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Upper limits to gas emission from lunar transient phenomena sites
Physics of the Earth and Planetary Interiors, 14(1977) 293—298 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
293
UPPER LIMITS TO GAS EMISSION FROM LUNAR TRANSIENT PHENOMENA SITES RICHARD R. VONDRAK Stanford Research Institute, Menlo Park, Calif 94025 (U.S.A.)
(Accepted for publication February 10, 1977)
Vondrak, R.R., 1977. Upper limits to gas emission from lunar transient phenomena sites. Phys. Earth Planet. Inter., 14: 293— 298. Lunar transient phenomena have been attributed to the release ofgases from within the moon. The failure of the Apollo surface experiments to detect significant atmospheric enhancements can be used to establish upper limits to the amount ofgases now being released from the various sites associated with lunar transient phenomena. An analysis of the sensitivity of the network of Apollo Suprathermal Ion Detector Experiments (deployed initially in 1969 and still operating) indicates that they would have detected any contemporary gas release greater than 6,500 kg from Aiphonsus, 28,000 kg from Aristarchus, and similar quantities from other craters. The quantity of gas required to cause such phenomena as obscurations or glow discharge is probably much greater than these values. Consequently, if transient phenomena are real lunar surface events, they must originate from a mechanism other than simple gas emission.
I. Introduction Transient events on the lunar surface are generally detected by ground-based observers as the obscuration of surface detail or localized changes in surface brightness (cf. reviews by Middlehurst, 1967; Cameron, 1972, and references therein). A common explanation of these phenomena is the emission or venting of gases from active sites on the lunar surface. The obscuration of a surface feature would then result from scattering of sunlight by a cloud of haze or mist, while optical emission spectra or changes in surface brightness could result from glow discharge in this gas cloud. Other possible sources of the lunar transient phenomena have been suggested: luminescence of the surface material (Cameron, 1972), possibly associated with the flow of hot lava (Kozyrev, 1962a, b; Hartmann and Harris, 1968); alteration of surface albedo by dust clouds raised by gas emission (Geake and Mills, 1976); and electrostatic glow discharge in dust clouds (Mills, 1970). Three different experiments designed to measure the lunar atmosphere and to search for transient gas releases were included as part of the Apollo Lunar Surface Experiments Package (ALSEP) on various missions.
In this paper I discuss the detectability of lunar gas releases by these instruments. In particular, the sensitivity of the Suprathermal Ion Detector Experiment (sIDE) is used to compute the smallest gas release at distant locations on the lunar surface that would be detectable by this instrument. Since none of the Apollo experin-ients have detected any unambiguous transient increases in the lunar atmosphere, upper limits can be placed on the amount of gas now being released from sites of lunar transient phenomena.
2. Sensitivity of the
ALSEP atmospheric
monitors
Three kinds of atmospheric monitors were placed on the lunar surface as part of ALSEP. The Cold Cathode Gauge Experiment (CcGE) and the Lunar Atmospheric Composition Experiment (LACE) measured the density and flux of neutral gases and the SIDE measured the flux of atmospheric ions. The CCGE and SIDE were deployed at three sites: Apollo 12 (selenographic location 3.2°S,23.4°W)in November, 1969; Apollo 14 (3.7°S,17.5°W)inFebruary, 1971; and Apollo 15 (26.l°N,3.7°E)in July, 1971. The LACE
294
was deployed only at the Apollo 17 site (20.2°N, 30.8°E)in December, 1972. The CCGE is a total pressure monitor that measures the number density of neutral gases at the lunar surface (Johnson et al., 1972a, b). Large fluctuations in the measured density have been reported, although these are attributed to outgassing from within the instrument or from the nearby Apollo lunar module. No transient increases that can be attributed to natural lunar venting have been reported. The most sensitive instrument for detection of lunar atmospheric gases is the LACE. This instrument was able to detect masses with excellent resolution and sensitivity (Hoffman et al., 1973). Unfortunately, only nine months of data were collected by LACE and useful data for most gases were obtained only during the lunar night. Gases that condense on the lunar darkside, such as CH 4, H20 and C02, were measured only for a limited time just prior to sunrise (Hoffman and Hodges, 1975). Evidence waspossibly found for a timetovariation in the 40Ar, related lunar teleseisabundance of mic events (Hodges and Hoffman, 1974). No unambiguous transient increase in the abundance of other gases was detected, except for a possible event involving the release of less than 50 kg of N 2 from a source within 300 km of Apollo 17 (R.R. Hodges, private communication, 1974). However, because of the limited operation of LACE, no definitive implications can be derived from its negative results. Each SIDE contains two ion detectors that monitor ions originating from the lunar atmosphere and from gases released during Apollo missions, as well as ions from the earth’s magnetosphere and the solar wind (Freeman et al., 1971, 1973; Lindeman et al., 1973). Important features of the SIDE network are its continued operation since November 1969, and its ability to monitor the local atmospheric density by a variety of methods (Freeman and Benson, 1977). Also, since the detectors rely on an exterior electric field to accelerate the ions, they are less sensitive to contamination due to internal instrument degassing. An extensive anal ysis of SIDE data has failed to uncover any transient increase in the lunar atmosphere (Freeman and Benson, 1977). The only exception is the March 7, 1971 event, which appears to be a singular case of unknown origin (Freeman et a!., 1973). Since an increase of atmospheric density to a level approximately five times ambient would be easily found in an examination of SIDE data,
that value can be used as an upper limit to the atmospheric increase resulting from transient gas releases since the deployment of the Apollo-12 SIDE. The SIDE measures ion flux, which is proportional to the product of ionization rate and height.integrated neutral number density (column density). The column density of the normal lunar dayside atmosphere is about 1012 atoms/cm2. A factor of five increase is 5 10~~ atoms/cm2 or 8 1012 mole/cm2. For approximate calculations, the transient gases can be assumed to have a mass of 32 a.m.u., since most vented gases should be within a factor of two of that mass. Thus, it is found that the threthold sensitivity for a sporadic gas emission to be easily detectable by SIDE is a column density of 3 10— 10 g/cm2 at any of the three SIDE sites. .
.
3. Gas transport on the lunar surface to derive usefulany implications the failureIn oforder the SIDE to detect significant from atmospheric increases, it is necessary to compute the way in which gases spread outward from a source on the lunar surface. By means of this calculation the column-density sensitivity of SIDE can be related to a total amount of gas released at any particular location. This amount will be an upper limit to gas venting since the deployment of the Apollo-12 SIDE in November, 1969. The manner in which neutral gases are transported across the lunar surface has been analyzed by several methods (Vogel, 1966; Milford and Pomilla, 1967; Hodges, 1972; Hodges et al., 1972; Hall, 1973; Vondrak, 1977). The dispersal rate and spatial distribution depend on the source intensity and duration, and the surface adhesion characteristics. Calculations also indicate that the propagation of gas clouds can be separated into two regions: the direct flux near the source, which describes the initial gas flow, and the diffusive flux whereby gases propagate to greater distances. An example of the direct flux can be simply computed for an impulsive point source that releases S particles with an isotropic Maxwellian distribution of velocities. At a time t the column density p ofpartides at a distance R is given by the expression (Lindeman et al., 1974, Appendix 1; Vondrak, 1977): 2]/27rkTt2 =
mS exp(_mR2/2kTt2)erfc~gt(m/8kfl~
(1)
295
where erfc is the error function complement defined as: erfc(x) = 1
—
(2/7r”2)
J” exp(—y2)dy
(2)
0
T is the temperature at which the gas is released, k is the Boltzmann constant, m is the molecular mass of the emitted gas, and a flat lunar surface has been assumed. This relation gives the distribution of particles until they collide with the surface. If they completely adhere to the surface, this expression is exact. However, if the emitted gases do not stick to the surface for an appreciable length of time, a diffusive description of gas transport becomes appropriate. In the case of diffusive propagation, the column density is given by (Hall, 1973): —
2
2 3
3 3
p 4 Sg exp(—4 gR hr v t)/ir v t (3) where g is the acceleration due to lunar gravity, v2 is equal to 8 kT/irm, and other quantities are as previously defined. This expression is exact only for planar, isothermal diffusion. However, more detailed computations (Hall, 1973) using a relation appropriate for spherical diffusion results in only small modifications for distances less than a lunar semicircumference. The direct flux and diffusive descriptions of gas transport differ in many respects. For example, the radial distance to the apparent density peak increases linearly with time for direct flux and is proportional to the square-root of time for diffusive transport. Also, the distance to which a gas enhancement will propagate by direct flux is severely limited by the error function complement term, which represents the fraction of particles lost through collisions with the surface. In contrast, if diffusion is possible, then particles can propagate to great distances. Measurements of the returned lunar soil samples indicate that absorption of most gases is reversible, and permanent retention (chemisorption) does not occur. This conclusion is consistent with LACE measurements of the lunar 40 Ar distribution (Hodges, 1975). Thus, it is reasonable to assume that emitted gases will propagate to great distances by diffusive transport. The transport parameter of greatest interest for present purposes is the maximum column density Pmax at an arbitrary distance from the source. This can be obtamed by differentiation of eqs. 1 and 3, and the iden—
.
.
tical result is found for both the direct flux and the diffusive transport: 2 (4) Pma=S!e7rR where e is the base of the natural logarithms. In the differentiation of eq. 1 it was assumed that mg2t2 <8kT, or that few particles had collided with the sur~ face. This result indicates that the maximum density observed at a distant point is nearly independent of propagation characteristics. Also, the relation in eq. 4 is simply exp(—1) times the result computed for cylindrical expansion at a uniform density (the factor exp(—l) arises because the greatest concentration always remains at the source). From the expression for maximum density we can compute the necessary quantity of gas released at a specific location in order to produce a column density at one of the ALSEP sites greater than the SIDE detection threshold. Such an evaluation is given in Table I for several sites where transient phenomena have been observed. Aristarchus (47°W,23°N)and its environs (Schröters Valley, Herodotus, and the Cobra Head) is the location at which more than one-third of all lunar transient events have been detected (Cameron, 1972). Alphonsus (4°W,14°S)is the site of many lunar transient events, most notably Kozyrev’s (1962a, b) observations of emission spectra and red enhancements and Alter’s (1967) photographic evidence of an obscuration covering the entire crater floor. Alter also reports many instances in which the small craters and features within the crater Plato (9°W,51°N)were obscured by a haze or mist. Finally, gaseous phenomena have been reported numerous times in the craters Gassendi (40°W,l7°S),Agrippa (ll°E,5°N),and Proclus
TABLE I Minimum gas release from lunar transient phenomena that would be detectable at the SIDE locations Location
sites
Gas release (kg) Apollo 12
Apollo 14
Apollo 15
2.8 1.1 7.1 1.0
i04 io~ i04 i04
3.6 6.5 7.0
io~ i03 10~ 10~
5.0 3.8 .. i04 i04
Plato Gassendi Agrippa Proclus
2.9 1.2
i04 i05
2.0 1.0
~ io~
1.1 3.8
Aristarchus Alphonsus
. . . . .
1.5
. . . . . .
1.6 . i04 8.4~ .
i04
296
(46°E,16°N).It is seen in Table I that for all these locations any gas venting greater than approximately
crater Alphonsus by Kozyrev (1962a, b). Kozyrev found an emission band of C2 localized in~the central
l0~kg would be detectable by one of the SIDE instruments. These estimates are only approximate because of uncertainties and simplifications made in the gas transport estimate. However, it is thought that they are accurate to within an order of magnitude.
portion of the crater. This feature was absent on other spectrographs made during the same evening. Kozyrev (l962b) estimated the quantity of gas release during this event tobe of the order of 1031 molecules contaming C2, or a minimum mass of 4. iO~kg. This quantity is an amount large enough to be easily detectable by SIDE. Had a similar event occurred in Alphonsus since the deployment of the Apollo- 12 SIDE in 1969, it would probably have been detected. The final type of transient phenomenon that will be considered is a red glow or brightening within a localized region. A possible interpretation of such an event is the heating of a small part of the lunar surface, as would result froñi a lava flow (Kozyrev, 1962a). Hartmann and Harris (1968) suggested that such an observation by Greenacre and Barr (1963) in the crater Aristarchus could be explained the heating of 2 toby a temperature of an 1 ,60( areaIfof approximately 1 kmto such a temperature, most K. lunar soils are heated of the trapped gases will be released. The mass fraction of gas trapped within lunar material is typically l0~—10~(compared to the volatile mass fraction of 3 102 in a terrestrial material). If we assume that a 1 m depth of material covering 1 km2 is heated, then about 2 1 ~ kg of gases will be released. This amount is near the threshold of detectability by the SIDE network, so such an event in Aristarchus or Alphonsus since 1969 cannot be definitely ruled out. The evaluations of each of the three types of transient phenomena are summarized in Table II.
4. Gas quantities associated with lunar transient phenomena The significance of these upper limits to gas emission can be found by evaluating the amount of gas needed for transient lunar phenomena. Three types of transient phenomena will be considered: obscurations (haze or mist), emission spectra, and gas releases associated with thermal luminescence (red glows), Lunar obscurations are frequently observed. These consistof ofthe a loss of detail or a fuzziness in a localized region moon. Examples are the “disappearance” of the small craters in the floor of the large crater Plato (area approximately 100 km2) and the photographs made by Alter (1967) of haze in the crater Aiphonsus. The localization of the obscuration is cited as evidence for a lunar origin rather than a simple degradation of “seeing” conditions in the terrestrial atmosphere. This obscuration is often described as a haze or mist. If the obscuration is due to nonresonant Rayleigh scattering, then the column density of gas above the lunar surface must be greater than about 1024 particles/cm2 (Kozyrev, 1962a). If such a quantity of mass 32 a.m.u. exists over a region of only 20 km2 then at least lO~~ kg of gas must have been released. This amount is large enough to be easily detectable by SIDE. Another famous observation of lunar transient phenomena is th~.detection of emission spectra in the
.
.
5. Discussion Since the results given here are based to some exten on a computation of the manner by which gases dis-
TABLE II Gas quantities required for transient phenomena Type of phenomenon
Gas quantity (kg)
Example
SIDE detection
10i0
Alphonsus(Alter, 1962);Plato (Alter, 1962) Alphonsus (Kozyrev, Aristarchus (Greenacre1962a, and Barr, b) 1963); Alphonsus (Kozyrev, 1962a, b)
definite probable possible
Haze (20 km2) or mist Emission spectra 2) Red glow (heating of 1 km
24
.
tO4
297
perse on the lunar surface, a few caveats are in order. For example, if the lunar materials are strongly adsorptive then lateral transport of gases may not be as efficient as assumed here. Measurements of localized concentrations of 210Po at the lunar surface by the orbital experiments on Apollo 15 and 16 (Gorenstein et al., 1973) imply that Rn, the Po parent, spreads at about 10% of the expected diffusion rate. Similarly, the LACE measurements of the diurnal concentration of 40Ar (Hodges and Hoffman, 1974) indicate that Ar spends about 80% of the time temporarily bound to the lunar surface. Such adsorption effectively reduces the column density of gases above the lunar surface by a factor of ten for Rn and five for Ar. As a result the quantities in Table I should be increased by the same factors for these gases. In the unlikely event that the surface is perfectly absorptive (chemisorptive) for the released gases such that they are permanently bound after a single collision with the surface, then only the direct flux of particles will be detectable. Also, these computations assume that the gases would be released with a velocity less than the lunar gravitational escape velocity (2.4 km/s). If all of the gases are released at a higher velocity, then they will be shot upward as a narrow conical jet and would escape detection by detectors on the lunar surface. Such a gas source could be detected only by an orbiting atmospheric monitor. The limits evaluated in this paper are for the detection of a contemporary increase in the lunar atmosphere. Since the exponential residence time for gases added to the lunar atmosphere is at least one month (Freeman et al., 1973), residual gases from a very large increase greatly exceeding the total present atmospheric mass (about iO~kg) will be observed for a long time. For this reason, upper limits can be placed in principle on the past size of the lunar atmosphere before deployment of the ALSEP instruments (Vondrak et a!., 1974). However, establishing such limits requires the modeling of the decline in density of a lunar atmosphere of arbitrary size. Such a model is currently being evaluated and will be reported elsewhere. During its seven years of operation, the SIDE network has detected only one event that is suggestive of a transient atmospheric increase of lunar origin. During a 14-h period on March 7, 1971, an intermittent series of ion bursts were detected by the SIDE instruments of Apollo 12 and 14 (Freeman et al., 1971, 1973). The fluxes were typically a factor of ten greater than that
normally observed and during a single counting cycle of 24-s duration the increase was approximately a factor of 1,000 greater than normal. The Apollo-14 SIDE mass analyzer indicated that these were water vapor ions. It is difficult to explain certain features of the March 7, event, such as the intermittent and monoenergetic nature of the detected fluxes and the singular intensity of the isolated burst. In addition, water is a substance that generally has been conspicuously absent in the returned lunar samples. However, a detailed anal. ysis of possible sources of the detected ions concluded that the water must have been of lunar origin and had not been brought to the moon by the American or Soviet lunar exploration programs (Freeman et al., 1973). Because Table I was constructed for the case of a transient increase five times greater than normal, quantities released from specific locations on the lunar surface would be twice those indicated in Table I. Thus, conclusions given in Table II are unaffected. For the single short (—24 s) intense burst during which the flux increased by a factor of 1,000, the required mass releases are 200 times that of Table I, or about io~kg for most of the sites listed. This is more than the amount needed to produce observable emission spectra but still about three orders of magnitude less than that needed to be visible as haze. Therefore, even this singular event was not sufficiently intense to require gas emission at the lunar surface in quantities sufficient to be directly visible by terrestrial observers. Also, the uniqueness of this event during over seven years of ALSEP—SIDE operation indicates that the frequency of occurrence is less than the several per year commonly reported for transient lunar phenomena (Middlehurst, 1967; Cameron, 1972).
6. Conclusions The failure of the SIDE network to detect any substantial transient increase in the lunar atmosphere (other than the exceptional event of March 7, 1971) has been used to set upper limits to the size of contemporary gas releases from specific lunar craters. These upper limits are of the order of i04 kg for most transient phenomena sites. A conclusion reached after comparing these limits to the amount of gases associated with transient phenomena is that the occurrence of haze caused by Rayleigh scattering on the lunar sur
298
face can definitely be ruled out. Other types of events, such as gas emissions that can be recorded spectroscopically but that are not so dense as to cause visual obscuration, or gas releases associated with thermal luminescence, cannot be as firmly excluded. The exclusion of simple gas emission as a mechanism for the production of lunar transient phenomena should not be interpreted as disproving the occurrence of such phenomena. For example, obscurations could arise from a gas—dust mixture, brightenings could be produced by a method that is not accompanied by a release of gas, and emission could result from an anomalous gas excitation more efficient than that considered here. However, now that atmospheric monitors have been deployed on the lunar surface, it is essential that any proposed mechanism that involves gas emission be evaluated quantitatively. The evaluations in this paper indicate that if transient phenomena are real lunar events, they must originate from a mechanism other than simple gas emission.
Hodges, R.R., 1972. Response of lunar atmosphere to volcanic gas releases. Planet. Space Sci., 20: 1849—1864. Hodges, R.R., 1975. Formation of the lunar atmosphere. Moon, 14: 139—157. Hodges, R.R. and Hoffman, J.H., 1974. Implications of atmospheric ‘WAr escape on the interior structure of the moon. In: Proc. 6th Lunar Sc Conf., Vol. 3, Pergamon, New York, N.Y., pp. 3039—3047. Hodges, R.R., Hoffman, J.H., Yeh, T.T. and Chang, G.K., 1972. Orbital search for lunar volcanism. J. Geophys. Res., 77: 4079—4085. Hoffman, J.H. and Hodges, R.R., 1975. Molecular gas species in the lunar atmosphere. Moon, 14: 159—168. Hoffman, J.H., Hodges, R.R., Johnson, F.S. and Evans, D.E., 1973. Lunar atmospheric composition results from Apollo 17. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2865—2875. Johnson, F.S., Evans, D.E. and Carroll, J.M., 1972a. Cold cathode gage experiment (lunar-atmosphere detector). In: Apollo 15 Prellm. Sci. Rep., NASA, Washington, D.C., Ch. 13. Johnson, F.S., Carroll, J.M. and Evans, D.E., 1972b. Lunar atmosphere measurements. In: D. Criswell (Editor), Proc. 3rd Lunar Sci. Conf., Vol. 3, MIT Press, Cambridge, Mass., pp. 2231—2242. Kozyrev, N.A., 1962a. Physical observations of the lunar sur-
References Alter, D., 1962. Pictorial Guide to the Moon. Crowell, New York, N.Y. Cameron,W.S., 1972. Comparative analyses of observations of lunar transient phenomena. Icarus, 16: 339—387. Freeman, J.W. and Benson, J.L., 1977. A search for gaseous emissions from the moon. Phys. Earth Planet Inter., 14: 276—281 (this volume). Freeman, J.W., Fenner, M.A., Hills, H.K., Lindeman, R.A., Medrano, R. and Meister, J., 1971. Suprathermal ions near the moon. Icarus, 16: 328. Freeman, J.W., Hills, H.K., Lindeman, R.A. and Vondrak, R.R., 1973. Observations of water vapor ions at the lunar surface. Moon, 8: 115—128. Geake, J.E. and Mifis, A.A., 1976. Possible physical processes causing transient lunar events. 7th Lunar Sci. Conf., Houston, Texas, March, 1976. Gorenstein, P., Golub, L. and Bjorkholm, P.J., 1973. Spatial features and temporal variability in the emission of radon from the moon: an interpretation of results from the alpha particle spectrometer. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 28032809. Greenacre, J.A. and Barr, E., 1963. A recent observation of lunar color phenomena. Sky Telesc., 26: 316. Hall, E.G., 1973. Role of pressure transients in the detection and identification of lunar surface gas sources. J. Geophys. Res., 78: 2111—2132. Hartmann, W.K. and Harris, D.H., 1968. Lunar volcanic eruptions near Aristarchus. Commun. Lunar Planet. Lab., 7: 161— 167.
face. In: Z. Kopal (Editor), Physics and Astronomy of the Moon. Academic Press, New York, N.Y., pp. 361—383. Kozyrev, N.A., 1962b. Spectroscopic proofs for existence of volcanic processes on the moon. In: Z. Kopal and Z.K. Mikhaiov (Editors), The Moon. Academic Press, New York, N.Y., pp. 263—271. Lindeman, R.A., Freeman, J.W. and Vondrak, R.R., 1973. Ions from the lunar atmosphere. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2889— 2896. Lindeman, R.A., Vondrak, R.R., Freeman, J.W. and Snyder, C.W., 1974. The interaction between an impact-produced neutral gas cloud and the solar wind at the lunar surface. J. Geophys. Res., 79: 2287—2296. Middlehurst, B.M., 1967. An analysis of lunar events. Rev. Geophys., 5: 173—189. Milford, S.N. and Pomilla, F.R., 1967. A diffusion model for the propagation of gases in the lunar atmosphere. J. Geophys. Res., 72: 4533—4545. Mills, A.A., 1970. Transient lunar phenomena and electrostatic glow discharges. Nature (London), 225: 929—930. Vogel, U., 1966. Molecular fluxes in the lunar atmosphere. Planet. Space Sci., 14: 1233—1252. Vondrak, R.R., 1977. Dispersal of gases released at the lunar surface. Moon (submitted). Vondrak, R.R., Freeman, J.W. and Lindeman, R.A., 1974. Measurements of lunar atmospheric loss rate. In: Proc. 5th Lunar sc. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2945—2954.
293
UPPER LIMITS TO GAS EMISSION FROM LUNAR TRANSIENT PHENOMENA SITES RICHARD R. VONDRAK Stanford Research Institute, Menlo Park, Calif 94025 (U.S.A.)
(Accepted for publication February 10, 1977)
Vondrak, R.R., 1977. Upper limits to gas emission from lunar transient phenomena sites. Phys. Earth Planet. Inter., 14: 293— 298. Lunar transient phenomena have been attributed to the release ofgases from within the moon. The failure of the Apollo surface experiments to detect significant atmospheric enhancements can be used to establish upper limits to the amount ofgases now being released from the various sites associated with lunar transient phenomena. An analysis of the sensitivity of the network of Apollo Suprathermal Ion Detector Experiments (deployed initially in 1969 and still operating) indicates that they would have detected any contemporary gas release greater than 6,500 kg from Aiphonsus, 28,000 kg from Aristarchus, and similar quantities from other craters. The quantity of gas required to cause such phenomena as obscurations or glow discharge is probably much greater than these values. Consequently, if transient phenomena are real lunar surface events, they must originate from a mechanism other than simple gas emission.
I. Introduction Transient events on the lunar surface are generally detected by ground-based observers as the obscuration of surface detail or localized changes in surface brightness (cf. reviews by Middlehurst, 1967; Cameron, 1972, and references therein). A common explanation of these phenomena is the emission or venting of gases from active sites on the lunar surface. The obscuration of a surface feature would then result from scattering of sunlight by a cloud of haze or mist, while optical emission spectra or changes in surface brightness could result from glow discharge in this gas cloud. Other possible sources of the lunar transient phenomena have been suggested: luminescence of the surface material (Cameron, 1972), possibly associated with the flow of hot lava (Kozyrev, 1962a, b; Hartmann and Harris, 1968); alteration of surface albedo by dust clouds raised by gas emission (Geake and Mills, 1976); and electrostatic glow discharge in dust clouds (Mills, 1970). Three different experiments designed to measure the lunar atmosphere and to search for transient gas releases were included as part of the Apollo Lunar Surface Experiments Package (ALSEP) on various missions.
In this paper I discuss the detectability of lunar gas releases by these instruments. In particular, the sensitivity of the Suprathermal Ion Detector Experiment (sIDE) is used to compute the smallest gas release at distant locations on the lunar surface that would be detectable by this instrument. Since none of the Apollo experin-ients have detected any unambiguous transient increases in the lunar atmosphere, upper limits can be placed on the amount of gas now being released from sites of lunar transient phenomena.
2. Sensitivity of the
ALSEP atmospheric
monitors
Three kinds of atmospheric monitors were placed on the lunar surface as part of ALSEP. The Cold Cathode Gauge Experiment (CcGE) and the Lunar Atmospheric Composition Experiment (LACE) measured the density and flux of neutral gases and the SIDE measured the flux of atmospheric ions. The CCGE and SIDE were deployed at three sites: Apollo 12 (selenographic location 3.2°S,23.4°W)in November, 1969; Apollo 14 (3.7°S,17.5°W)inFebruary, 1971; and Apollo 15 (26.l°N,3.7°E)in July, 1971. The LACE
294
was deployed only at the Apollo 17 site (20.2°N, 30.8°E)in December, 1972. The CCGE is a total pressure monitor that measures the number density of neutral gases at the lunar surface (Johnson et al., 1972a, b). Large fluctuations in the measured density have been reported, although these are attributed to outgassing from within the instrument or from the nearby Apollo lunar module. No transient increases that can be attributed to natural lunar venting have been reported. The most sensitive instrument for detection of lunar atmospheric gases is the LACE. This instrument was able to detect masses with excellent resolution and sensitivity (Hoffman et al., 1973). Unfortunately, only nine months of data were collected by LACE and useful data for most gases were obtained only during the lunar night. Gases that condense on the lunar darkside, such as CH 4, H20 and C02, were measured only for a limited time just prior to sunrise (Hoffman and Hodges, 1975). Evidence waspossibly found for a timetovariation in the 40Ar, related lunar teleseisabundance of mic events (Hodges and Hoffman, 1974). No unambiguous transient increase in the abundance of other gases was detected, except for a possible event involving the release of less than 50 kg of N 2 from a source within 300 km of Apollo 17 (R.R. Hodges, private communication, 1974). However, because of the limited operation of LACE, no definitive implications can be derived from its negative results. Each SIDE contains two ion detectors that monitor ions originating from the lunar atmosphere and from gases released during Apollo missions, as well as ions from the earth’s magnetosphere and the solar wind (Freeman et al., 1971, 1973; Lindeman et al., 1973). Important features of the SIDE network are its continued operation since November 1969, and its ability to monitor the local atmospheric density by a variety of methods (Freeman and Benson, 1977). Also, since the detectors rely on an exterior electric field to accelerate the ions, they are less sensitive to contamination due to internal instrument degassing. An extensive anal ysis of SIDE data has failed to uncover any transient increase in the lunar atmosphere (Freeman and Benson, 1977). The only exception is the March 7, 1971 event, which appears to be a singular case of unknown origin (Freeman et a!., 1973). Since an increase of atmospheric density to a level approximately five times ambient would be easily found in an examination of SIDE data,
that value can be used as an upper limit to the atmospheric increase resulting from transient gas releases since the deployment of the Apollo-12 SIDE. The SIDE measures ion flux, which is proportional to the product of ionization rate and height.integrated neutral number density (column density). The column density of the normal lunar dayside atmosphere is about 1012 atoms/cm2. A factor of five increase is 5 10~~ atoms/cm2 or 8 1012 mole/cm2. For approximate calculations, the transient gases can be assumed to have a mass of 32 a.m.u., since most vented gases should be within a factor of two of that mass. Thus, it is found that the threthold sensitivity for a sporadic gas emission to be easily detectable by SIDE is a column density of 3 10— 10 g/cm2 at any of the three SIDE sites. .
.
3. Gas transport on the lunar surface to derive usefulany implications the failureIn oforder the SIDE to detect significant from atmospheric increases, it is necessary to compute the way in which gases spread outward from a source on the lunar surface. By means of this calculation the column-density sensitivity of SIDE can be related to a total amount of gas released at any particular location. This amount will be an upper limit to gas venting since the deployment of the Apollo-12 SIDE in November, 1969. The manner in which neutral gases are transported across the lunar surface has been analyzed by several methods (Vogel, 1966; Milford and Pomilla, 1967; Hodges, 1972; Hodges et al., 1972; Hall, 1973; Vondrak, 1977). The dispersal rate and spatial distribution depend on the source intensity and duration, and the surface adhesion characteristics. Calculations also indicate that the propagation of gas clouds can be separated into two regions: the direct flux near the source, which describes the initial gas flow, and the diffusive flux whereby gases propagate to greater distances. An example of the direct flux can be simply computed for an impulsive point source that releases S particles with an isotropic Maxwellian distribution of velocities. At a time t the column density p ofpartides at a distance R is given by the expression (Lindeman et al., 1974, Appendix 1; Vondrak, 1977): 2]/27rkTt2 =
mS exp(_mR2/2kTt2)erfc~gt(m/8kfl~
(1)
295
where erfc is the error function complement defined as: erfc(x) = 1
—
(2/7r”2)
J” exp(—y2)dy
(2)
0
T is the temperature at which the gas is released, k is the Boltzmann constant, m is the molecular mass of the emitted gas, and a flat lunar surface has been assumed. This relation gives the distribution of particles until they collide with the surface. If they completely adhere to the surface, this expression is exact. However, if the emitted gases do not stick to the surface for an appreciable length of time, a diffusive description of gas transport becomes appropriate. In the case of diffusive propagation, the column density is given by (Hall, 1973): —
2
2 3
3 3
p 4 Sg exp(—4 gR hr v t)/ir v t (3) where g is the acceleration due to lunar gravity, v2 is equal to 8 kT/irm, and other quantities are as previously defined. This expression is exact only for planar, isothermal diffusion. However, more detailed computations (Hall, 1973) using a relation appropriate for spherical diffusion results in only small modifications for distances less than a lunar semicircumference. The direct flux and diffusive descriptions of gas transport differ in many respects. For example, the radial distance to the apparent density peak increases linearly with time for direct flux and is proportional to the square-root of time for diffusive transport. Also, the distance to which a gas enhancement will propagate by direct flux is severely limited by the error function complement term, which represents the fraction of particles lost through collisions with the surface. In contrast, if diffusion is possible, then particles can propagate to great distances. Measurements of the returned lunar soil samples indicate that absorption of most gases is reversible, and permanent retention (chemisorption) does not occur. This conclusion is consistent with LACE measurements of the lunar 40 Ar distribution (Hodges, 1975). Thus, it is reasonable to assume that emitted gases will propagate to great distances by diffusive transport. The transport parameter of greatest interest for present purposes is the maximum column density Pmax at an arbitrary distance from the source. This can be obtamed by differentiation of eqs. 1 and 3, and the iden—
.
.
tical result is found for both the direct flux and the diffusive transport: 2 (4) Pma=S!e7rR where e is the base of the natural logarithms. In the differentiation of eq. 1 it was assumed that mg2t2 <8kT, or that few particles had collided with the sur~ face. This result indicates that the maximum density observed at a distant point is nearly independent of propagation characteristics. Also, the relation in eq. 4 is simply exp(—1) times the result computed for cylindrical expansion at a uniform density (the factor exp(—l) arises because the greatest concentration always remains at the source). From the expression for maximum density we can compute the necessary quantity of gas released at a specific location in order to produce a column density at one of the ALSEP sites greater than the SIDE detection threshold. Such an evaluation is given in Table I for several sites where transient phenomena have been observed. Aristarchus (47°W,23°N)and its environs (Schröters Valley, Herodotus, and the Cobra Head) is the location at which more than one-third of all lunar transient events have been detected (Cameron, 1972). Alphonsus (4°W,14°S)is the site of many lunar transient events, most notably Kozyrev’s (1962a, b) observations of emission spectra and red enhancements and Alter’s (1967) photographic evidence of an obscuration covering the entire crater floor. Alter also reports many instances in which the small craters and features within the crater Plato (9°W,51°N)were obscured by a haze or mist. Finally, gaseous phenomena have been reported numerous times in the craters Gassendi (40°W,l7°S),Agrippa (ll°E,5°N),and Proclus
TABLE I Minimum gas release from lunar transient phenomena that would be detectable at the SIDE locations Location
sites
Gas release (kg) Apollo 12
Apollo 14
Apollo 15
2.8 1.1 7.1 1.0
i04 io~ i04 i04
3.6 6.5 7.0
io~ i03 10~ 10~
5.0 3.8 .. i04 i04
Plato Gassendi Agrippa Proclus
2.9 1.2
i04 i05
2.0 1.0
~ io~
1.1 3.8
Aristarchus Alphonsus
. . . . .
1.5
. . . . . .
1.6 . i04 8.4~ .
i04
296
(46°E,16°N).It is seen in Table I that for all these locations any gas venting greater than approximately
crater Alphonsus by Kozyrev (1962a, b). Kozyrev found an emission band of C2 localized in~the central
l0~kg would be detectable by one of the SIDE instruments. These estimates are only approximate because of uncertainties and simplifications made in the gas transport estimate. However, it is thought that they are accurate to within an order of magnitude.
portion of the crater. This feature was absent on other spectrographs made during the same evening. Kozyrev (l962b) estimated the quantity of gas release during this event tobe of the order of 1031 molecules contaming C2, or a minimum mass of 4. iO~kg. This quantity is an amount large enough to be easily detectable by SIDE. Had a similar event occurred in Alphonsus since the deployment of the Apollo- 12 SIDE in 1969, it would probably have been detected. The final type of transient phenomenon that will be considered is a red glow or brightening within a localized region. A possible interpretation of such an event is the heating of a small part of the lunar surface, as would result froñi a lava flow (Kozyrev, 1962a). Hartmann and Harris (1968) suggested that such an observation by Greenacre and Barr (1963) in the crater Aristarchus could be explained the heating of 2 toby a temperature of an 1 ,60( areaIfof approximately 1 kmto such a temperature, most K. lunar soils are heated of the trapped gases will be released. The mass fraction of gas trapped within lunar material is typically l0~—10~(compared to the volatile mass fraction of 3 102 in a terrestrial material). If we assume that a 1 m depth of material covering 1 km2 is heated, then about 2 1 ~ kg of gases will be released. This amount is near the threshold of detectability by the SIDE network, so such an event in Aristarchus or Alphonsus since 1969 cannot be definitely ruled out. The evaluations of each of the three types of transient phenomena are summarized in Table II.
4. Gas quantities associated with lunar transient phenomena The significance of these upper limits to gas emission can be found by evaluating the amount of gas needed for transient lunar phenomena. Three types of transient phenomena will be considered: obscurations (haze or mist), emission spectra, and gas releases associated with thermal luminescence (red glows), Lunar obscurations are frequently observed. These consistof ofthe a loss of detail or a fuzziness in a localized region moon. Examples are the “disappearance” of the small craters in the floor of the large crater Plato (area approximately 100 km2) and the photographs made by Alter (1967) of haze in the crater Aiphonsus. The localization of the obscuration is cited as evidence for a lunar origin rather than a simple degradation of “seeing” conditions in the terrestrial atmosphere. This obscuration is often described as a haze or mist. If the obscuration is due to nonresonant Rayleigh scattering, then the column density of gas above the lunar surface must be greater than about 1024 particles/cm2 (Kozyrev, 1962a). If such a quantity of mass 32 a.m.u. exists over a region of only 20 km2 then at least lO~~ kg of gas must have been released. This amount is large enough to be easily detectable by SIDE. Another famous observation of lunar transient phenomena is th~.detection of emission spectra in the
.
.
5. Discussion Since the results given here are based to some exten on a computation of the manner by which gases dis-
TABLE II Gas quantities required for transient phenomena Type of phenomenon
Gas quantity (kg)
Example
SIDE detection
10i0
Alphonsus(Alter, 1962);Plato (Alter, 1962) Alphonsus (Kozyrev, Aristarchus (Greenacre1962a, and Barr, b) 1963); Alphonsus (Kozyrev, 1962a, b)
definite probable possible
Haze (20 km2) or mist Emission spectra 2) Red glow (heating of 1 km
24
.
tO4
297
perse on the lunar surface, a few caveats are in order. For example, if the lunar materials are strongly adsorptive then lateral transport of gases may not be as efficient as assumed here. Measurements of localized concentrations of 210Po at the lunar surface by the orbital experiments on Apollo 15 and 16 (Gorenstein et al., 1973) imply that Rn, the Po parent, spreads at about 10% of the expected diffusion rate. Similarly, the LACE measurements of the diurnal concentration of 40Ar (Hodges and Hoffman, 1974) indicate that Ar spends about 80% of the time temporarily bound to the lunar surface. Such adsorption effectively reduces the column density of gases above the lunar surface by a factor of ten for Rn and five for Ar. As a result the quantities in Table I should be increased by the same factors for these gases. In the unlikely event that the surface is perfectly absorptive (chemisorptive) for the released gases such that they are permanently bound after a single collision with the surface, then only the direct flux of particles will be detectable. Also, these computations assume that the gases would be released with a velocity less than the lunar gravitational escape velocity (2.4 km/s). If all of the gases are released at a higher velocity, then they will be shot upward as a narrow conical jet and would escape detection by detectors on the lunar surface. Such a gas source could be detected only by an orbiting atmospheric monitor. The limits evaluated in this paper are for the detection of a contemporary increase in the lunar atmosphere. Since the exponential residence time for gases added to the lunar atmosphere is at least one month (Freeman et al., 1973), residual gases from a very large increase greatly exceeding the total present atmospheric mass (about iO~kg) will be observed for a long time. For this reason, upper limits can be placed in principle on the past size of the lunar atmosphere before deployment of the ALSEP instruments (Vondrak et a!., 1974). However, establishing such limits requires the modeling of the decline in density of a lunar atmosphere of arbitrary size. Such a model is currently being evaluated and will be reported elsewhere. During its seven years of operation, the SIDE network has detected only one event that is suggestive of a transient atmospheric increase of lunar origin. During a 14-h period on March 7, 1971, an intermittent series of ion bursts were detected by the SIDE instruments of Apollo 12 and 14 (Freeman et al., 1971, 1973). The fluxes were typically a factor of ten greater than that
normally observed and during a single counting cycle of 24-s duration the increase was approximately a factor of 1,000 greater than normal. The Apollo-14 SIDE mass analyzer indicated that these were water vapor ions. It is difficult to explain certain features of the March 7, event, such as the intermittent and monoenergetic nature of the detected fluxes and the singular intensity of the isolated burst. In addition, water is a substance that generally has been conspicuously absent in the returned lunar samples. However, a detailed anal. ysis of possible sources of the detected ions concluded that the water must have been of lunar origin and had not been brought to the moon by the American or Soviet lunar exploration programs (Freeman et al., 1973). Because Table I was constructed for the case of a transient increase five times greater than normal, quantities released from specific locations on the lunar surface would be twice those indicated in Table I. Thus, conclusions given in Table II are unaffected. For the single short (—24 s) intense burst during which the flux increased by a factor of 1,000, the required mass releases are 200 times that of Table I, or about io~kg for most of the sites listed. This is more than the amount needed to produce observable emission spectra but still about three orders of magnitude less than that needed to be visible as haze. Therefore, even this singular event was not sufficiently intense to require gas emission at the lunar surface in quantities sufficient to be directly visible by terrestrial observers. Also, the uniqueness of this event during over seven years of ALSEP—SIDE operation indicates that the frequency of occurrence is less than the several per year commonly reported for transient lunar phenomena (Middlehurst, 1967; Cameron, 1972).
6. Conclusions The failure of the SIDE network to detect any substantial transient increase in the lunar atmosphere (other than the exceptional event of March 7, 1971) has been used to set upper limits to the size of contemporary gas releases from specific lunar craters. These upper limits are of the order of i04 kg for most transient phenomena sites. A conclusion reached after comparing these limits to the amount of gases associated with transient phenomena is that the occurrence of haze caused by Rayleigh scattering on the lunar sur
298
face can definitely be ruled out. Other types of events, such as gas emissions that can be recorded spectroscopically but that are not so dense as to cause visual obscuration, or gas releases associated with thermal luminescence, cannot be as firmly excluded. The exclusion of simple gas emission as a mechanism for the production of lunar transient phenomena should not be interpreted as disproving the occurrence of such phenomena. For example, obscurations could arise from a gas—dust mixture, brightenings could be produced by a method that is not accompanied by a release of gas, and emission could result from an anomalous gas excitation more efficient than that considered here. However, now that atmospheric monitors have been deployed on the lunar surface, it is essential that any proposed mechanism that involves gas emission be evaluated quantitatively. The evaluations in this paper indicate that if transient phenomena are real lunar events, they must originate from a mechanism other than simple gas emission.
Hodges, R.R., 1972. Response of lunar atmosphere to volcanic gas releases. Planet. Space Sci., 20: 1849—1864. Hodges, R.R., 1975. Formation of the lunar atmosphere. Moon, 14: 139—157. Hodges, R.R. and Hoffman, J.H., 1974. Implications of atmospheric ‘WAr escape on the interior structure of the moon. In: Proc. 6th Lunar Sc Conf., Vol. 3, Pergamon, New York, N.Y., pp. 3039—3047. Hodges, R.R., Hoffman, J.H., Yeh, T.T. and Chang, G.K., 1972. Orbital search for lunar volcanism. J. Geophys. Res., 77: 4079—4085. Hoffman, J.H. and Hodges, R.R., 1975. Molecular gas species in the lunar atmosphere. Moon, 14: 159—168. Hoffman, J.H., Hodges, R.R., Johnson, F.S. and Evans, D.E., 1973. Lunar atmospheric composition results from Apollo 17. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2865—2875. Johnson, F.S., Evans, D.E. and Carroll, J.M., 1972a. Cold cathode gage experiment (lunar-atmosphere detector). In: Apollo 15 Prellm. Sci. Rep., NASA, Washington, D.C., Ch. 13. Johnson, F.S., Carroll, J.M. and Evans, D.E., 1972b. Lunar atmosphere measurements. In: D. Criswell (Editor), Proc. 3rd Lunar Sci. Conf., Vol. 3, MIT Press, Cambridge, Mass., pp. 2231—2242. Kozyrev, N.A., 1962a. Physical observations of the lunar sur-
References Alter, D., 1962. Pictorial Guide to the Moon. Crowell, New York, N.Y. Cameron,W.S., 1972. Comparative analyses of observations of lunar transient phenomena. Icarus, 16: 339—387. Freeman, J.W. and Benson, J.L., 1977. A search for gaseous emissions from the moon. Phys. Earth Planet Inter., 14: 276—281 (this volume). Freeman, J.W., Fenner, M.A., Hills, H.K., Lindeman, R.A., Medrano, R. and Meister, J., 1971. Suprathermal ions near the moon. Icarus, 16: 328. Freeman, J.W., Hills, H.K., Lindeman, R.A. and Vondrak, R.R., 1973. Observations of water vapor ions at the lunar surface. Moon, 8: 115—128. Geake, J.E. and Mifis, A.A., 1976. Possible physical processes causing transient lunar events. 7th Lunar Sci. Conf., Houston, Texas, March, 1976. Gorenstein, P., Golub, L. and Bjorkholm, P.J., 1973. Spatial features and temporal variability in the emission of radon from the moon: an interpretation of results from the alpha particle spectrometer. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 28032809. Greenacre, J.A. and Barr, E., 1963. A recent observation of lunar color phenomena. Sky Telesc., 26: 316. Hall, E.G., 1973. Role of pressure transients in the detection and identification of lunar surface gas sources. J. Geophys. Res., 78: 2111—2132. Hartmann, W.K. and Harris, D.H., 1968. Lunar volcanic eruptions near Aristarchus. Commun. Lunar Planet. Lab., 7: 161— 167.
face. In: Z. Kopal (Editor), Physics and Astronomy of the Moon. Academic Press, New York, N.Y., pp. 361—383. Kozyrev, N.A., 1962b. Spectroscopic proofs for existence of volcanic processes on the moon. In: Z. Kopal and Z.K. Mikhaiov (Editors), The Moon. Academic Press, New York, N.Y., pp. 263—271. Lindeman, R.A., Freeman, J.W. and Vondrak, R.R., 1973. Ions from the lunar atmosphere. In: Proc. 4th Lunar Sci. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2889— 2896. Lindeman, R.A., Vondrak, R.R., Freeman, J.W. and Snyder, C.W., 1974. The interaction between an impact-produced neutral gas cloud and the solar wind at the lunar surface. J. Geophys. Res., 79: 2287—2296. Middlehurst, B.M., 1967. An analysis of lunar events. Rev. Geophys., 5: 173—189. Milford, S.N. and Pomilla, F.R., 1967. A diffusion model for the propagation of gases in the lunar atmosphere. J. Geophys. Res., 72: 4533—4545. Mills, A.A., 1970. Transient lunar phenomena and electrostatic glow discharges. Nature (London), 225: 929—930. Vogel, U., 1966. Molecular fluxes in the lunar atmosphere. Planet. Space Sci., 14: 1233—1252. Vondrak, R.R., 1977. Dispersal of gases released at the lunar surface. Moon (submitted). Vondrak, R.R., Freeman, J.W. and Lindeman, R.A., 1974. Measurements of lunar atmospheric loss rate. In: Proc. 5th Lunar sc. Conf., Vol. 3, Pergamon, New York, N.Y., pp. 2945—2954.