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 provided the first data and photos of the phenomenon. Astronauts in lunar orbit observed it during the Apollo 15 and Apollo 17 missions.
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. [5] 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 impacts. The term "Moon fountain" has been used to describe this effect.
During the lunar day, infrared rays and ultraviolet rays 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. Particles at the night side achieve greater electrical tension differences than the day side, launching dust particles to even higher altitudes. [6] This dust eventually falls back to the surface, and the cycle repeats. [7]
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 asteroids 433 Eros and comet 67P/Churyumov–Gerasimenko. [8]
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 a 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 in his short story "Dust Rag", published in Astounding Science Fiction . [6]
Coronal photographs of the Moon from Apollo 15 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 and Apollo 10. These might have been similar to crepuscular rays on Earth. [6] The glow is also believed to appear in the star tracker data from the Clementine mission, although it would be masked by coronal and zodiacal light. The Apollo missions 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. [9]
On 20 July 2011, scientists performed experiments with the Lunar Reconnaissance Orbiter Camera (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 by the Moon 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 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. [10]
Apollo 14 was the eighth crewed mission in the United States Apollo program, the third to land on the Moon, and the first to land in the lunar highlands. It was the last of the "H missions", landings at specific sites of scientific interest on the Moon for two-day stays with two lunar extravehicular activities.
Apollo 16 was the tenth crewed mission in the United States Apollo space program, administered by NASA, and the fifth and penultimate to land on the Moon. It was the second of Apollo's "J missions", with an extended stay on the lunar surface, a focus on science, and the use of the Lunar Roving Vehicle (LRV). The landing and exploration were in the Descartes Highlands, a site chosen because some scientists expected it to be an area formed by volcanic action, though this proved not to be the case.
Apollo 17 was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon or traveled beyond low Earth orbit. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, while Command Module Pilot Ronald Evans orbited above. Schmitt was the only professional geologist to land on the Moon; he was selected in place of Joe Engle, as NASA had been under pressure to send a scientist to the Moon. The mission's heavy emphasis on science meant the inclusion of a number of new experiments, including a biological experiment containing five mice that was carried in the command module.
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.
Surveyor 7 was sent to the Moon in 1968 on a scientific and photographic mission as the seventh and last lunar lander of the American uncrewed Surveyor program. With two previous unsuccessful missions in the Surveyor series, and with Surveyor 7's landing success, Surveyor 7 became the fifth and final spacecraft in the series to achieve a lunar soft landing. A total of 21,091 pictures were transmitted from Surveyor 7 back to Earth.
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.
A transient lunar phenomenon (TLP) or lunar transient phenomenon (LTP) is a short-lived change in light, color or appearance on the surface of the Moon. The term was created by Patrick Moore in his co-authorship of NASA Technical Report R-277 Chronological Catalog of Reported Lunar Events, published in 1968.
A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon.
In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.
During the 1971 Apollo 15 mission to the Moon, and its three days of exploration on the lunar surface by David Scott and James Irwin, Command Module Pilot (CMP) Al Worden had a busy schedule of observations. Apollo 15 was the first mission to carry the Scientific Instrument Module (SIM) bay, which contained a panoramic camera, gamma ray spectrometer, mapping camera, laser altimeter and mass spectrometer. Worden had to operate the shutter and lenses on the cameras and turn on and off the various instruments. During the coast back to Earth, he would perform an EVA to retrieve film cassettes from the cameras.
The interplanetary dust cloud, or zodiacal cloud, consists of cosmic dust that pervades the space between planets within planetary systems, such as the Solar System. This system of particles has been studied for many years in order to understand its nature, origin, and relationship to larger bodies. There are several methods to obtain space dust measurement.
Lunar regolith is the unconsolidated material found on the surface of the Moon and in the Moon's tenuous atmosphere. Sometimes referred to as Lunar soil, Lunar soil specifically refers to the component of regolith smaller than 1 cm. It differs substantially in properties from terrestrial soil.
The magnetic field of the Moon is very weak in comparison to that of the Earth; the major difference is the Moon does not have a dipolar magnetic field currently, so that the magnetization present is varied and its origin is almost entirely crustal in location; so it's difficult to compare as a percentage to Earth. But, one experiment discovered that lunar rocks formed 1 - 2.5 billion years ago were created in a field of about 5 microtesla (μT), compared to present day Earth's 50 μT. During the Apollo program several magnetic field strength readings were taken with readings ranging from a low of 6γ (6nT) at the Apollo 15 site to a maximum of 313γ (0.31μT) at the Apollo 16 site, note these readings were recorded in gammas(γ) a now outdated unit of magnetic flux density equivalent to 1nT.
The atmosphere of the Moon is a very sparse layer of gases surrounding the Moon. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity compared to interplanetary medium, referred to as "lunar atmosphere" for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar System. The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and in total mass less than 10 metric tonnes. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its gases are lost into space.
The Lunar Atmosphere and Dust Environment Explorer was a NASA lunar exploration and technology demonstration mission. It was launched on a Minotaur V rocket from the Mid-Atlantic Regional Spaceport on September 7, 2013. During its seven-month mission, LADEE orbited the Moon's equator, using its instruments to study the lunar exosphere and dust in the Moon's vicinity. Instruments included a dust detector, neutral mass spectrometer, and ultraviolet-visible spectrometer, as well as a technology demonstration consisting of a laser communications terminal. The mission ended on April 18, 2014, when the spacecraft's controllers intentionally crashed LADEE into the far side of the Moon, which, later, was determined to be near the eastern rim of Sundman V crater.
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 the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.
Lunar swirls are enigmatic features found across the Moon's surface, which are characterized by having a high albedo, appearing optically immature, and (often) having a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls. They appear to overlay the lunar surface, superposed on craters and ejecta deposits, but impart no observable topography. Swirls have been identified on the lunar maria and on highlands - they are not associated with a specific lithologic composition. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain appear less prominent and exhibit simpler shapes, such as single loops or diffuse bright spots.
Dust ponds are a phenomenon where pockets of dust are seen in celestial bodies without a significant atmosphere, like asteroids, comets and some minor planets. These are smooth deposits of dust accumulated in depressions on the surface of the body, contrasting from the rocky terrain around them. They typically have different color and albedo compared to the surrounding areas. As there are no air around them, their method of formation is still debated. The phenomenon was discovered on 2000 October 28, by the spaceprobe NEAR Shoemaker on Asteroid 433 Eros.
The Lunar Ejecta and Meteorites Experiment (LEAM) was a lunar science experiment that flew to the Moon on board Apollo 17 in 1972. It collected information on dust particles produced as a result of meteoroid impacts on the surface of the Moon.