due to high surface temperatures. The energy, derived from the radiation emitted from the warm surface, pushes those elements up- and sidewards, so that they either escape the Moons gravity or they settle on the colder night side, where there`s not so much radiation pushing them around. Argon behaves the other way around. It show the highest abundance on the day side. It`s bigger and heavier than the other three elements and therefore doesn`t care so much for the radiation bumping everyone around during the day. On the cold night side however, there is not enough energy around to keep the Argon in gaseous state, it condensates on the lunar surface and therefore, its abundance in the atmosphere drops.
The Apollo Missions brought up some instruments to study the lunar atmosphere. The first things those instruments detected were – well – the Apollo missions. The extremely sensitive Cold Cathode Gage experiments went crazy whenever an astronaut approached due to the waste gas cloud they emitted from their suites (see Fig. 4). But what was far more important: Every Apollo mission dumped about 10 tons of rocket exhaust in the lunar atmosphere which then remained like a haze for weeks to months over the landing side. 10 tons! Yes that means every Apollo mission added the equivalent of one lunar atmosphere to the lunar atmosphere. Due to the various loss mechanism described above it`s unlikely we`ll find anything of those gases left there though. Or to put it other way: The lunar atmosphere should have recovered its natural state by now.
However, the experiences with the Apollo missions showed that the lunar atmosphere can be easily affected by human interaction. For example detonating about ten 1-kton nuclear bombs underground on the Moon might create enough gas to give the Moon an (on human timescale) long lived atmosphere (> 3000 year). [Vondrak, R. R., 1974. Creation of an Artificial Lunar Atmosphere. Nature 248, 657-659.]
This atmosphere would still be much thinner than Earth’s though. And whether it would be sensible to produce it, is another question. But just in the case: If you have ten nuclear bombs in your basement, this might be something to think about. We`re f****n around with Earth’s atmosphere so much, I don`t see why the Moon shouldn`t get a fair go as well.
So the lunar atmosphere is quite distinct from Earth`s and shows a lot of interesting effects. From my point of view that is enough to go back there and have a closer look at it with LADEE. Just in case you`re not persuaded by that yet, there is something else to solve:
The Mysterious Twilight Rays
Don`t worry, that has nothing to do with crappy vampires (Fig. 5).
Apollo astronauts reported bands of corona/zodiacal light when their spacecraft approached orbital sunrise. A sketch of the made observations by Apollo 17 astronauts is shown in Figure 6. Orbital sunrise means that, when they made this observation, the astronauts were orbiting the Moon in their spacecraft. As all Apollo landing missions were set on lunar daytime, no human being so far has ever seen a sunrise (or sunset) on the
surface of the Moon. The Lunar Ejecta and Meteorites experiment set up by the Apollo 17 crew however, recorded increased particle counts during the change of day and night time. The line between day and night is called the (lunar) terminator (Fig. 7). The interpretation so far for the twilight rays goes like this: The ultraviolet (UV) radiation from the sun charges lunar dust particles on the day side. Then these electrostatically charged particles get set into motion caused by the resulting drop in UV flux during the passing of the terminator. The resulting dust clouds scatter the sunlight. This scattered light is then being seen as the twilight rays.
The question is, whether that really is the right explanation? And if yes, what exactly is the nature of the dust particles and their interactions to form those dust clouds? And last but not least, how could that affect humans that are on the lunar surface as the terminator swaps over them?
Hopefully LADEE will help us to answer those questions.
For more information on LADEE have a look on the official website. The information about the lunar atmosphere used in this post is mainly from the Lunar Sourcebook [Heiken, G. H., Vaniman, D. T., and French, B. M., 1991. Lunar Sourcebook. Apollo The International Magazine Of Art And Antiques.].