This map, based on a survey of 300 TLPs by Barbara Middlehurst and Patrick Moore
, shows the approximate distribution of observed events. Red-hued events are in red; the remainder are in yellow.
A transient lunar phenomenon (TLP) or lunar transient phenomenon (LTP) is a short-lived light, color, or change in appearance on the surface of the Moon.
Claims of short-lived lunar phenomena go back at least 1,000 years, with some having been observed independently by multiple witnesses or reputable scientists. Nevertheless, the majority of transient lunar phenomenon reports are irreproducible and do not possess adequate control experiments that could be used to distinguish among alternative hypotheses to explain their origins.
Most lunar scientists will acknowledge that transient events such as outgassing and impact cratering do occur over geologic time: the controversy lies in the frequency of such events.
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.
Description of events
Reports of transient lunar phenomena range from foggy patches to permanent changes of the lunar landscape. Cameron classifies these as (1) gaseous, involving mists and other forms of obscuration, (2) reddish colorations, (3) green, blue or violet colorations, (4) brightenings, and (5) darkening. Two extensive catalogs of transient lunar phenomena exist, with the most recent tallying 2,254 events going back to the 6th century. Of the most reliable of these events, at least one-third come from the vicinity of the Aristarchus plateau.
A few of the more famous historical accounts of transient phenomena include the following:
- On June 18, 1178, five or more monks from Canterbury reported an upheaval on the Moon shortly after sunset. "There was a bright new moon, and as usual in that phase its horns were tilted toward the east; and suddenly the upper horn split in two. From the midpoint of this division a flaming torch sprang up, spewing out, over a considerable distance, fire, hot coals, and sparks. Meanwhile the body of the moon which was below writhed, as it were, in anxiety, and, to put it in the words of those who reported it to me and saw it with their own eyes, the moon throbbed like a wounded snake. Afterwards it resumed its proper state. This phenomenon was repeated a dozen times or more, the flame assuming various twisting shapes at random and then returning to normal. Then after these transformations the moon from horn to horn, that is along its whole length, took on a blackish appearance." This description appears outlandish perhaps due to lacking public understanding of astronomical phenomena. In 1976, Jack Hartung proposed that this described the formation of the Giordano Bruno crater. However, more recent studies suggest that it appears very unlikely the 1178 event was related to the formation of Crater Giordano Bruno, or was even a true transient lunar phenomenon at all. The millions of tons of lunar debris ejected from an impact large enough to leave a 22-km-wide crater would have resulted in an unprecedentedly intense, week-long meteor storm on Earth. No accounts of such a memorable storm have been found in any known historical records, including several astronomical archives from around the world. In light of this, it is suspected that the group of monks (the event's only known witnesses) saw the atmospheric explosion of a directly oncoming meteor in chance alignment, from their specific vantage point, with the far more distant moon.
- During the night of April 19, 1787, the British astronomer Sir William Herschel noticed three red glowing spots on the dark part of the Moon. He informed King George III and other astronomers of his observations. Herschel attributed the phenomena to erupting volcanoes and perceived the luminosity of the brightest of the three as greater than the brightness of a comet that had been discovered on April 10. His observations were made while an aurora borealis (northern lights) rippled above Padua, Italy. Aurora activity that far south from the Arctic Circle was very rare. Padua's display and Herschel's observations had happened a few days before the number of sunspots had peaked in May 1787.
- In 1866, the experienced lunar observer and mapmaker J. F. Julius Schmidt claimed that the Linné crater had changed its appearance. Based on drawings made earlier by J. H. Schröter, as well as personal observations and drawings made between 1841 and 1843, he stated that the crater "at the time of oblique illumination cannot at all be seen" (his emphasis), whereas at high illumination, it was visible as a bright spot. Based on repeat observations, he further stated that "Linné can never be seen under any illumination as a crater of the normal type" and that "a local change has taken place". Today, Linné is visible as a normal young impact crater with a diameter of about 1.5 miles (2.4 km).
- On November 2, 1958, the Russian astronomer Nikolai A. Kozyrev observed an apparent half-hour "eruption" that took place on the central peak of Alphonsus crater using a 48-inch (122-cm) reflector telescope equipped with a spectrometer. During this time, the obtained spectra showed evidence for bright gaseous emission bands due to the molecules C2 and C3. While exposing his second spectrogram, he noticed "a marked increase in the brightness of the central region and an unusual white colour." Then, "all of a sudden the brightness started to decrease" and the resulting spectrum was normal.
- On October 29, 1963, two Aeronautical Chart and Information Center cartographers, James Clarke Greenacre and Edward M. Barr, at the Lowell Observatory, Flagstaff, Arizona, manually recorded very bright red, orange, and pink colour phenomena on the southwest side of Cobra Head; a hill southeast of the lunar valley Vallis Schröteri; and the southwest interior rim of the Aristarchus crater. This event sparked a major change in attitude towards TLP reports. According to Willy Ley: "The first reaction in professional circles was, naturally, surprise, and hard on the heels of the surprise there followed an apologetic attitude, the apologies being directed at a long-dead great astronomer, Sir William Herschel." A notation by Winifred Sawtell Cameron states (1978, Event Serial No. 778): "This and their November observations started the modern interest and observing the Moon." The credibility of their findings stemmed from Greenacre's exemplary reputation as an impeccable cartographer, rather than from any photographic evidence.
- On the night of November 1-2, 1963, a few days after Greenacre's event, at the Observatoire du Pic-du-Midi in the French Pyrenees, Zden?k Kopal and Thomas Rackham made the first photographs of a "wide area lunar luminescence". His article in Scientific American transformed it into one of the most widely publicized TLP events. Kopal, like others, had argued that Solar Energetic Particles could be the cause of such a phenomenon.
- During the Apollo 11 mission in 1969, Houston radioed to Apollo 11: "We've got an observation you can make if you have some time up there. There's been some lunar transient events reported in the vicinity of Aristarchus." Astronomers in Bochum, West Germany, had observed a bright glow on the lunar surface--the same sort of eerie luminescence that has intrigued Moon watchers for centuries. The report was passed on to Houston and thence to the astronauts. Almost immediately, Michael Collins reported back: "Hey, Houston, I'm looking north up toward Aristarchus now, and there's an area that is considerably more illuminated than the surrounding area. It seems to have a slight amount of fluorescence." 
- In 1992, Audouin Dollfus of the Observatoire de Paris reported anomalous features on the floor of Langrenus crater using a one-meter (3.2-foot) telescope. While observations on the night of December 29, 1992, were normal, unusually high albedo and polarization features were recorded the following night that did not change in appearance over the six minutes of data collection. Observations three days later showed a similar, but smaller, anomaly in the same vicinity. While the viewing conditions for this region were close to specular, it was argued that the amplitude of the observations were not consistent with a specular reflection of sunlight. The favored hypothesis was that this was the consequence of light scattering from clouds of airborne particles resulting from a release of gas. The fractured floor of this crater was cited as a possible source of the gas.
Explanations for the transient lunar phenomena fall in four classes: outgassing, impact events, electrostatic phenomena, and unfavorable observation conditions.
Some TLPs may be caused by gas escaping from underground cavities. These gaseous events are purported to display a distinctive reddish hue, while others have appeared as white clouds or an indistinct haze. The majority of TLPs appear to be associated with floor-fractured craters, the edges of lunar maria, or in other locations linked by geologists with volcanic activity. However, these are some of the most common targets when viewing the Moon, and this correlation could be an observational bias.
In support of the outgassing hypothesis, data from the Lunar Prospector alpha particle spectrometer indicate the recent outgassing of radon to the surface. In particular, results show that radon gas was emanating from the vicinity of the craters Aristarchus and Kepler during the time of this two-year mission. These observations could be explained by the slow and visually imperceptible diffusion of gas to the surface, or by discrete explosive events. In support of explosive outgassing, it has been suggested that a roughly 3 km- (1.9 mi-) diameter region of the lunar surface was "recently" modified by a gas release event. The age of this feature is believed to be about 1 million years old, suggesting that such large phenomena occur only infrequently.
Impact events are continually occurring on the lunar surface. The most common events are those associated with micrometeorites, as might be encountered during meteor showers. Impact flashes from such events have been detected from multiple and simultaneous Earth-based observations. Tables of impacts recorded by video cameras exist for years since 2005 many of which are associated with meteor showers. Furthermore, impact clouds were detected following the crash of ESA's SMART-1 spacecraft, India's Moon Impact Probe and NASA's LCROSS. Impact events leave a visible scar on the surface, and these could be detected by analyzing before and after photos of sufficiently high resolution. No impact craters formed between the Clementine (global resolution 100 metre, selected areas 7-20 metre) and SMART-1 (resolution 50 metre) missions have been identified.
Eight individual frames taken from a video of the lunar crater Clavius showing the effect of the Earth's atmosphere on astronomical images
It has been suggested that effects related to either electrostatic charging or discharging might be able to account for some of the transient lunar phenomena. One possibility is that electrodynamic effects related to the fracturing of near-surface materials could charge any gases that might be present, such as implanted solar wind or radiogenic daughter products. If this were to occur at the surface, the subsequent discharge from this gas might be able to give rise to phenomena visible from Earth. Alternatively, it has been proposed that the triboelectric charging of particles within a gas-borne dust cloud could give rise to electrostatic discharges visible from Earth. Finally, electrostatic levitation of dust near the terminator could potentially give rise to some form of phenomenon visible from Earth.
Unfavourable observation conditions
It is possible that many transient phenomena might not be associated with the Moon itself but could be a result of unfavourable observing conditions or phenomena associated with the Earth. For instance, some reported transient phenomena are for objects near the resolution of the employed telescopes. The Earth's atmosphere can give rise to significant temporal distortions that could be confused with actual lunar phenomena (an effect known as astronomical seeing). Other non-lunar explanations include the viewing of Earth-orbiting satellites and meteors or observational error.
Debated status of TLPs
The most significant problem that faces reports of transient lunar phenomena is that the vast majority of these were made either by a single observer or at a single location on Earth (or both). The multitude of reports for transient phenomena occurring at the same place on the Moon could be used as evidence supporting their existence. However, in the absence of eyewitness reports from multiple observers at multiple locations on Earth for the same event, these must be regarded with caution. As discussed above, an equally plausible hypothesis for some of these events is that they are caused by the terrestrial atmosphere. If an event were to be observed at two different places on Earth at the same time, this could be used as evidence against an atmospheric origin.
One attempt to overcome the above problems with transient phenomena reports was made during the Clementine mission by a network of amateur astronomers. Several events were reported, of which four of these were photographed both beforehand and afterward by the spacecraft. However, careful analysis of these images shows no discernible differences at these sites. This does not necessarily imply that these reports were a result of observational error, as it is possible that outgassing events on the lunar surface might not leave a visible marker, but neither is it encouraging for the hypothesis that these were authentic lunar phenomena.
Observations are currently being coordinated by the Association of Lunar and Planetary Observers and the British Astronomical Association to re-observe sites where transient lunar phenomena were reported in the past. By documenting the appearance of these features under the same illumination and libration conditions, it is possible to judge whether some reports were simply due to a misinterpretation of what the observer regarded as an abnormality. Furthermore, with digital images, it is possible to simulate atmospheric spectral dispersion, astronomical seeing blur and light scattering by our atmosphere to determine if these phenomena could explain some of the original TLP reports.
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