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Photographic evidence of a short duration: Strong flash from the surface of the moon
ICARUS 76, 5 2 5 - 5 3 2 (1988)
Photographic Evidence of a Short Duration" Strong Flash from the Surface of the Moon G. K O L O V O S , J. H. S E I R A D A K I S , H. V A R V O G L I S , AND S. AVGOLOUPIS University of Thessaloniki, Department of Physics, Section of Astrophysics, Astronomy, and Mechanics, GR-54006 Thessaloniki, Greece R e c e i v e d F e b r u a r y 3, 1988; r e v i s e d April 22, 1988 W e present photographic evidence of a very short duration, strong flash from the surface of the Moon (near an irregularly shaped crater in Palus Somni). The flash covered a region roughly 22 by 18 km wide with a total energy of the order of 1017 erg. The event is established to be slightly above the surface of the Moon. An explanation is proposed
involving outgassing and a subsequent electrical discharge caused by a piezoelectric effect.
© 1988AcademicPress,Inc.
sess although several attempts have been made to classify and catalog those which appear to be most reliable (e.g., Cameron 1978). Hilbrecht and Kuveler (1984) estimate the reliability of the reports using four criteria:
1. I N T R O D U C T I O N
Lunar transient phenomena (LTPs) have often been observed and monitored since 1540 AD or earlier (Cameron 1972, 1978). After about 1920 most observations have been contributed by amateur astronomers and made by chance. In the past, however, reputable astronomers, e.g., Herschel, Bode, Hevelius, Olbers, Baily, and Struve, have reported "flashes" and changes in lunar formations. Early in the century Darhey (1929) draws attention to Proclus as being especially interesting for LTP events. In 1958 Kozyrev (1959) observed volcanic activity in the lunar crater Alphonsus and photographed its spectrum, which showed emission bands of CE. Bright transient luminescence of the region near the crater Kepler was photographed by Kopal and Rackham (1964) and was connected to solar activity. In her Lunar Transient P h e n o m ena Catalog Cameron (1978) listed 1468 events, including many permanently recorded by photographic or photometric methods. The origin of these events is still unclear, although several mechanisms have been proposed, e.g., Cameron (1975, 1980) and references therein or Robinson (1986). The reliability of LTPs is difficult to as-
(1) multiple, independent observations of the same event, (2) accuracy of description, (3) experience of the reporting observer, and (4) aperture, power, kind of telescope and tests of aberration, and seeing conditions during the event. Robinson (1986) has summarized and commented on possible causes of LTPs. He mentions I1 possible causes and ventures to conclude that the most promising answer to the LTP riddle is connected to tidal strains or thermal shocks causing outgassing and producing a piezoelectric effect. In this paper we report and give details of an LTP which was monitored by one of us (G.K.), by chance, during a routine test of the performance of a well-equipped, small portable refractor. In Section 2 we describe the observing equipment and the weather conditions, in Section 3 we concentrate on the results and the methods of analysis
525 0019-1035/88 $3.00 Copyright ~ 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
526
KOLOVOS ET AL.
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used, and in Section 4 we attempt to estimate some physical parameters of the event and discuss their implications.
TABLE I EXPOSURE TIMES
Frame 2. O B S E R V A T I O N S
The observations took place at a remote village (of about 1500 inhabitants) in North Greece (Nea Bafra, Serrai) with geographic longitude )~ = -23054.2 ' and latitude ~ +40058.5 ' (altitude, h = 300 m). The nearest city (population 35,000) was at a distance of about 27 km. On May 23, 1985, at about 17h41m UT a 108-ram refractor w i t h f = 1600 mm, ]714.8 equipped with a motor drive and a Miranda camera (loaded with Kodak 2415 film) was used for a series of seven white-light focal photographs of the Moon with exposure times varying between 6~o and 1 sec (Table I). The film was subsequently developed using an HC-110 K o d a k solution (T = 20°C, t = 6 rain) achieving a fairly high contrast. The weather at the time was excellent with very few atmospheric fluctuations. The Moon was 3.8 days old (0.13 of the M o o n ' s disk could be seen at the time) and its semidiameter was 15'08.53". Some of the major features on the terminator at the time were (from North to South) the craters Atlas, Proclus, Gutenberg, and Janssen. The E a r t h - M o o n distance was 61864 RE. 3. R E S U L T S
Table 1 gives the details of the exposure times of the seven photographs (frames). The exact time of the last photograph (May 23, 1985, 17h41m50~ was noted with an accuracy of a few seconds, whereas for the rest a time interval of 8 sec between the exposures was estimated later (after repeated attempts trying to simulate the original setup). The overall accuracy of timing is better than -+10 sec. The exposure time of the second photograph was not noted down. Comparing it with the rest, we estimate it to be about 0.8 sec. Frames 2, 4, 5, and 7 are presented in Fig. 1. In frame 4 a striking "bright s p o t " is
Exposure time (sec)
!
2 3 4 5 6
(0.8) ,~ ~ (flash) .', .I,
7
I
immediately visible, which is totally absent from all the others. The exact position of the spot in selenographic coordinates is / = 43o06.5 ' , b = +13°4.2 ', roughly between Proclus C and an irregularly shaped, unnamed crater in the region of Palus Somni. The first and third frames (not shown in Fig. 1) were rather underexposed but show no spot. The sixth flame also shows no spot. The original film has been archived in our laboratory. A quick examination of the film at the Kodak Laboratories in Athens, Greece, immediately excluded a fault of the film. The grain structure in the spot image is completely undisturbed and very similar to the grain structure of other randomly selected areas of the film. Later analysis at the laboratory of our observatory corroborated this result, as will become obvious in what follows. The diameter of the image of the Moon on the film was estimated using the characteristics of the telescope and measured on the film using a Carl Zeiss measuring microscope Model B K 70 × 50. Both methods yielded very similar results, i.e., D = 14.392 + 0.003 ram. Using the same microscope we measured the dimensions of the spot, which is slightly elliptical with a = 0.093 -+ 0.003 mm and b = 0.075 -+ 0.003 mm. Assuming that the diameter of the Moon is 3476 km, the true dimensions of the spot are a = 22.5 -+ 0.1 km and b = 18.0 _+ 0.I km (ratio a/b = 1.24, area S ' = 318
527
STRONG FLASH FROM THE MOON
® --7 Cleomedes
obius
® "undius
bS m
Frame
2
Frame
4
Frame
5
Frame
7
® Fie. 1. Frames 2, 4, 5 and 7. Points of the compass are indicated. The outlines of a few well-known craters are also indicated. Note the very bright spot on the terminator in frame 4. kin2). The ellipticity follows the meridian lines of the M o o n and can, therefore, be roughly explained by the projection of the local surface of the M o o n at the time of the event. We m e a s u r e d a similar a/b ratio for the crater Macrobius, a p p r o x i m a t e l y lying on the same meridian. Assuming that the spot is circular on the surface of the Moon, it occupies an area S of about 530 km 2 (S = S ' • c o s e c ( a r c s i n ( R / R o ) ) , where R is the distance of the spot f r o m the center of the disk of the M o o n and Ro the radius of the Moon). The brightness of the spot was c o m p a r e d to the brightness of the edge of the lunar image on the original film using an Askania irisphotometer and a specially constructed n a r r o w - b e a m p h o t o d e n s i t o m e t e r . Several m e a s u r e m e n t s along the radius of the M o o n ' s disk passing through the spot were taken; all m e t h o d s gave a ratio of 0.85 +0.02 for the brightness of the spot to the brightness of the edge of the Moon. This c o r r e s p o n d s to a magnitude difference of
Am = 0.22 + 0.02. M e a s u r e m e n t s along the same radius on f r a m e 5 does not show any indication of the spot. Similar measurements on an enlargement of the original film were made using a J o y c e and Loebl photodensitometer. The results are shown in Fig. 2. The spot is clearly visible in Fig. 2a (frame 4). Several features of the M o o n ' s surface are identified in Fig. 2. The fact that the brightness of the spot does not reach the saturation level of the brightly illuminated portion of the lunar cresent shows that the origin of the spot cannot be attributed to a flaw or p r e e x p o s u r e of that part of the emulsion. Figure 3a shows a 200x (photographically smoothed) magnification of the spot. In Fig. 3b the positions of Proclus C and the u n n a m e d crater are also indicated. It is obvious that the shape of the spot is confined by the irregularities of the surrounding region, indicating that the spot is connected to the m o r p h o l o g y of the area. Close examination of f r a m e 4 in Fig. 1 shows that the
528
KOLOVOS ET AL.
a)
b) 1
Flash
Proclus P
Proclus P
....... I 0.8
sky I 0.9
...... R/Ro
~k~G¢ 0 ,
.
.
.
.
. I CL8
.
.
.
sky] 0.9
........ R/Ro
~
]
•
FIG. 2. The intensity of the lunar features measured along the radius of the Moon passing through the center of the spot. (a) For frame 4. (b) For frame 5.
o p p o s i t e i n n e r e d g e o f the u n n a m e d c r a t e r , a b o u t 50 k m a w a y f r o m the c e n t e r o f the s p o t , is i l l u m i n a t e d . S u c h i l l u m i n a t i o n is not o b s e r v e d in the o t h e r f r a m e s . This m e a n s t h a t t h e s p o t , w h i c h f r o m n o w on w e shall call a " f l a s h , " i l l u m i n a t e d t h e r e g i o n f r o m a p o i n t slightly a b o v e t h e s u r f a c e o f
the M o o n (~< 1 k m , o f t h e o r d e r o f t h e height of the crater). 4. DISCUSSION T a k i n g into a c c o u n t t h e r e s u l t s o f o u r a n a l y s i s up to n o w , it b e c o m e s o b v i o u s that on M a y 23, 1985, a r o u n d 17h41m26 s, a
FIG. 3. (a) A 200× magnification of the region of the spot. (b) Figure 3a with the outline of some surface features superimposed (a, unnamed crater; b, Proclus C).
STRONG FLASH FROM THE MOON
529
FIG. 4. A detailed (Apollo 17) photograph of the region. The exact position of the flash seen in Fig. 1 (frame 4) is indicated. strong, short duration flash illuminated the region around crater Proclus C. Figure 4 is a detailed photograph of the region. The exact position of the flash lies in an area with several small craters scattered along two
parallel lines on a rather flat or slightly irregular ground. Several are funnel-shaped, suggesting volcanic origin. To the best of our knowledge this is the first time that such an event has been photographed and
530
KOLOVOS ET AL.
clearly documented. The characteristics of the flash show many similarities with previously reported lunar transient phenomena, although due to its very short duration it would probably have passed unnoticed if it was observed only optically. Any attempt to explain the new data presented here should take into account its short duration (maximum duration 16 sec), its occurrence slightly above the lunar surface, the absence of a dust cloud after the event (within the limits of our resolution), and its intensity. Taking into account the solar constant (1.374 × l06 erg/cmZ/sec), the albedo of the surface of the Moon (0.073), and the ratio of the brightness of the flash to the brightness of the edge of the Moon (see Section 3), we can estimate the flux of the flash, E = 1.374 x 10 6 × 0.073 × 0.85 = 0.85 x 105 erg/cm2/ sec. Its total power then is E~ = 5.3 x l0 lz x E = 4.5 x 1017 erg/sec. The available solar activity data show no particularly prominent events for a few days around May 23. The possibility of reflection of sunlight from the lunar surface is ruled out because it would need extremely peculiar morphology of the region and the illumination of the region indicates that the flash did not occur o n the surface of the Moon but slightly a b o v e it. The absence of any sign of obscuration in the subsequent photographs seems to rule out any volcanic eruption. A matter-antimatter annihilation (which could easily explain the amount of energy released) is also ruled out because in this case the energy is mainly released in the form of relativistic pions and short wavelength gamma rays. Could the flash be explained by a meteor impact on the surface of the Moon? Again the height of the flash seems to rule out a scenario involving the transformation of a meteorite's kinetic energy into heat. An impact would result in mechanical deformation of the lunar crust (breaking of the upper layer and penetration of the meteor) instead of heating the surface to the temperature of at least 2000 K necessary to radiate
at visual wavelengths. A meteor impact, of course, could eject a column of dust, which in turn could reflect the sunlight. However, the free fall time of dust particles from a height of 1 km above the lunar surface is about 30 sec, much longer than the recorded duration of the event. Excluding the case of man-made intervention, we favor an explanation involving outgassing from the surface of the Moon combined with a subsequent discharge over it, caused by a piezoelectric effect (Robinson 1986). Any reasonably intense stress applied on the lunar surface could have been responsible for the above scenario. The cause of the stress could be either a meteor impact or, more probably, the steep temperature gradient along the lunar surface due to the rising Sun (the flash is very close to the terminator). We note that according to Cameron (1972) a large percentage of L T P s occur within 1 day of sunrise in the region where they are observed. Her explanation of the difference in the rate of occurrence of LTPs between sunrise and sunset is attributed to the poor statistics of sunset observations. We favor a more fundamental mechanism involving a steeper temperature gradient during sunrise than during sunset. An intense stress could trigger a " c r a c k i n g " of the lunar surface through which gas (probably radon or some other product of radioactive decay trapped below the surface) escapes as well as a potential difference due to piezoelectric effects. This potential, in turn, can initiate a current discharge through the expanding gas, which may be easily ionized. In what follows we investigate the above scenario and understand its details. Radon emission at the edges of lunar maria was detected with Apollo 15 and 16 alpha-particle spectrometers. According to the data presented by Gorenstein e t al. (1974), the highest count was recorded at the edge of Mare Crisium, where crater Proclus (and the flash reported here) lies. The resistivity of a one-species, fully ion-
STRONG FLASH FROM THE MOON ized plasma is given by the relation (Spitzer 1962) Ulna p = 5 x 10 -3 T3/Z(eV----~ ohm • cm, where U is the number of unbound electrons (for fully ionized atoms U is equal to the atomic n u m b e r Z), T is the temperature of the electrons, and A is the plasma parameter. Parameter A does not vary very much for different plasmas and a value of l0 should be within a factor of 2 of the actual value (Spitzer 1962). U is more difficult to calculate. F o r fully ionized radon U = 86, and it becomes less for partially ionized atoms. Since the ionization potential of radon for any electron b e y o n d the first dozen or so is extremely high (e.g., for the tenth electron it is 110 = 200 eV), we may take an " e f f e c t i v e " U = 10. The concept of temperature in such a dynamic event, as a discharge, is not well defined. We adopt a value of T = I00 eV and we estimate a resistivity p = 5 x I0 -4 ohm • cm. If we assume that the discharge occurred in a loop reaching a height of h = I000 m above the lunar surface with a cross section of A = 100 x 100 m z, then the resistance of the loop can be calculated as R = p x 2 h / A = I 0 - 4 ohm.
If t is the duration of the flash, we can calculate the electrical p o w e r of the discharge W = V 2 / R = 10J7/t erg,
from which for 0.01 sec < t < l0 sec we estimate 3 x 102 < V < 104. Therefore the voltage necessary to drive a discharge releasing 1017 erg is of the order of 1000 V, which can easily be obtained from a piezoelectric effect. Here we should note that similar calculations for a weakly ionized gas give very similar results (Smirnov 1981). Once we have established that the voltage required to produce a flash of the ob-
531
served energy can be attributed to a piezoelectric effect, the next question is whether the elastic deformation of lunar material (due to differential heating) can store enough mechanical energy to produce such a flash. The energy stored in a rod of crosssection A and length l, which has been stretched by AI is given by I YA
E-2
1 (Al)2'
where Y is the Young modulus (Sears and Z e m a n s k y 1967). Al can be calculated from the law of linear expansion AI = I a A T
where AT is the temperature difference and c~ is the linear expansion coefficient. Thus E = 1yAlo~2(AT)2.
The temperature difference between the day and night sector of the surface of the Moon is AT ~ 300 K, whereas the Young modulus and expansion coefficient for basalt are Y = 1012 dyn/cm 2 and c~ = 10 5. For a slab of lunar rock near the terminator of dimensions 1 km x 1 km x 10 cm the mechanical energy stored is E = 1018 erg. This energy, therefore, can comfortably drive an electrical discharge of the order of 1017 erg. In conclusion we point out that the outgassing and subsequent electrical discharge scenario is the only one involving a flash of light a b o v e the lunar surface. We acknowledge the controversial nature of LTPs. Our observation certainly does not serve as a confirmation of all previously reported events. However, to the best of our knowledge, it gives the first, and only up to now, hard evidence of a very short duration, strong flash which occurred close to the surface of the Moon. We present our results with caution and we hope that additional data (e.g., lunar seismography and/or new high-resolution photographs) may lead to their indisputable explanation.
532
K O L O V O S E T AL. ACKNOWLEDGMENTS
We thank J. I. Vette, F. J. Doyle, and the National Space Science Data Center (World Data Center-A, Rockets and Satellites) for kindly forwarding relevant photographs from the Apollo 15 and 17 missions, W. S. Cameron for helpful comments on the original manuscript, and E. Vanidis for his help in obtaining Fig. 2. REFERENCES CAMERON, W. S. 1972. Comparative analyses of observations of lunar transient phenomena. Icarus 16, 339. CAMERON, W. S. 1975. Manifestations and possible sources of lunar transient phenomena (LTP). Moon 14, 187. CAMERON, W. S. 1978. Lunar Transient Phenomena Catalog, NSSDC 78-03. CAMERON, W. S. 1980. New results from old data: Lunar photometric anomalies in Wildey and Pohn's 1962 observations. Astron. J. 85, 314. DARNEY, M. 1929. Etudes Lunaires: Proclus et ses
Rayonnements. Bull. Soc. Astron. France, F6vrier, 86. GORENSTEIN, P., L. GOLUB, AND P. BJORKHOLM 1974. Detection of radon emission at the edges of lunar maria with the Apollo particle spectrometer. Science 183, 411. HILBRECHT, H., AYD G. K/3VELER 1984. Observations of Lunar Transient Phemonena (LTP) in 1972 and 1973. Earth Moon Planets 30, 53. KOPAL, Z., AND T. W. RA¢•HAM 1964. Excitation of lunar luminescence by solar flares. Nature 201, 239. KOZYREV, N. A. 1959. Observation of a volcanic process on the Moon. Sky Telesc. 18, 184. ROBINSON, J. H. 1986. Possible causes of LTP. J. Brit. Astron. Assoc. 97~ 13. SEARS, F. W., AND M. W. ZEMANSKY 1967. University Physics, pp. 250, 345. Addison-Wesley, Reading, MA. SM1RNOV, B. M. 1981. Physics of weakly ionized gases. Mir, Moscow, p. 147. SPITZER, L., JR. 1962. Physics of Fully Ionized Gases. Interscience, New York.
Photographic Evidence of a Short Duration" Strong Flash from the Surface of the Moon G. K O L O V O S , J. H. S E I R A D A K I S , H. V A R V O G L I S , AND S. AVGOLOUPIS University of Thessaloniki, Department of Physics, Section of Astrophysics, Astronomy, and Mechanics, GR-54006 Thessaloniki, Greece R e c e i v e d F e b r u a r y 3, 1988; r e v i s e d April 22, 1988 W e present photographic evidence of a very short duration, strong flash from the surface of the Moon (near an irregularly shaped crater in Palus Somni). The flash covered a region roughly 22 by 18 km wide with a total energy of the order of 1017 erg. The event is established to be slightly above the surface of the Moon. An explanation is proposed
involving outgassing and a subsequent electrical discharge caused by a piezoelectric effect.
© 1988AcademicPress,Inc.
sess although several attempts have been made to classify and catalog those which appear to be most reliable (e.g., Cameron 1978). Hilbrecht and Kuveler (1984) estimate the reliability of the reports using four criteria:
1. I N T R O D U C T I O N
Lunar transient phenomena (LTPs) have often been observed and monitored since 1540 AD or earlier (Cameron 1972, 1978). After about 1920 most observations have been contributed by amateur astronomers and made by chance. In the past, however, reputable astronomers, e.g., Herschel, Bode, Hevelius, Olbers, Baily, and Struve, have reported "flashes" and changes in lunar formations. Early in the century Darhey (1929) draws attention to Proclus as being especially interesting for LTP events. In 1958 Kozyrev (1959) observed volcanic activity in the lunar crater Alphonsus and photographed its spectrum, which showed emission bands of CE. Bright transient luminescence of the region near the crater Kepler was photographed by Kopal and Rackham (1964) and was connected to solar activity. In her Lunar Transient P h e n o m ena Catalog Cameron (1978) listed 1468 events, including many permanently recorded by photographic or photometric methods. The origin of these events is still unclear, although several mechanisms have been proposed, e.g., Cameron (1975, 1980) and references therein or Robinson (1986). The reliability of LTPs is difficult to as-
(1) multiple, independent observations of the same event, (2) accuracy of description, (3) experience of the reporting observer, and (4) aperture, power, kind of telescope and tests of aberration, and seeing conditions during the event. Robinson (1986) has summarized and commented on possible causes of LTPs. He mentions I1 possible causes and ventures to conclude that the most promising answer to the LTP riddle is connected to tidal strains or thermal shocks causing outgassing and producing a piezoelectric effect. In this paper we report and give details of an LTP which was monitored by one of us (G.K.), by chance, during a routine test of the performance of a well-equipped, small portable refractor. In Section 2 we describe the observing equipment and the weather conditions, in Section 3 we concentrate on the results and the methods of analysis
525 0019-1035/88 $3.00 Copyright ~ 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
526
KOLOVOS ET AL.
"
used, and in Section 4 we attempt to estimate some physical parameters of the event and discuss their implications.
TABLE I EXPOSURE TIMES
Frame 2. O B S E R V A T I O N S
The observations took place at a remote village (of about 1500 inhabitants) in North Greece (Nea Bafra, Serrai) with geographic longitude )~ = -23054.2 ' and latitude ~ +40058.5 ' (altitude, h = 300 m). The nearest city (population 35,000) was at a distance of about 27 km. On May 23, 1985, at about 17h41m UT a 108-ram refractor w i t h f = 1600 mm, ]714.8 equipped with a motor drive and a Miranda camera (loaded with Kodak 2415 film) was used for a series of seven white-light focal photographs of the Moon with exposure times varying between 6~o and 1 sec (Table I). The film was subsequently developed using an HC-110 K o d a k solution (T = 20°C, t = 6 rain) achieving a fairly high contrast. The weather at the time was excellent with very few atmospheric fluctuations. The Moon was 3.8 days old (0.13 of the M o o n ' s disk could be seen at the time) and its semidiameter was 15'08.53". Some of the major features on the terminator at the time were (from North to South) the craters Atlas, Proclus, Gutenberg, and Janssen. The E a r t h - M o o n distance was 61864 RE. 3. R E S U L T S
Table 1 gives the details of the exposure times of the seven photographs (frames). The exact time of the last photograph (May 23, 1985, 17h41m50~ was noted with an accuracy of a few seconds, whereas for the rest a time interval of 8 sec between the exposures was estimated later (after repeated attempts trying to simulate the original setup). The overall accuracy of timing is better than -+10 sec. The exposure time of the second photograph was not noted down. Comparing it with the rest, we estimate it to be about 0.8 sec. Frames 2, 4, 5, and 7 are presented in Fig. 1. In frame 4 a striking "bright s p o t " is
Exposure time (sec)
!
2 3 4 5 6
(0.8) ,~ ~ (flash) .', .I,
7
I
immediately visible, which is totally absent from all the others. The exact position of the spot in selenographic coordinates is / = 43o06.5 ' , b = +13°4.2 ', roughly between Proclus C and an irregularly shaped, unnamed crater in the region of Palus Somni. The first and third frames (not shown in Fig. 1) were rather underexposed but show no spot. The sixth flame also shows no spot. The original film has been archived in our laboratory. A quick examination of the film at the Kodak Laboratories in Athens, Greece, immediately excluded a fault of the film. The grain structure in the spot image is completely undisturbed and very similar to the grain structure of other randomly selected areas of the film. Later analysis at the laboratory of our observatory corroborated this result, as will become obvious in what follows. The diameter of the image of the Moon on the film was estimated using the characteristics of the telescope and measured on the film using a Carl Zeiss measuring microscope Model B K 70 × 50. Both methods yielded very similar results, i.e., D = 14.392 + 0.003 ram. Using the same microscope we measured the dimensions of the spot, which is slightly elliptical with a = 0.093 -+ 0.003 mm and b = 0.075 -+ 0.003 mm. Assuming that the diameter of the Moon is 3476 km, the true dimensions of the spot are a = 22.5 -+ 0.1 km and b = 18.0 _+ 0.I km (ratio a/b = 1.24, area S ' = 318
527
STRONG FLASH FROM THE MOON
® --7 Cleomedes
obius
® "undius
bS m
Frame
2
Frame
4
Frame
5
Frame
7
® Fie. 1. Frames 2, 4, 5 and 7. Points of the compass are indicated. The outlines of a few well-known craters are also indicated. Note the very bright spot on the terminator in frame 4. kin2). The ellipticity follows the meridian lines of the M o o n and can, therefore, be roughly explained by the projection of the local surface of the M o o n at the time of the event. We m e a s u r e d a similar a/b ratio for the crater Macrobius, a p p r o x i m a t e l y lying on the same meridian. Assuming that the spot is circular on the surface of the Moon, it occupies an area S of about 530 km 2 (S = S ' • c o s e c ( a r c s i n ( R / R o ) ) , where R is the distance of the spot f r o m the center of the disk of the M o o n and Ro the radius of the Moon). The brightness of the spot was c o m p a r e d to the brightness of the edge of the lunar image on the original film using an Askania irisphotometer and a specially constructed n a r r o w - b e a m p h o t o d e n s i t o m e t e r . Several m e a s u r e m e n t s along the radius of the M o o n ' s disk passing through the spot were taken; all m e t h o d s gave a ratio of 0.85 +0.02 for the brightness of the spot to the brightness of the edge of the Moon. This c o r r e s p o n d s to a magnitude difference of
Am = 0.22 + 0.02. M e a s u r e m e n t s along the same radius on f r a m e 5 does not show any indication of the spot. Similar measurements on an enlargement of the original film were made using a J o y c e and Loebl photodensitometer. The results are shown in Fig. 2. The spot is clearly visible in Fig. 2a (frame 4). Several features of the M o o n ' s surface are identified in Fig. 2. The fact that the brightness of the spot does not reach the saturation level of the brightly illuminated portion of the lunar cresent shows that the origin of the spot cannot be attributed to a flaw or p r e e x p o s u r e of that part of the emulsion. Figure 3a shows a 200x (photographically smoothed) magnification of the spot. In Fig. 3b the positions of Proclus C and the u n n a m e d crater are also indicated. It is obvious that the shape of the spot is confined by the irregularities of the surrounding region, indicating that the spot is connected to the m o r p h o l o g y of the area. Close examination of f r a m e 4 in Fig. 1 shows that the
528
KOLOVOS ET AL.
a)
b) 1
Flash
Proclus P
Proclus P
....... I 0.8
sky I 0.9
...... R/Ro
~k~G¢ 0 ,
.
.
.
.
. I CL8
.
.
.
sky] 0.9
........ R/Ro
~
]
•
FIG. 2. The intensity of the lunar features measured along the radius of the Moon passing through the center of the spot. (a) For frame 4. (b) For frame 5.
o p p o s i t e i n n e r e d g e o f the u n n a m e d c r a t e r , a b o u t 50 k m a w a y f r o m the c e n t e r o f the s p o t , is i l l u m i n a t e d . S u c h i l l u m i n a t i o n is not o b s e r v e d in the o t h e r f r a m e s . This m e a n s t h a t t h e s p o t , w h i c h f r o m n o w on w e shall call a " f l a s h , " i l l u m i n a t e d t h e r e g i o n f r o m a p o i n t slightly a b o v e t h e s u r f a c e o f
the M o o n (~< 1 k m , o f t h e o r d e r o f t h e height of the crater). 4. DISCUSSION T a k i n g into a c c o u n t t h e r e s u l t s o f o u r a n a l y s i s up to n o w , it b e c o m e s o b v i o u s that on M a y 23, 1985, a r o u n d 17h41m26 s, a
FIG. 3. (a) A 200× magnification of the region of the spot. (b) Figure 3a with the outline of some surface features superimposed (a, unnamed crater; b, Proclus C).
STRONG FLASH FROM THE MOON
529
FIG. 4. A detailed (Apollo 17) photograph of the region. The exact position of the flash seen in Fig. 1 (frame 4) is indicated. strong, short duration flash illuminated the region around crater Proclus C. Figure 4 is a detailed photograph of the region. The exact position of the flash lies in an area with several small craters scattered along two
parallel lines on a rather flat or slightly irregular ground. Several are funnel-shaped, suggesting volcanic origin. To the best of our knowledge this is the first time that such an event has been photographed and
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clearly documented. The characteristics of the flash show many similarities with previously reported lunar transient phenomena, although due to its very short duration it would probably have passed unnoticed if it was observed only optically. Any attempt to explain the new data presented here should take into account its short duration (maximum duration 16 sec), its occurrence slightly above the lunar surface, the absence of a dust cloud after the event (within the limits of our resolution), and its intensity. Taking into account the solar constant (1.374 × l06 erg/cmZ/sec), the albedo of the surface of the Moon (0.073), and the ratio of the brightness of the flash to the brightness of the edge of the Moon (see Section 3), we can estimate the flux of the flash, E = 1.374 x 10 6 × 0.073 × 0.85 = 0.85 x 105 erg/cm2/ sec. Its total power then is E~ = 5.3 x l0 lz x E = 4.5 x 1017 erg/sec. The available solar activity data show no particularly prominent events for a few days around May 23. The possibility of reflection of sunlight from the lunar surface is ruled out because it would need extremely peculiar morphology of the region and the illumination of the region indicates that the flash did not occur o n the surface of the Moon but slightly a b o v e it. The absence of any sign of obscuration in the subsequent photographs seems to rule out any volcanic eruption. A matter-antimatter annihilation (which could easily explain the amount of energy released) is also ruled out because in this case the energy is mainly released in the form of relativistic pions and short wavelength gamma rays. Could the flash be explained by a meteor impact on the surface of the Moon? Again the height of the flash seems to rule out a scenario involving the transformation of a meteorite's kinetic energy into heat. An impact would result in mechanical deformation of the lunar crust (breaking of the upper layer and penetration of the meteor) instead of heating the surface to the temperature of at least 2000 K necessary to radiate
at visual wavelengths. A meteor impact, of course, could eject a column of dust, which in turn could reflect the sunlight. However, the free fall time of dust particles from a height of 1 km above the lunar surface is about 30 sec, much longer than the recorded duration of the event. Excluding the case of man-made intervention, we favor an explanation involving outgassing from the surface of the Moon combined with a subsequent discharge over it, caused by a piezoelectric effect (Robinson 1986). Any reasonably intense stress applied on the lunar surface could have been responsible for the above scenario. The cause of the stress could be either a meteor impact or, more probably, the steep temperature gradient along the lunar surface due to the rising Sun (the flash is very close to the terminator). We note that according to Cameron (1972) a large percentage of L T P s occur within 1 day of sunrise in the region where they are observed. Her explanation of the difference in the rate of occurrence of LTPs between sunrise and sunset is attributed to the poor statistics of sunset observations. We favor a more fundamental mechanism involving a steeper temperature gradient during sunrise than during sunset. An intense stress could trigger a " c r a c k i n g " of the lunar surface through which gas (probably radon or some other product of radioactive decay trapped below the surface) escapes as well as a potential difference due to piezoelectric effects. This potential, in turn, can initiate a current discharge through the expanding gas, which may be easily ionized. In what follows we investigate the above scenario and understand its details. Radon emission at the edges of lunar maria was detected with Apollo 15 and 16 alpha-particle spectrometers. According to the data presented by Gorenstein e t al. (1974), the highest count was recorded at the edge of Mare Crisium, where crater Proclus (and the flash reported here) lies. The resistivity of a one-species, fully ion-
STRONG FLASH FROM THE MOON ized plasma is given by the relation (Spitzer 1962) Ulna p = 5 x 10 -3 T3/Z(eV----~ ohm • cm, where U is the number of unbound electrons (for fully ionized atoms U is equal to the atomic n u m b e r Z), T is the temperature of the electrons, and A is the plasma parameter. Parameter A does not vary very much for different plasmas and a value of l0 should be within a factor of 2 of the actual value (Spitzer 1962). U is more difficult to calculate. F o r fully ionized radon U = 86, and it becomes less for partially ionized atoms. Since the ionization potential of radon for any electron b e y o n d the first dozen or so is extremely high (e.g., for the tenth electron it is 110 = 200 eV), we may take an " e f f e c t i v e " U = 10. The concept of temperature in such a dynamic event, as a discharge, is not well defined. We adopt a value of T = I00 eV and we estimate a resistivity p = 5 x I0 -4 ohm • cm. If we assume that the discharge occurred in a loop reaching a height of h = I000 m above the lunar surface with a cross section of A = 100 x 100 m z, then the resistance of the loop can be calculated as R = p x 2 h / A = I 0 - 4 ohm.
If t is the duration of the flash, we can calculate the electrical p o w e r of the discharge W = V 2 / R = 10J7/t erg,
from which for 0.01 sec < t < l0 sec we estimate 3 x 102 < V < 104. Therefore the voltage necessary to drive a discharge releasing 1017 erg is of the order of 1000 V, which can easily be obtained from a piezoelectric effect. Here we should note that similar calculations for a weakly ionized gas give very similar results (Smirnov 1981). Once we have established that the voltage required to produce a flash of the ob-
531
served energy can be attributed to a piezoelectric effect, the next question is whether the elastic deformation of lunar material (due to differential heating) can store enough mechanical energy to produce such a flash. The energy stored in a rod of crosssection A and length l, which has been stretched by AI is given by I YA
E-2
1 (Al)2'
where Y is the Young modulus (Sears and Z e m a n s k y 1967). Al can be calculated from the law of linear expansion AI = I a A T
where AT is the temperature difference and c~ is the linear expansion coefficient. Thus E = 1yAlo~2(AT)2.
The temperature difference between the day and night sector of the surface of the Moon is AT ~ 300 K, whereas the Young modulus and expansion coefficient for basalt are Y = 1012 dyn/cm 2 and c~ = 10 5. For a slab of lunar rock near the terminator of dimensions 1 km x 1 km x 10 cm the mechanical energy stored is E = 1018 erg. This energy, therefore, can comfortably drive an electrical discharge of the order of 1017 erg. In conclusion we point out that the outgassing and subsequent electrical discharge scenario is the only one involving a flash of light a b o v e the lunar surface. We acknowledge the controversial nature of LTPs. Our observation certainly does not serve as a confirmation of all previously reported events. However, to the best of our knowledge, it gives the first, and only up to now, hard evidence of a very short duration, strong flash which occurred close to the surface of the Moon. We present our results with caution and we hope that additional data (e.g., lunar seismography and/or new high-resolution photographs) may lead to their indisputable explanation.
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K O L O V O S E T AL. ACKNOWLEDGMENTS
We thank J. I. Vette, F. J. Doyle, and the National Space Science Data Center (World Data Center-A, Rockets and Satellites) for kindly forwarding relevant photographs from the Apollo 15 and 17 missions, W. S. Cameron for helpful comments on the original manuscript, and E. Vanidis for his help in obtaining Fig. 2. REFERENCES CAMERON, W. S. 1972. Comparative analyses of observations of lunar transient phenomena. Icarus 16, 339. CAMERON, W. S. 1975. Manifestations and possible sources of lunar transient phenomena (LTP). Moon 14, 187. CAMERON, W. S. 1978. Lunar Transient Phenomena Catalog, NSSDC 78-03. CAMERON, W. S. 1980. New results from old data: Lunar photometric anomalies in Wildey and Pohn's 1962 observations. Astron. J. 85, 314. DARNEY, M. 1929. Etudes Lunaires: Proclus et ses
Rayonnements. Bull. Soc. Astron. France, F6vrier, 86. GORENSTEIN, P., L. GOLUB, AND P. BJORKHOLM 1974. Detection of radon emission at the edges of lunar maria with the Apollo particle spectrometer. Science 183, 411. HILBRECHT, H., AYD G. K/3VELER 1984. Observations of Lunar Transient Phemonena (LTP) in 1972 and 1973. Earth Moon Planets 30, 53. KOPAL, Z., AND T. W. RA¢•HAM 1964. Excitation of lunar luminescence by solar flares. Nature 201, 239. KOZYREV, N. A. 1959. Observation of a volcanic process on the Moon. Sky Telesc. 18, 184. ROBINSON, J. H. 1986. Possible causes of LTP. J. Brit. Astron. Assoc. 97~ 13. SEARS, F. W., AND M. W. ZEMANSKY 1967. University Physics, pp. 250, 345. Addison-Wesley, Reading, MA. SM1RNOV, B. M. 1981. Physics of weakly ionized gases. Mir, Moscow, p. 147. SPITZER, L., JR. 1962. Physics of Fully Ionized Gases. Interscience, New York.