Transient Lunar Phenomena

[Kiwi]

I remember hearing about transient phenomena on the moon at the time, but nothing ever seemed to come of them. What was the consensus? Was this just overactive imaginations, or could at least some been impacts? I know that a number of definite impact fireballs have been seen on the moon with video cameras in recent years.

Wikipedia article:
https://en.wikipedia.org/wiki/Transient_lunar_phenomenon

Patrick Moore did a great job of explaining the history and conclusions regarding TLP in his 1976-77 book Guide to the Moon.

Guide to the Moon, Patrick Moore, Book Club Associates, London (1977), pages 202-212

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...So let us move on to what I have christened TLP, or Transient Lunar Phenomena—a term which now seems to have become generally accepted.
   Lunar features change rapidly under the influence of altering illumination. A feature which shows up as a well-marked crater one evening may look like a nebulous patch the next, and there are even cases in which the actual forms appear to vary, Messier being the best example. Yet in general all the details on the Moon are sharp and clear-cut, as is only to be expected upon a world which lacks atmosphere. Local obscurations, blurring the detail over a restricted area, could not be explained as ‘clouds’ or ‘mists’, but so many reports of them have come in over the years that there can be little doubt of their reality. They include glows, usually reddish in colour; colourless obscurations over limited regions, and—occasionally—larger obscurations which do not hide the surface details, but merely make them less distinct.
   In the days when it was still thought that the Moon had an atmosphere, albeit a tenuous one, some of the phenomena were put down to what might be called lunar twilight. Comparisons were made with Venus, which has of course a very dense atmosphere rising high above the surface of the planet. When Venus is at the crescent stage, the horns are often extended, and pioneer lunar observers, including Schroter and Madler, described similar prolongations at the horns of the crescent Moon. Later reports were also made, but I am frankly sceptical, partly—I admit—because my own searches have been so completely fruitless, but mainly because no reliable reports of horn prolongation have come in since the start of the Space Age. If accurate, they would be very hard to explain, but I am sure that they can be put down to errors in interpretation.
   Localized obscurations inside or near walled formations come into a different category, and records of them were not confined to amateur astronomers. Thus in 1892 Edward Emerson Barnard, the great American observer who was noted for his keen eyes (for instance, he discovered the fifth satellite of Jupiter), recorded that on one occasion the bright ray-crater Thales was ‘filled with pale luminous haze’, though the surrounding features were perfectly sharp and clear-cut. In 1902
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the French astronomer Charbonneaux, using one of the world’s largest refractors (the Meudon 33-inch, at the Paris Observatory) described how he saw a small but unmistakable white ‘cloud’ form close to Theaetetus, in the region of the Apennines, and various localized obscurations were reported by W. H. Pickering.
   Variations were reported, too, inside the 60-mile, dark-floored Plato, one of the most-studied formations on the Moon. The craterlets on the floor are sometimes invisible when they should be obvious. I can cite a personal case here. Before midnight on 3 April, 1952 I was unable to see them at all, though I too was using the Meudon refractor under good conditions; four hours later T. A. Cragg, now of the Mount Wilson Observatory, looked at Plato with a 12½-inch reflector and saw that the floor looked blank. He was in no doubt that some local obscuration was responsible. Plato has a long history of similar anomalies.
   Despite the professional observations, it was true that the bulk of the reports came from amateurs, and the local obscurations were officially dismissed as being due to imagination or tricks of the eye. Those who believed otherwise, such as myself, were not taken very seriously. This was understandable enough, because to see one genuine event means many hours of fruitless checking—something which no professional, busy with more important matters, has the time to do. So the amateurs, notably those of the Lunar Section of the British Astronomical Association, went on with their patient work and waited to see what would emerge from it.
   The whole situation was transformed by one episode, in which I played a minor and totally undistinguished part. During 1955 Dr. Dinsmore Alter, using the 60-inch reflector at Mount Wilson, took some photographs of the two large walled plains Alphonsus and Arzachel, which are members of the Ptolemaeus chain, and which lie near the centre of the Moon’s disk as seen from Earth. Alter took pictures in both infra-red light and in blue-violet. As almost everyone knows, infra-red will penetrate haze or mist, while light of shorter wavelength will be blocked or scattered by it. (This is why our sky is blue; the shorter wavelengths of the Sun’s radiation are spread around, while the longer wavelengths are not—at least, not to
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the same extent.) Alter naturally expected that in his sets of photographs Alphonsus and Arzachel would he similarly clear-cut, but he found that on several occasions part of the floor of Alphonsus was blurred in the blue-violet pictures. He wrote: ‘For some reason the blue-violet photographs lose more detail in the east side of Alphonsus than they do in the floor of Arzachel. This is not true of the infra-red ones... There is a temptation to interpret these results immediately as being due to a thin atmosphere, either temporary or permanent, over the floor of Alphonsus. The theoretical difficulties inherent in such a hypothesis are, however, strong enough to forbid whole-hearted acceptance of it.’
   This was by no means the first time that local effects had been reported in Alphonsus. I had some correspondence with Alter and various other professional astronomers, including N. A. Kozyrev, of the Crimean Astrophysical Observatory in the U.S.S.R. I suggested that it would be worth while to keep a careful watch on the area, and Kozyrev was among those who did so, using the 50-inch reflector at the Crimea. His method was to take regular spectrograms (that is to say, photographic spectra), and since the Crimean reflector has no separate guiding telescope he had to watch during the exposure time to make sure that there was no drift of the image. While doing this, at 0100 hours G.M.T. on 3 November, 1958, he noticed that the central peak of Alphonsus had become blurred, and was apparently engulfed in a reddish ‘cloud’. Another spectrogram, taken between 0300 and 0330 hours G.M.T., proved to be remarkably interesting. While guiding the telescope, Kozyrev kept his eyes on Alphonsus, and noticed that the central peak had become abnormally bright. Suddenly the brilliance began to fade; Kozyrev immediately stopped the exposure and started a new one, which was completed at 0345 G.M.T. By the time it was finished, everything was normal once more, and Alphonsus looked the same as it usually does.
   The announcement of Kozyrev’s results took many astronomers by surprise. Kozyrev himself was quite definite: ‘On the spectrogram recorded on 3 November, 1 hour U.T., the central peak of the crater appears redder than normal. Probably at this time the peak was being observed and illuminated by the Sun through the dust (ashes) being thrown up by the eruption.’ In a
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letter to me written shortly afterwards, he said that the spectrograms showed that ‘hot carbon gas had been sent out, causing a rise in temperature of perhaps 2,000 degrees’.
   The hot carbon gas and the marked rise in temperature were questioned by other theorists, but at least one thing was evident: there had been a disturbance of some kind. ‘Eruption’ is a misleading term, but certainly what Kozyrev had seen was a TLP. Checking the records, I recalled that as long ago as 1882 a well-known German selenographer, H. Klein, had claimed that volcanic phenomena were still going on inside Alphonsus, though I would again stress the danger of putting too much faith in last-century reports.
   The next step was to see if there had been any permanent change in the area. During the following weeks and months several observers reported red patches near the site, and these were interpreted as being due to coloured material thrown out at the time of the disturbance; for instance Brian Warner, now Professor of Astronomy at the University of Cape Town but then working at the University of London Observatory, used the 18-inch refractor there, and described the patch as ‘bright red’. I was less successful, and I admit that I was never able to see any red patch at all; it certainly seems to be absent now, but the 1958-9 reports are not at all easy to explain away.
   Since then there have been other records of red TLP in the crater, plus one more spectrographic observation by Kozyrev on 23 October, 1959, though on that occasion nothing was seen visually. The floor has been photographed from the Orbiters, and of course Ranger 9 actually landed there. There can be little serious doubt of extensive past vulcanism. Kozyrev maintains that the central peak of Alphonsus is a genuine volcano with a funnel, but I admit to having grave doubts, largely because I question whether the Moon has sufficient internal energy at the present time to produce a bona-fide eruption. Neither can I accept a temperature-rise of thousands of degrees—or, indeed, any increased heat at all. E. J. Opik questioned the reality of the phenomenon as a true disturbance, and attributed it to fluorescence, but recent work has shown that no fluorescent effects on the Moon can be strong enough to cause glows visible from Earth.
   The next really major development came in 1963, when, on
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30 October, J. Greenacre and E. Barr, at the Lowell Observatory in Arizona, observed colour in the region of Aristarchus. The phenomena included red and pink patches, and were quite unmistakable. In the following month similar events were seen, and were confirmed by P. Boyce at the Perkins Observatory, using a 69-inch reflector—a giant telescope by any standards. Detailed accounts were published; and if any lingering doubts about the reality of TLP remained after the Kozyrev episode, they were finally dispelled.
   Aristarchus is the brightest crater on the Moon. It is only 23 miles in diameter and 6,000 feet deep, but it is so brilliant that it is always instantly recognizable even when lit only by earthshine; in 1787 no less a person that [sic] Sir William Herschel mistook it for a volcano in eruption. It has a central mountain; the walls are terraced, and are crossed by dark bands which are easy objects. The bands are due to differences in level and in surface texture, as we now know from the Orbiter and Apollo photographs. Close beside Aristarchus is Herodotus, of similar size but with a darkish floor, and from Herodotus extends the great valley often known as Schroter’s Valley in honour of its discoverer. Aristarchus is the most event-prone crater on the Moon, and it is responsible for more than half the total number of reported TLP. Gaseous emissions from it have been confirmed spectrographically, and on 19 July, 1969 activity was seen by the astronauts of Apollo 11, who were then together in the command module orbiting the Moon. Armstrong, Aldrin and Collins used binoculars, and reported a luminous north-west wall, ‘more active’ than anywhere else on the surface. Added confirmation came from observers on Earth, who reported TLP in Aristarchus at and around that time.*
   To run ahead of the story: there were further developments with the flight of Apollo 15, in 1971, when the command module carried a special device intended to detect what are known as alpha particles. These are produced by the decay of the radioactive gas radon, which in turn comes from uranium and thorium. If radon gas diffuses through the regolith, it will release atoms into the tenuous lunar atmo-

* Unfortunately I could take no part in the observational programme; I was in the B.B.C. television studio throughout the mission, carrying out ‘live’ commentaries.
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sphere; when these atoms decay, alpha particles will be emitted. As Apollo 15 passed seventy miles above Aristarchus, there was a significant rise in the numbers of alpha particles emitted by radon-222 (that is to say, the radon isotope with an atomic weight of 222). Particles associated with the decay of the other main kind of radon, radon-220, were not found, and this showed that the effect was not due merely to a local excess on the surface of uranium and thorium. The radon isotopes must have diffused through the regolith from below. Radon-222 could be (and was) detected, but radon-220 atoms have a very brief existence; they last for less than one minute, and would not persist for long enough to come through the regolith. Therefore, there seems no doubt that radioactivity in the Aristarchus area is responsible, and the scientists who carried out the analyses—P. Gorenstein and P. Bjorkholm—added that: ‘The observed radon emanation is associated with the same internal processes which will on occasion emit volatiles in sufficient quantity to produce observable optical effects.’
   There is a clear link here with TLP, and it is worth noting that similar radon emissions have been found in the region of Grimaldi, another event-prone area. At the time of the Lowell observations, in 1963, Apollo lay in the future, but the whole problem of ‘lunar events’ was taken up energetically, and the patient amateurs were at last joined by professional astronomers. One method of detecting coloured phenomena involves the use of rotating filters, and the Lunar Section of the British Astronomical Association introduced what has become known as the ‘Moonblink’ device, described in the Appendix. Various positive reports came in, notably that of 30 April, 1966, when a red TLP in the area of the walled plain Gassendi was seen by several completely independent observers. This was, in fact, the most unmistakable red event that I have ever seen on the Moon, and it persisted for about four hours. The main feature was a wedge-shaped, reddish-orange streak extending from the wall of Gassendi right across to the central peak.
   In America, Barbara Middlehurst and her colleagues at the University of Arizona and the Goddard Space Flight Center began compiling a catalogue of all lunar events which had been reported since the start of serious telescopic observation of the Moon. I was doing the same thing independently at Armagh
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Observatory, and when we compared notes we found that our results were similar, so that the catalogue was eventually published under our joint authorship. It took the story up to 1967, and there were 579 reports altogether, though I would be the last to claim that all are reliable (in fact, I am quite certain that they are not!). In 1971 I extended the catalogue, bringing it up to date; the last entry was No. 713.
   When we began to analyse the reports, some curious facts emerged. The event sites were not distributed at random, but lay around the edges of the regular maria and in regions rich in rills. The chart given here (Fig. 47), drawn up in 1975, shows all the areas in which events have been reliably reported; red TLP are starred, others indicated by black dots. The chart speaks for itself. For instance, the boundaries of the Mare Imbrium and the Mare Serenitatis are clearly outlined by event-prone areas, and so is the smaller Mare Crisium. I have also found that most of the red events are seen in the western hemisphere, while the others are concentrated in the Mare Tranquillitatis area to the east, so that different areas on the Moon seem to produce different kinds of events.
   One has always to be on guard against observational bias. Because Aristarchus is so notorious as an event area it is closely watched, and a TLP there is more likely to be noticed than one in—say—Petavius, where no events have ever been reported. Also, I have to admit to being sceptical about some of the entries in the American catalogues, because, in my view, it is dangerous to trust anything except reports made by observers of proved skill and using adequate telescopes. Even then, independent confirmation is—to put it mildly—highly desirable.
   What, then, causes these strange, elusive phenomena? I have an utter lack of faith in the idea of volcanic eruptions of any sort; the Moon is simply not that kind of world nowadays. Fluorescence, as we have noted, was seriously considered, but recent work carried out by Dr. J. Geake at Manchester University has shown that it cannot possibly be the answer. The theory of gas release from below the lunar crust has much more to recommend it, and there are various reasons for believing this to be the right explanation.
   Professor S. Miyamoto of the Kwasan Observatory in Japan
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[Kiwi]

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Fig. 47. Distribution of Lunar Events, as drawn up from the catalogues by Barbara Middlehurst and myself. Stars indicate red events: dots, all others. Only areas in which events have been reliably recorded are shown

made the first really useful comment when, in 1960, he considered the possibility of gas release; and pointed out that in such a case the events would be more frequent in areas rich in rills, which go down for some way into the crust. Miyamoto also wondered whether the Earth’s gravitational pull could produce any observable effects. Then, in 1963, Dr. Jack Green in America analyzed the records available and found that TLP are commonest near lunar perigee, when the Moon is at its closest to the Earth and its surface is under maximum
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gravitational strain. Later analyses by Barbara Middlehurst and her colleagues have confirmed this. However, when we tried to link the frequency of events with the eleven-year solar cycle we met with total failure, and in this respect at least there seems to be no connection between the Moon and the Sun.
   That gases do seep out from below the Moon’s crust has been proved without a shadow of doubt. There are the Aristarchus spectrogram records, and similar data from the dark-floored Grimaldi, while positive results have also come from the equipment left in the Fra Mauro area by Astronauts Shepard and Mitchell from Apollo 14. And on the basis of his fluidized bed theory of crater formation (which becomes more and more impressive the closer it is examined), Dr. Allan Mills suggests that the phenomena are due to friction produced by particles moving around after the gas ejection, thereby making a perceptible glow.
   Yet all in all, the most satisfactory feature of the whole investigation so far is the connection between TLP and moonquakes. When we began our analyses we had no positive information about lunar seismic activity—or the lack of it; but when Barbara Middlehurst and I wrote a paper on the subject in 1966 we suggested that when data could be obtained, a link would be found. The first seismometer was taken to the Moon by Armstrong and Aldrin in Apollo 11, and it registered definite activity, though unfortunately it did not operate for long. Since then seismic equipment has been set up by the astronauts of Apollos 12 and 14 to 17 inclusive, and all this equipment is still working. Moonquakes do occur, and are very frequent. It is from them that the present picture of the Moon’s structure has been drawn up—a 35-mile crust, below which is a mantle extending down to 600 miles, below which again are the asthenosphere and finally the molten core, at a depth of about 900 miles.
   The moonquakes seem to be of two kinds (Fig. 48). The shallow ones occur at depths of from 15 to 150 miles, and the deep ones at up to over 600 miles, with a peak frequency at just less than 600 miles. In other words, moonquakes occur in a region half-way between the lunar surface and the core, though in the region between 180 and 500 miles below ground level very few shocks are recorded. Each seismic station records one
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Fig. 48. Depths of Moonquakes

or two moonquakes daily, though I hasten to add that by terrestrial standards they are feeble. It has been said that an observer on the Moon standing right above the focal point of an earthquake would hardly notice anything at all, so that there is no danger to a future Lunar Base on this score. From the seismic point of view, the Mare Imbrium is far safer than Tokyo or San Francisco. Because the Moon is so relatively ‘quiet’ seismically, the recording devices can be made a thousand times more sensitive than is practical on the turbulent Earth.
   There is no doubt that the moonquakes are genuine, and that they are produced by the same sort of mechanism as our own shocks. (Meteorite impacts have also been recorded by the lunar seismometers, but luckily they produce different kinds of wave-patterns, so that they can be weeded out from the
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analyses.) And when the moonquake areas are plotted, there is an obvious similarity with the pattern of event-prone areas. Moreover, we were right in predicting that moonquakes, like TLP, are commonest near perigee. So far as it goes, everything fits, and at least the observers who were reporting lunar events many years ago have been completely vindicated.
   Yet there is one final point to bear in mind. Even though the Moon is not so inert as used to be thought, major changes there belong to the remote past. If we could go back to the days of the cavemen, or even the dinosaurs, and turn a telescope toward the Moon, we would see the mountains, the valleys, the seas and the craters looking to all intents and purposes exactly the same as they do today.
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[smartcooky]

"When Venus is at the crescent stage, the horns are often extended, and pioneer lunar observers, including Schroter and Madler, described similar prolongations at the horns of the crescent Moon. Later reports were also made, but I am frankly sceptical, partly—I admit—because my own searches have been so completely fruitless, but mainly because no reliable reports of horn prolongation have come in since the start of the Space Age. If accurate, they would be very hard to explain,"

The "horns" are simply the ends of the terminator viewed at an oblique angle, so with the suggestion that there may be clouds of dust formed by electro-statically charged dust pushed across the terminator, we may finally have a plausible explanation for this apparent extension.

[ka9q]

Fascinating.

So I guess it's not unreasonable to think there could be localized "belching" of various gases produced by radioactive decay; helium comes to mind, as well as radon.

I'd wondered why radon was such a problem on earth since it seemed too short-lived (3.8 day half-life) to get to the surface before it decays. Turns out it's carried out by helium, carbon dioxide, methane and other gases also below the surface. There's probably little or no CO₂ and CH₄ on the moon but there certainly has to be a fair bit of He produced by alpha decay and maybe even some Ar-40 from K-40 decay. (Our atmosphere is about 1% Ar-40 from the same source.)

[onebigmonkey]

The problem with radon is how, where, and with what we build our houses.

A double glazed house built from granite on top of a granite substrate can be a problem!

[ka9q]

It seems to vary a lot from one house to the next, even on the same street. Probably has to do where the small subterranean cracks and channels form that let the gas vent to the surface.

[bknight]

It seems to vary a lot from one house to the next, even on the same street. Probably has to do where the small subterranean cracks and channels form that let the gas vent to the surface.

That seems like a reasonable explanation in the variances observed.

[Dalhousie]

The problem with radon is how, where, and with what we build our houses.

A double glazed house built from granite on top of a granite substrate can be a problem!

Legislatively yes, although probably not in real life, given the over-estimation of low level radiation risk.

[Zakalwe]

Radon sumps are easy to install and very effective.
http://www.ukradon.org/information/reducelevels_sump

[ka9q]

I'm wondering if there is a difference between UK and US construction methods. That document seems to suggest that UK houses usually have empty space under their ground floors; is that true? This seems to be common practice only in older US houses built before the mid 1900s, usually with wooden ground floors.

In the US, the lowest floor (basement, or ground floor in a house without a basement) is usually a reinforced concrete slab poured over gravel over dirt. In areas with lots of rain (e.g., the east coast where most of my family lives, but not here in California) houses with basements in flood-prone areas often have sump pumps. A sump (a hole dug into the slab to below the gravel layer) accumulates water from storm ("French") drains buried around the outside periphery of the house, and a pump periodically empties the accumulated water from the sump into the sewer system or simply to the street. Any water in the basement itself can similarly be drained into the sump to be pumped out.

When my parents discovered high radon levels in their basement, they had a contractor cover this sump with plastic and install a continuous pump to exhaust the air within the sump. Although there's no actual gap between the slab and soil, the gravel between them is porous enough for radon that would otherwise diffuse through the slab and into the house to be drawn into the sump and exhausted outside.

It is kind of surprising that radon can diffuse through solid concrete quickly enough to get into a house before it decays (3.8 day half life) but apparently it does.

[Zakalwe]

I'm wondering if there is a difference between UK and US construction methods. That document seems to suggest that UK houses usually have empty space under their ground floors; is that true?

No, no underfloor space required. The sump is installed under the concrete floor pad. The pump can pull the radon from the soil, up to 9 metres from the sump location. obviously this will depend on the type of soil, so more than one sump may be required.

[ka9q]

No, no underfloor space required. The sump is installed under the concrete floor pad. The pump can pull the radon from the soil, up to 9 metres from the sump location. obviously this will depend on the type of soil, so more than one sump may be required.

Isn't the concrete floor pad poured over gravel? That's the US practice, and gravel is porous enough to allow the gases to be drawn over to the pump instead of diffusing up through the slab into the living space.