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.
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