Lunar horizon glow is a phenomenon in which dust particles in the Moon's thin atmosphere create a glow during lunar sunset. The

Surveyor program The Surveyor program was a NASA program that, from June 1966 through January 1968, sent seven robotic spacecraft to the surface of the Moon. Its primary goal was to demonstrate the feasibility of Soft landing (rocketry), soft landings on the Moo ...

provided the first data and photos of the phenomenon. Astronauts in lunar orbit observed it during the

Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the Apollo program and the fourth Moon landing. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a longer stay on the Moon and a greate ...

and

Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...

missions.

Cause

Dust kicked off from the surface of the Moon will stay in the atmosphere for around 3 hours. Apart from this, electrically charged particles could be levitated by electrostatic fields with a strength of >500 V cm⁻¹. This cloud of dust, near the lunar terminator line, forward scatters the light, creating a glow near the horizon during lunar sunset. The dust, 10 micrometers in diameter, is thought to be above 10 km from the lunar surface. The levitation mechanism is thought to eject 10⁷ more particles per unit time into the cloud than those caused by

micrometeoroid A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface. The term "micrometeoro ...

impacts. The term "Moon fountain" has been used to describe this effect. During the lunar day, infrared rays and

ultraviolet rays Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the ...

from the Sun are strong enough to knock electrons off the dust present on lunar surface. These positively charged particles get repelled from the surface kilometers high. On the night side, the dust is negatively charged by electrons from the

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

. Particles at the night side achieve greater electrical tension differences than the day side, launching dust particles to even higher altitudes. This dust eventually falls back to the surface, and the cycle repeats. In celestial bodies without any significant atmosphere, electrostatic transport is believed to be the leading cause of dust transport. Laboratory experiments show that dusty surfaces tend to become smooth as a result of dust mobilization. This phenomenon is thought to explain the process of dust ponds in the

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

433 Eros 433 Eros is a stony asteroid of the Amor group, and the first discovered, and second-largest near-Earth object. It has an elongated shape and a volume-equivalent diameter of approximately . Visited by the NEAR Shoemaker space probe in 1998, ...

and comet

67P/Churyumov–Gerasimenko 67P/Churyumov–Gerasimenko (abbreviated as 67P or 67P/C–G) is a Jupiter-family comet. It is originally from the Kuiper belt and has an orbital period of 6.45 years as of 2012, a rotation period of approximately 12.4 hours, and a maximum velo ...

. Strange glowing lights on the Moon are recorded from Earth for centuries. This phenomenon, known as " transient lunar phenomena", is now generally accepted to be visible evidence of meteoroids impacting the lunar surface. But others with an amorphous reddish or whitish glows or even as dusky hazy regions that change shape or disappear over seconds or minutes, are thought to be because of sunlight reflecting from suspended lunar dust. In 1956, this effect was anticipated by science fiction author

Hal Clement Harry Clement Stubbs (May 30, 1922 – October 29, 2003), better known by the pen name Hal Clement, was an American people, American science fiction writer and a leader of the hard science fiction subgenre. He also painted astronomically oriented ...

in his short story "Dust Rag", published in ''

Astounding Science Fiction ''Analog Science Fiction and Fact'' is an American science fiction magazine published under various titles since 1930. Originally titled ''Astounding Stories of Super-Science'', the first issue was dated January 1930, published by William C ...

''.

Exploration

Coronal photographs of the Moon from

and 17 showed excessive brightness. The glow was also observed by astronauts in lunar orbit during sunrise for about 10 seconds. Such rays were also reported by astronauts aboard

Apollo 8 Apollo 8 (December 21–27, 1968) was the first crewed spacecraft to leave Sphere of influence (astrodynamics), Earth's gravitational sphere of influence, and the first human spaceflight to reach the Moon. The crew orbited the Moon ten times ...

and

Apollo 10 Apollo 10 (May 18–26, 1969) was the fourth human spaceflight in the United States' Apollo program and the second to orbit the Moon. NASA, the mission's operator, described it as a "dress rehearsal" for the first Moon landing (Apollo 11, two ...

. These might have been similar to

crepuscular ray Crepuscular rays, sometimes colloquially referred to as god rays, are sunbeams that originate when the Sun appears to be just above or below a layer of clouds, during the twilight period. Crepuscular rays are noticeable when the contrast (vision ...

s on Earth. The glow is also believed to appear in the

star tracker A star tracker is an optical device that measures the positions of stars using photocells or a camera. As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may ...

data from the

Clementine A clementine (''Citrus × clementina'') is a tangor, a citrus fruit hybrid between a willowleaf mandarin orange ( ''C.'' × ''deliciosa'') and a sweet orange (''C. × sinensis''), named in honor of Clément Rodier, a French missionary who f ...

mission, although it would be masked by coronal and

zodiacal light The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direct ...

. The

Apollo missions The Apollo program was a United States human spaceflight program carried out from 1961 to 1972 by the National Aeronautics and Space Administration (NASA), which landed the first astronauts on the Moon. The program used the Saturn IB and Saturn V ...

placed laser retroreflectors on the lunar surface. The dust is believed to be the cause of the degradation of the instruments. Apollo 17 also placed an experiment on the Moon's surface called LEAM (Lunar Ejecta and Meteorites). It looked for dust kicked up by small meteoroids hitting the Moon's surface, and recorded the speed, energy, and direction of tiny particles. LEAM saw a large increase in the number of particles every morning, coming from the east or west and slower than speeds expected for lunar ejecta. The experiment's temperature increased to near 100 degrees Celsius a few hours after each lunar sunrise, so the unit had to be turned off temporarily to prevent overheating. It is thought that this is a result of electrically charged moondust sticking to LEAM, darkening its surface so the experiment package absorbed rather than reflected sunlight. Scientists were unable to make a definite explanation of the problem, as LEAM operated only briefly before the Apollo program ended. On 20 July 2011, scientists performed experiments with the

Lunar Reconnaissance Orbiter Camera The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to ...

(LROC) to attempt to detect a weak signal of lunar horizon glow. The experiment was done jointly with the Lyman-Alpha Mapping Project (LAMP), Narrow Angle Camera (NAC), and Wide Angle Camera (WAC). Both of these captured pictures with long exposure times over 50 times longer than their normal exposure times. During the experiment, the Lunar Reconnaissance Orbiter was positioned in a way that it shadowed the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

by the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and was looking back across space to observe material. The NAC found a glow of 0.03 DN, and the lunar horizon was found to have a glow of 0.2 DN. A

spectral radiance In radiometry, spectral radiance or specific intensity is the radiance of a surface per unit frequency or wavelength, depending on whether the Spectral radiometric quantity, spectrum is taken as a function of frequency or of wavelength. The Interna ...

of 0.01 W/m²/sr/um was predicted to be detected by the NAC. So for the given observing geometry, the lunar horizon glow must be dimmer than 0.01 W/m²/sr/um.

References

{{reflist Lunar science

Cause

Exploration

See also

References