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A new mechanism for lunar transient phenomena
ICARUS 82, 419--422 (1989)
A New Mechanism for Lunar Transient Phenomena RICHARD R. ZITO Lockheed Missiles and Space Company, Inc., 0/62-47, B/076, 1111 Lockheed Way, Sunnyvale, California 94086 Received August 19, 1988; revised February 6, 1989
A new mechanism for lunar transient phenomena is proposed. Electrodynamic effects associated with rock fracturing seem to be able to account for sporadic optical pulses seen near certain lunar features from Earth. Only mild seismic activity or perhaps thermal cracking is required. © 1989AcademicPress,Inc.
Lunar transient phenomena (LTP) are local changes in the lunar surface brightness which have been observed and reported over the past 4 centuries. Such changes are frequently flashes (Cameron 1975), whose rise time may be less than k sec, or so-called "sudden brightenings," whose rise time may vary from 1 to 5 sec (Cameron 1975). Such emissions are usually described as reddish or bluish. Furthermore, photographic, photoelectric, and polarization observations also exist for LTP. More than 900 observations have been reported and discussed (Cameron 1972, Cameron and Gilheany 1967, Burley and Middlehurst 1966, Green 1965, Middlehurst 1967, Middlehurst and Moore 1967, Anonymous 1969). LTP have been reported from at least 100 lunar sites with 33% of all these reports associated with the crater Aristarchus (Cameron 1975). During the Apollo 12 mission Aristarchus was observed continuously from November 14 to 24, 1969. Within this time period about 60% of all positive LTP reports were associated with passage of the terminator over the crater (Anonymous 1969). Many mechanisms have been suggested for LTP (Cameron 1972, Srnka 1977). These include stimulated light emission from the lunar surface by solar ultraviolet photons, accelerated particles from the Earth's magneto-tail, and solar flare parti-
cles as well as emissions from plasma turbulence in the Earth's bow shock, and the interaction of the solar wind with occasional gaseous outbursts from the lunar surface. Tidal mechanisms (Green 1965) and low-angle illumination effects (Sidran 1968) have also been proposed as the source of LTP. In this paper the amount of optical energy released during electrodynamic effects associated with rock fracturing will be calculated. It has recently been observed that flashes of light are emitted during the laboratory fracturing of rocks (Cress et al. 1987, Brady and Rowell 1986). This emission appears to be due to excitation of atmospheric gases by energetic electrons emitted from freshly fractured surfaces (Cress et al. 1987, Brady and Rowell 1986). A similar effect is known to occur when Wint-O-Green Life Savers are cracked, however, in this case the emission is primarily due to excitation of molecules of wintergreen instead of atmospheric gases (Raloff 1988). On a larger-than-laboratory scale, light emissions were noted during cracking events of the 1965 Matsushiro earthquake swarm and the 1976 Tangshan earthquake (Brady and Rowell 1986). In the lunar environment, surface cracking cannot excite atmospheric gases. However, rocks are known to contain rare gases of radiogenic and primordial origin. Radiogenic sources of gas include gases evolving from 419 0019-1035/89 $3.00 Copyright © 1989by Academic Press, Inc. All rights of reproduction in any form reserved.
420
RICHARD R. ZITO
radioactive decay, gases absorbed from the solar wind, and gases produced by the interaction o f radioactive emissions from uranium, thorium, and their decay daughters with light elements in rock. In general, basaltic rocks may be expected to contain 6 × 10 -8 m 3 of He at STP per kilogram of rock, 2.2 x 10 -8 m 3 of Ar at STP per kilogram of rock, 7.7 × 10 -I1 m 3 of N e at STP per Kg of rock, and less than 6 x 10 -16 ppm (by weight) of Rn (Fairbridge 1972). It will be shown that excitation of these gases (in particular He and Ar) are sufficient to produce the observed outbursts. The small traces of Ne and Rn can be ignored. In order to observe a L T P in a small aperture telescope on earth the criterion of Srnka (1977) will be adopted. It will be assumed that the emission must be at least 8% of the average lunar surface brightness. This criterion is equivalent to an emitted power o f 1 W/m 2 (optical at 5550 .A). In this report calculations will be performed for a model basalt having typical properties with a density D of 3.0 x 103 kg/m 3 and a porosity of 10% (Stephens and LiUey 1970). The pores will be modeled as randomly scattered spherical cavities with a diameter of 5 >( 10 -4 m and a n u m b e r density of 1.5 x l09 pores/m 3 (S. N. Davis 1987, private communications, Department of Hydrology, Univ. of Arizona, Tucson, AZ). It will also be assumed that diffusion has unif o r m l y distributed gases throughout the rock over tens of thousands to billions of years so that free gas atoms are available in all pores prior to cracking. Of course, only gas atoms from fractured cavities are capable of radiating energy into space. Consider a cube of rock, I m on a side, which is split in half in 1 sec so that each of the two pieces has a width of ½ m, a length of 1 m, and a height of I m. Such a crack will break open pores whose centers lie within 2.5 x 10 -4 m on each side of itself. Therefore, pore gases belonging to a slab of rock 0.5 x 10 -3 m wide, 1 m long, and 1 m deep will be effective in radiating. Such a slab has a volume of 0.5 x 10 -3 m 3. The
total volume (at STP) of H e plus Ar trapped in the slab is just the product of the volume V of the slab (in m3), the density D (in kg/ m3), and the gas concentration C (in m 3 of H e plus Ar per kilogram of rock). However, to get the STP volume of gas trapped in the p o r e s of the slab the total STP gas volume of the slab must be multiplied by a dimensionless porosity factor P of 0.1 since only 10% of the rock volume exists as pores. The number N of gas atoms capable of radiating energy into space can then be calculated by dividing the STP volume of pore gases in the slab by the gram molecular volume (or GMV equal to 22.4 liters/ mole or 2.24 × 10 -2 m3/mole) to get the number of STP moles of gas involved in radiation and then multiplying by Avogadro's number No. In symbols N = V D C P No/(GMV).
(I)
Therefore
N=
(0.0005)(3 × 103)(8.2 × 10-8)(0.1)(6.02 × 10 23) 2.24 × 10 -2
N=3.3
× 1017.
If a crack of 1 m length penetrates down into rock to a depth of h meters than N = 3.3 × 1017 × h. F r o m Planck's law the energy of a single 5550-,~ photon is 3.6 x 10 -19 J. The number of 5550-,~ emissions per second required to produce a 1-W output is 2.8 x 1018. If each gas atom in a broken pore of a split rock column of depth h emits one photon whose wavelength is close to 5550 A, then the depth h of the crack needed to produce a I-W output is given by 2.8 × 1018 = 3.3 × 1017 × h.
(3)
Therefore, h = 8.5 m. It seems that only modest surface cracking is required to produce the observed optical LTP. Such cracking may be due to weak seismic activity or temperature changes associated with passage of the terminator. Many L T P sites, such as Aristarchus, are associated with systems of
NEW MECHANISM FOR LTP faults (Firsoff 1961). Also, extreme lunar surface temperature changes of about 300°C (Short 1975) would cause individual mineral grains in a rock to expand and contract under alternate heating and cooling. As different minerals swell and shrink at different rates at least some portions of the rock will experience compressive strains, similar to those observed in laboratory experiments, which may cause eventual cracking (Ordway 1966). The effect has been Observed in rocks subject to extreme, rapid temperature changes in forest fires (Ordway 1966). At this point it is important to note that the generation of small surface cracks may not result in optical flashes visible from Earth due to obstruction by the soil-like lunar regolith. However, infrared studies of the Moon have shown that lunar regions where large amounts of bare rock are present (e.g., younger craters, regions in or adjacent to several maria where volcanic activity may have taken place well after the main features were formed, Aristarchus region, etc.) coincide well with LTP sites (Short 1975). Of course, several assumptions regarding the composition and geometry of lunar rocks have been made, and deviations from these assumptions will result in concomitant changes in the minimum depth of surface cracks required to produce the minimum detectable optical output. The most important assumption concerns the abundance of gas contained within lunar rocks. Surface rocks returned from the Moon by Apollo 11 show inert gas concentrations 20 to 10,000 times larger than those of the terrestrial values (Heymann et al. 1970). However, the source of much of this gas is believed to be the solar wind. In that case, surface rock gas concentrations would not be indicative of concentrations found in deeper subsurface rock. Consequently, the more conservative terrestrial rare gas concentrations have been used. Nevertheless, the thermal cracking of gas-rich surface rocks during the passage of the terminator may be important in the production of LTP.
421
Another problem associated with He concentration estimates is that He is highly mobile in the subsurface (S. N. Davis 1987, private communications). If rock pores are well sealed, then He emissions will dominate in the optical spectra during rock fracturing, resulting in a pinkish-violet light. If rock is poorly sealed, the blue light of argon will dominate. This may explain the color variations of LTP seen by different observers. It has also been assumed that all gas atoms of broken pores are .excited during the fracturing process. The true fraction of atoms that will experience excitation is uncertain. However, it is known that collision between energetic electrons (0.1 to 10 keV; a common energy for fracture-released electrons) and gas atoms results in atomic excitation about 80% of the time (McDowell and Coleman 1970). The actual number of pores broken into during the fracture process is also somewhat uncertain. The model calculation presented above used a flat planar crack. This is the most pessimistic assumption. It is more realistic to assume that a crack opens up on the surface of a rock with the fractal geometry of a dendritic Julia set (Peitgen and Richter 1986). In that case the fracture surface area and, therefore, the number of broken pores, would increase by a factor of 9 (an estimate based on the square of the increase in path length due to the dendritic geometry and ignoring branches whose length is smaller than 0.5 × 10-3 m). The increased number of broken pores causes a corresponding increase in radiated output power. Finally, there is one last fact which may be of some importance. During rock fracture a curious radio emission occurs, together with the optical pulse, in a band from 900 Hz to 5 kHz (Cress et al. 1987). Radiation of this frequency is believed to be due to the rotational, vibrational, and linear motions of charged fresh surfaces created upon cracking (Cress et al. 1987). This radio frequency burst might provide important corroboration of the proposed LTP
422
RICHARD R. ZITO
mechanism. Unfortunately, 5 kHz is far bework During the Apollo 12 Mission. Smithsonian Inst., Cambridge, MA. low the plasma frequency for the Earth's ionosphere, making Earth-based observa- BRADY, B. T., AND G. A. ROWELL 1986. A laboratory investigation of the electromagnetics of rock fraction impossible. Detection from a probe in ture. Nature (London) 321, 488-492. low lunar orbit (say 100 km above the sur- BURLEY, J. M., AND B. M. MIDDLEHURST 1966. Apface) is more promising. In laboratory exparent lunar activity: Historical review. Proc. Nat. Acad. Sci. (U.S.A.)55, 1007-1011. periments (Cress et al. 1987) samples of rock with dimensions of approximately 0.01 CAMERON, W. S. 1972. Comparative analysis of observations of lunar transient phenomena. Icarus 16, m were cracked at a distance of 0.1 m from 339-387. a loop antenna whose diameter was on the CAMERON, W. S. 1975. Manifestations and possible order of 0.01 m. The radio emissions were sources of lunar transient phenomena (LTP). Moon 14, 187-199. easily detected with a signal-to-noise ratio of about 10. Since the Poynting vector falls CAMERON, W. S., AND J. J. GILHEANY 1967. Operation Moon Blink and report of observations of lunar off as the square of the distance from the transient phenomena. Icarus 7, 29-41. source, it is clear that the detected intensity CRESS, G. O., B. T. BRADY, AND G. A. ROWELL 1987. is reduced by a factor of 1012 if a rock is Sources of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys. Res. cracked at a distance of 100 km. If a large Lett. 14, 331-334. cubic block of rock I0 m on a side cracks on FAIRBRIDGE, R. W. 1972. The Encyclopedia of Geothe surface of the Moon, 106 times more chemistry and Environmental Sciences, pp. 278radiation will emerge than that from the 1 x 284. Van Nostrand, New York. 10 -6 m 3 laboratory sample. Furthermore, if FIRSOFF, V. A. 1961. Moon Atlas, p. 32 and selenographic map. Viking Press, New York. a large antenna, about 10 m in diameter, is used to detect the radio signal then, gener- GREEN, J. 1965. Tidal and gravity effects intensifying lunar defluidization and volcanism. Ann. N.Y. ally speaking, the antenna gain will increase Acad. Sci. 123, 403-469. by another factor of 106 (Stutzman and HEYMAN, D., A. YANIV, J. A. S. ADAMS, AND G. E. Thiele 1981). Therefore, the detection from FRYER 1970. Inert gases in lunar samples. Science 167, 555-558. low lunar orbit should be just as successful as detection in the laboratory. Of course, if McDOWELL, M. R. C., AND J. P. COLEMAN 1970. Introduction to the Theory of Ion Atom Collisions, state of the art detectors, amplifiers, antenpp. 306-372. North-Holland, Amsterdam. nas, and signal processing are used, then MIDDLEHURST, B. M. 1967. An analysis of lunar detection will be even easier. It is also useevents. Rev. Geophys. 5, 173-189. ful to note that the radio emissions of inter- MIDDLEHURST, B. M., AND P. A. MOORE 1967. Lunar transient phenomena: Topographical distribution. est have a wavelength between 60 and 330 Science 155, 449-451. km so that a low-flying lunar probe would ORDWAY,R. J. 1966. Earth Science, p. 78. Van Nossee a near-field radiation pattern from the trand, New York. source similar to that observed in the labo- PEITGEN, H.-O., AND P. H. RICHTER 1986. The Beauty ofFractals, p. 14. Springer-Verlag, Berlin. ratory. ACKNOWLEDGMENTS The author thanks S. N. Davis of the University of Arizona and M. J. Price of Science Applications International Corporation for their contributions to this research. S. Stanic and M. Dines of the Lockheed fibrary helped with the literature search. REFERENCES ANONYMOUS 1969. Transient Lunar Phenomena Reports from the Lunar International Observers Net-
RALOEF, J. 1988. The electric life saver effect. Science News 134(5), 78-78. SHORT, N. M. 1975. Planetary Geology, pp. 63-65. Prentice-Hall, Englewood Cliffs, NJ. SIDRAN, M. 1968. Thermoluminescence of the Moon. J. Geophys. Res. 73, 5195-5206. SmqKA, L. J. 1977. Critical velocity phenomena and the LTP. Phys. Earth Planet. Inter. 14, 321-329. STEPHENS, D. R., AND E. M. LILLEY 1970. Compressibilities of lunar crystalline rock, microbreccia, and fines to 40 kilobars. Science 167, 731-732. STUTZMAN, W. L , AND G. A. THIELE 1981. Antenna Theory and Design, p. 395. Wiley, New York.
A New Mechanism for Lunar Transient Phenomena RICHARD R. ZITO Lockheed Missiles and Space Company, Inc., 0/62-47, B/076, 1111 Lockheed Way, Sunnyvale, California 94086 Received August 19, 1988; revised February 6, 1989
A new mechanism for lunar transient phenomena is proposed. Electrodynamic effects associated with rock fracturing seem to be able to account for sporadic optical pulses seen near certain lunar features from Earth. Only mild seismic activity or perhaps thermal cracking is required. © 1989AcademicPress,Inc.
Lunar transient phenomena (LTP) are local changes in the lunar surface brightness which have been observed and reported over the past 4 centuries. Such changes are frequently flashes (Cameron 1975), whose rise time may be less than k sec, or so-called "sudden brightenings," whose rise time may vary from 1 to 5 sec (Cameron 1975). Such emissions are usually described as reddish or bluish. Furthermore, photographic, photoelectric, and polarization observations also exist for LTP. More than 900 observations have been reported and discussed (Cameron 1972, Cameron and Gilheany 1967, Burley and Middlehurst 1966, Green 1965, Middlehurst 1967, Middlehurst and Moore 1967, Anonymous 1969). LTP have been reported from at least 100 lunar sites with 33% of all these reports associated with the crater Aristarchus (Cameron 1975). During the Apollo 12 mission Aristarchus was observed continuously from November 14 to 24, 1969. Within this time period about 60% of all positive LTP reports were associated with passage of the terminator over the crater (Anonymous 1969). Many mechanisms have been suggested for LTP (Cameron 1972, Srnka 1977). These include stimulated light emission from the lunar surface by solar ultraviolet photons, accelerated particles from the Earth's magneto-tail, and solar flare parti-
cles as well as emissions from plasma turbulence in the Earth's bow shock, and the interaction of the solar wind with occasional gaseous outbursts from the lunar surface. Tidal mechanisms (Green 1965) and low-angle illumination effects (Sidran 1968) have also been proposed as the source of LTP. In this paper the amount of optical energy released during electrodynamic effects associated with rock fracturing will be calculated. It has recently been observed that flashes of light are emitted during the laboratory fracturing of rocks (Cress et al. 1987, Brady and Rowell 1986). This emission appears to be due to excitation of atmospheric gases by energetic electrons emitted from freshly fractured surfaces (Cress et al. 1987, Brady and Rowell 1986). A similar effect is known to occur when Wint-O-Green Life Savers are cracked, however, in this case the emission is primarily due to excitation of molecules of wintergreen instead of atmospheric gases (Raloff 1988). On a larger-than-laboratory scale, light emissions were noted during cracking events of the 1965 Matsushiro earthquake swarm and the 1976 Tangshan earthquake (Brady and Rowell 1986). In the lunar environment, surface cracking cannot excite atmospheric gases. However, rocks are known to contain rare gases of radiogenic and primordial origin. Radiogenic sources of gas include gases evolving from 419 0019-1035/89 $3.00 Copyright © 1989by Academic Press, Inc. All rights of reproduction in any form reserved.
420
RICHARD R. ZITO
radioactive decay, gases absorbed from the solar wind, and gases produced by the interaction o f radioactive emissions from uranium, thorium, and their decay daughters with light elements in rock. In general, basaltic rocks may be expected to contain 6 × 10 -8 m 3 of He at STP per kilogram of rock, 2.2 x 10 -8 m 3 of Ar at STP per kilogram of rock, 7.7 × 10 -I1 m 3 of N e at STP per Kg of rock, and less than 6 x 10 -16 ppm (by weight) of Rn (Fairbridge 1972). It will be shown that excitation of these gases (in particular He and Ar) are sufficient to produce the observed outbursts. The small traces of Ne and Rn can be ignored. In order to observe a L T P in a small aperture telescope on earth the criterion of Srnka (1977) will be adopted. It will be assumed that the emission must be at least 8% of the average lunar surface brightness. This criterion is equivalent to an emitted power o f 1 W/m 2 (optical at 5550 .A). In this report calculations will be performed for a model basalt having typical properties with a density D of 3.0 x 103 kg/m 3 and a porosity of 10% (Stephens and LiUey 1970). The pores will be modeled as randomly scattered spherical cavities with a diameter of 5 >( 10 -4 m and a n u m b e r density of 1.5 x l09 pores/m 3 (S. N. Davis 1987, private communications, Department of Hydrology, Univ. of Arizona, Tucson, AZ). It will also be assumed that diffusion has unif o r m l y distributed gases throughout the rock over tens of thousands to billions of years so that free gas atoms are available in all pores prior to cracking. Of course, only gas atoms from fractured cavities are capable of radiating energy into space. Consider a cube of rock, I m on a side, which is split in half in 1 sec so that each of the two pieces has a width of ½ m, a length of 1 m, and a height of I m. Such a crack will break open pores whose centers lie within 2.5 x 10 -4 m on each side of itself. Therefore, pore gases belonging to a slab of rock 0.5 x 10 -3 m wide, 1 m long, and 1 m deep will be effective in radiating. Such a slab has a volume of 0.5 x 10 -3 m 3. The
total volume (at STP) of H e plus Ar trapped in the slab is just the product of the volume V of the slab (in m3), the density D (in kg/ m3), and the gas concentration C (in m 3 of H e plus Ar per kilogram of rock). However, to get the STP volume of gas trapped in the p o r e s of the slab the total STP gas volume of the slab must be multiplied by a dimensionless porosity factor P of 0.1 since only 10% of the rock volume exists as pores. The number N of gas atoms capable of radiating energy into space can then be calculated by dividing the STP volume of pore gases in the slab by the gram molecular volume (or GMV equal to 22.4 liters/ mole or 2.24 × 10 -2 m3/mole) to get the number of STP moles of gas involved in radiation and then multiplying by Avogadro's number No. In symbols N = V D C P No/(GMV).
(I)
Therefore
N=
(0.0005)(3 × 103)(8.2 × 10-8)(0.1)(6.02 × 10 23) 2.24 × 10 -2
N=3.3
× 1017.
If a crack of 1 m length penetrates down into rock to a depth of h meters than N = 3.3 × 1017 × h. F r o m Planck's law the energy of a single 5550-,~ photon is 3.6 x 10 -19 J. The number of 5550-,~ emissions per second required to produce a 1-W output is 2.8 x 1018. If each gas atom in a broken pore of a split rock column of depth h emits one photon whose wavelength is close to 5550 A, then the depth h of the crack needed to produce a I-W output is given by 2.8 × 1018 = 3.3 × 1017 × h.
(3)
Therefore, h = 8.5 m. It seems that only modest surface cracking is required to produce the observed optical LTP. Such cracking may be due to weak seismic activity or temperature changes associated with passage of the terminator. Many L T P sites, such as Aristarchus, are associated with systems of
NEW MECHANISM FOR LTP faults (Firsoff 1961). Also, extreme lunar surface temperature changes of about 300°C (Short 1975) would cause individual mineral grains in a rock to expand and contract under alternate heating and cooling. As different minerals swell and shrink at different rates at least some portions of the rock will experience compressive strains, similar to those observed in laboratory experiments, which may cause eventual cracking (Ordway 1966). The effect has been Observed in rocks subject to extreme, rapid temperature changes in forest fires (Ordway 1966). At this point it is important to note that the generation of small surface cracks may not result in optical flashes visible from Earth due to obstruction by the soil-like lunar regolith. However, infrared studies of the Moon have shown that lunar regions where large amounts of bare rock are present (e.g., younger craters, regions in or adjacent to several maria where volcanic activity may have taken place well after the main features were formed, Aristarchus region, etc.) coincide well with LTP sites (Short 1975). Of course, several assumptions regarding the composition and geometry of lunar rocks have been made, and deviations from these assumptions will result in concomitant changes in the minimum depth of surface cracks required to produce the minimum detectable optical output. The most important assumption concerns the abundance of gas contained within lunar rocks. Surface rocks returned from the Moon by Apollo 11 show inert gas concentrations 20 to 10,000 times larger than those of the terrestrial values (Heymann et al. 1970). However, the source of much of this gas is believed to be the solar wind. In that case, surface rock gas concentrations would not be indicative of concentrations found in deeper subsurface rock. Consequently, the more conservative terrestrial rare gas concentrations have been used. Nevertheless, the thermal cracking of gas-rich surface rocks during the passage of the terminator may be important in the production of LTP.
421
Another problem associated with He concentration estimates is that He is highly mobile in the subsurface (S. N. Davis 1987, private communications). If rock pores are well sealed, then He emissions will dominate in the optical spectra during rock fracturing, resulting in a pinkish-violet light. If rock is poorly sealed, the blue light of argon will dominate. This may explain the color variations of LTP seen by different observers. It has also been assumed that all gas atoms of broken pores are .excited during the fracturing process. The true fraction of atoms that will experience excitation is uncertain. However, it is known that collision between energetic electrons (0.1 to 10 keV; a common energy for fracture-released electrons) and gas atoms results in atomic excitation about 80% of the time (McDowell and Coleman 1970). The actual number of pores broken into during the fracture process is also somewhat uncertain. The model calculation presented above used a flat planar crack. This is the most pessimistic assumption. It is more realistic to assume that a crack opens up on the surface of a rock with the fractal geometry of a dendritic Julia set (Peitgen and Richter 1986). In that case the fracture surface area and, therefore, the number of broken pores, would increase by a factor of 9 (an estimate based on the square of the increase in path length due to the dendritic geometry and ignoring branches whose length is smaller than 0.5 × 10-3 m). The increased number of broken pores causes a corresponding increase in radiated output power. Finally, there is one last fact which may be of some importance. During rock fracture a curious radio emission occurs, together with the optical pulse, in a band from 900 Hz to 5 kHz (Cress et al. 1987). Radiation of this frequency is believed to be due to the rotational, vibrational, and linear motions of charged fresh surfaces created upon cracking (Cress et al. 1987). This radio frequency burst might provide important corroboration of the proposed LTP
422
RICHARD R. ZITO
mechanism. Unfortunately, 5 kHz is far bework During the Apollo 12 Mission. Smithsonian Inst., Cambridge, MA. low the plasma frequency for the Earth's ionosphere, making Earth-based observa- BRADY, B. T., AND G. A. ROWELL 1986. A laboratory investigation of the electromagnetics of rock fraction impossible. Detection from a probe in ture. Nature (London) 321, 488-492. low lunar orbit (say 100 km above the sur- BURLEY, J. M., AND B. M. MIDDLEHURST 1966. Apface) is more promising. In laboratory exparent lunar activity: Historical review. Proc. Nat. Acad. Sci. (U.S.A.)55, 1007-1011. periments (Cress et al. 1987) samples of rock with dimensions of approximately 0.01 CAMERON, W. S. 1972. Comparative analysis of observations of lunar transient phenomena. Icarus 16, m were cracked at a distance of 0.1 m from 339-387. a loop antenna whose diameter was on the CAMERON, W. S. 1975. Manifestations and possible order of 0.01 m. The radio emissions were sources of lunar transient phenomena (LTP). Moon 14, 187-199. easily detected with a signal-to-noise ratio of about 10. Since the Poynting vector falls CAMERON, W. S., AND J. J. GILHEANY 1967. Operation Moon Blink and report of observations of lunar off as the square of the distance from the transient phenomena. Icarus 7, 29-41. source, it is clear that the detected intensity CRESS, G. O., B. T. BRADY, AND G. A. ROWELL 1987. is reduced by a factor of 1012 if a rock is Sources of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys. Res. cracked at a distance of 100 km. If a large Lett. 14, 331-334. cubic block of rock I0 m on a side cracks on FAIRBRIDGE, R. W. 1972. The Encyclopedia of Geothe surface of the Moon, 106 times more chemistry and Environmental Sciences, pp. 278radiation will emerge than that from the 1 x 284. Van Nostrand, New York. 10 -6 m 3 laboratory sample. Furthermore, if FIRSOFF, V. A. 1961. Moon Atlas, p. 32 and selenographic map. Viking Press, New York. a large antenna, about 10 m in diameter, is used to detect the radio signal then, gener- GREEN, J. 1965. Tidal and gravity effects intensifying lunar defluidization and volcanism. Ann. N.Y. ally speaking, the antenna gain will increase Acad. Sci. 123, 403-469. by another factor of 106 (Stutzman and HEYMAN, D., A. YANIV, J. A. S. ADAMS, AND G. E. Thiele 1981). Therefore, the detection from FRYER 1970. Inert gases in lunar samples. Science 167, 555-558. low lunar orbit should be just as successful as detection in the laboratory. Of course, if McDOWELL, M. R. C., AND J. P. COLEMAN 1970. Introduction to the Theory of Ion Atom Collisions, state of the art detectors, amplifiers, antenpp. 306-372. North-Holland, Amsterdam. nas, and signal processing are used, then MIDDLEHURST, B. M. 1967. An analysis of lunar detection will be even easier. It is also useevents. Rev. Geophys. 5, 173-189. ful to note that the radio emissions of inter- MIDDLEHURST, B. M., AND P. A. MOORE 1967. Lunar transient phenomena: Topographical distribution. est have a wavelength between 60 and 330 Science 155, 449-451. km so that a low-flying lunar probe would ORDWAY,R. J. 1966. Earth Science, p. 78. Van Nossee a near-field radiation pattern from the trand, New York. source similar to that observed in the labo- PEITGEN, H.-O., AND P. H. RICHTER 1986. The Beauty ofFractals, p. 14. Springer-Verlag, Berlin. ratory. ACKNOWLEDGMENTS The author thanks S. N. Davis of the University of Arizona and M. J. Price of Science Applications International Corporation for their contributions to this research. S. Stanic and M. Dines of the Lockheed fibrary helped with the literature search. REFERENCES ANONYMOUS 1969. Transient Lunar Phenomena Reports from the Lunar International Observers Net-
RALOEF, J. 1988. The electric life saver effect. Science News 134(5), 78-78. SHORT, N. M. 1975. Planetary Geology, pp. 63-65. Prentice-Hall, Englewood Cliffs, NJ. SIDRAN, M. 1968. Thermoluminescence of the Moon. J. Geophys. Res. 73, 5195-5206. SmqKA, L. J. 1977. Critical velocity phenomena and the LTP. Phys. Earth Planet. Inter. 14, 321-329. STEPHENS, D. R., AND E. M. LILLEY 1970. Compressibilities of lunar crystalline rock, microbreccia, and fines to 40 kilobars. Science 167, 731-732. STUTZMAN, W. L , AND G. A. THIELE 1981. Antenna Theory and Design, p. 395. Wiley, New York.