A permanent, asymmetric dust cloud around the Moon

• Published in: Nature (London) 2015-06, Vol.522 (7556), p.324-326
• Main Authors: Horányi, M., Szalay, J. R., Kempf, S., Schmidt, J., Grün, E., Srama, R., Sternovsky, Z.
• Format: Article
• Language: English
• Subjects: 639/33/445/847; Clouds; Dust; Humanities and Social Sciences; letter; Meteors & meteorites; Moon; multidisciplinary; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14479

Observations are reported of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. A permanent dust ring around the Moon Before its planned demise on lunar impact in April 2014, NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spent some seven months orbiting the Moon's equator, collecting dust particles for spectroscopic analysis. Sketches made by the Apollo 17 astronauts famously showed a lunar horizon glow, triggering suggestions that electrostatic lofting might be generating dense clouds of small dust particles high above the lunar surface. In this first report on the observations made by the Lunar Dust Experiment (LDEX) onboard LADEE, Mihaly Horànyi et al . find no evidence for such clouds. However, they have detected a permanent asymmetric dust cloud around the Moon, supplied by secondary ejecta dust particles produced by the continual surface impacts of high-speed cometary dust particles in eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths and striking the Moon at lower speeds. The lunar surface is exposed to the same stream of interplanetary dust particles as the Earth, and the LDEX data show that the density of the lunar ejecta cloud increases during meteor showers such as the Geminids. Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged 1 and neutral 2 gas clouds, as well as secondary ejecta dust particles 3 . Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter 4 and Saturn 5 , but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators 6 , 7 , although later orbital observations 8 , 9 yielded upper limits on the abundance of such particles that were a factor of about 10 4 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds.