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The lunar atmosphere
Ic~vs
2 1 , 4 1 5 - 4 2 6 (1974)
The Lunar Atmosphere R. R. HODGES, Jr., J. H. HOFFMAN, AND FRANCIS S. J O H N S O N The University of Texas at Dallas, Dallas, Texas 75230 R e c e i v e d O c t o b e r 24, 1973; r e v i s e d D e c e m b e r , 11 1973 I n c o n t r a s t t o e a r t h , t h e a t m o s p h e r e of t h e m o o n is e x c e e d i n g l y t e n u o u s a n d a p p e a r s to consist m a i n l y of n o b l e gases. T h e solar w i n d i m p i n g e s o n t h e l u n a r surface, s u p p l y i n g d e t e c t a b l e a m o u n t s of h e l i u m , n e o n a n d 3~Ar. I n f l u x e s of solar w i n d p r o t o n s a n d c a r b o n a n d n i t r o g e n ions are significant, b u t a t m o s p h e r i c gases c o n t a i n i n g t h e s e e l e m e n t s h a v e n o t b e e n p o s i t i v e l y identified. R a d i o g e n i c 4°Ar a n d 222Rn p r o d u c e d w i t h i n t h e m o o n h a v e b e e n d e t e c t e d . T h e p r e s e n t r a t e o f effusion of a r g o n f r o m t h e m o o n a c c o u n t s for a b o u t 0 . 4 % of t h e t o t a l p r o d u c t i o n of 4°Ar d u e to d e c a y o f 4°K i f t h e a v e r a g e a b u n d a n c e of p o t a s s i u m i n t h e m o o n is 1 0 0 0 p p m . L a c k of w e a t h e r i n g processes i n t h e r e g o l i t h suggests t h a t m o s t of t h e a t m o s p h e r i c 4°Ar o r i g i n a t e s deep in t h e l u n a r interior, p e r h a p s i n a p a r t i a l l y m o l t e n core. I f so, o t h e r gases m a y b e v e n t e d a l o n g w i t h t h e argon.
The atmosphere of the moon is so tenuous t h a t it can be regarded as a collisionless exosphere in which atoms and molecules are gravitationally bound in ballistic trajectories between encounters with the lunar surface. Despite the small amount of gas, the vestige of atmosphere is an important indicator of lunar processes which produce atmospheric gases. Direct measurements of lunar gases by means of the Apollo cold cathode gage and mass spectrometer experiments, along with anciliary data from alpha particle and ultraviolet spectrometers and analyses of returned lunar samples, provide a basis for study of the relationship between the moon and its atmosphere. Lack of a significant amount of lunar atmosphere can be attributed mainly to an efficient escape mechanism for particles t h a t are too h ea vy to escape thermally. Neutral gas molecules or atoms are photoionized by solar radiation and then, in the absence of significant magnetization of the moon, the v × B field of the impinging solar wind accelerates the resultant ions. Roughly half of these ions impact the lunar surface, but the other half escape. The geomagnetic field inhibits this escape Copyright © 1974by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
415
process over the earth, while induced ionospheric currents apparently cause a deflection of the solar wind around both Mars and Venus, thus causing ions produced below the deflection boundary to remain in the planetary atmosphere. The present tenuous state of the lunar atmosphere is not inherent to the size, mass or orbit of the moon. Given a rate of volcanic release of gases in excess of the photoionization loss rate, a dense atmosphere would form. To understand this possibility, it is useful to consider the physics of a hypothetical dense atmosphere on the moon, of sufficient depth to form an ionosphere t h a t is not in contact with the lunar surface. Gases which cannot escape thermally (i.e., gases other than hydrogen and helium) would presumably make up the bulk of this atmosphere. The photoescape rate would be limited to photoions formed above the level where ionospheric currents produce an ionosphere-solar wind boundary. This escape rate is also a lower bound on the rate of degassing of the moon needed to form a dense atmosphere, since surface chemical processes could also act as a sink. The relationship of the surface loss processes to the excess gas release rate
416
HODGES, HOFFI~IAN, AND JOHNSON
would determine the surface concentration and hence the height of the region where the solar wind deflection occurs. An a p p r o x i m a t i o n to the m a x i m u m rate of loss of lunar atmosphere is found b y integration of the rate of photoionization over the sunlit side of the moon. The resulting mass outflow of a specific gas is 27rR 2 n m H / r where R is the radius of the ionosphere-solar wind b o u n d a r y , n is concentration at t h a t level, m is molecular mass, H is d a y t i m e scale height, and r is photoionization time. Since the p r o d u c t m H is i n d e p e n d e n t of molecular mass, the t o t a l loss rate is also given b y the above expression if n is total neutral concentration and r is an average photoionization time. Assuming t h a t the solar wind is deflected near the base of the exosphere of the h y p o t h e t i c a l atmosphere in question, the concentration n should be a b o u t l08 cm -3. Making the f u r t h e r assumptions of an average exobase t e m p e r a t u r e of 1000°K and a photoionization time of 107sec, the net mass loss rate would be 1.7 × 103g/sec, or a global average loss rate per unit area of roughly 4 × 10-~Sgcm -2 sec -I. F o r purposes of comparison, the volcanic release rate, if at the same rate per unit p l a n e t a r y mass as for the earth, would be a b o u t 3 × 10-1:gcm-2sec-~ of H 2 0 and 5 × 10-13gcm-2sec -l of CO 2 (Johnson, 1969 ; 1971). Thus the absence of a lunar atmosphere is evidence t h a t volcanic a c t i v i t y on the moon is at least several orders of m a g n i t u d e less intense per unit mass t h a n on earth. The above a r g u m e n t does not rule out presently active lunar volcanism, b u t it does set a limit on the degassing rate over geologic time scales. However, based on Apollo 14 and 15 cold cathode gage d a t a (Johnson et al., 1972) a present upper b o u n d of a b o u t 10-16gcm-2sec -1 has been established b y Hodges et al. (1972a). Since the cold cathode gage d a t a obtained during lunar d a y t i m e m a y be m a i n l y due to artifact gases released from r e m n a n t spaceflight hardware, it is entirely possible t h a t this u p p e r b o u n d greatly over-estimates the actual degassing rate of the moon. Subsequent discussion of spectrometric d a t a will show t h a t some gases are present-
ly evolving from the moon, and hence t h a t a lower b o u n d can be placed on the release rate. I n addition to the release of lunar gases, the impinging solar wind is an i m p o r t a n t and predictable source of atmosphere. Solar wind ions i m p a c t the m o o n with energies the order of 1 keV per a m u and become imbedded in surface rocks. The soil is p r o b a b l y s a t u r a t e d with most solar wind gases, and a balance of ion inflow and neutral effusion from the lunar surface p r o b a b l y exists. This hypothesis is verified b y the close balance between the solar wind influx and the lunar atmospheric content of helium, 2°Ne, and 36At which will be discussed later. Owing to the small a m o u n t of ambient gas on the moon, species identification is greatly complicated b y c o n t a m i n a n t s of spacecraft origin. A distinguishing feature of a native gas is the diurnal tidal oscillation of its concentration, a collective effect t h a t reflects statistics of encounters of particles with the moon b u t not with each other. Atmospheric atoms and molecules travel in ballistic trajectories between encounters with the lunar surface. Neglecting adsorption or chemical reaction at the surface, global t r a n s p o r t of gases is similar to a two-dimensional r a n d o m walk process, the mean step size being a p p r o x i m a t e l y equal to two scale heights, and therefore proportional to t e m p e r a t u r e . Fig. 1 shows a portion of the p a t h of a h y p o t h e t i c a l a t o m
F~G. 1. P a t h of a h y p o t h e t i c a l a t o m of t h e lunar atmosphere.
LUNAR ATMOSPHERE
on the moon. Trajectories are generally longer over the warm sunlit side, resulting in the tendency for migration of particles to the colder nighttime side to be greater than t h a t from night to day. Equilibrium of lateral transport requires t h a t the distribution of a gas vary in concentration roughly as the -5/2 power of temperature (Hodges and Johnson, 1968; see also recent extensions of the theory by Hodges, 1972 and 1973a). Since the day-to-night temperature ratio is about 4 : 1, the expected nighttime gas concentration is approximately 32 times that in daytime. Gases which are adsorbed on the nighttime side of the moon have both nighttime and daytime minima, the former being due to surface adsorption, and the latter, to the tendency of gases to avoid a temperature maximum. This leaves a meridional maximum at the terminator. Near sunrise, where the adsorbed gases are released as the surface warms, the maximum concentration is significantly greater than t h a t at sunset.
417
ATMOSPHERIC CONSTITUENTS A summary of present knowledge of lunar atmospheric parameters is given in Table I. Theoretical values for hydrogen and helium are based on calculations by Hodges (1973b), which are in close agreement with independent calculations of Hartle and Thomas (1973). Daytime concentrations of hydrogen and helium may be expressed in two ways--in terms of downcoming bound particles which have completed at least one ballistic trajectory or as the total of bound and newly created upgoing molecules in initial trajectories. Hodges has related the bound particle definition of concentration to the downcoming molecular flux measured by the Apollo 17 lunar surface mass spectrometer, while Hartle and Thomas were concerned with the total concentration which is appropriate to analyses of optical data from the Apollo 17 orbital ultraviolet spectrometer. Owing to long residence
TABLE
I
SUMMARY OF LUNAR ATMOSPttERE PARAMETERS H Solar wind influx (ions/see) Lunar venting (atoms/see) Photoionization time (see) Residence time
H2
2.8 × 102s
4He
2° N e
36Ar
4°Ar
1.3 x 1024
2.2 x 1021
8.0 x 1019
--
--
--
.
10 7
10 7
10 v
6 x l 06
1.2 × 103
7 x 103
8 x 104
4 x 10 v
1.7 x 10 3 1.9 x 10 3 4 x 10 4
4 x 10 3 1.1 x lO s
.
.
.
8.7 x 1020 1.6 x 106
1.6 x 106
10 v
10 v
1.3 x 10 z 3 × 10 3a
1.6 × 10 3 4 x 10 aa
3 x l O 3a
4 x I 0 4a
(see)" Concentration
(cm-3): (lay Theory
bound total
night Experiment b day night
6 x 10 *c 2.7 × 10 ac 1.6 × 103c
2 3.5 1.2 <6 <3.5
× × x × x
l0 3 10 a 10* l0 3 10*
2 x I0 a
--
4 × I0 +
lO s
" Hydrogen and helium escape thermally, while photoionization controls lifetimes of the other gases. b D a y t i m e u p p e r b o u n d s o n H a n d H 2 a r e A p o l l o 17 o r b i t a l u l t r a v i o l e t s p e c t r o m e t e r r e s u l t s ( F a s t i e et al., 1973) w h i l e t h e r e m a i n i n g d a t a a r e f r o m A p o l l o m a s s s p e c t r o m e t e r e x p e r i m e n t s . c Amounts that would be present if released in atoinic rather than molecular form. a Sunrise terminator maxima are given for argon. Surface adsorption removes most of the nighttime argon. 15
418
ttODGES~ HOFFMAlg~ AND JOI-INSON
times for neon and argon, differences between their respective bound and total concentrations are negligible. Amounts of helium, a°Ne and 36Ar, which are known from Apollo 16 (Hodges et al., 1973), and Apollo 17 mass spectrometer data (Hoffman et al., 1973), are in balance with the solar wind influxes of these species. The lack of a large accumulation of hydrogen in the lunar soil suggests t h a t the solar wind influx of protons is similarly converted to a neutral gas, presumably H2, to equalize rates of accretion and escape. B y analogy, it is reasonable to expect t h a t the carbon and nitrogen influxes from the solar wind are also balanced by atmospheric escape, probably as methane and ammonia, respectively. The presence of excess amounts of 4°Ar trapped in returned lunar samples has been recognized as evidence of 4°Ar as an atmospheric gas (Manka and Michel, 1971 ). More recently, 4°Ar has been identified in the lunar atmosphere by the Apollo 17 mass spectrometer (Hodges et al., 1973; Hoffinan et al., 1973). Since the only known source of 4°Ar is radiogenic decay of 4°K within the moon, its presence in the atmosphere is evidence of a venting or degassing process which m ay involve other gases. The a-particle experiments on Apollo 15 and Apollo 16 have shown evidence of atmospheric 222Rn and its long lived daughter zl°po (Bjorkholm et al., 1973; Gorenstein et al., 1973). One interpretation of an imbalance of radon and polonium in the a-particle data is t h a t sporadic venting of other gases m a y cause spatial and temporal fluctuations in the rate of effusion of radon. Whether venting rates of 4°Ar and 222Rn are related is speculative at this time. Subsequent sections on hydrogen, helium, neon and argon delineate the origins and geophysical implications of the data summarized in Table I. Additional discussion is presented on other candidate gases of the lunar atmosphere which have not been detected. Reasons to expect other gases include the large, mainly artifact, daytime gas levels detected by the Apollo 14 and Apollo 15 cold cathode gages and b y the Apollo 17 mass spectrometer, which
could hide some ambient, constituents.
condensible
HYDROGEN
Existence of atomic or molecular hydrogen in the lunar atmosphere seems necessary to balance the escape of hydrogen with the large solar wind influx of protons (~3 x 108em-/see-l) which impinges on the daytime side of the moon. Johnson et al. (1972), made a rough estimate that the surface concentration of H should be 5 x 103cm -3 if hydrogen remained in atomic form. A more detailed calculation by Hodges (1973b) gave a nighttime maximum concentration of 1.6 x l03 cm -3 and a daytime minimum of 6 × 102cm -3. An independent calculation by Hartle and Thomas (1973) is in close agreement if account is taken of the subtle difference in the definition of concentration adopted in the former study and t hat used by Hodges. However, the Apollo 17 orbital ultraviolet spectrometer showed t hat the daytime concentration of H is less than l0 atoms cm -3 (Fastie et al., 1973), suggesting t h a t the bulk of the solar wind hydrogen influx must reappear in molecular form. Figure 2 shows the global distribution of bound H 2 from the Monte Carlo calculations of Hodges (1973b). The results are displayed as the average concentration in each of 75 equi-area zones covering one hemisphere of the moon. These zones are arranged in circumferential bands which are centered at the latitudes indicated on the graphs. The global temperature model used in these calculations was developed b y M. G. Langseth and S. J. Keihm (private communication). Line widths in Fig. 2 give the range of concentrations resulting from a ±5°K uncertainty in the nighttime temperature model. The amount of atmospheric H 2 was established by balancing the total rate of thermal escape of hydrogen with the rate of impact of solar wind protons on the moon. New molecules were assumed to evolve only from the daytime lunar surface, with the outflow rate of H 2 molecules equal to the local solar wind influx of proton pairs. In Fig. 2 the daytime minimum concentration
419
LUNAR ATMOSPHERE ~DA~"
:]:
SUNSET
15
NIGHT~DAY SUNRISE
~ 71.2 ° "
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o
I
I
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%
5 o
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l
I 39.8"
IO 5
o
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l
~,o5~ 0
(..)
23.6*-
15
o
HELIUM I
I
I
15
7.7"-
5 0
O*
level, rather t han as an upper bound possibly influenced by contamination, must be tempered by the subsequent discussion of the distribution of lunar helium, which is quite accurately predicted by the theory. I f H 2 does exist on the moon, the average molecular residence time against thermal escape is only about 7 × 103sec. This is short enough t h a t the response of atmospheric H a to sudden changes in the solar wind should reveal the surface reaction time for formation and release of H 2. Hence, it is important t h a t continued attempts be made to obtain realistic in situ measurements of molecular hydrogen.
90"
180"
270"
360"
LONGITUDE
Fro. 2. C o m p u t e d global d i s t r i b u t i o n of H2 on t h e m o o n (from H o d g c s , 1973b). L a t i t u d e s of t h e graphs correspond to geographic centers of data a c c u m u l a t i o n zones u s e d in t h e calculation. T h e line w i d t h s s h o w t h e r a n g e of values r e s u l t i n g f r o m a _ 5 ° K u n c e r t a i n t y in t h e n i g h t t i m e temperature model.
is about 2 × 103 cm -3, while at night a level of 1.2 × 104cm -3 is reached. These results in Fig. 2 agree fairly well with those of Hartle and Thomas (1973) if differences in definitions of concentration are taken into account. Near the subsolar point the solar wind contribution of new molecules amounts to an equivalent concentration of 1.5 × l03 cm -3, making the theoretical total amount of H 2 somewhat less than the uv upper limit of 6 × l03 cm -3 established by Feldman and Fastie (1973). At night the lowest concentration of H 2 yet recorded b y the Apollo 17 mass spectrometer is 3.5 × 104cm -3, or about three times the theoretical level. Temptation to accept this measurement as a true atmospheric
The Apollo 17 mass spectrometer data clearly show helium to be an ambient lunar atmosphere gas (Hoffman et al., 1973). Figure 3 shows preliminary mass spectrometer data superimposed on a theoretical graph of the diurnal variation of helium appropriate to the Apollo 17 landing site (from Hodges, 1973b). The small amount of daytime data is due to limited usage of the instrument during daytime, when artifact gas levels are quite high and extended operation could have resulted in irreversible contamination of the ion source electrodes. Owing to the low ambient level of He in daytime, it is probable t h a t the daytime data points may include a significant artificial bias due to contamination. The important facet of Fig. 3 is the close agreement of experiment and theory over the night hemisphere, where the bulk of the gas is found. This is strong support for the hypothesis t hat the total solar wind influx of helium--assumed to be 0.045 times the proton flux in accordance with Johnson et al. (1972)--is balanced by thermal escape. Figure 4 shows the theoretical global distribution of bound helium calculated by Hodges (1973b). Average residence time for this model is about one day. The pattern is almost an ideal example of exospheric equilibrium. The nighttime a s y m m e t r y about the antisolar meridian is
420
HODGES, HOF~MAN, AND JOHNSON I0: APOLLO 17 LUNATION
SYMBOL
3
×
to,
g t~
I0 ~
o
:E
..__r--
V
k_l_._ 9JO*
I 180" LONGITUDE
I 270*
360"
FIG. 3. P r e l i m i n a r y d a t a f r o m t h e Apollo 17 m a s s s p e c t r o m e t e r s u p e r i m p o s e d on a c a l c u l a t e d d i u r n a l v a r i a t i o n o f h e l i u m (from H o d g e s , 1973b).
related to a continual decrease in surface t e m p e r a t u r e t h r o u g h the night. io 4
I~EON
Calculations b y H i n t o n and Taeusch (1964), J o h n s o n (1971), J o h n s o n et al. (1972), and Hodges et al. (1973) have shown 2°Ne to be the most probable d o m i n a n t gas of solar wind origin on the moon, although successive refinements of t h e o r y have resulted in a significant decrease in the e x p e c t e d amount. At present there is a fairly good agreement between t h e o r y and experimental results. Figure 5 is a superposition of a theoretical global model distribution of Z°Ne and the existing experimental results. The p a u c i t y of d a t a points reflects the difficulties t h a t have plagued a t t e m p t s to measure neon. D a t a shown on the 7.7 ° latitude graph are surface values e x t r a p o l a t e d from the Apollo 16 orbital mass spectrometer measurements at latitudes between 7 ° and 10 ° (Hodges et al., 1972b; 1973). These points are the only d a t a in which the spectral peak at mass 20 a m u was not overwhelmed b y H21so arising from a spacecraft source of water. Scatter of the points is well within the large statistical uncertainties of the d a t a t h a t result from subtraction of a b o u t a 90% w a t e r contribution from the mass 2 0 a m u measurements. Available measurements from the Apollo
io 3
g
tO 3
104
z
0
t03
1
I
I /
I
1
I
I 90"
I 180 ° LONGITUDE
I ZTO"
IO 4
J
I03
103 o"
d |
~0"
FIG. 4. C o m p u t e d global d i s t r i b u t i o n of h e l i u m on t h e m o o n (from H o d g e s , 1973b). L a t i t u d e s of t h e g r a p h s c o r r e s p o n d to g e o g r a p h i c c e n t e r s of d a t a a c c u m u l a t i o n zones u s e d in t h e calculation. The line w i d t h s s h o w t h e r a n g e of values r e s u l t i n g from a +_5°K u n c e r t a i n t y in t h e n i g h t t i m e t e m p e r a t u r e model.
4~ 1
LUNAR ATMOSPHERE "---DAY
-_L ,SUNSET
106
NIGHT
_L =tDAY SUNRISE7 |. 2 =
I0 6
104 10:5
I
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l__
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106
55,8 °
105 I0 4 10 3
Z h-
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-
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l
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106
2 3 . 6 = --
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~ (SURFACE) A ~
104 1031
I
I 7.7 °
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n4L. '- ~
io~l
0=
r~1 APOLLO 16 (ORBIT)
I
90 =
180= LONGITUDE
I ::,70 =
360 =
FIG. 5. C o m p u t e d g l o b a l d i s t r i b u t i o n o f 2°Ne o n t h e m o o n b a s e d o n a s o l a r w i n d f l u x o f 2.4 × 1 0 4 c m - 2 s e c -1 a n d a p h o t o i o n i z a t i o n l i f e t i m e o f 6 × 106sec. L a t i t u d e s o f t h e g r a p h s c o r r e s p o n d to geographic centers of data accumulation zones used in the calculation. Experimental r e s u l t s f r o m t h e A p o l l o 16 o r b i t a l m a s s s p e c t r o m e t e r a n d t h e A p o l l o 17 l u n a r s u r f a c e m a s s spectrometer are superimposed on the calculated
graphs at appropriate latitudes. 17 mass spectrometer at 20°N (Hoffman et al., 1973) are shown on the computed graph for 23.6 ° latitude. Each point was obtained by a complex process in which the instrument was turned off and allowed to cool sufficiently to condense a significant mass 20amu contaminant, H F, which is produced in the ion source, probably from decomposition of vestiges of contaminant halogen and hydrogen compounds ingrained in materials from which the source was constructed. These measurements also provide an isotopic abundance ratio of
2ONe to :2Ne of about 14, which is in reasonable agreement with the solar wind ratio of 13.7 measured by Geiss et al. (1972). The theoretical model results from application of the Monte Carlo technique of Hodges (1973b). I t employs the assumption of no surface adsorption and complete conversion of the solar wind influx of neon ions to neutral, atmospheric atoms. A solar wind flux of 2.4 × 104cm-Zsec-I was adopted on the basis of the measurements by Geiss et al. (1972) which show the ratio of 4He to 2°Ne in the solar wind to be about 570. This flux has been corrected for the fraction of time the moon spends in the geomagnetic tail, and hence not in the solar wind, about 4 days per lunation. It has also been assumed t hat the dominant loss mechanism for atmospheric neon is photoionization with a lifetime of 6 × l06 sec, as suggested by Manka (1972). These photoious are accelerated by the v × B field of the solar wind so t h a t about half escape while the other half impact the moon and are subsequently recycled into the atmosphere. Close agreement of theory and experiment suggests t hat the assumptions of the model are essentially correct. The failure of the Apollo 17 data to rise late in the night may be interpreted as an indication of a very slight amount of surface adsorption. Comparison with subsequent argon calculations indicates t hat the fraction of surface encounters which result in adsorption is probably the order of l0 -4. ARGON The dominant isotope of argon in the lunar atmosphere is 4°Ar, which is radiogenic and produced within the moon from 4°K. In addition, 36Ar of solar wind origin is present at a level of about 10%. One of the interesting features of argon is t h a t it is adsorbed by the lunar surface at night and released just after sunrise. Its residence time as an atmospheric atom includes long periods on the nighttime surface, and hence, technically not in the atmosphere. Figure 6 gives a global atmospheric model for 4°Ar t hat was computed by use of a modification of the Monte Carlo
422
HODGES~ HOFFMAN, AND JOHNSON ~DA'¢
'1'
NIGHT
technique of H o d g e s (1973b) which included n i g h t t i m e surface a d s o r p t i o n a n d sunrise release processes. This model is the c u l m i n a t i o n of a series of calculations in which the fit at 23.6 ° l a t i t u d e w i t h the Apollo 17 mass s p e c t r o m e t e r d a t a was o p t i m i z e d b y successive iteration of the t e m p e r a t u r e dependence of a d s o r p t i o n p r o b a b i l i t y . The resulting a d s o r p t i o n function is shown in Fig. 7 where the solid line represents the t e m p e r a t u r e r a n g e of t h e Apollo ]7 d a t a , a n d t h e dashed lines are extrapolations. Of course such syntheses do n o t give unique answers, a n d t h u s o t h e r a d s o r p t i o n functions m a y give equally good a g r e e m e n t w i t h the e x p e r i m e n t a l data. I n an e x t r e m e case in which the cold t e m p e r a t u r e limit of the a d s o r p t i o n prob a b i l i t y was increased from 0.05 (used above) to unity, the only significant change was a slight increase in the a m o u n t of gas in polar regions where the cold n i g h t t i m e t e m p e r a t u r e insures adsorption. A s s u m i n g a p h o t o i o n i z a t i o n lifetime of 1.6 × 106sec for argon (Manka, 1972) a n d the escape of h a l f the p h o t o i o n s due to solar wind v × B acceleration, the global a v e r a g e r a t e of loss of argon f r o m the a t m o s p h e r i c distribution shown in Fig. 6 is 2.3 × ]03 a t o m s e m - 2 s e e -~. I f the a v e r a g e l u n a r a b u n d a n c e of p o t a s s i u m is a b o u t 1000 p p m ,
D~*Y~ •
712 °
104
IO~ I0 ~ 55 8 °
104
IO" Io; £¢: Z~IO 4 UJ
~
10 ~
0 0
2 "~
102 23 6 ° 104
I0: IO 2
0°
i
i
90 °
;
!
i
180 °
270 °
56,0 °
LONGITUDE
FIG. 6. Computed global distribution of 4°At on the moon. Latitudes of individual graphs correspond to geographic centers of data accumulation zones used in the calculation. Preliminary experimental results from the Apollo 17 mass spectrometer are superimposed on the graph for 23.6 ° latitude.
SUN-REFERENCE i0-1
LONGITUDE AT APOLLO
2 7 0 ° 180 °
I"
I
~
17
90 e
'
I
SUNRISE
SUNSET
(~ I0 -z n," 0,.
O.
~
10 - ~
io"
t 80
I !00
I
I
I
120
SURFACE TEMPERATURE
I 140
(°K)
Fro. 7. Solid part of graph shows synthesized probability of adsorption of argon as a funeNon of surface temperature and sun referenced longitude at the Apollo 17 site (lower and upper abscissas, respectively). Dashed lines are arbitrary extrapolations used for other nigbttime temperatures.
LU:NAR ATMOSPHERE
the total rate of production of 4°Ar amounts to roughly 6 × l0 s atomscm-2sec -1 at the surface. Thus, the rate of loss of atmospheric 4°Ar from the moon corresponds to about 0.4% of the total production. Another way to view this rate of loss is t h a t it corresponds to release of the entire argon production from the upper 3 km of the lunar soil. The mechanism for conduction of such a large fraction of the 4°Ar production into the atmosphere from its point of origin within rocks is obscure. On earth, where potassium is probably concentrated mainly in the crust, the weathering of rocks has released the observed amount of atmospheric argon. Assuming the earth to be similar in composition to chondrites, Turekian (1964) has estimated t hat the present terrestrial atmospheric 4°Ar represents release of 10% of the total produced over geologic time. However, there is no evidence of ongoing weathering on the moon to a sufficient depth to account for the argon release there. It is not reasonable to postulate an enhanced abundance of 4°K below the surface of the moon because of heat flow considerations. The energy generated by the assumed 1000ppm amount would correspond to a heat outflow of about l0 -6 watts cm -2 which is a substantial fraction of the measured heat flows--the order of 3 × 10-6wattscm -2 according to Langseth et al. (1973). As mentioned previously, the amount of 36Ar on the moon is roughly 10% of the 4°Ar. The solar wind flux of 36Ar ions is about 8 × 102cm-:sec-l(Johnson et al., 1972). Since these ions impinge only on the side of the moon facing the sun, the average global influx is 1/4 the solar wind flux, or 2 × 102cm-:sec -~, which is about 10% of the 4°Ar effusion rate calculated above. Thus, the sustenance of 36Ar at about 10% of the amount of 4°Ar requires t h a t the rate of supply of 36Ar to the atmosphere be essentially in balance with the solar wind influx. The average rate of embedding of 36Ar in lunar surface materials is the sum of the averages of solar wind influx and half the photoionization rate, which amounts to
423
twice the average solar wind influx, or about 4 × 10:cm-2sec -1. Embedding of photoionized atmospheric 4°Ar occurs at just half the average photoionization rate, which is also equal to the previously calculated escape rate of 2.3 × 103cm -2 sec -1. Balance of atmospheric supply and solar wind influx of 36Ar implies saturation of the surface materials with argon, and the balance of effusion and embedding rates requires t h a t the ratio of trapped 4°Ar to 36Ar be the order of 6. This presents a slight dilemma since the ratio of trapped 4°Ar and 36Ar varies from .5 to 2 in most returned lunar soil samples. However, gas release rates obtained during linear heating of soil samples typically show a fairly high ratio of 4°Ar to 36Ar at low temperatures (Baur etal., 1972; Frick et al., 1973). This m ay be interpreted to imply a higher ratio of 4°Ar to 36Ar near grain surfaces, where exchange of effusing neutral atoms and impacting ions is most likely to occur. The isotope of argon at mass 38amu is mainly of solar wind origin, and thus its abundance in the atmosphere should be in the same ratio to 36Ar as the abundance ratio of these isotopes in lunar soil samples, which is generally the same as the terrestrial ratio, i.e., 3SAr is about 20% as abundant as 36Ar. In the Apollo 17 mass spectrometer data there are interferences at masses 36 and 38amu due to an HCI contaminant, which apparently is produced in the same reaction that forms .HF (discussed earlier in connection with neon). The argon diurnal effect is sufficiently above the contaminant background at 36amu that the 36Ar presence is obvious. However, a more detailed analysis of the data is needed to positively verify the existence of 3SAr in the lunar atmosphere. OTHER SOLAR WIND GASES
Solar abundances ofO, C and N probably exceed t h a t of Ne (Cameron, 1968), but atomic and molecular forms of these elements derived from the solar wind do not show evidence of a recognizable diurnal atmospheric oscillation in the Apollo 17 mass spectrometer data. The only reasonable interpretation of this fact is that
424
I-IODGES~ tIOFFMA:N~ AND JOHNSON
ambient levels are small compared to the artifact background. Atomic oxygen ions of the solar wind probably react with lunar materials, as the moon is less than fully oxidized, even though oxygen is the major constituent of the moon. Thus the lack of evidence of O or O 2 in the atmosphere is understandable. Atmospheric forms of C and N are more reasonably expected. B y analogy with the previously discussed hypothesis of the formation of H2, it is expected t ha t CH 4 and N H 3should also be formed and released to the atmosphere (Hodges et al., 1973). The large amount of oxygen in the soil may lead to production of CO, CO 2 and NO, but these reactions m a y also be reversible. The alternative to atmospheric forms of C or N is t h a t most of the solar wind influx of these elements is now being permanently implanted in the soil. OTHER GASES OF LUNAR ORIGIN
The only gases of lunar origin t ha t have been positively identified, 4°Ar and 222Rn, were anticipated on the basis of geophysical considerations. However, the rate of effusion of 4°Ar is significantly more than might be expected in view of the lack of evidence of a crustal excess of potassium or an efficient weathering mechanism. Since the outflow of 4°Ar may be accompanied by other gases, it is important to consider possible argon release mechanisms. One possibility is t h a t most of the atmospheric 4°Ar is produced deep in the lunar interior. L a t h a m et al. (1973) have identifled a highly attenuating zone for seismic shear waves beginning at a depth of ]000kin to l l 0 0 k m . I f the attenuation is due to high temperatures and partial melting, then an outflow of argon from the core could result. This would undoubtedly result in trapping of the gas in some regions and in a global venting rate t ha t depends on the distribution of deep fissures. Alternatively, radiogenic argon could have diffused into voids within the moon over a long period and some of these reservoirs could now be venting argon into the atmosphere. An attractive feature of
either hypothesis is t hat it can explain, through seismic changes in venting rates, a great variability in the level of 4°Ar in the lunar atmosphere over geologic time. Yaniv and H e y m a n n (1972) have proposed such variations to explain the dispersion of the 4°Ar to 36Ar ratio in the lunar soil samples. The regolith must be ruled out as an important source of atmospheric argon simply because of the magnitude of the loss rate, which corresponds to release of 4°Ar at its rate of production throughout the upper 3km of soil. Impact gardening m ay release a small fraction of tile trapped argon from the soil, but the depth of this effect cannot be great enough to make a substantial contribution. Diffusion of argon out of small grains to a great depth also seems an unlikely explanation because this mechanism would not have produced a varying supply of argon. I f argon is indeed vented from the lunar interior, then it m ay carry with it other common volcanic gases. By analogy with earth, water vapor would seem to be a candidate gas, although there is no evidence of atmospheric water in the Apollo 17 mass spectrometer data. The photodissociation time for H20 is quite short, and its dissociation products are chemically active, so t h a t the rate of effusion of water vapor from the moon may in fact be significant without being detectable. A transient gas event was detected by the Apollo 15 orbital mass spectrometer (Hodges et al., 1973) as the spacecraft passed northwest of Mare Orientale (110.3 ° W, 4.1°S) in lunar night. Briefly, this event consisted of a sudden burst of gases at masses 14, 28 and 32ainu. Similar excursions could have been detected at all other parts of the spectrum between 12 amu and 67ainu except at 16, 17, 18 and 44 amu, which unfortunately were dominated by contaminants. The absence of other mass numbers in this event may have been a temporal effect caused by a duration of the disturbance t hat was shorter than the time required to scan the mass spectrum (62sec). The ratio of masses 14 and 18ainu was greater than the cracking pattern of
L ~ N A R ATMOSPHERE
N2, w h i l e t h e a b s e n c e o f m a s s 12 a m u r u l e s o u t a s i g n i f i c a n t a m o u n t o f CO or CO 2. A m i x t u r e o f N, N z a n d a s m a l l a m o u n t o f CO or CO 2 w o u l d fit t h e o b s e r v e d s p e c t r a l d i s t r i b u t i o n of m a s s p e a k s . T h e e x c u r s i o n a t 32 a m u c o u l d h a v e b e e n 02, or m o r e l i k e l y a f r a g m e n t o f SO 2 f r o m a v e r y s h o r t b u r s t t h a t l a s t e d less t h a n t h e 20 s e c o n d s n e e d e d to scan the s p e c t r u m from 3 2 a m u to 64 a m u . T h e t o t a l a m o u n t o f gas n e e d e d to produce this e v e n t has been e s t i m a t e d to b e a b o u t 20kg. T h e v o l c a n i c o r i g i n o f s o m e o f t h e gases t h a t m i g h t h a v e b e e n i n v o l v e d i n t h e a b o v e e v e n t , i.e. N, N z a n d 02, were n o t g e n e r a l l y a n t i c i p a t e d . H o w ever, CO, CO 2, SO z a n d H z 0 s e e m l i k e l y candidates, based on a terrestrial analogy. ACKNOWLEDGMENTS
The assistance of Mr. H. D. Hammack in programming of numerical computations is grateflilly acknowledged. This research was supported by NASA under contracts NAS95964, NAS9-10410 and NAS9-12074. REFERENCES BAIYR,H., FRICK, U., FUNK, H., SCHULTZ,L., AND SIGNER, P. (1972). Thermal Release of Helium, Neon, and Argon from L u n a r Fines and Minerals. Proc. Third Lunar Sci. Conf., Geochim Cosmochim Acta, Suppl. 3, Vol. 2, 1947. BJORKHOLM, P., GOLUB, L., AND GO:RENSTEIN, P. (1973). Detection of radon emanation from the lunar regolith during Apollo 15 and 16 (abstract), I n "Lunar Science IV." (J. W. Chamberlain and Carolyn Watkins, eds.). Lunar Science Institute, Houston, Texas, 78. CAMERON, A. G. V¢. (1968). A new table of abundances of the elements in the solar system, I u "Origin and :Distribution of the Elements," (L. H. Ahrens, ed.). Pergamon, 124. FASTIE, W. G., FELD~[AN, P. D., HENRY, R. C., MOOS, ]~. ~vV., BARTH, C. A. THOMAS, G. E., AND DONAHUE, W. M. (1973). A search for far ultraviolet emissions from the lunar atmosphere. Science 182, 710. FELDMAN, P. D., AND FASTIE, W. (~. (1973). Fluorescence of molecular hydrogen excited by solar extreme ultraviolet radiation. Submitted to ,4 strophys. J. Letters. FI~ICK, U., BAUR, I-L, FUNK, H., PIIINNEY, D., SCHULTZ, L., AND SIGNER, P. (]973). Diffusion of argon in lunar fines, or little orphan argon retrapped (abstract), In Lunar Science IV, (J. W. Chamberlain and Carolyn Watkins,
425
eds.). Lunar Science Institute, Houston, Texas, 266. GEISS, J., BUEHLER, F., CERVTTI, H., EBERHARDT, P., AND Ch. FILLEUX (1972). Solar Wind Composition Experiment. Apollo 16 Preliminary Science Report, NASA SP-315, 14-1. GORENSTEIN, P., GOLUB, L., AND BJORKHOLM, P. (1973). Spatial nonhomogeneity and temporal variability in the emanation of radon from the lunar surface: interpretation (abstract). I n " L u n a r Science IV," (J. W. Chamberlain and Carolyn Watkins, eds.). Lunar Science Institute, Houston, Texas, 307. HARTLE, R. E., AND THOMAS, G. E. (1973). Neutral and ion-exosphere models for lunar hydrogen and helium. Submitted to J. Geophys. Res. HINTON, F. L., AND TAEUSCIt, D. R. (1964). Variation of the lunar atmosphere with the strength of the solar wind. J. Geophys. Res. 69, 1341. HODGES, R. R. ( 1972). Applicability of a diffusion model to lateral transport in the terrestrial and lunar exospheres. Planet. Space Sci. 20, 103. HODGES, R. R. (1973a). The differential equation of exospheric lateral transport and its application to terrestrial hydrogen. J. Geophys. Res. 78, 7340. HODGES, R. 1~. (1973b). Helium and hydrogen in the lunar atmosphere, J. Geophys. Res., 78, 8055. HODGES, R. R., AND JOHNSON, F. S. (1968). Lateral transport in planetary exospheres. J. Geophys. Res. 73, 7307. HODGES, R. R., HOFFMAN, J . H . , YEH, T. T. J. AND CHANG, G. K. (1972a). Orbital search for lunar volcanism. J. Geophys. Res. 77, 4079. HOI)GES, R. R., ttOFFMAN, J. H., AND EVANS, D. E. (1972b). Lunar Orbital Mass Spectrometer Experiment. Apollo 16 Preliminary Science Report, NASA SP-315, 21-1. HODGES, R. R., HOFFMAN, J. H., JOHNSON, F. S., AND EVANS, D. E. (1973) Composition and dynamics of lunar atmosphere. Proc. Fourth Lunar Sci. Conf., Geochim Cosmochim Acta, Suppl. 4, 3, 2855. HOFFMAN, J. H., HODGES, R.. R., AND EVANS, D. E. (1973). Lunar atmospheric composition results from Apollo 17, Proc. Fourth Lunar Sci. Conf., Geochim Cosmochim Acta., Suppl. 4, 3, 2865. JOHNSON, F. S. (1969). Origin of planetary atmosphere. Space Sci. Rev., 9, 303 JOHNSON, F. S. (1971). Lunar atmosphere. Rev. Geophys. Space Phys., 9, 813. JOHNSON, F. S., CARROLL, J. M., AND EVANS,
426
ItODGES, HOFFI~IAN:N, AND JOHNSON
D. E. (1972). Lunar atmospheric measurements. Proc. Third Lunar Sci. Conf., Geochim Cosmochim Acta, Suppl. 3, Vol. 3, 2231. LANGSETH, M. G., CHUTE, J. L., AND KEIHM, S. (1973). Direct measurements of heat flow from the moon (abstract), I n " L u n a r Science I V , " (J. W. Chamberlain and Carolyn Watkins, eds.). Lunar Science Institute, Houston, Texas, 455. LATItAM, G., DORMAN, J., DUENNEBIER, F., EWING, M., LAMMLEIN,D., AND NAKAMLrRA,Y. (1973). Moonquakes, meteoroids, and the state of the lunar interior (abstract). I n " L u n a r Science IV," (J. W. Chamberlain and Carolyn Watkins, eds.) Lunar Science Institute, Houston, Texas, 457.
1V~ANKA, R. I-I. (1972). Lunar Atmosphere and Ionosphere. Ph.D. Thesis, Rice University. MANKA, ]:~. H., AND MICH~'L, F. C. (1971). Lunar atmosphere as a source of lunar surface elements. Proc. S'econd Lunar ~ci. Conf., Geochim Cosmochim Acta, Suppl. 2, Vol. 2, 1717. TUREKIAN, K. K. (1964). Degassing of argon and helium from the earth. I n "The Origin and Evolution of Atmospheres and Oceans," (P. J. Brancazio and A. G. W. Cameron, eds.). J o h n Wiley, N. Y., 74. YANIV, A., AND HEYlVIANN, D. (1972). Atmospheric Ar 4° in lunar fines. Proc. Third Lunar Sci. Conf., Geochim. Cosmochim Acta, Suppl. 3, Vol. 2, 1967.
2 1 , 4 1 5 - 4 2 6 (1974)
The Lunar Atmosphere R. R. HODGES, Jr., J. H. HOFFMAN, AND FRANCIS S. J O H N S O N The University of Texas at Dallas, Dallas, Texas 75230 R e c e i v e d O c t o b e r 24, 1973; r e v i s e d D e c e m b e r , 11 1973 I n c o n t r a s t t o e a r t h , t h e a t m o s p h e r e of t h e m o o n is e x c e e d i n g l y t e n u o u s a n d a p p e a r s to consist m a i n l y of n o b l e gases. T h e solar w i n d i m p i n g e s o n t h e l u n a r surface, s u p p l y i n g d e t e c t a b l e a m o u n t s of h e l i u m , n e o n a n d 3~Ar. I n f l u x e s of solar w i n d p r o t o n s a n d c a r b o n a n d n i t r o g e n ions are significant, b u t a t m o s p h e r i c gases c o n t a i n i n g t h e s e e l e m e n t s h a v e n o t b e e n p o s i t i v e l y identified. R a d i o g e n i c 4°Ar a n d 222Rn p r o d u c e d w i t h i n t h e m o o n h a v e b e e n d e t e c t e d . T h e p r e s e n t r a t e o f effusion of a r g o n f r o m t h e m o o n a c c o u n t s for a b o u t 0 . 4 % of t h e t o t a l p r o d u c t i o n of 4°Ar d u e to d e c a y o f 4°K i f t h e a v e r a g e a b u n d a n c e of p o t a s s i u m i n t h e m o o n is 1 0 0 0 p p m . L a c k of w e a t h e r i n g processes i n t h e r e g o l i t h suggests t h a t m o s t of t h e a t m o s p h e r i c 4°Ar o r i g i n a t e s deep in t h e l u n a r interior, p e r h a p s i n a p a r t i a l l y m o l t e n core. I f so, o t h e r gases m a y b e v e n t e d a l o n g w i t h t h e argon.
The atmosphere of the moon is so tenuous t h a t it can be regarded as a collisionless exosphere in which atoms and molecules are gravitationally bound in ballistic trajectories between encounters with the lunar surface. Despite the small amount of gas, the vestige of atmosphere is an important indicator of lunar processes which produce atmospheric gases. Direct measurements of lunar gases by means of the Apollo cold cathode gage and mass spectrometer experiments, along with anciliary data from alpha particle and ultraviolet spectrometers and analyses of returned lunar samples, provide a basis for study of the relationship between the moon and its atmosphere. Lack of a significant amount of lunar atmosphere can be attributed mainly to an efficient escape mechanism for particles t h a t are too h ea vy to escape thermally. Neutral gas molecules or atoms are photoionized by solar radiation and then, in the absence of significant magnetization of the moon, the v × B field of the impinging solar wind accelerates the resultant ions. Roughly half of these ions impact the lunar surface, but the other half escape. The geomagnetic field inhibits this escape Copyright © 1974by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
415
process over the earth, while induced ionospheric currents apparently cause a deflection of the solar wind around both Mars and Venus, thus causing ions produced below the deflection boundary to remain in the planetary atmosphere. The present tenuous state of the lunar atmosphere is not inherent to the size, mass or orbit of the moon. Given a rate of volcanic release of gases in excess of the photoionization loss rate, a dense atmosphere would form. To understand this possibility, it is useful to consider the physics of a hypothetical dense atmosphere on the moon, of sufficient depth to form an ionosphere t h a t is not in contact with the lunar surface. Gases which cannot escape thermally (i.e., gases other than hydrogen and helium) would presumably make up the bulk of this atmosphere. The photoescape rate would be limited to photoions formed above the level where ionospheric currents produce an ionosphere-solar wind boundary. This escape rate is also a lower bound on the rate of degassing of the moon needed to form a dense atmosphere, since surface chemical processes could also act as a sink. The relationship of the surface loss processes to the excess gas release rate
416
HODGES, HOFFI~IAN, AND JOHNSON
would determine the surface concentration and hence the height of the region where the solar wind deflection occurs. An a p p r o x i m a t i o n to the m a x i m u m rate of loss of lunar atmosphere is found b y integration of the rate of photoionization over the sunlit side of the moon. The resulting mass outflow of a specific gas is 27rR 2 n m H / r where R is the radius of the ionosphere-solar wind b o u n d a r y , n is concentration at t h a t level, m is molecular mass, H is d a y t i m e scale height, and r is photoionization time. Since the p r o d u c t m H is i n d e p e n d e n t of molecular mass, the t o t a l loss rate is also given b y the above expression if n is total neutral concentration and r is an average photoionization time. Assuming t h a t the solar wind is deflected near the base of the exosphere of the h y p o t h e t i c a l atmosphere in question, the concentration n should be a b o u t l08 cm -3. Making the f u r t h e r assumptions of an average exobase t e m p e r a t u r e of 1000°K and a photoionization time of 107sec, the net mass loss rate would be 1.7 × 103g/sec, or a global average loss rate per unit area of roughly 4 × 10-~Sgcm -2 sec -I. F o r purposes of comparison, the volcanic release rate, if at the same rate per unit p l a n e t a r y mass as for the earth, would be a b o u t 3 × 10-1:gcm-2sec-~ of H 2 0 and 5 × 10-13gcm-2sec -l of CO 2 (Johnson, 1969 ; 1971). Thus the absence of a lunar atmosphere is evidence t h a t volcanic a c t i v i t y on the moon is at least several orders of m a g n i t u d e less intense per unit mass t h a n on earth. The above a r g u m e n t does not rule out presently active lunar volcanism, b u t it does set a limit on the degassing rate over geologic time scales. However, based on Apollo 14 and 15 cold cathode gage d a t a (Johnson et al., 1972) a present upper b o u n d of a b o u t 10-16gcm-2sec -1 has been established b y Hodges et al. (1972a). Since the cold cathode gage d a t a obtained during lunar d a y t i m e m a y be m a i n l y due to artifact gases released from r e m n a n t spaceflight hardware, it is entirely possible t h a t this u p p e r b o u n d greatly over-estimates the actual degassing rate of the moon. Subsequent discussion of spectrometric d a t a will show t h a t some gases are present-
ly evolving from the moon, and hence t h a t a lower b o u n d can be placed on the release rate. I n addition to the release of lunar gases, the impinging solar wind is an i m p o r t a n t and predictable source of atmosphere. Solar wind ions i m p a c t the m o o n with energies the order of 1 keV per a m u and become imbedded in surface rocks. The soil is p r o b a b l y s a t u r a t e d with most solar wind gases, and a balance of ion inflow and neutral effusion from the lunar surface p r o b a b l y exists. This hypothesis is verified b y the close balance between the solar wind influx and the lunar atmospheric content of helium, 2°Ne, and 36At which will be discussed later. Owing to the small a m o u n t of ambient gas on the moon, species identification is greatly complicated b y c o n t a m i n a n t s of spacecraft origin. A distinguishing feature of a native gas is the diurnal tidal oscillation of its concentration, a collective effect t h a t reflects statistics of encounters of particles with the moon b u t not with each other. Atmospheric atoms and molecules travel in ballistic trajectories between encounters with the lunar surface. Neglecting adsorption or chemical reaction at the surface, global t r a n s p o r t of gases is similar to a two-dimensional r a n d o m walk process, the mean step size being a p p r o x i m a t e l y equal to two scale heights, and therefore proportional to t e m p e r a t u r e . Fig. 1 shows a portion of the p a t h of a h y p o t h e t i c a l a t o m
F~G. 1. P a t h of a h y p o t h e t i c a l a t o m of t h e lunar atmosphere.
LUNAR ATMOSPHERE
on the moon. Trajectories are generally longer over the warm sunlit side, resulting in the tendency for migration of particles to the colder nighttime side to be greater than t h a t from night to day. Equilibrium of lateral transport requires t h a t the distribution of a gas vary in concentration roughly as the -5/2 power of temperature (Hodges and Johnson, 1968; see also recent extensions of the theory by Hodges, 1972 and 1973a). Since the day-to-night temperature ratio is about 4 : 1, the expected nighttime gas concentration is approximately 32 times that in daytime. Gases which are adsorbed on the nighttime side of the moon have both nighttime and daytime minima, the former being due to surface adsorption, and the latter, to the tendency of gases to avoid a temperature maximum. This leaves a meridional maximum at the terminator. Near sunrise, where the adsorbed gases are released as the surface warms, the maximum concentration is significantly greater than t h a t at sunset.
417
ATMOSPHERIC CONSTITUENTS A summary of present knowledge of lunar atmospheric parameters is given in Table I. Theoretical values for hydrogen and helium are based on calculations by Hodges (1973b), which are in close agreement with independent calculations of Hartle and Thomas (1973). Daytime concentrations of hydrogen and helium may be expressed in two ways--in terms of downcoming bound particles which have completed at least one ballistic trajectory or as the total of bound and newly created upgoing molecules in initial trajectories. Hodges has related the bound particle definition of concentration to the downcoming molecular flux measured by the Apollo 17 lunar surface mass spectrometer, while Hartle and Thomas were concerned with the total concentration which is appropriate to analyses of optical data from the Apollo 17 orbital ultraviolet spectrometer. Owing to long residence
TABLE
I
SUMMARY OF LUNAR ATMOSPttERE PARAMETERS H Solar wind influx (ions/see) Lunar venting (atoms/see) Photoionization time (see) Residence time
H2
2.8 × 102s
4He
2° N e
36Ar
4°Ar
1.3 x 1024
2.2 x 1021
8.0 x 1019
--
--
--
.
10 7
10 7
10 v
6 x l 06
1.2 × 103
7 x 103
8 x 104
4 x 10 v
1.7 x 10 3 1.9 x 10 3 4 x 10 4
4 x 10 3 1.1 x lO s
.
.
.
8.7 x 1020 1.6 x 106
1.6 x 106
10 v
10 v
1.3 x 10 z 3 × 10 3a
1.6 × 10 3 4 x 10 aa
3 x l O 3a
4 x I 0 4a
(see)" Concentration
(cm-3): (lay Theory
bound total
night Experiment b day night
6 x 10 *c 2.7 × 10 ac 1.6 × 103c
2 3.5 1.2 <6 <3.5
× × x × x
l0 3 10 a 10* l0 3 10*
2 x I0 a
--
4 × I0 +
lO s
" Hydrogen and helium escape thermally, while photoionization controls lifetimes of the other gases. b D a y t i m e u p p e r b o u n d s o n H a n d H 2 a r e A p o l l o 17 o r b i t a l u l t r a v i o l e t s p e c t r o m e t e r r e s u l t s ( F a s t i e et al., 1973) w h i l e t h e r e m a i n i n g d a t a a r e f r o m A p o l l o m a s s s p e c t r o m e t e r e x p e r i m e n t s . c Amounts that would be present if released in atoinic rather than molecular form. a Sunrise terminator maxima are given for argon. Surface adsorption removes most of the nighttime argon. 15
418
ttODGES~ HOFFMAlg~ AND JOI-INSON
times for neon and argon, differences between their respective bound and total concentrations are negligible. Amounts of helium, a°Ne and 36Ar, which are known from Apollo 16 (Hodges et al., 1973), and Apollo 17 mass spectrometer data (Hoffman et al., 1973), are in balance with the solar wind influxes of these species. The lack of a large accumulation of hydrogen in the lunar soil suggests t h a t the solar wind influx of protons is similarly converted to a neutral gas, presumably H2, to equalize rates of accretion and escape. B y analogy, it is reasonable to expect t h a t the carbon and nitrogen influxes from the solar wind are also balanced by atmospheric escape, probably as methane and ammonia, respectively. The presence of excess amounts of 4°Ar trapped in returned lunar samples has been recognized as evidence of 4°Ar as an atmospheric gas (Manka and Michel, 1971 ). More recently, 4°Ar has been identified in the lunar atmosphere by the Apollo 17 mass spectrometer (Hodges et al., 1973; Hoffinan et al., 1973). Since the only known source of 4°Ar is radiogenic decay of 4°K within the moon, its presence in the atmosphere is evidence of a venting or degassing process which m ay involve other gases. The a-particle experiments on Apollo 15 and Apollo 16 have shown evidence of atmospheric 222Rn and its long lived daughter zl°po (Bjorkholm et al., 1973; Gorenstein et al., 1973). One interpretation of an imbalance of radon and polonium in the a-particle data is t h a t sporadic venting of other gases m a y cause spatial and temporal fluctuations in the rate of effusion of radon. Whether venting rates of 4°Ar and 222Rn are related is speculative at this time. Subsequent sections on hydrogen, helium, neon and argon delineate the origins and geophysical implications of the data summarized in Table I. Additional discussion is presented on other candidate gases of the lunar atmosphere which have not been detected. Reasons to expect other gases include the large, mainly artifact, daytime gas levels detected by the Apollo 14 and Apollo 15 cold cathode gages and b y the Apollo 17 mass spectrometer, which
could hide some ambient, constituents.
condensible
HYDROGEN
Existence of atomic or molecular hydrogen in the lunar atmosphere seems necessary to balance the escape of hydrogen with the large solar wind influx of protons (~3 x 108em-/see-l) which impinges on the daytime side of the moon. Johnson et al. (1972), made a rough estimate that the surface concentration of H should be 5 x 103cm -3 if hydrogen remained in atomic form. A more detailed calculation by Hodges (1973b) gave a nighttime maximum concentration of 1.6 x l03 cm -3 and a daytime minimum of 6 × 102cm -3. An independent calculation by Hartle and Thomas (1973) is in close agreement if account is taken of the subtle difference in the definition of concentration adopted in the former study and t hat used by Hodges. However, the Apollo 17 orbital ultraviolet spectrometer showed t hat the daytime concentration of H is less than l0 atoms cm -3 (Fastie et al., 1973), suggesting t h a t the bulk of the solar wind hydrogen influx must reappear in molecular form. Figure 2 shows the global distribution of bound H 2 from the Monte Carlo calculations of Hodges (1973b). The results are displayed as the average concentration in each of 75 equi-area zones covering one hemisphere of the moon. These zones are arranged in circumferential bands which are centered at the latitudes indicated on the graphs. The global temperature model used in these calculations was developed b y M. G. Langseth and S. J. Keihm (private communication). Line widths in Fig. 2 give the range of concentrations resulting from a ±5°K uncertainty in the nighttime temperature model. The amount of atmospheric H 2 was established by balancing the total rate of thermal escape of hydrogen with the rate of impact of solar wind protons on the moon. New molecules were assumed to evolve only from the daytime lunar surface, with the outflow rate of H 2 molecules equal to the local solar wind influx of proton pairs. In Fig. 2 the daytime minimum concentration
419
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level, rather t han as an upper bound possibly influenced by contamination, must be tempered by the subsequent discussion of the distribution of lunar helium, which is quite accurately predicted by the theory. I f H 2 does exist on the moon, the average molecular residence time against thermal escape is only about 7 × 103sec. This is short enough t h a t the response of atmospheric H a to sudden changes in the solar wind should reveal the surface reaction time for formation and release of H 2. Hence, it is important t h a t continued attempts be made to obtain realistic in situ measurements of molecular hydrogen.
90"
180"
270"
360"
LONGITUDE
Fro. 2. C o m p u t e d global d i s t r i b u t i o n of H2 on t h e m o o n (from H o d g c s , 1973b). L a t i t u d e s of t h e graphs correspond to geographic centers of data a c c u m u l a t i o n zones u s e d in t h e calculation. T h e line w i d t h s s h o w t h e r a n g e of values r e s u l t i n g f r o m a _ 5 ° K u n c e r t a i n t y in t h e n i g h t t i m e temperature model.
is about 2 × 103 cm -3, while at night a level of 1.2 × 104cm -3 is reached. These results in Fig. 2 agree fairly well with those of Hartle and Thomas (1973) if differences in definitions of concentration are taken into account. Near the subsolar point the solar wind contribution of new molecules amounts to an equivalent concentration of 1.5 × l03 cm -3, making the theoretical total amount of H 2 somewhat less than the uv upper limit of 6 × l03 cm -3 established by Feldman and Fastie (1973). At night the lowest concentration of H 2 yet recorded b y the Apollo 17 mass spectrometer is 3.5 × 104cm -3, or about three times the theoretical level. Temptation to accept this measurement as a true atmospheric
The Apollo 17 mass spectrometer data clearly show helium to be an ambient lunar atmosphere gas (Hoffman et al., 1973). Figure 3 shows preliminary mass spectrometer data superimposed on a theoretical graph of the diurnal variation of helium appropriate to the Apollo 17 landing site (from Hodges, 1973b). The small amount of daytime data is due to limited usage of the instrument during daytime, when artifact gas levels are quite high and extended operation could have resulted in irreversible contamination of the ion source electrodes. Owing to the low ambient level of He in daytime, it is probable t h a t the daytime data points may include a significant artificial bias due to contamination. The important facet of Fig. 3 is the close agreement of experiment and theory over the night hemisphere, where the bulk of the gas is found. This is strong support for the hypothesis t hat the total solar wind influx of helium--assumed to be 0.045 times the proton flux in accordance with Johnson et al. (1972)--is balanced by thermal escape. Figure 4 shows the theoretical global distribution of bound helium calculated by Hodges (1973b). Average residence time for this model is about one day. The pattern is almost an ideal example of exospheric equilibrium. The nighttime a s y m m e t r y about the antisolar meridian is
420
HODGES, HOF~MAN, AND JOHNSON I0: APOLLO 17 LUNATION
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related to a continual decrease in surface t e m p e r a t u r e t h r o u g h the night. io 4
I~EON
Calculations b y H i n t o n and Taeusch (1964), J o h n s o n (1971), J o h n s o n et al. (1972), and Hodges et al. (1973) have shown 2°Ne to be the most probable d o m i n a n t gas of solar wind origin on the moon, although successive refinements of t h e o r y have resulted in a significant decrease in the e x p e c t e d amount. At present there is a fairly good agreement between t h e o r y and experimental results. Figure 5 is a superposition of a theoretical global model distribution of Z°Ne and the existing experimental results. The p a u c i t y of d a t a points reflects the difficulties t h a t have plagued a t t e m p t s to measure neon. D a t a shown on the 7.7 ° latitude graph are surface values e x t r a p o l a t e d from the Apollo 16 orbital mass spectrometer measurements at latitudes between 7 ° and 10 ° (Hodges et al., 1972b; 1973). These points are the only d a t a in which the spectral peak at mass 20 a m u was not overwhelmed b y H21so arising from a spacecraft source of water. Scatter of the points is well within the large statistical uncertainties of the d a t a t h a t result from subtraction of a b o u t a 90% w a t e r contribution from the mass 2 0 a m u measurements. Available measurements from the Apollo
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FIG. 5. C o m p u t e d g l o b a l d i s t r i b u t i o n o f 2°Ne o n t h e m o o n b a s e d o n a s o l a r w i n d f l u x o f 2.4 × 1 0 4 c m - 2 s e c -1 a n d a p h o t o i o n i z a t i o n l i f e t i m e o f 6 × 106sec. L a t i t u d e s o f t h e g r a p h s c o r r e s p o n d to geographic centers of data accumulation zones used in the calculation. Experimental r e s u l t s f r o m t h e A p o l l o 16 o r b i t a l m a s s s p e c t r o m e t e r a n d t h e A p o l l o 17 l u n a r s u r f a c e m a s s spectrometer are superimposed on the calculated
graphs at appropriate latitudes. 17 mass spectrometer at 20°N (Hoffman et al., 1973) are shown on the computed graph for 23.6 ° latitude. Each point was obtained by a complex process in which the instrument was turned off and allowed to cool sufficiently to condense a significant mass 20amu contaminant, H F, which is produced in the ion source, probably from decomposition of vestiges of contaminant halogen and hydrogen compounds ingrained in materials from which the source was constructed. These measurements also provide an isotopic abundance ratio of
2ONe to :2Ne of about 14, which is in reasonable agreement with the solar wind ratio of 13.7 measured by Geiss et al. (1972). The theoretical model results from application of the Monte Carlo technique of Hodges (1973b). I t employs the assumption of no surface adsorption and complete conversion of the solar wind influx of neon ions to neutral, atmospheric atoms. A solar wind flux of 2.4 × 104cm-Zsec-I was adopted on the basis of the measurements by Geiss et al. (1972) which show the ratio of 4He to 2°Ne in the solar wind to be about 570. This flux has been corrected for the fraction of time the moon spends in the geomagnetic tail, and hence not in the solar wind, about 4 days per lunation. It has also been assumed t hat the dominant loss mechanism for atmospheric neon is photoionization with a lifetime of 6 × l06 sec, as suggested by Manka (1972). These photoious are accelerated by the v × B field of the solar wind so t h a t about half escape while the other half impact the moon and are subsequently recycled into the atmosphere. Close agreement of theory and experiment suggests t hat the assumptions of the model are essentially correct. The failure of the Apollo 17 data to rise late in the night may be interpreted as an indication of a very slight amount of surface adsorption. Comparison with subsequent argon calculations indicates t hat the fraction of surface encounters which result in adsorption is probably the order of l0 -4. ARGON The dominant isotope of argon in the lunar atmosphere is 4°Ar, which is radiogenic and produced within the moon from 4°K. In addition, 36Ar of solar wind origin is present at a level of about 10%. One of the interesting features of argon is t h a t it is adsorbed by the lunar surface at night and released just after sunrise. Its residence time as an atmospheric atom includes long periods on the nighttime surface, and hence, technically not in the atmosphere. Figure 6 gives a global atmospheric model for 4°Ar t hat was computed by use of a modification of the Monte Carlo
422
HODGES~ HOFFMAN, AND JOHNSON ~DA'¢
'1'
NIGHT
technique of H o d g e s (1973b) which included n i g h t t i m e surface a d s o r p t i o n a n d sunrise release processes. This model is the c u l m i n a t i o n of a series of calculations in which the fit at 23.6 ° l a t i t u d e w i t h the Apollo 17 mass s p e c t r o m e t e r d a t a was o p t i m i z e d b y successive iteration of the t e m p e r a t u r e dependence of a d s o r p t i o n p r o b a b i l i t y . The resulting a d s o r p t i o n function is shown in Fig. 7 where the solid line represents the t e m p e r a t u r e r a n g e of t h e Apollo ]7 d a t a , a n d t h e dashed lines are extrapolations. Of course such syntheses do n o t give unique answers, a n d t h u s o t h e r a d s o r p t i o n functions m a y give equally good a g r e e m e n t w i t h the e x p e r i m e n t a l data. I n an e x t r e m e case in which the cold t e m p e r a t u r e limit of the a d s o r p t i o n prob a b i l i t y was increased from 0.05 (used above) to unity, the only significant change was a slight increase in the a m o u n t of gas in polar regions where the cold n i g h t t i m e t e m p e r a t u r e insures adsorption. A s s u m i n g a p h o t o i o n i z a t i o n lifetime of 1.6 × 106sec for argon (Manka, 1972) a n d the escape of h a l f the p h o t o i o n s due to solar wind v × B acceleration, the global a v e r a g e r a t e of loss of argon f r o m the a t m o s p h e r i c distribution shown in Fig. 6 is 2.3 × ]03 a t o m s e m - 2 s e e -~. I f the a v e r a g e l u n a r a b u n d a n c e of p o t a s s i u m is a b o u t 1000 p p m ,
D~*Y~ •
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FIG. 6. Computed global distribution of 4°At on the moon. Latitudes of individual graphs correspond to geographic centers of data accumulation zones used in the calculation. Preliminary experimental results from the Apollo 17 mass spectrometer are superimposed on the graph for 23.6 ° latitude.
SUN-REFERENCE i0-1
LONGITUDE AT APOLLO
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Fro. 7. Solid part of graph shows synthesized probability of adsorption of argon as a funeNon of surface temperature and sun referenced longitude at the Apollo 17 site (lower and upper abscissas, respectively). Dashed lines are arbitrary extrapolations used for other nigbttime temperatures.
LU:NAR ATMOSPHERE
the total rate of production of 4°Ar amounts to roughly 6 × l0 s atomscm-2sec -1 at the surface. Thus, the rate of loss of atmospheric 4°Ar from the moon corresponds to about 0.4% of the total production. Another way to view this rate of loss is t h a t it corresponds to release of the entire argon production from the upper 3 km of the lunar soil. The mechanism for conduction of such a large fraction of the 4°Ar production into the atmosphere from its point of origin within rocks is obscure. On earth, where potassium is probably concentrated mainly in the crust, the weathering of rocks has released the observed amount of atmospheric argon. Assuming the earth to be similar in composition to chondrites, Turekian (1964) has estimated t hat the present terrestrial atmospheric 4°Ar represents release of 10% of the total produced over geologic time. However, there is no evidence of ongoing weathering on the moon to a sufficient depth to account for the argon release there. It is not reasonable to postulate an enhanced abundance of 4°K below the surface of the moon because of heat flow considerations. The energy generated by the assumed 1000ppm amount would correspond to a heat outflow of about l0 -6 watts cm -2 which is a substantial fraction of the measured heat flows--the order of 3 × 10-6wattscm -2 according to Langseth et al. (1973). As mentioned previously, the amount of 36Ar on the moon is roughly 10% of the 4°Ar. The solar wind flux of 36Ar ions is about 8 × 102cm-:sec-l(Johnson et al., 1972). Since these ions impinge only on the side of the moon facing the sun, the average global influx is 1/4 the solar wind flux, or 2 × 102cm-:sec -~, which is about 10% of the 4°Ar effusion rate calculated above. Thus, the sustenance of 36Ar at about 10% of the amount of 4°Ar requires t h a t the rate of supply of 36Ar to the atmosphere be essentially in balance with the solar wind influx. The average rate of embedding of 36Ar in lunar surface materials is the sum of the averages of solar wind influx and half the photoionization rate, which amounts to
423
twice the average solar wind influx, or about 4 × 10:cm-2sec -1. Embedding of photoionized atmospheric 4°Ar occurs at just half the average photoionization rate, which is also equal to the previously calculated escape rate of 2.3 × 103cm -2 sec -1. Balance of atmospheric supply and solar wind influx of 36Ar implies saturation of the surface materials with argon, and the balance of effusion and embedding rates requires t h a t the ratio of trapped 4°Ar to 36Ar be the order of 6. This presents a slight dilemma since the ratio of trapped 4°Ar and 36Ar varies from .5 to 2 in most returned lunar soil samples. However, gas release rates obtained during linear heating of soil samples typically show a fairly high ratio of 4°Ar to 36Ar at low temperatures (Baur etal., 1972; Frick et al., 1973). This m ay be interpreted to imply a higher ratio of 4°Ar to 36Ar near grain surfaces, where exchange of effusing neutral atoms and impacting ions is most likely to occur. The isotope of argon at mass 38amu is mainly of solar wind origin, and thus its abundance in the atmosphere should be in the same ratio to 36Ar as the abundance ratio of these isotopes in lunar soil samples, which is generally the same as the terrestrial ratio, i.e., 3SAr is about 20% as abundant as 36Ar. In the Apollo 17 mass spectrometer data there are interferences at masses 36 and 38amu due to an HCI contaminant, which apparently is produced in the same reaction that forms .HF (discussed earlier in connection with neon). The argon diurnal effect is sufficiently above the contaminant background at 36amu that the 36Ar presence is obvious. However, a more detailed analysis of the data is needed to positively verify the existence of 3SAr in the lunar atmosphere. OTHER SOLAR WIND GASES
Solar abundances ofO, C and N probably exceed t h a t of Ne (Cameron, 1968), but atomic and molecular forms of these elements derived from the solar wind do not show evidence of a recognizable diurnal atmospheric oscillation in the Apollo 17 mass spectrometer data. The only reasonable interpretation of this fact is that
424
I-IODGES~ tIOFFMA:N~ AND JOHNSON
ambient levels are small compared to the artifact background. Atomic oxygen ions of the solar wind probably react with lunar materials, as the moon is less than fully oxidized, even though oxygen is the major constituent of the moon. Thus the lack of evidence of O or O 2 in the atmosphere is understandable. Atmospheric forms of C and N are more reasonably expected. B y analogy with the previously discussed hypothesis of the formation of H2, it is expected t ha t CH 4 and N H 3should also be formed and released to the atmosphere (Hodges et al., 1973). The large amount of oxygen in the soil may lead to production of CO, CO 2 and NO, but these reactions m a y also be reversible. The alternative to atmospheric forms of C or N is t h a t most of the solar wind influx of these elements is now being permanently implanted in the soil. OTHER GASES OF LUNAR ORIGIN
The only gases of lunar origin t ha t have been positively identified, 4°Ar and 222Rn, were anticipated on the basis of geophysical considerations. However, the rate of effusion of 4°Ar is significantly more than might be expected in view of the lack of evidence of a crustal excess of potassium or an efficient weathering mechanism. Since the outflow of 4°Ar may be accompanied by other gases, it is important to consider possible argon release mechanisms. One possibility is t h a t most of the atmospheric 4°Ar is produced deep in the lunar interior. L a t h a m et al. (1973) have identifled a highly attenuating zone for seismic shear waves beginning at a depth of ]000kin to l l 0 0 k m . I f the attenuation is due to high temperatures and partial melting, then an outflow of argon from the core could result. This would undoubtedly result in trapping of the gas in some regions and in a global venting rate t ha t depends on the distribution of deep fissures. Alternatively, radiogenic argon could have diffused into voids within the moon over a long period and some of these reservoirs could now be venting argon into the atmosphere. An attractive feature of
either hypothesis is t hat it can explain, through seismic changes in venting rates, a great variability in the level of 4°Ar in the lunar atmosphere over geologic time. Yaniv and H e y m a n n (1972) have proposed such variations to explain the dispersion of the 4°Ar to 36Ar ratio in the lunar soil samples. The regolith must be ruled out as an important source of atmospheric argon simply because of the magnitude of the loss rate, which corresponds to release of 4°Ar at its rate of production throughout the upper 3km of soil. Impact gardening m ay release a small fraction of tile trapped argon from the soil, but the depth of this effect cannot be great enough to make a substantial contribution. Diffusion of argon out of small grains to a great depth also seems an unlikely explanation because this mechanism would not have produced a varying supply of argon. I f argon is indeed vented from the lunar interior, then it m ay carry with it other common volcanic gases. By analogy with earth, water vapor would seem to be a candidate gas, although there is no evidence of atmospheric water in the Apollo 17 mass spectrometer data. The photodissociation time for H20 is quite short, and its dissociation products are chemically active, so t h a t the rate of effusion of water vapor from the moon may in fact be significant without being detectable. A transient gas event was detected by the Apollo 15 orbital mass spectrometer (Hodges et al., 1973) as the spacecraft passed northwest of Mare Orientale (110.3 ° W, 4.1°S) in lunar night. Briefly, this event consisted of a sudden burst of gases at masses 14, 28 and 32ainu. Similar excursions could have been detected at all other parts of the spectrum between 12 amu and 67ainu except at 16, 17, 18 and 44 amu, which unfortunately were dominated by contaminants. The absence of other mass numbers in this event may have been a temporal effect caused by a duration of the disturbance t hat was shorter than the time required to scan the mass spectrum (62sec). The ratio of masses 14 and 18ainu was greater than the cracking pattern of
L ~ N A R ATMOSPHERE
N2, w h i l e t h e a b s e n c e o f m a s s 12 a m u r u l e s o u t a s i g n i f i c a n t a m o u n t o f CO or CO 2. A m i x t u r e o f N, N z a n d a s m a l l a m o u n t o f CO or CO 2 w o u l d fit t h e o b s e r v e d s p e c t r a l d i s t r i b u t i o n of m a s s p e a k s . T h e e x c u r s i o n a t 32 a m u c o u l d h a v e b e e n 02, or m o r e l i k e l y a f r a g m e n t o f SO 2 f r o m a v e r y s h o r t b u r s t t h a t l a s t e d less t h a n t h e 20 s e c o n d s n e e d e d to scan the s p e c t r u m from 3 2 a m u to 64 a m u . T h e t o t a l a m o u n t o f gas n e e d e d to produce this e v e n t has been e s t i m a t e d to b e a b o u t 20kg. T h e v o l c a n i c o r i g i n o f s o m e o f t h e gases t h a t m i g h t h a v e b e e n i n v o l v e d i n t h e a b o v e e v e n t , i.e. N, N z a n d 02, were n o t g e n e r a l l y a n t i c i p a t e d . H o w ever, CO, CO 2, SO z a n d H z 0 s e e m l i k e l y candidates, based on a terrestrial analogy. ACKNOWLEDGMENTS
The assistance of Mr. H. D. Hammack in programming of numerical computations is grateflilly acknowledged. This research was supported by NASA under contracts NAS95964, NAS9-10410 and NAS9-12074. REFERENCES BAIYR,H., FRICK, U., FUNK, H., SCHULTZ,L., AND SIGNER, P. (1972). Thermal Release of Helium, Neon, and Argon from L u n a r Fines and Minerals. Proc. Third Lunar Sci. Conf., Geochim Cosmochim Acta, Suppl. 3, Vol. 2, 1947. BJORKHOLM, P., GOLUB, L., AND GO:RENSTEIN, P. (1973). Detection of radon emanation from the lunar regolith during Apollo 15 and 16 (abstract), I n "Lunar Science IV." (J. W. Chamberlain and Carolyn Watkins, eds.). Lunar Science Institute, Houston, Texas, 78. CAMERON, A. G. V¢. (1968). A new table of abundances of the elements in the solar system, I u "Origin and :Distribution of the Elements," (L. H. Ahrens, ed.). Pergamon, 124. FASTIE, W. G., FELD~[AN, P. D., HENRY, R. C., MOOS, ]~. ~vV., BARTH, C. A. THOMAS, G. E., AND DONAHUE, W. M. (1973). A search for far ultraviolet emissions from the lunar atmosphere. Science 182, 710. FELDMAN, P. D., AND FASTIE, W. (~. (1973). Fluorescence of molecular hydrogen excited by solar extreme ultraviolet radiation. Submitted to ,4 strophys. J. Letters. FI~ICK, U., BAUR, I-L, FUNK, H., PIIINNEY, D., SCHULTZ, L., AND SIGNER, P. (]973). Diffusion of argon in lunar fines, or little orphan argon retrapped (abstract), In Lunar Science IV, (J. W. Chamberlain and Carolyn Watkins,
425
eds.). Lunar Science Institute, Houston, Texas, 266. GEISS, J., BUEHLER, F., CERVTTI, H., EBERHARDT, P., AND Ch. FILLEUX (1972). Solar Wind Composition Experiment. Apollo 16 Preliminary Science Report, NASA SP-315, 14-1. GORENSTEIN, P., GOLUB, L., AND BJORKHOLM, P. (1973). Spatial nonhomogeneity and temporal variability in the emanation of radon from the lunar surface: interpretation (abstract). I n " L u n a r Science IV," (J. W. Chamberlain and Carolyn Watkins, eds.). Lunar Science Institute, Houston, Texas, 307. HARTLE, R. E., AND THOMAS, G. E. (1973). Neutral and ion-exosphere models for lunar hydrogen and helium. Submitted to J. Geophys. Res. HINTON, F. L., AND TAEUSCIt, D. R. (1964). Variation of the lunar atmosphere with the strength of the solar wind. J. Geophys. Res. 69, 1341. HODGES, R. R. ( 1972). Applicability of a diffusion model to lateral transport in the terrestrial and lunar exospheres. Planet. Space Sci. 20, 103. HODGES, R. R. (1973a). The differential equation of exospheric lateral transport and its application to terrestrial hydrogen. J. Geophys. Res. 78, 7340. HODGES, R. 1~. (1973b). Helium and hydrogen in the lunar atmosphere, J. Geophys. Res., 78, 8055. HODGES, R. R., AND JOHNSON, F. S. (1968). Lateral transport in planetary exospheres. J. Geophys. Res. 73, 7307. HODGES, R. R., HOFFMAN, J . H . , YEH, T. T. J. AND CHANG, G. K. (1972a). Orbital search for lunar volcanism. J. Geophys. Res. 77, 4079. HOI)GES, R. R., ttOFFMAN, J. H., AND EVANS, D. E. (1972b). Lunar Orbital Mass Spectrometer Experiment. Apollo 16 Preliminary Science Report, NASA SP-315, 21-1. HODGES, R. R., HOFFMAN, J. H., JOHNSON, F. S., AND EVANS, D. E. (1973) Composition and dynamics of lunar atmosphere. Proc. Fourth Lunar Sci. Conf., Geochim Cosmochim Acta, Suppl. 4, 3, 2855. HOFFMAN, J. H., HODGES, R.. R., AND EVANS, D. E. (1973). Lunar atmospheric composition results from Apollo 17, Proc. Fourth Lunar Sci. Conf., Geochim Cosmochim Acta., Suppl. 4, 3, 2865. JOHNSON, F. S. (1969). Origin of planetary atmosphere. Space Sci. Rev., 9, 303 JOHNSON, F. S. (1971). Lunar atmosphere. Rev. Geophys. Space Phys., 9, 813. JOHNSON, F. S., CARROLL, J. M., AND EVANS,
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ItODGES, HOFFI~IAN:N, AND JOHNSON
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