The Moon has an impermanent hold on its tenuous atmosphere. Its relatively weak gravity (1.62 m s-2), one sixth of Earth’s, struggles to hold onto the gas species that do exist in its exosphere environment. Though the Moon readily contributes gasses to its exosphere, external forces strip these away into a streaming tail. Fine dust particles undergoing electrostatic levitation also make their way into the exosphere and were most notably observed by Apollo astronauts in lunar orbit as the dust was illuminated by sunlight near the terminator. This dust was also encountered by the astronauts on the lunar surface where it posed problems to spacesuit operations and astronaut health. As lunar exploration interests are rekindled through efforts in both civil and private spaceflight, imagined futures are presented with a lunar industrial revolution including mining, manufacturing, and habitation. The effects of these activities on the lunar exospheric environment should be considered to help better inform planning of future lunar operations and to mitigate any projected hazards to human activity near or on the Moon.
The increased introduction of gas and dust into the lunar exosphere by human activities, if the envisioned industrialization of the Moon comes to fruition over the next several centuries, could lead to a more substantial lunar atmosphere. This multicomponent atmosphere of rarefied gasses, plasma, and charged dust particles could form conditions adverse to human activities on the Moon, requiring mitigation techniques or the creation of systems to cope with the altered lunar environment.
Exposure to solar photons, particles, and dynamic magnetosphere environments provide the energy to drive current lunar exosphere processes. A description of the lunar day cycle, the Moon’s interaction with the solar magnetosphere, and transits of Earth’s magnetotail provide the basis for modelling the current lunar exosphere and the processes that drive its composition, growth, and depletion. Sources of gas loss and gain have been identified from remote sensing and in situ surface and orbital observations. Sputtering processes from solar particles produce the bulk of gasses that are observed in the Moon’s exosphere. The sublimation of ices from the near-permanently shadowed craters of the poles provides another source of gaseous species to the lunar exosphere, along with other processes such as meteorite impacts. These gases are lost from the Moon’s gravitational grasp through solar radiation pressure acting on neutral gas species, and the solar magnetosphere acting on ionized, plasma, species.,
To model the dust component of the lunar exosphere, we must understand the dust transportation processes that create the current environment, including electrostatic and kinetic forces. Exploration of the Moon during the Apollo missions yielded a wealth of information about its dust environment from lunar orbit to the surface. Observations by crewmembers have provided information about dust adhesion to surfaces such as their suits and equipment, as well as physiological responses to dust exposure once those suits were brought back inside the Lunar Module. Samples returned to Earth provided for further studies on the mineralogy, morphology, adhesion, and transport processes of these regolith particles.
With the current lunar gas and dust exospheric system modeled, anticipated disturbances stemming from industrial activities can be introduced to the model to determine the extent of any impacts that these activities will have. Among the currently proposed industrial processes to be undertaken on the lunar surface is the production of water, and rocket fuel (notably H2 and O2) from local resources. These efforts will require the extraction of water ice deposits buried in loose regolith or hygroscopically bound to hard rock. The extraction activities might resemble that of open-pit mining on Earth and could create analogous dust conditions. Refining processes of any extracted rock or metal ore material could also be a significant source of outgassing byproducts that would contribute to any anthropogenic lunar atmosphere. Beyond these industrial activities, frequent visits by chemical rocket propelled spacecraft introduce gaseous rocket exhaust products of various species, most significantly CO2 and H2O. Surface and near surface operations of these launch vehicles, and wheeled or legged surface transport vehicles could mobilize dust particles with enough kinetic energy to persist in the exosphere for extended periods of time, lengthened by electrostatic forces. Finally, biological waste from human, plant, and animal life support systems could represent another source of outgassed byproducts.
A variety of outcomes could result from the introduction of gas and dust into the lunar exosphere by industrialization and other human activities. There may not be any significant change to current lunar exospheric conditions, attributed to the limited amount of dust mobilization and gas emission from industrial processes, or the strength of external forces that keep the space around the Moon clear. Other outcomes may result in large dust and gas contributions to the lunar exosphere forming a dusty, rarefied atmosphere. An outcome such as this could influence natural processes dependent on incoming solar radiation as well as human activity that relies on vacuum conditions and optically clear space near the lunar surface. Exploring the current and imagined future lunar exosphere will also aid the study of other “airless” bodies such as small terrestrial planets (Mercury, Ceres), rock/ice moons (Phobos, Ganymede), and asteroids (Vesta, Bennu).
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