The lunar dust environment: concerns for Moon-based astronomy

Abstract

The Moon has no atmosphere, hence, it offers a unique opportunity to place telescopes on its surface for astronomical observations. It is phase-locked with Earth, and its far side remains free from ground-based interference, enabling the optimal use of radio telescopes. However, the surface of the Moon, as any other airless planetary object in the solar system, is continually bombarded by interplanetary dust particles that cause impact damage and generate secondary ejecta particles that continually overturn the top layer of the lunar regolith. In addition, there is evidence, that small particles comprising the lunar regolith can be electrically charged, mobilized and transported, also representing a hazard for covering sensitive surfaces and interfering with exposed mechanical structures. In addition to the naturally occurring dust transport, rocket firings during landings and take-offs, pedestrian and motorized vehicle traffic will also liberate copious amounts of dust, representing a potential hazard for the safe and optimal use of optical platforms. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.

Keywords: dust hazard; moon; near-surface dusty plasmas.

Figure 1.

(a) The flux of interplanetary meteoroids at 1 AU as a function of their size (mass), the labels indicate space missions, and $\beta$-meteoroids are IDPs on escaping orbits driven by radiation pressure [9]; (b) the modelled speed distribution, independent of the size of a meteoroid, scaled with the mass flux at the Moon (black solid line) and at Earth (blue solid line) [11,12].

Figure 2.

(a) The daily running average of impacts per minute of particles with radii $>\hspace{0.17em}0.3$ µm and $a>0.7$ µm recorded by LDEX. Four annual meteoroid showers generated elevated impact rates lasting several days. The labelled annual meteor showers (blue vertical lines) are the Northern Taurids (NTa); the Geminids (Gem); the Quadrantids (Qua); and the Omicron Centaurids (oCe). Towards the end of March LDEX data indicated a meteor shower that remained unidentified by ground-based observers [19]. (b) The average dust ejecta cloud density observed by LDEX for each calendar month LADEE was operational in 2014. Each colour ring corresponds to the density every 20 km. The plot is in a reference frame where the Sun is on the left ($-x$ direction) and the apex motion of the Moon about the Sun is towards the top of the page ($+y$ direction) [21,22]. The missing bottom left quadrants represent data gaps, as LDEX could not make measurements while the Sun was in its FOV.

Figure 3.

(a) Images of lunar horizon glow. The contribution of Zodiacal light is evident in Surveyor 5 and 6 but not in Surveyor 7 images, perhaps because of the different camera iris settings. (b) Apollo 12 Charles ‘Pete’ Conrad Jr. gestures near the Surveyor 3 spacecraft on the lunar surface on 21 November 1969. The Surveyor 3 television mirror shows a finger mark made by Conrad in a layer of dust on the mirror. Surveyor 3 landed on the Moon on 20 April 1967. Dust was likely deposited on the mirror both during the landing of the Surveyor and also during the landing of the Apollo 12 Lunar Module 155 m away (photos are from NASA’s Science Data Center).

Figure 4.

(a) The schematic of the experimental set-up to investigate dust charging, mobilization and transport under UV and/or plasma exposure of a regolith surface [–62]. (b) Stacked images of the trajectory of a single dust particle lofted from the surface, using a narrow focal plane normal to the boresight of the camera. Images like this are used to measure the initial speed of the particle [62]. (c) Initial launch velocities as a function of dust size for irregularly shaped lunar simulant particles (circles) and $10\hspace{0.17em}\text{μ}\text{m}$ radius silica microspheres (triangles). The theoretical curves (solid lines), obtained from energy conservation, are shown with $\gamma =1$ and 5 that parameterize the cohesion between particles [62]. (d) Cartoon of the ‘patched charge model’ [59].