In the minds of most people, space is a vacuum – a dark, empty expanse of nothingness, where planets remain neatly fixed in their orbits as they travel around the Sun. However, at least within the “empty” space of our Solar System, this seeming void is filled with invisible magnetic and electrical fields, streams of high velocity gases emitted from the Sun, high-energy atomic nuclei spewed out from distant stars, and debris leftover from planetary assembly. It is an active, dynamic environment, if one knows what to look for and how to measure it.
We’ve long known that the Moon has no atmosphere and hence, no hydrosphere. This is largely a consequence of the small size of the Moon (about 1/4 the diameter and only one percent of the mass of the Earth). Its gravity is too weak to retain an appreciable atmosphere. As a result, there is nothing to slow down or stop material – usually traveling at cosmic (extremely high) velocities – from striking the Moon. Meteoroids hit the lunar surface at about 20 km per second, vaporizing particles on impact and throwing large clouds of lunar dust into space. The lack of atmosphere means these particles follow pure ballistic trajectories and unless ejected at greater than orbital velocity (1.6 km/s), over time, they will travel through space and land back on the Moon. This “churning” of the surface by micrometeorite impact permits us to sample distant areas on the Moon from a single place (although determining the point of origin of such a sample is virtually impossible).
The dust thrown through space is in flight and thus, a transient phenomenon. But debris is hitting the Moon constantly and thus, the ejecta thrown up from these impacts is constant. In principle, the Moon should be surrounded by a “cloud” of dust ejecta, thrown out into space by the constant “rain” of micrometeorites. I put the term “cloud” in quotes because it is somewhat misleading – a cloud on the Earth is a collection of dust and water droplets, suspended in the air by buoyant forces provided by thermal differences. Lunar dust “clouds” are not suspended, but are in constant ballistic motion, some particles ascending and some descending, depending upon their crater of origin. But collectively, these particles make up a dust exosphere that varies in density and position with time.
New studies indicate that the Moon possesses a permanent dust cloud, one generated by impact and constantly filling the space surrounding that body. Dust exospheres have been clearly observed and measured around the icy moons of Jupiter and Saturn, but the history of observations of the lunar dust cloud is murkier. We first had an inkling that dust might exist in the space above the lunar surface during the Surveyor missions (the series of unmanned soft landers flown in the 1960s, designed to pave the way for the Apollo astronauts). Surveyor found that the Moon’s surface is composed of very fine dust, yet is cohesive enough to bear the weight of both a heavily laden astronaut and a loaded Lunar Module.
During its mission, the Surveyor spacecraft collected images and data during the long lunar day (14 Earth days). As sunset approached, and the spacecraft began shutting down to survive the long (14 Earth days), extremely cold (-150° C) lunar night (all the while, taking pictures at its landing site), a faint glow of light was observed along the horizon around the location where the Sun had set.
Since the Moon is an airless body, there is no atmosphere to scatter light and no “horizon glow” was expected. This phenomenon was puzzling, but the Surveyor scientists thought that fine dust was being stirred above the surface by micrometeorite impacts. A few years later, while orbiting the Moon in the Apollo 17 Command Module, mission commander Gene Cernan looked out the window just before sunrise and saw an awesome vista – a spectacular set of “streamers” radiating above the limb of the Moon. He was so impressed that he made a sketch of his observations.
Those sketches resulted in 40 years of consternation and confusion by lunar scientists. Cernan not only observed the solar corona (glowing, outer atmosphere of the Sun, also visible from the surface of the Earth during total solar eclipses) and the zodiacal light (vast cloud of fine dust that orbits the Sun in the ecliptic plane), he had also observed a strange, lunar horizon-hugging glow, similar to that seen by the Surveyor spacecraft but on a much larger scale (Surveyor was on the surface, meaning its “horizon glow” was only a meter or two above the surface while Apollo 17 was 100 km above the Moon). During the flight, Cernan’s observations were duplicated by his fellow crewman Ron Evans, who also observed the glow and made his own real-time sketches of the phenomena.
The solar corona and zodiacal light are clearly visible in these drawings. What is curious is their depiction of a horizon-hugging glow of light. Other crews did not report seeing a horizon glow; between 1968 and 1972, we sent 22 men into lunar orbit (two went twice) and 3 additional astronauts flew around the Moon during the aborted Apollo 13 mission. Only two reported seeing the lunar horizon glow. Twenty years later, the unmanned Clementine mission orbited the Moon for 71 days. The spacecraft took a variety of images using its Star Tracker camera (more light sensitivity) as well as the normal visible mapping camera. Despite repeated attempts to catch the elusive horizon glow, no evidence for it was found.
Now, published results from the Lunar Atmosphere and Dust Detector Explorer (LADEE) mission indicates that the Moon is surrounded by a cloud of dust, just as the satellites of Jupiter (and at about the same density). From their data, it was also found that the density of the dust notably spikes during known annual meteor showers. These showers occur when the Earth and Moon fly through the orbital paths of short period comets (icy bodies that shed debris as they get close to the Sun). This debris orbits the same path around the Sun as the comet nucleus and thus forms “corridors” of dust in space. As the Earth-Moon system passes through the corridors, the dust hits both bodies, forming a spectacular “shower” of high velocity meteors on Earth, where they burn up in the dense atmosphere, but hit the unprotected lunar surface at high speed. These lunar impacts kick up ejecta, adding to the dust cloud that surrounds the Moon.
Is this dust cloud responsible for lunar horizon glow? An interesting find in these new measurements is that the density of exospheric dust on the Moon is much lower than had been expected from the Apollo data. Studies of the Apollo dust suggested that there was too much present to be the result of impact – the dust must be levitated and suspended above the surface by electrical force. This effect may be at least partly responsible for the dust, as measurements show higher dust concentrations near the morning terminator on the Moon (where surface electrical charging is expected to be highest). The authors of the new study infer that most of the dust is being kicked up by high-velocity cometary debris, not the more common (but slower) asteroid meteoroids. This cometary connection is strengthened by the correlation of spikes in dust density with the annual meteor showers.
So where does this leave the horizon glow observations? Unfortunately, we still aren’t sure of their origin. A lower density of dust means that any horizon glow from light scattered by a dust exosphere would be extremely weak. No horizon glow was observed by Clementine, Lunar Reconnaissance Orbiter or LADEE, despite attempts to detect it. It is possible that the Apollo 17 crew saw a transient event, possibly the remnant effects of a single, large impact that locally threw a great quantity of dust into the sky. The sketches by Cernan and Evans are labeled “GET (i.e., Ground Elapsed Time since mission began) 232-55”, meaning that they were observed and drawn at the same time. LM Pilot Jack Schmitt made his drawing after the mission. He tells me that he was focused more on the streamers that appear to emerge from the below-horizon Sun and in his rendering, horizon glow is not prominent, if present at all (see Figure 4 in McCoy and Criswell, 1974).
Clearly, the Surveyor observations are different (reflecting lofted dust well below the kilometer altitudes observed by Apollo and LADEE). So we are still left with a lunar mystery – where is this horizon glow coming from? Is it a one-off, transient event or a constant and recurring phenomenon? We must learn more in order to fully understand the lunar dust exosphere, as the high vacuum environment of the Moon’s surface makes it a desirable future laboratory for experimentation.