Lunar Transient Phenomena
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KeywordsSolar Wind Lunar Surface Luminous Efficiency Lunar Prospector Dust Plume
A transient lunar phenomenon (or TLP) is a short-term change on, or above, the lunar surface and can take the form of a colored glow, a brightness variability, an obscuration of detail, gray components to a shadow, or flashes. As far as Earth-based astronomers can tell, no permanent lunar surface changes result, hence why the phenomena are “transient” in nature. In the USA, TLP is sometimes referred to as LTP or lunar transient phenomenon.
The topic of TLP is controversial for three reasons. Firstly, the Moon is essentially a geologically dead world (Heiken et al. 1991), and so astronomers should not expect to see visibly active kilometer-scale surface activity occurring in this modern era. Secondly, the majority of TLPs were discovered by visual Earth-based telescopes which some critics explain away as terrestrial atmospheric, telescope optical, or even psychological factors (Dobbins and Sheehan 2014). Thirdly, TLPs must be extremely rare (Hynek et al. 1976), which makes them difficult to search for. Nevertheless, among the nearly 3,000 claims of TLP sightings (O’Connell and Cook 2013), there have been a few well-authenticated cases. It is also possible that at least some flashes seen on the Moon, by visual observers in the past, could be attributed to impact events of the same type that we have modern-day video confirmation of (Cudnik 2010). Several plausible theories, explaining the mechanisms behind TLP, have been published and will be outlined below.
The majority of TLPs have been catalogued in two NASA publications (Middlehust et al. 1968; Cameron 1978) and one online catalogue extension (Cameron 2006a). Cameron introduces a weighting system for TLP, namely, 1 for a report by an inexperienced observer and up to 5 for a highly authenticated observation of a TLP. While the above are fairly comprehensive catalogues, they do have some typographical errors and probable observational mistakes, and the Cameron catalogues may have over optimistically high weights associated with some of the TLPs (Dobbins and Sheehan 2014). Nevertheless, these catalogues do form a useful starting point for initial studies of this topic. A combined and revised catalogue, with more rigorously assigned weights, is being constructed from the above and also from the archives of the Association of Lunar and Planetary Observers (ALPO) and the British Astronomical Association (BAA), by the Department of Physics at the University of Aberystwyth (Cook et al. 2010).
The earliest TLP noted was from a naked-eye sighting of a light on the Moon from around 557 AD (Newton 1972), though a better documented and more topical pre-telescopic sighting in 1178 comes from the writings of Gervase of Canterbury; this has been attributed to the formation of the geologically young, bright ray crater, Giordano Bruno (Hartung 1976); however, recent age estimates for this crater suggest that it may be too old (Basilevsky and Head 2012). In the telescopic age, there have been a number of famous accounts of TLP. For example, in the eighteenth century, Sir William Herschel described in some of his observations (Klado 1961) what he referred to as lunar volcanoes seen in Earthshine. It has been argued that these may have been misidentifications of bright ray craters, in particular Aristarchus; however, this does not explain the reddish color seen on 1783 May 4. Another topical debate from the eighteenth century was a reported change in appearance of the crater Linne, but this turned out to be due to a combination of earlier descriptive inaccuracies and map errors (Moore 1977). Of the more notable TLPs of modern times, these have included Kozyrev’s spectra of gas emissions from the central peak of Alphonsus in 1958 (Kozyrev 1962; Kalinyak and Kamionko 1962), the pseudopeak effect seen in the crater Herodotus from the 1950s onwards (Cook and Dobbins 2012), bright flashes seen on the Moon in the 1940/50’s era (e.g., Thornton 1947; Stuart 1957), various claimed observations of lunar luminescence (Kopal and Rackham 1963; Link 1972), the Lowell Observatory sighting of red spots in the Aristarchus area in 1963 (Greenacre 1965; O’Connell and Cook 2013), the 1983 Torricelli B event (Cook 2000), and the Langrenus polarized light events of 1992 (Dollfus 2000).
Several observing programs have spent time looking for TLP. The two NASA-backed ones from the 1960s were Project Moon-Blink (Trident Engineering Associates 1966), organized by Winifred Sawtell Cameron, operating out of NASA’s Goddard Space Flight Center, and another team led by Allen Hynek, from Northwestern University, using the Corralitos Observatory (Hynek et al. 1976) in New Mexico. Both projects utilized electronic imaging cameras behind rotating filter wheels. If a colored area was present on the lunar surface, say red, then through a red filter it would be bright on a monochrome cathode ray tube viewing screen and through a blue filter it would be dark, and the net result would be an obvious blink effect on the viewing screen. The Project Moon-Blink system equipment were supplied to 22 observatories with a minimum telescope aperture of 38 cm and detected several TLPs over its lifetime from 1964 to 1966 (Trident Engineering Associates 1966). The Corralitos team detected no TLP, despite putting in over 6,466 h of observing time, between 1966 and 1973 (Hynek et al. 1976). They did however detect very large area “blue clearing” effects on a few occasions, where a UV excess was observed, but this was discounted as a TLP because of the large surface area of the Moon involved and the fact that they noted it would occur close to the full Moon and also when the Moon was at a high altitude above the horizon. However, a full explanation of the “blue clearing effect” was never given.
During the run-up to the Apollo missions, a large number of amateur astronomers participated in two further projects: ARGUS-ASTRONET and LION (Schneider 1970). During this time and subsequently, the lunar sections of the Association of Lunar and Planetary Observers (ALPO), the British Astronomical Association (BAA), and the American Lunar Society have continued to monitor the Moon but at a lower level of interest, though there was again some extra support during the Clementine (Buratti et al. 2000) and Lunar Prospector missions (Darling 1998). Mobberley (2013) has questioned the reliability of amateur-based networks, in particular the number of small telescopes used and the problems associated in the influx of inexperienced/overenthusiastic observers. Amateur astronomers still work on TLP projects in 2014 but are more pragmatic, concentrating on disproving past TLPs by reobserving the same sites under similar illumination and where possible similar (topocentric) libration. Their aim is to establish the normal appearance of a lunar formation, and if what was reported for a past TLP repeats, then it was probably not a TLP originally but something like a natural color (McCord 1968), a low-texture area appearing fuzzy (Cook 2013), a sunlit terrain protruding from a shadow (Lena and Cook 2004), etc.
The AEOLUS (atmosphere from Earth, orbit, and lunar surface) project, led by Arlin Crotts, built and operates a dual monitoring telescope system operating at Cerro Tololo, Chile, and Rutherford Observatory, New York, USA. The system captures the whole nearside lunar disk in white light and compares images taken every 20 s. When a change is found in one telescope, it can be checked for on the second telescope, to make sure that it is not a result of some local effect. It was reported (Crotts et al. 2009) that one month’s worth of continuous observation had been made with a sensitivity to changes at the 1–2 % level. A later publication (Crotts 2010) mentions that 200,000 images had been taken and some plausible optical transients found but does not elaborate on what these might be.
Middlehurst (1966) was able to show that there was no correlation between TLP and the solar cycle. Chapman (1967) had suggested that there may be a correlation between TLP in Aristarchus and the Earth’s tidal pull, though Cook (2011), using a larger dataset, shows this not to be the case. Middlehurst and Moore (1967) plot the locations of TLP sites and deduce that these tend to be distributed around mare edges, something which is confirmed later by Crotts (2008). Cameron (2006b) investigated many physical parameters which might have been associated with TLP, e.g., magnetopause crossing, perigee, apogee, etc., and deduced that the only one that showed any correlation was that TLPs seem to occur more frequently near the terminator. However, this view is contradicted by Cook et al. (2010) who show after normalizing for observational bias, i.e., where astronomers prefer to look on the Moon, that TLPs occur more frequently toward local noon on the lunar surface.
In another statistical analysis, using TLP reports from the Middlehurst et al. (1968) catalogue, Crotts (2008) attempts to remove observational bias that favors observers concentrating on TLP site craters. He did this by comparing pre-1930 and post-1930 TLP reports. 1930 was picked as a division point in time, to avoid overreporting artifacts when TLPs start to become overly interesting to the astronomical community. He was able to show that seven lunar features (Aristarchus, Plato, Mare Crisium, Tycho, Kepler, and Copernicus) were statistically significant sites for TLPs and also confirmed the Middlehurst and Moore (1967) finding that TLPs were more likely to be located near mare edges than elsewhere.
Volcanism: Although no longer a tenable theory since the modern era of spaceflight, it was one of the earliest explanations for TLP. This theory became popular after Sir William Herschel reported the presence of lunar volcanoes in Earthshine (Klado 1961). However, with modern hindsight, we know that the last throes of endogenic lunar volcanism were about 1 billion years ago (Ziethe et al. 2009), as determined by crater count age estimates. Interestingly, in the 1960s, Hartman and Harris (1968) suggested that the red glow from a 1963 TLP observation by Greenacre and Barr was due to the incandescent black-body radiation from a fire fountain effect near Aristarchus − though there is no evidence for any resulting annular surface deposit effect seen in modern-era spacecraft images (O’Connell and Cook 2013). Small-scale volcanic flows are still possible on the Moon, via impact melt (Carter et al. 2012); however, with the present low cratering rate, any new craters would be too small to be seen from Earth, and any resulting impact melt incandescent glow is unlikely to be seen either, unless on the nightside of the Moon, and for an impact larger than those observed so far (e.g., Madiedo et al. 2014).
Specular Reflection: is another early TLP theory and makes use of the Sun’s glint off of shiny components to rocks on the lunar surface. A variation on the theory involves internal reflection through volcanic glass beads. The net result is that at a specific viewing and illumination angle (equal in the case of reflection), the surface will appear to brighten as the Sun moves through its angular diameter across the lunar sky. Attempts have been made to test this theory on at least three TLP sites: Aristarchus (Cook et al. 2011), Herodotus (Cook and Dobbins 2012), and Torricelli B (Tost 2001), but in all three instances, there was no repeat occurrence evidence to support the specular reflection theory.
Impacts: are the only instance, so far, of TLP that have been proven (Cudnik 2010). Although all confirmed impact flashes have been seen in Earthshine, one of the brightest (Madiedo et al. 2014) could in theory have been detected against the daylight side of the Moon, and this might explain the Thornton and Stuart flashes (Thornton 1947; Stuart 1957). Unfortunately, impact events do not account for the nonrandom distribution of TLP across the lunar surface (Middlehurst and Moore 1967) nor do the vast majority of impacts account for the typical TLP duration of half an hour (Cameron 2006b).
Luminescence: has been proposed to explain some colored TLPs and the observational measurements on the filling in of absorption lines in reflected solar spectra. Early measurements of the latter inferred lunar surface luminous efficiencies of anywhere between <1 % and 40 % (Potter and Mendell 1984). If TLPs were as a result of luminescence, say from solar proton bombardment, or X-ray/UV, then TLPs should follow the solar cycle; however, a study by Middlehurst (1966) revealed no such correlation. Thermoluminescence has also been considered as variant on the standard models of luminescence (Chanin et al. 1982), but an analysis of the rocks returned by the Apollo missions revealed considerably less luminous efficiency (Geake and Mills 1977) than would be needed to make a visible TLP. The problem of the observed reflected solar absorption line filling was solved eventually by Potter et al. (1984) who suggested that the line filling was most likely caused by inelastic scattering of light resulting in wavelength shifts which in turn filled in narrower absorption lines more than broader ones. This explained at least some of the earlier examples of observations of luminescence. One especially interesting claim of an observation of luminescence though came from a sequence of three narrowband interference pair photographs of the Moon taken by Kopal and Rackham (1963) on 1963 Nov 1/2 from the Pic du Midi Observatory. Each image pair consisted of a red light image at 672.5 nm (FWHM 4.5 nm) and a green light image taken at 545.0 nm (FWHM 9.5 nm). In the first and last pair, an enhancement of up to 85 % through the red filter, in the vicinity of Kepler, was noticed. Ney et al. (1966) criticize this observation though on the grounds that they believed that not all sources of systematic error were accounted for in Kopal and Rackham’s paper. However, an earlier report by Rackham (1964) seem to show that the Manchester University team at Pic du Midi had a good understanding of the sources of error and what was real on their photographs.
Outgassing: has remained a popular explanation for TLP, especially after Nikolai Kozyrev (Kozyrev 1962; Kalinyak and Kamionko 1962) captured spectral emission from the central peak of the Alphonsus crater, which he attributed to molecular carbon, C2. The Swan bands from molecular carbon idea were rejected though in the last published attempt to analyze the spectra by Phillips and Arpigny (1967); however, they were unable to find an alternative identification of the gas involved. Dobbins and Sheehan (2014) even express an opinion that the anomaly on the Kozyrev spectrograph may have been as a result of a spectroscope slit tracking error. The Apollo orbital experiments (Feldman and Morrison 1991), Apollo surface experiments (Hoffman et al. 1974), Lunar Prospector (Lawson et al. 2005), LRO (Cook et al. 2014), and LADEE (Elphic et al. 2014) have identified atomic and molecular species present in the lunar exosphere and can be used to place a limit on their population. The lunar exosphere is a complicated environment to understand though because it can vary by more than an order of magnitude over a lunar day due to hydrogen and helium contributions from the solar wind (Wurz et al. 2007), can be added to close to full Moon from terrestrial magnetotail particles (Poppe et al. 2013), and there is radiogenic outgassing of argon and radon from the surface (Heiken et al. 1991). The lower mass gases have a large-scale height and consequently are more likely to be lost to interactions with the solar wind, whereas heavier gases are more likely to hop around the surface on ballistic trajectories, freezing/absorbed to the nightside, and in the case of argon, can generate a detectable breeze at dawn (Hoffman et al. 1974). Evidence for the concentration of another endogenic gas, radon (Gorenstein et al. 1974), was found around the mare edges and at higher quantities in the vicinity of the crater Aristarchus. Mechanisms for the purging of reservoirs of radiogenic, and other, gases from beneath the surface of the Moon could be as a result of shallow moon quakes (Binder 1980). There are various means by which gas released from the Moon can emit light, for example, Srnka’s (1977) critical velocity explanation. However, any gas optical emission theory must meet two requirements: a) the quantity of emitted gas must not exceed detectable variations of gas as already measured from the lunar surface, or from orbit, and b) the optical emission should be bright enough to be seen visually by Earth-based astronomers against the lunar surface.
Lunar Frosts: Although suggested (a misinterpretation of evidence) originally by Pickering (1916) and images showing frosts have been recorded on the airless moons of Jupiter (Lebofsky 1977), no published evidence has been found for observations of lunar frosts being imaged by Earth-based astronomers. Besides, in order to get a local concentration area of frost that would be visible from Earth (say an area of 1 km2), an underground gas needs to be released from a source on the nightside, and it would have to break through without freezing itself before reaching the surface. If it were possible (e.g., Crotts and Hummels 2009), then it would neatly explain some bright or colored patches that have been seen shortly after sunrise.
Meteoroid Origin: Ejecta from meteorite impacts moves at the speed of the order of a kilometer per second along initial cone-shaped paths similar to what is predicted in models (Richardson 2011). NASA’s LADEE mission has detected the presence of dust plumes (Elphic et al. 2014), the most probable source being from meteoroid impacts against the lunar surface. Although ejecta dust plumes might explain some TLPs that resemble long duration impact flares, they do not account for the nonrandom distribution of majority of TLPs across the lunar surface (Middlehurst and Moore 1967).
Outgassing: Crotts and Hummel (2009) consider the possibility of occasional subsurface gas eruptions that would kick up a dust cloud. Seen against the low albedo lunar surface and allowing for scattering and absorption of light, this could cause a whole number of effects such as brightness changes, and even color, depending upon the particle size distribution. However, a permanent change at the site of the source would result but perhaps only at very small scales visible to spacecraft. Note that any outgassing must be comparatively small, else it would have contributed significantly to the already tenuous lunar atmosphere whose mass is estimated at around 104 kg Heiken et al. 1991) and this would probably have been detected by numerous lunar spacecraft over the last few decades.
Electrostatic Levitation: Levitated dust particles (presumably charged) were detected by the Apollo surface LEAM experiments (Berg and Perkins 1979) by a Lunokhod photometer (Severny et al. 1975) and inferred from a horizon glow recorded by Surveyor 7 near the crater Tycho (Criswell 1972). The near-surface dust transport mechanisms are probably due to negative charging on the nightside due to the ion/electron charge separation as the solar wind streams past the terminator creating electric fields (Nitter et al. 1998). Laboratory models (Wang et al. 2007) and computer simulations (Stubbs et al. 2006) both demonstrate how particles can become charged, repel, and levitate. In the case of shadowed areas, the experiments show that the direction of dust motion is from the shadow toward sunlit areas. Interestingly, Mills (1980) suggests that triboelectric discharge may occur between closely spaced charged dust particles if present within a rarefied gas. It has been suggested that the horizon glow effects seen from orbit by the Apollo 17 astronauts (McCoy and Criswell 1974) and by the Clementine star tracker cameras (Glenar et al. 2010) were due to high-altitude dust particles. Smith and Smith (2012) have even proposed that an altitude/size distribution model of dust particles could account for the observed Herschel volcano effect, though if their mechanism were to exist, then frequent sunlight scatter brightening beyond the terminator could be expected also, and this has not been observed (Cook 2013). A dust fountain model was proposed by Stubbs (2006) to explain Apollo 17 and Clementine horizon glow effects seen at several tens of kilometers in altitude; however, it is difficult to see, from the preliminary analysis of LADEE star tracker camera lunar horizon images (Grossman 2014), how the dust fountain model can explain these earlier observations.
Electrodynamic Effects: as a mechanism for TLP were suggested by Zitto (1989). He proposed that when rocks occasionally split, during the diurnal day/night thermal cycle, or from tectonic activity, flashes of light could be produced from photoemission from helium and argon freed from rock pores, being excited by energetic electrons released from the fractured rock surfaces. For helium-rich rocks, a pink-violet flash would be seen and from argon-rich rocks, a bluish flash. Energy calculations showed that these could be visible from Earth providing that these occurred from exposed rock outcrops. Although not proposed for the lunar environment yet, quake lights here on Earth have been recorded, but these need a combination of both electric fields set up by tectonic activity and an atmosphere (St-Laurent et al. 2006). Of course, much of the lunar surface is covered in dust and regolith layers (Heiken et al. 1991), but steep slopes, tectonic features, and fresh craters might exhibit the necessary uncovered rock outcrops.
Scattered Light and Earthshine: are both observing environments where unsuspecting observers may be tricked occasionally into thinking that they have seen a TLP. For example, light scattered off a sunlit rim of a shadow-filled bright ray crater can sometimes illuminate the dark floor sufficiently to permit interior detail to be seen (Major 2011) which many observers might not expect to see. However, the effect will always repeat at the same lunar phase, so it is possible to eliminate these as TLP, given enough observing time. Secondly, cloud cover and the color and reflectivity of the Earth’s land and sea will vary with the terrestrial day, with the season, and with the fraction of the Earth’s disk (as seen from the Moon) and this affects Earthshine brightness (Langford et al. 2009). The net result of variable Earth illumination on faint detail within Earthshine is that features just on the limit of detectability can sometimes appear to brighten or fade over a few tens of minutes as the Earth’s rotation brings bright cloud systems into visibility over the terrestrial limb. The appearance of disappearance of ray craters on the limits of detectability may trick observers into thinking that they have seen a TLP. Only by comparing a suspected brightness varying feature, in Earthshine, against other similar size/brightness features, can one be sure about the reality of the apparent change seen (Cook 2014).
Effects on Our Side of the Atmosphere: have been suggested as a probable explanation for many TLPs. This category encompasses human error and misinterpretations (Dobbins and Sheehan 2014; Mobberley 2013), atmospheric spectral dispersion adding color onto contrasty edges (Peach 2012), color tinges on brightness gradients perpendicular to the horizon, atmospheric turbulence causing obscuration of surface detail, telescope chromatic aberration, stray light issues inside the telescope, and scintillation effects in our atmosphere making tiny bright craterlets occasionally flash (Cook 2014). While there may be some instances where the above have been the case with TLP, many archive observational reports mention checks for the above artifacts on other lunar features. Simulations using repeat illumination imagery can be used to test the above hypotheses for specific TLP (O’Connell and Cook 2013).
The topic of TLP remains controversial because despite there being nearly 3,000 reports (O’Connell and Cook 2013), non-lunar effects could explain some of these (Mobberley 2013; Dobbins and Sheehan 2014). However, there are also several double or multiple confirmed observations of TLP (Greenacre 1965), as well as polarimetric (Dollfus 2000) and spectroscopic evidence (Kozyrev 1962; Kalinyak and Kamionko 1962; Phillips and Arpigny 1967). Unfortunately, our knowledge of lunar surface geology suggests that the Moon should be a geologically quiet place (Heiken et al. 1991) with the exception of lunar impact-related phenomenon. New spacecraft data, such as from LADEE (Elphic et al. 2014; Grossman 2014), will help to constrain, or eliminate, some of the proposed TLP theories. At least, impact flashes, despite being a controversial topic for many years until confirmation in the era of low-cost light-sensitive CCTV cameras, have now become mainstream scientifically useful phenomena to study (Cudnik 2010).
Although TLPs are at present a mystery, it is likely that their reality or at least a limit on their rate of occurrence will either be solved by long-duration monitoring of the type employed by the AEOLUS team (Crotts et al. 2009), by long-term spacecraft exospheric gas/dust population monitoring, or by the examination of temporal imagery, comparing early Apollo era with more recent space mission imagery (Crotts 2011; Speyerer et al. 2014), e.g., LRO.
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