- Email: [email protected]
Trapped solar wind helium and neon in Surveyor 3 material
EARTH AND PLANETARY SCIENCE LETTERS 10 (1971) 297-306. NORTH-HOLLAND PUBLISHING COMPANY
TRAPPED
SOLAR WIND HELIUM AND NEON
IN SURVEYOR
3 MATERIAL
F. BOHLER, P. EBERHARDT, J. GEISS and J. SCHWARZMOLLER Physikalisches Institut, University o f Bern, 3000 Bern, Switzerland
Received 5 January 1971
The Apollo 12 astronauts salvaged an unpainted Al-tube from the Surveyor 3 spacecraft. We measured He and Ne in sectors cut from a small ring from this tube. The lower side of the tube is contaminated by lunar dust, which cannot be completely removed by ultrasonic cleaning. The upper half shows very little or no dust contamination and contains trapped solar wind He and Ne with 4He/2°Ne = 295 in the trapped4gas. This value is lower than the ratios measured by the Apollo 11 and 12 SWC experiments. This could be due to He diffusion loss or to a small residual dust contamination. The 4He distribution around the Al-tube is in agreement with the theoretically expected distribution and corresponds to an average solar wind 4 H e flux of 7 X 10 6 c m -2 sec -1 . I f 4 H e diffusion loss had occurred the average 4 H e flux could be as high 13 X 10 6 c m -2 sec -1 . Neglecting the small influences of a possible dust contamination or of diffusion loss we obtain the following average isotopic compositions for the solar wind during the exposure of the Surveyor 3 material: 4He/3He = 2700 + 130; ~°Ne/22Ne = 13.3 + 0.4 and 22Ne/21Ne = 31 + 5. Compared with the Apollo 11 and 12 SWC results the 4He/3He ratio is unexpectedly high.
On April 20, 1967, the unmanned spacecraft Surveyor 3 landed on the lunar surface in the Oceanum Procellarum (3.2°S; 23.4 ° W). 2½ years later, on November 20, 1969, the Apollo 12 astronauts Charles Conrad and Alan L. Bean recovered several pieces o f this Surveyor spacecraft and returned them to Earth. Among these was a section of a support strut, a 12.7 m m o.d. tube o f polished, unpainted aluminum (alloy 2024) with 1.2 m m wall thickness. This unpainted tube was salvaged with the specific aim of investing implanted solar wind particles. The location of the returned section of the strut on the Surveyor spacecraft is shown in fig. l . We have received from the Jet Propulsion Laboratory, Pasadena, California, a small ring from this tube. The ring, designated B-1, was approximately 2 m m wide and was located about 41 mm from the A-end o f the Al-tube. Our measurements on section B-1 represent a preliminary investigation with the aim o f establishing the presence of trapped noble gases from the solar wind in the aluminum surface and o f measuring the
abundances of the light noble gases. Furthermore, contamination by lunar dust and the distribution o f the trapped solar wind around the tube had to be studied. Our measurements give the necessary information for subsequent, more detailed studies o f the implanted solar wind gases in Surveyor 3 materials. Our analytical sensitivity is sufficiently high to allow 4He and 2°Ne determinations in a I m m 2 sample. Consequently, we cut the ring B-1 into two rings o f about equal width (cf. fig. 2). A wire saw with a 0.2 m m diamond-impregnated stainless steel wire was used. We removed the aluminum from the inside of these tings, reducing the weight by about 50%. This was done in order to lower the noble gas blank from the aluminum. The rings were then cut into sectors (cf. figs. 2, 3 and 4) and thoroughly cleaned b y repeated ultrasonic treatment in acetone. Inspection under the optical microscope and investigation with a scanning electron microscope revealed that approximately one half o f the B-1 ring shows surface alterations and is, moreover, contaminated
298
F. Bi~hler et aL, Trapped solar wind helium and neon in Surveyor 3 material
Fig. 1. Original location in spacecraft of returned part of Surveyor 3 strut (enlargement from NASA photograph AS 12-48-7114). with fine crystalline particles, presumably of lunar origin. The ultrasonic treatment reduced this contamination, but did not completely eliminate it. No dust particles were found after the ultrasonic treatment on sector 16 d taken from the uncontaminated side of B-1. So far, the noble gases He and Ne have been determinated in a number of sectors. The measurement pro-
cedure and the analytical blanks were the same as for the foil analyses of the Solar Wind Composition (SWC) experiment [2]. The results are given in tables 1 and 2, and the distribution around the ring is shown in figs. 3 and 4. No corrections for He and Ne from lunar dust contamination or for blanks in the aluminum were applied.
F. B~thler et al. Trapped solar wind helium and neon in Surveyor 3 material
SCRATCH
~
299
SCRIBE
R B-1-26a
SCRIBE SCRATCH
e "~
B- I -2
j~ J J J B-1-1
Fig. 2. Orientation of sectors cut from ring section B-1.
The probable orientation of the ring B-1 at the lunar surface given by Carroll [ 1] was adopted for figs. 3 and 4. Accordingly, the contaminated side of the aluminum tube essentially faced the lunar surface, whereas the clean side was sun-lit and, thus, exposed to the solar wind. We assume that the contamination and surface alteration occurred during landing as the vernier-engines were cut off only 34 sec after the initial touchdown. Our He and Ne data do essentially support the probable orientation given by Carroll [ 1]. In the Apollo 12 lunar fine material [3] and also in lunar dust adhering to the Apollo 12 SWC foil, the He/Ne ratios are below 100. Thus, He and Ne found on the lower part of the ring (cf. fig. 3) are readily explained as resulting from residual dust contamination. On the sun-lit side we find 4He/2°Ne ~ 300, which is a much higher ratio than in the lunar fine material and closer to the 4He/2°Ne ratio in the solar wind [2]. Taking into account the observed high He/Ne ratio, the smoothness of the He distribution obtained on the sun-lit side, and the fact that a thorough investigation
with the scanning electron microscope on sector 16 d did not reveal any lunar dust particles after ultrasonic treatment, we conclude that the He data obtained on the sun-lit side of the ring represent the solar wind particles implanted in the aluminum and that in this area the contamination from the dust is minimal or absent. In table 3 averages for the trapped solar wind He and Ne in the Surveyor 3 material are given. The maximum 4 He surface concentration is the average of sectors 1 lc, I ld and 16a (cf. fig. 3). The 4He/2°Ne ratio is the average of the six sectors with the highest 4He/2°Ne ratios (cf. tables 1 and 2). All other ratios are averages of the two large sectors 21b and 26a (cf. table 2). The values in table 3 were corrected for the Ne blank of the aluminum tube. This blank correction is based on the noble gas concentration found in aluminum turnings removed from the inside of ring B-1 (measured concentrations: (19 -+ 5) × 10 -8 cm a STP 4He/g; (0.4 +- 0.1) × 10 -8 cm 3 STP 2°Ne/g). As the 4He/2°Ne ratio of 50 indicates contamination with lunar dust, only 30% of the measured 2°Ne
300
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
Table t Results of 4He and 2°Ne measurements in sectors cut from unpainted Surveyor 3 Al-tube (cf. figs. 1, 2 and 3). Values not corrected for Ne blank of Al-tube. Sample no.
Area (mm 2)
Weight (mg)
4 He 20 Ne (10 -8 cm a STP/cm 2)
B-l-lla
1.06 +0.0.3
1.94 +0.02
+
140 6
0.66 -+0.17
210 + 50
B-l-llc
0.98 +0.04
1.49 -+0.02
180 + 10
0.62 +0.04
290 + 10
B-l-lld
0.88 +-0.06
1.42 -+0.02
165 + 13
0.58 +0.04
285 -+ 15
B-l-12a
1.29 +0.04
2.00 +0.02
-+
204 9
2.15 +0.30
95 + 13
1.38 +0.05
2.00 -+0.02
570 + 30
5.60 +0.30
-+
102 5
1.48 +0.08
2.00 -+0.02
590 -+ 40
5.50 +0.40
-+
107 4
1.16 +0.05
1.65 +0.02
630 -+ 30
5.90 -+0.40
+
107 4
1.43 +0.06
1.97 +0.02
-+
80 5
0.98 +0.06
+
83 5
1.33 +0.04
1.82 -+0.02
+
1.28 5
1.54 +0.06
+
83 5
1.26 + 0.05
1.71 -+0.02
780 + 40
8.50 + 0.40
+
93 4
1.34 •+0.04
1.93 +-0.02
-+
104 5
0.50 -+0.04
210 + 15
1.55 -+0.06
2.06 +0.02
-+
40 2
0.31 +0.02
+-
1.28 +0.05
1.95 +-0.02
+
194 7
0.71 +0.04
275 -+ 15
1.68 -+0.06
2.31 -+0.02
148 -+ 13
0.52 -+0.03
285 -+ 30
B-l-13a
B-l-13b
B-l-13c
B-l-14a
B-l-14b *
B-l-14e
B-l-15a
B-l-15c
B-1-16a
B-l-16c
* No ultrasonic cleaning used.
4 He/Z° Ne
130 7
301
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
ZENITH TOWARDS EDGE COMPARTMENT A OF
I i
ZEMTH
TOWARDS EDGE OF COMPARTMENT A
\
SUNRISE DIRECTION
~
x ~-200 0 100
0.5
rnm
I
,
[o
10"a
R
I
cm3$TPHe4on12
E
-200 x 10-8 cm3 STP He4 cm-2 0
He4//Ne20- rotio
0.5 mm I
Fig. 3.4He concentrations and 4He/2°Ne ratios measured in small sectors cut from ring section B-l-1 (same view as in fig. 2). No corrections for Ne blanks of the Al-tube have been made. Orientation of tube on lunar surface according to Carroll [ 1 ].
S
r 1°° i
He4/Ne2°-rati°
It O
Fig. 4. 4He concentrations and 4He/2°Ne ratios measured in larger sectors cut from ring section B-l-2 (same view as in fig. 2). No corrections for Ne blanks of the Al-tube have been made. Orientation of tube on lunar surface according to Carroll [ 1 ].
Table 2. Results of noble gas measurements on two larger sectors cut from unpainted Surveyor 3 Al-tube (cf. figs. 1, 2 and 4). Values not corrected for Ne blank of Al-tube. 4He/aHe
2°Ne/22Ne
22Ne/21Ne
4He/2°Ne
0.51 +0.03
2780 + 140
13.2 +- 0.4
30 + 5
280 + 10
0.42 +0.03
2770 + 120
13.3 -+ 0.4
32 -+ 6
315 + 15
Sample no.
Area (mm 2)
Weight (mg)
4He 2°Ne (10 -8 cm a STP/cm z)
B-l-21b
8.00 +0.25
6.21 +0.02
+
143 6
8.45 +0.40
7.23 +0.02
-+
133 7
B-1-26a
c o n c e n t r a t i o n was assumed to be blank. The blank correction was always smaller than 7%. The 4 He/2O Ne ratio we find in the trapped solar wind gas in the S u r v e y o r 3 material is nearly a factor of two lower than the ratio observed in the a l u m i n u m
of the A p o l l o 12 SWC foil [2]. Differences in the trapping efficiencies o f the SWC Al-foil and the Surveyor 3 Al-tube are e x p e c t e d to be small and c a n n o t account for the low 4 H e / 2 ° N e ratio found. A surface c o n t a m i n a t i o n by terrestrial 2 ° N e on the polished Surveyor 3 Al-tube could explain the difference.
302
F. Bfthler et aL, Trapped solar wind hefium and neon in Surveyor 3 material
Table 3. Maximum surface concentration and elemental and isotopic abundance ratios of trapped solar wind He and Ne in section B-1 of returned Surveyor 3 Al-tube. The figures given are corrected for the Ne blank in the aluminum. No correction for a possible residual lunar dust contamination or for diffusion loss was applied. 4Hema x
(180
+ 20) X 10 -8 cm3 STP/cm 2
4He/SHe
2770
+ 120
4He/2°Ne
295
+ 15
2°Ne/22Ne
13.3 +
22Ne/21Ne
31
ZENITH I
TOWARDS EDGE OF COMPARTMENT A
\
L/',, 5UNRtSE DIRECTION
0.4
U
-+ 5
However, the required Ne concentrations of more than 10 -9 cm 3 STP/cm 2 are orders of magnitude larger than the surface blanks observed on different kinds of Al-foils investigated in connection with the SWC experiment. Furthermore, the necessary correction for such a high terrestrial Ne blank would lead to a 2°Ne/22Ne ratio for the trapped solar wind particles considerably higher than the values observed in the Apollo 11 and 12 SWC experiments [2]. Diffusion loss of He or a residual contamination with lunar dust could explain the low ratio 4He/2°Ne = 295 found on the clean, sun-lit side of the Surveyor 3 strut. We estimate that around lunar noon the temperature of the strut has reached 120°C to 140°C. Trapped solar wind He begins to diffuse out of aluminum at these temperatures [4] and we cannot exclude that a sizable fraction of the trapped 4 He was lost. The subsequent discussion o f our Surveyor 3 results must thus consider the possibility o f preferential 4 He diffusion loss or a residual dust contamination. For comparison we have calculated the expected 4 He distribution around the tube and plotted it in fig. 5, tak!ng into account the oblique and variable angle of incidence of the solar wind ions and the passage of the moon through the earth's tail. The following assumptions were made: (1) The trapping probability of He is proportional to the cosine o f the angle of incidence a [4]. (2) The aberration and corotation of the solar wind is 3 ° [5,6].
)
THEORETICALTRAPPED SOLAR WIND He 4 D I S T R I ~ (FLUX 7* 10e;H#'CnTZSec'I) t
O,5mm
I
I00~ 10~ cm3 STP He~ om-2
I0
Fig. 5. Comparison of theoretical trapped 4He distribution with measured concentrations. Orientation of tube changed from orientation given by Carroll [ 1] to obtain best fit. For details of calculations and corrections used see text. The shape of the left side of the theoretical curve depends strongly on the exact location of compartment A. (3) The earth tail was assumed to be 50 earth radii wide at the lunar orbit; i.e. the Surveyor 3 landhag site is in the earth tail for solar wind zenith angles from 54 ° to 5 ° [7]. A negligible 4He flux was assumed for the earth's tail ([8] ;cf. also our subsequent discussion). (4) The 4 He solar wind flux was adjusted to agree with the observed maximum concentration o f 4He. The measured 4He concentration was corrected for a possible lunar dust contamination by assuming 4He/2°Ne = 460 (average o f Apollo 11 and 12 SWC experiments) in the trapped solar wind, and 4 He/2ONe 90 for the lunar dust. The resulting correction for possible lunar dust 4 He is small ( < 30%) for all sectors except 15 c. Virtually the same relative angular distribution is obtained if a different 4 He/2O Ne ratio for the trapped solar wind is assumed. 4 He diffusion loss
303
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
,~. . . . . . . . . . . ~ .... ~ ~ Table 4 ' Comparison of average #He solar wind fluxes. For details of the calculation of Surveyor 3 flux limits see text. Time period
Average 4 He flux
Ref.
Surveyor 3
April 20, 1967November 20, 1969
Between 7 X 106 and 13 X 106 cm-2 see-1
this paper
Apollo 11 SWC
July 21, 1969 03:35-04:52 GMT
(6.2 + 1.2) X 1 0 6
c m -2 see -1
[21
Apollo 12 SWC
November 19, 1969 12:35 GMTNovember 20, 1969 07:17 GMT
(8.1 + 1.0) X
em -2 see-1
[2]
Vela 3A and 3B
July 1965-July 1967
9 X 10 6
would also not change the shape of the 4 He angular distribution curve as the temperature of the Al-ring is estimated to be uniform within I°C. To obtain the best possible fit of the experimental data with the theoretical curve the probable orientation of the returned Al-tube on the lunar surface had to be changed somewhat. The best agreement is obtained by rotating the tube clockwise (as seen from end G) by 20 ° relative to the probable orientation given by Carroll [2]. The scribe line corresponds then to a solar zenith angle of 48 ° towards lunar east. The agreement obtained between the theoretical and the measured angular distribution of the implanted solar wind 4He is satisfactory, especially if one takes into account the uncertainty in the exact location of the earth's taft during the 2½ years of exposure of the Al-tube. For the most strongly irradiated sectors (1 lc, 1 ld and 16 a) the integrated e~posure is equivalent to 75 days exposure at orthogonal incidence. The average implanted 4He concentration of sectors 1 l c, 11 d and 16a, corrected for a possible lunar dust contamination as outlined above, corresponds to an average solar wind 4He flux of 7 X 1 0 6 cm -2 sec -1 (trapping efficiency 0.9 cos a). Because of the specific assumption made for the correction of a possible residual lunar dust contamination and because diffusion loss may have occurred, this flux value has to be considered as a lower limit of the true average 4 He flux during the exposure time. An appropriate upper limit of 13 X 106 cm -2 sec -1 is obtained if we assume that the low 4He/2°Ne ratio is due to preferential diffusion loss of 4 He from the Al-tube, with virtually no Ne loss, and that the true ratio for the trapped solar wind particles
10 6
c m -2 see -1
[91
is 4 He/2ONe = 460 (average of Apollo 11 and 12 SWC experiments). The solar wind 4 He fluxes measured by other experiments fall well within the possible flux range deduced from the Surveyor 3 material (cf. table
4). In table 5 the isotopic compositions of the solar wind during the exposure of the Surveyor 3.material, as derived from our measurements compiled in table 3, are given. It was assumed that the Surveyor 3 aluminum had the same trapping properties as the SWC Al-foil [2]. Isotopic fractionation due to diffusion loss and the effects of a possible residual lunar dust contamination were neglected and will be discussed in detail. For comparison the solar wind compositions as measured by the Apollo 11 and 12 SWC experiments are given. The 4 He/aHe ratio obtained from the Surveyor 3 material is higher than the ratios measured with the Apollo 11 and 12 SWC experiments. It could be that time variations of this ratio [2,10] are responsible for this difference, i.e. the long-time average of the 4He/a He ratio is higher than the two values found during the Apollo 11 and 12 missions. However, the 4He/a He ratio in the Surveyor 3 strut could very well have been altered and, thus, does not necessarily represent a true solar wind average. The following effects have to be considered: (a) Spallation by cosmic rays or solar protons; (b) Stripping of cosmic-ray or energetic solar a particles; (c) Recycling of solar wind He and radiogenic Ne; (d) He from the terrestrial atmosphere; (e) Mass discrimination near the moon; (f) Mass dependence of trapping probability; (g) Diffusion; (h) Contamination by lunar dust.
304
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
Table 5 Isotopic composition of solar wind derived from trapped gases in the unpainted Al-tube recovered from Surveyor 3. SWC-data from Geiss et al. [2]. Surveyor 3 data were not corrected for effects of diffusion loss or for possible residual lunar dust contamination (see text). All values represent averages over respective exposure periods. SWC experiments Surveyor 3 4He/aHe
2700
-+ 130
2°Ne/22Ne
13.3 -+ 0.4
22Ne/21Ne
31
-+
5
(a) The spallation rate induced by cosmic rays at the lunar surface is ~ 10 -14 cm 3 3He/g year. Even if we assume that solar protons produce on the average 10 times more aHe, the relative contribution to the observed aHe is still only of the order o f 10 -4 . Also for 21 Ne the contribution from spallation is negligible. (b) It is readily estimated that 3He produced from 4 He in stripping reactions in the Surveyor 3 material can be neglected. (c) Released lunar radiogenic 4 He and trapped solar wind He could be recycled and retrapped in solid material at the lunar surface in the same way as 4°At is retrapped [ 1 1 - 1 3 ] . Estimates show that the influence o f this process on the 4He/3 He ratio in the Surveyor strut should be negligible. The efficiency of the process for He is even smaller than for Ar since most of the He is lost from the moon by gravitational escape before it is ionized. Moreover, the orientation of the strut is such that it is a poor collector for accelerated lunar ions which should preferentially arrive nearly horizontally from the southern or northern direction. (d) It is possible that the terrestrial atmosphere loses most o f its helium by way o f the polar wind [14]. If these helium ions escape from the earth through the magnetospheric tail, then the moon would encounter a flux of 4He* of terrestrial origin for a few days every month. With Axford's [ 14] estimate o f the helium flux in the polar wind, we obtain an upper limit for the terrestrial 4He÷ flux in the tail o f 104 cm -2 sec -1 . The effect o f this flux in the Surveyor 3 material is negligible. (e) Mass discrimination could result from disturbances o f the electromagnetic field in the solar wind
Apollo 11
Apollo 12
1860
2450
-+ 140
13.5 +-
1.0
+- 100
13.1 -+ 0.6 26
-
12
near the moon and also from the static 35 3' field found near the Apollo 12 site by Dyal et al. [15]. However, the equality of the 4 He/3He ratios found in the sectors 21b and 26a renders any significant mass discrimination unlikely. (f) For small angles of incidence the trapping probability of 4 He in aluminum is 90% [4], i.e. 10% of the incoming 4 He ions are backscattered. In evaluating the 4 He/3 He ratio, we have assumed for 3 He a somewhat larger backscattering coefficient (12%). An error in the estimate of such a small backscattering coefficient does not affect the trapping probability very much. However, at large angles o f incidence, the trapping probability for helium falls below 50%, and here the difference between 4Ite and 3 He might be appreciable. (g) We have concluded above that some of the helium on the sun-lit side o f the strut may have been lost by thermal diffusion. This could have led to a depletion o f a He relative to 4 He. The reasons are twofold: The average depth of implantation d is smaller for 3He than for 4He, and the diffusion constant o f a He is larger than that o f 4 He. From the range formula given by Nielsen [16], we estimate d 3 ~ 0.91 d 4. Assuming diffusion constants inversely proportional to the square root of the mass, the characteristic parameter d/Dl/2 for 3He is 15% smaller than for 4He. The resulting isotopic enrichment is a function of the loss fraction and o f diD 1/2. For a loss of 50% o f 4He we estimate an isotopic fractionation of 5 to 10%. (h) The lunar fine material at the Apollo 12 landing site has a ratio 4 He/3 He ~ 2300 ([17] and unpublished Bern data). We expect that this ratio will depend on the grain size, similar to the observations
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
made for the Apollo 11 fine material [ 12]. The very fine material, which has to be considered as possible source of a remaining lunar dust contamination, should have a higher 4 He/3 He ratio. The maximum possible lunar dust contamination, as deduced from the 4 He/ 2°Ne ratios, would necessitate a correction o f approximately 3% of the measured 4 He/3 He ratios (sectors 21b and 26 a, lunar dust 4He/3He = 2300 assumed). The true solar wind 4 He/3 He ratio, averaged over the Surveyor 3 exposure time could thus be as high as 2800 -+ 130. The effects (d) to (g) all enrich 4 He relative to 3 He. The combined effect can hardly be more than 10% and the true solar wind 4 He/3 He ratio, averaged over the Surveyor 3 exposure period must be higher than 2400. As upper limit, for the case o f the maximum possible lunar dust contamination we obtain a value of 2800. The Surveyor 4 He/a He ratio is thus distinctly higher than the value measured during the Apollo 11 EVA and probably also higher than the value determined during the Apollo 12 mission. The Surveyor 4He/3 He ratio agrees quite well with the value derived from the ilmenite o f the Apollo 11 lunar fine material [12]. However, the Surveyor 3 exposure time is only a small fraction o f a solar cycle. We may expect that the 4He/a He ratio varies with the solar cycle and the Surveyor results are not necessarily a good long time average o f the present day solar wind 4 He/a He ratio. The neon isotopic composition obtained in the Surveyor 3 material agrees within the limits o f error with the results o f the Apollo 11 and 12 SWC experiments. If the relatively low 4 He/2O Ne ratio found should be due to a residual dust contamination, then the resulting correction would raise the 2°Ne/22Ne ratio by a few percent. A comparison of the 2°Ne: 21Ne: 22Ne ratios shows that the large difference in the isotopic abundances o f neon in the terrestrial atmosphere and in the solar wind is mainly due to mass fractionation and not to nuclear reactions. This confirms a conclusion which was drawn from data obtained in lunar fine material [12].
Acknowledgments We would like to thank the National Aeronautics
305
and Space Administration for providing Surveyor 3 material for analyses and in particular the Apollo 12 astronauts Charles Conrad, Jr., Alan L. Bean and Richard F. Gordon. We are grateful to Drs. N.L. Nickle and B. Carroll for their support. We would like to thank Dr. H.U. Nissen and R. Wessicken from the Laboratorium ftir Elektronenmikroskopie, ETH, Ziirich and Dr. N. Gr6gler from our institute for the supporting scanning electron microscope investigations. The help of H. Wyniger in sample preparation is acknowledged. This work was supported b y the Swiss National Science Foundation (grants N F 5079.2 and 2.213.69).
References [ 1] W. Carroll, Communication by N.L. Nickle, October 1970. [2] J. Geiss, P. Eberhardt, F. Bfihler, J. Meister and P. Signer, Apollo 11 and 12 solar wind composition experiments: Fluxes of He and Ne isotopes, J. Geophys. Res. 75 (1970) 5972. [3] The Lunar Sample Preliminary Examination Team, Preliminary examination of lunar samples, Apollo 12 Prelim. Sci. Rept., NASA SP-235 (1970) 189. [4] J. Meister, F. Biihler, P. Eberhardt and J. Geiss, Apollo solar wind composition experiment: Trapping of low energy ions in aluminum, to be published (1971). [5] A.J. Hundhausen, Direct observations of solar wind particles, Space Sci. Rev. 8 (1968) 690. [6] W.I. Axford, Observations of the interplanetary plasma, Space Sci. Rev. 8 (1968) 331. [7] J.D. Mihalov, D.S. Colburn and C.P. Sonett, Observations of magnetopause geometry and waves at the lunar distance, Planet. Space Sci. 18 (1970) 239. [8] C.W. Snyder, D.R. Clay and M. Neugebauer, The solarwind spectrometer experiment, Apollo 12 Prelim. Sci. Rept., NASA SP-235 (1970) 75. [9] D.E. Robbins, A.J. Hundhausen and S.J. Bame, Helium in the solar wind, J. Geophys. Res. 75 (1970) 1178. [10] S.J. Bame, A.J. Hundhausen, I.R. Asbridge and I.B. Strong, Solar wind ion composition, Phys. Rev. Letters 20 (1968) 393. [11] D. Heymann and A. Yaniv, 4°Ar anomaly in lunar samples from Apollo 11, Geochim. Cosmochim. Acta, Suppl. 1, 2 (1970) 1261. [12] P. Eberhardt, J. Geiss, H. Graf, N. Gr6gler, U. Kr~ihenbiihl, H. SchwaUer, J. Schwarzmiiller and A. Stettler, Trapped solar wind noble gases, exposure age and K/Atage in Apollo 11 lunar fine material, Geochim. Cosmochim. Acta, Suppl. 1, 2 (1970) 1037.
306
F. Bi~hler et al., Trapped solar wind helium and neon in Surveyor 3 material
[13] R.H. Manka and F.C. Michel, Lunar atmosphere as a source of argon-40 and other lunar surface elements, Science 169 (1970) 278. [14] W.I. Axford, The polar wind and the terrestrial helium budget, J. Geophys. Res. 73 (1968) 6855. [15] P. Dyal, C.W. Parkin and C.P. Sonett, Apollo 12 magnetometer:Measurement,of a steady magnetic field on the surface of the moon, Science 169 (1970) 762.
[16] K.O. Nielsen, The range of atomic particles with energies about 50 k,eV, in: Elect~omagnetically Enriched Isotopes and Mass Spectrometry, ed. by M.L. Smith, (Butterworths Scientific Publications, London, 1956) p. 68. [17] Lunar Sample Preliminary Examination Team, Preliminary examination of lunar samples from Apollo 12, Science 167 (1970) 1325.
TRAPPED
SOLAR WIND HELIUM AND NEON
IN SURVEYOR
3 MATERIAL
F. BOHLER, P. EBERHARDT, J. GEISS and J. SCHWARZMOLLER Physikalisches Institut, University o f Bern, 3000 Bern, Switzerland
Received 5 January 1971
The Apollo 12 astronauts salvaged an unpainted Al-tube from the Surveyor 3 spacecraft. We measured He and Ne in sectors cut from a small ring from this tube. The lower side of the tube is contaminated by lunar dust, which cannot be completely removed by ultrasonic cleaning. The upper half shows very little or no dust contamination and contains trapped solar wind He and Ne with 4He/2°Ne = 295 in the trapped4gas. This value is lower than the ratios measured by the Apollo 11 and 12 SWC experiments. This could be due to He diffusion loss or to a small residual dust contamination. The 4He distribution around the Al-tube is in agreement with the theoretically expected distribution and corresponds to an average solar wind 4 H e flux of 7 X 10 6 c m -2 sec -1 . I f 4 H e diffusion loss had occurred the average 4 H e flux could be as high 13 X 10 6 c m -2 sec -1 . Neglecting the small influences of a possible dust contamination or of diffusion loss we obtain the following average isotopic compositions for the solar wind during the exposure of the Surveyor 3 material: 4He/3He = 2700 + 130; ~°Ne/22Ne = 13.3 + 0.4 and 22Ne/21Ne = 31 + 5. Compared with the Apollo 11 and 12 SWC results the 4He/3He ratio is unexpectedly high.
On April 20, 1967, the unmanned spacecraft Surveyor 3 landed on the lunar surface in the Oceanum Procellarum (3.2°S; 23.4 ° W). 2½ years later, on November 20, 1969, the Apollo 12 astronauts Charles Conrad and Alan L. Bean recovered several pieces o f this Surveyor spacecraft and returned them to Earth. Among these was a section of a support strut, a 12.7 m m o.d. tube o f polished, unpainted aluminum (alloy 2024) with 1.2 m m wall thickness. This unpainted tube was salvaged with the specific aim of investing implanted solar wind particles. The location of the returned section of the strut on the Surveyor spacecraft is shown in fig. l . We have received from the Jet Propulsion Laboratory, Pasadena, California, a small ring from this tube. The ring, designated B-1, was approximately 2 m m wide and was located about 41 mm from the A-end o f the Al-tube. Our measurements on section B-1 represent a preliminary investigation with the aim o f establishing the presence of trapped noble gases from the solar wind in the aluminum surface and o f measuring the
abundances of the light noble gases. Furthermore, contamination by lunar dust and the distribution o f the trapped solar wind around the tube had to be studied. Our measurements give the necessary information for subsequent, more detailed studies o f the implanted solar wind gases in Surveyor 3 materials. Our analytical sensitivity is sufficiently high to allow 4He and 2°Ne determinations in a I m m 2 sample. Consequently, we cut the ring B-1 into two rings o f about equal width (cf. fig. 2). A wire saw with a 0.2 m m diamond-impregnated stainless steel wire was used. We removed the aluminum from the inside of these tings, reducing the weight by about 50%. This was done in order to lower the noble gas blank from the aluminum. The rings were then cut into sectors (cf. figs. 2, 3 and 4) and thoroughly cleaned b y repeated ultrasonic treatment in acetone. Inspection under the optical microscope and investigation with a scanning electron microscope revealed that approximately one half o f the B-1 ring shows surface alterations and is, moreover, contaminated
298
F. Bi~hler et aL, Trapped solar wind helium and neon in Surveyor 3 material
Fig. 1. Original location in spacecraft of returned part of Surveyor 3 strut (enlargement from NASA photograph AS 12-48-7114). with fine crystalline particles, presumably of lunar origin. The ultrasonic treatment reduced this contamination, but did not completely eliminate it. No dust particles were found after the ultrasonic treatment on sector 16 d taken from the uncontaminated side of B-1. So far, the noble gases He and Ne have been determinated in a number of sectors. The measurement pro-
cedure and the analytical blanks were the same as for the foil analyses of the Solar Wind Composition (SWC) experiment [2]. The results are given in tables 1 and 2, and the distribution around the ring is shown in figs. 3 and 4. No corrections for He and Ne from lunar dust contamination or for blanks in the aluminum were applied.
F. B~thler et al. Trapped solar wind helium and neon in Surveyor 3 material
SCRATCH
~
299
SCRIBE
R B-1-26a
SCRIBE SCRATCH
e "~
B- I -2
j~ J J J B-1-1
Fig. 2. Orientation of sectors cut from ring section B-1.
The probable orientation of the ring B-1 at the lunar surface given by Carroll [ 1] was adopted for figs. 3 and 4. Accordingly, the contaminated side of the aluminum tube essentially faced the lunar surface, whereas the clean side was sun-lit and, thus, exposed to the solar wind. We assume that the contamination and surface alteration occurred during landing as the vernier-engines were cut off only 34 sec after the initial touchdown. Our He and Ne data do essentially support the probable orientation given by Carroll [ 1]. In the Apollo 12 lunar fine material [3] and also in lunar dust adhering to the Apollo 12 SWC foil, the He/Ne ratios are below 100. Thus, He and Ne found on the lower part of the ring (cf. fig. 3) are readily explained as resulting from residual dust contamination. On the sun-lit side we find 4He/2°Ne ~ 300, which is a much higher ratio than in the lunar fine material and closer to the 4He/2°Ne ratio in the solar wind [2]. Taking into account the observed high He/Ne ratio, the smoothness of the He distribution obtained on the sun-lit side, and the fact that a thorough investigation
with the scanning electron microscope on sector 16 d did not reveal any lunar dust particles after ultrasonic treatment, we conclude that the He data obtained on the sun-lit side of the ring represent the solar wind particles implanted in the aluminum and that in this area the contamination from the dust is minimal or absent. In table 3 averages for the trapped solar wind He and Ne in the Surveyor 3 material are given. The maximum 4 He surface concentration is the average of sectors 1 lc, I ld and 16a (cf. fig. 3). The 4He/2°Ne ratio is the average of the six sectors with the highest 4He/2°Ne ratios (cf. tables 1 and 2). All other ratios are averages of the two large sectors 21b and 26a (cf. table 2). The values in table 3 were corrected for the Ne blank of the aluminum tube. This blank correction is based on the noble gas concentration found in aluminum turnings removed from the inside of ring B-1 (measured concentrations: (19 -+ 5) × 10 -8 cm a STP 4He/g; (0.4 +- 0.1) × 10 -8 cm 3 STP 2°Ne/g). As the 4He/2°Ne ratio of 50 indicates contamination with lunar dust, only 30% of the measured 2°Ne
300
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
Table t Results of 4He and 2°Ne measurements in sectors cut from unpainted Surveyor 3 Al-tube (cf. figs. 1, 2 and 3). Values not corrected for Ne blank of Al-tube. Sample no.
Area (mm 2)
Weight (mg)
4 He 20 Ne (10 -8 cm a STP/cm 2)
B-l-lla
1.06 +0.0.3
1.94 +0.02
+
140 6
0.66 -+0.17
210 + 50
B-l-llc
0.98 +0.04
1.49 -+0.02
180 + 10
0.62 +0.04
290 + 10
B-l-lld
0.88 +-0.06
1.42 -+0.02
165 + 13
0.58 +0.04
285 -+ 15
B-l-12a
1.29 +0.04
2.00 +0.02
-+
204 9
2.15 +0.30
95 + 13
1.38 +0.05
2.00 -+0.02
570 + 30
5.60 +0.30
-+
102 5
1.48 +0.08
2.00 -+0.02
590 -+ 40
5.50 +0.40
-+
107 4
1.16 +0.05
1.65 +0.02
630 -+ 30
5.90 -+0.40
+
107 4
1.43 +0.06
1.97 +0.02
-+
80 5
0.98 +0.06
+
83 5
1.33 +0.04
1.82 -+0.02
+
1.28 5
1.54 +0.06
+
83 5
1.26 + 0.05
1.71 -+0.02
780 + 40
8.50 + 0.40
+
93 4
1.34 •+0.04
1.93 +-0.02
-+
104 5
0.50 -+0.04
210 + 15
1.55 -+0.06
2.06 +0.02
-+
40 2
0.31 +0.02
+-
1.28 +0.05
1.95 +-0.02
+
194 7
0.71 +0.04
275 -+ 15
1.68 -+0.06
2.31 -+0.02
148 -+ 13
0.52 -+0.03
285 -+ 30
B-l-13a
B-l-13b
B-l-13c
B-l-14a
B-l-14b *
B-l-14e
B-l-15a
B-l-15c
B-1-16a
B-l-16c
* No ultrasonic cleaning used.
4 He/Z° Ne
130 7
301
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
ZENITH TOWARDS EDGE COMPARTMENT A OF
I i
ZEMTH
TOWARDS EDGE OF COMPARTMENT A
\
SUNRISE DIRECTION
~
x ~-200 0 100
0.5
rnm
I
,
[o
10"a
R
I
cm3$TPHe4on12
E
-200 x 10-8 cm3 STP He4 cm-2 0
He4//Ne20- rotio
0.5 mm I
Fig. 3.4He concentrations and 4He/2°Ne ratios measured in small sectors cut from ring section B-l-1 (same view as in fig. 2). No corrections for Ne blanks of the Al-tube have been made. Orientation of tube on lunar surface according to Carroll [ 1 ].
S
r 1°° i
He4/Ne2°-rati°
It O
Fig. 4. 4He concentrations and 4He/2°Ne ratios measured in larger sectors cut from ring section B-l-2 (same view as in fig. 2). No corrections for Ne blanks of the Al-tube have been made. Orientation of tube on lunar surface according to Carroll [ 1 ].
Table 2. Results of noble gas measurements on two larger sectors cut from unpainted Surveyor 3 Al-tube (cf. figs. 1, 2 and 4). Values not corrected for Ne blank of Al-tube. 4He/aHe
2°Ne/22Ne
22Ne/21Ne
4He/2°Ne
0.51 +0.03
2780 + 140
13.2 +- 0.4
30 + 5
280 + 10
0.42 +0.03
2770 + 120
13.3 -+ 0.4
32 -+ 6
315 + 15
Sample no.
Area (mm 2)
Weight (mg)
4He 2°Ne (10 -8 cm a STP/cm z)
B-l-21b
8.00 +0.25
6.21 +0.02
+
143 6
8.45 +0.40
7.23 +0.02
-+
133 7
B-1-26a
c o n c e n t r a t i o n was assumed to be blank. The blank correction was always smaller than 7%. The 4 He/2O Ne ratio we find in the trapped solar wind gas in the S u r v e y o r 3 material is nearly a factor of two lower than the ratio observed in the a l u m i n u m
of the A p o l l o 12 SWC foil [2]. Differences in the trapping efficiencies o f the SWC Al-foil and the Surveyor 3 Al-tube are e x p e c t e d to be small and c a n n o t account for the low 4 H e / 2 ° N e ratio found. A surface c o n t a m i n a t i o n by terrestrial 2 ° N e on the polished Surveyor 3 Al-tube could explain the difference.
302
F. Bfthler et aL, Trapped solar wind hefium and neon in Surveyor 3 material
Table 3. Maximum surface concentration and elemental and isotopic abundance ratios of trapped solar wind He and Ne in section B-1 of returned Surveyor 3 Al-tube. The figures given are corrected for the Ne blank in the aluminum. No correction for a possible residual lunar dust contamination or for diffusion loss was applied. 4Hema x
(180
+ 20) X 10 -8 cm3 STP/cm 2
4He/SHe
2770
+ 120
4He/2°Ne
295
+ 15
2°Ne/22Ne
13.3 +
22Ne/21Ne
31
ZENITH I
TOWARDS EDGE OF COMPARTMENT A
\
L/',, 5UNRtSE DIRECTION
0.4
U
-+ 5
However, the required Ne concentrations of more than 10 -9 cm 3 STP/cm 2 are orders of magnitude larger than the surface blanks observed on different kinds of Al-foils investigated in connection with the SWC experiment. Furthermore, the necessary correction for such a high terrestrial Ne blank would lead to a 2°Ne/22Ne ratio for the trapped solar wind particles considerably higher than the values observed in the Apollo 11 and 12 SWC experiments [2]. Diffusion loss of He or a residual contamination with lunar dust could explain the low ratio 4He/2°Ne = 295 found on the clean, sun-lit side of the Surveyor 3 strut. We estimate that around lunar noon the temperature of the strut has reached 120°C to 140°C. Trapped solar wind He begins to diffuse out of aluminum at these temperatures [4] and we cannot exclude that a sizable fraction of the trapped 4 He was lost. The subsequent discussion o f our Surveyor 3 results must thus consider the possibility o f preferential 4 He diffusion loss or a residual dust contamination. For comparison we have calculated the expected 4 He distribution around the tube and plotted it in fig. 5, tak!ng into account the oblique and variable angle of incidence of the solar wind ions and the passage of the moon through the earth's tail. The following assumptions were made: (1) The trapping probability of He is proportional to the cosine o f the angle of incidence a [4]. (2) The aberration and corotation of the solar wind is 3 ° [5,6].
)
THEORETICALTRAPPED SOLAR WIND He 4 D I S T R I ~ (FLUX 7* 10e;H#'CnTZSec'I) t
O,5mm
I
I00~ 10~ cm3 STP He~ om-2
I0
Fig. 5. Comparison of theoretical trapped 4He distribution with measured concentrations. Orientation of tube changed from orientation given by Carroll [ 1] to obtain best fit. For details of calculations and corrections used see text. The shape of the left side of the theoretical curve depends strongly on the exact location of compartment A. (3) The earth tail was assumed to be 50 earth radii wide at the lunar orbit; i.e. the Surveyor 3 landhag site is in the earth tail for solar wind zenith angles from 54 ° to 5 ° [7]. A negligible 4He flux was assumed for the earth's tail ([8] ;cf. also our subsequent discussion). (4) The 4 He solar wind flux was adjusted to agree with the observed maximum concentration o f 4He. The measured 4He concentration was corrected for a possible lunar dust contamination by assuming 4He/2°Ne = 460 (average o f Apollo 11 and 12 SWC experiments) in the trapped solar wind, and 4 He/2ONe 90 for the lunar dust. The resulting correction for possible lunar dust 4 He is small ( < 30%) for all sectors except 15 c. Virtually the same relative angular distribution is obtained if a different 4 He/2O Ne ratio for the trapped solar wind is assumed. 4 He diffusion loss
303
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
,~. . . . . . . . . . . ~ .... ~ ~ Table 4 ' Comparison of average #He solar wind fluxes. For details of the calculation of Surveyor 3 flux limits see text. Time period
Average 4 He flux
Ref.
Surveyor 3
April 20, 1967November 20, 1969
Between 7 X 106 and 13 X 106 cm-2 see-1
this paper
Apollo 11 SWC
July 21, 1969 03:35-04:52 GMT
(6.2 + 1.2) X 1 0 6
c m -2 see -1
[21
Apollo 12 SWC
November 19, 1969 12:35 GMTNovember 20, 1969 07:17 GMT
(8.1 + 1.0) X
em -2 see-1
[2]
Vela 3A and 3B
July 1965-July 1967
9 X 10 6
would also not change the shape of the 4 He angular distribution curve as the temperature of the Al-ring is estimated to be uniform within I°C. To obtain the best possible fit of the experimental data with the theoretical curve the probable orientation of the returned Al-tube on the lunar surface had to be changed somewhat. The best agreement is obtained by rotating the tube clockwise (as seen from end G) by 20 ° relative to the probable orientation given by Carroll [2]. The scribe line corresponds then to a solar zenith angle of 48 ° towards lunar east. The agreement obtained between the theoretical and the measured angular distribution of the implanted solar wind 4He is satisfactory, especially if one takes into account the uncertainty in the exact location of the earth's taft during the 2½ years of exposure of the Al-tube. For the most strongly irradiated sectors (1 lc, 1 ld and 16 a) the integrated e~posure is equivalent to 75 days exposure at orthogonal incidence. The average implanted 4He concentration of sectors 1 l c, 11 d and 16a, corrected for a possible lunar dust contamination as outlined above, corresponds to an average solar wind 4He flux of 7 X 1 0 6 cm -2 sec -1 (trapping efficiency 0.9 cos a). Because of the specific assumption made for the correction of a possible residual lunar dust contamination and because diffusion loss may have occurred, this flux value has to be considered as a lower limit of the true average 4 He flux during the exposure time. An appropriate upper limit of 13 X 106 cm -2 sec -1 is obtained if we assume that the low 4He/2°Ne ratio is due to preferential diffusion loss of 4 He from the Al-tube, with virtually no Ne loss, and that the true ratio for the trapped solar wind particles
10 6
c m -2 see -1
[91
is 4 He/2ONe = 460 (average of Apollo 11 and 12 SWC experiments). The solar wind 4 He fluxes measured by other experiments fall well within the possible flux range deduced from the Surveyor 3 material (cf. table
4). In table 5 the isotopic compositions of the solar wind during the exposure of the Surveyor 3.material, as derived from our measurements compiled in table 3, are given. It was assumed that the Surveyor 3 aluminum had the same trapping properties as the SWC Al-foil [2]. Isotopic fractionation due to diffusion loss and the effects of a possible residual lunar dust contamination were neglected and will be discussed in detail. For comparison the solar wind compositions as measured by the Apollo 11 and 12 SWC experiments are given. The 4 He/aHe ratio obtained from the Surveyor 3 material is higher than the ratios measured with the Apollo 11 and 12 SWC experiments. It could be that time variations of this ratio [2,10] are responsible for this difference, i.e. the long-time average of the 4He/a He ratio is higher than the two values found during the Apollo 11 and 12 missions. However, the 4He/a He ratio in the Surveyor 3 strut could very well have been altered and, thus, does not necessarily represent a true solar wind average. The following effects have to be considered: (a) Spallation by cosmic rays or solar protons; (b) Stripping of cosmic-ray or energetic solar a particles; (c) Recycling of solar wind He and radiogenic Ne; (d) He from the terrestrial atmosphere; (e) Mass discrimination near the moon; (f) Mass dependence of trapping probability; (g) Diffusion; (h) Contamination by lunar dust.
304
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
Table 5 Isotopic composition of solar wind derived from trapped gases in the unpainted Al-tube recovered from Surveyor 3. SWC-data from Geiss et al. [2]. Surveyor 3 data were not corrected for effects of diffusion loss or for possible residual lunar dust contamination (see text). All values represent averages over respective exposure periods. SWC experiments Surveyor 3 4He/aHe
2700
-+ 130
2°Ne/22Ne
13.3 -+ 0.4
22Ne/21Ne
31
-+
5
(a) The spallation rate induced by cosmic rays at the lunar surface is ~ 10 -14 cm 3 3He/g year. Even if we assume that solar protons produce on the average 10 times more aHe, the relative contribution to the observed aHe is still only of the order o f 10 -4 . Also for 21 Ne the contribution from spallation is negligible. (b) It is readily estimated that 3He produced from 4 He in stripping reactions in the Surveyor 3 material can be neglected. (c) Released lunar radiogenic 4 He and trapped solar wind He could be recycled and retrapped in solid material at the lunar surface in the same way as 4°At is retrapped [ 1 1 - 1 3 ] . Estimates show that the influence o f this process on the 4He/3 He ratio in the Surveyor strut should be negligible. The efficiency of the process for He is even smaller than for Ar since most of the He is lost from the moon by gravitational escape before it is ionized. Moreover, the orientation of the strut is such that it is a poor collector for accelerated lunar ions which should preferentially arrive nearly horizontally from the southern or northern direction. (d) It is possible that the terrestrial atmosphere loses most o f its helium by way o f the polar wind [14]. If these helium ions escape from the earth through the magnetospheric tail, then the moon would encounter a flux of 4He* of terrestrial origin for a few days every month. With Axford's [ 14] estimate o f the helium flux in the polar wind, we obtain an upper limit for the terrestrial 4He÷ flux in the tail o f 104 cm -2 sec -1 . The effect o f this flux in the Surveyor 3 material is negligible. (e) Mass discrimination could result from disturbances o f the electromagnetic field in the solar wind
Apollo 11
Apollo 12
1860
2450
-+ 140
13.5 +-
1.0
+- 100
13.1 -+ 0.6 26
-
12
near the moon and also from the static 35 3' field found near the Apollo 12 site by Dyal et al. [15]. However, the equality of the 4 He/3He ratios found in the sectors 21b and 26a renders any significant mass discrimination unlikely. (f) For small angles of incidence the trapping probability of 4 He in aluminum is 90% [4], i.e. 10% of the incoming 4 He ions are backscattered. In evaluating the 4 He/3 He ratio, we have assumed for 3 He a somewhat larger backscattering coefficient (12%). An error in the estimate of such a small backscattering coefficient does not affect the trapping probability very much. However, at large angles o f incidence, the trapping probability for helium falls below 50%, and here the difference between 4Ite and 3 He might be appreciable. (g) We have concluded above that some of the helium on the sun-lit side o f the strut may have been lost by thermal diffusion. This could have led to a depletion o f a He relative to 4 He. The reasons are twofold: The average depth of implantation d is smaller for 3He than for 4He, and the diffusion constant o f a He is larger than that o f 4 He. From the range formula given by Nielsen [16], we estimate d 3 ~ 0.91 d 4. Assuming diffusion constants inversely proportional to the square root of the mass, the characteristic parameter d/Dl/2 for 3He is 15% smaller than for 4He. The resulting isotopic enrichment is a function of the loss fraction and o f diD 1/2. For a loss of 50% o f 4He we estimate an isotopic fractionation of 5 to 10%. (h) The lunar fine material at the Apollo 12 landing site has a ratio 4 He/3 He ~ 2300 ([17] and unpublished Bern data). We expect that this ratio will depend on the grain size, similar to the observations
F. Bfthler et al., Trapped solar wind helium and neon in Surveyor 3 material
made for the Apollo 11 fine material [ 12]. The very fine material, which has to be considered as possible source of a remaining lunar dust contamination, should have a higher 4 He/3 He ratio. The maximum possible lunar dust contamination, as deduced from the 4 He/ 2°Ne ratios, would necessitate a correction o f approximately 3% of the measured 4 He/3 He ratios (sectors 21b and 26 a, lunar dust 4He/3He = 2300 assumed). The true solar wind 4 He/3 He ratio, averaged over the Surveyor 3 exposure time could thus be as high as 2800 -+ 130. The effects (d) to (g) all enrich 4 He relative to 3 He. The combined effect can hardly be more than 10% and the true solar wind 4 He/3 He ratio, averaged over the Surveyor 3 exposure period must be higher than 2400. As upper limit, for the case o f the maximum possible lunar dust contamination we obtain a value of 2800. The Surveyor 4 He/a He ratio is thus distinctly higher than the value measured during the Apollo 11 EVA and probably also higher than the value determined during the Apollo 12 mission. The Surveyor 4He/3 He ratio agrees quite well with the value derived from the ilmenite o f the Apollo 11 lunar fine material [12]. However, the Surveyor 3 exposure time is only a small fraction o f a solar cycle. We may expect that the 4He/a He ratio varies with the solar cycle and the Surveyor results are not necessarily a good long time average o f the present day solar wind 4 He/a He ratio. The neon isotopic composition obtained in the Surveyor 3 material agrees within the limits o f error with the results o f the Apollo 11 and 12 SWC experiments. If the relatively low 4 He/2O Ne ratio found should be due to a residual dust contamination, then the resulting correction would raise the 2°Ne/22Ne ratio by a few percent. A comparison of the 2°Ne: 21Ne: 22Ne ratios shows that the large difference in the isotopic abundances o f neon in the terrestrial atmosphere and in the solar wind is mainly due to mass fractionation and not to nuclear reactions. This confirms a conclusion which was drawn from data obtained in lunar fine material [12].
Acknowledgments We would like to thank the National Aeronautics
305
and Space Administration for providing Surveyor 3 material for analyses and in particular the Apollo 12 astronauts Charles Conrad, Jr., Alan L. Bean and Richard F. Gordon. We are grateful to Drs. N.L. Nickle and B. Carroll for their support. We would like to thank Dr. H.U. Nissen and R. Wessicken from the Laboratorium ftir Elektronenmikroskopie, ETH, Ziirich and Dr. N. Gr6gler from our institute for the supporting scanning electron microscope investigations. The help of H. Wyniger in sample preparation is acknowledged. This work was supported b y the Swiss National Science Foundation (grants N F 5079.2 and 2.213.69).
References [ 1] W. Carroll, Communication by N.L. Nickle, October 1970. [2] J. Geiss, P. Eberhardt, F. Bfihler, J. Meister and P. Signer, Apollo 11 and 12 solar wind composition experiments: Fluxes of He and Ne isotopes, J. Geophys. Res. 75 (1970) 5972. [3] The Lunar Sample Preliminary Examination Team, Preliminary examination of lunar samples, Apollo 12 Prelim. Sci. Rept., NASA SP-235 (1970) 189. [4] J. Meister, F. Biihler, P. Eberhardt and J. Geiss, Apollo solar wind composition experiment: Trapping of low energy ions in aluminum, to be published (1971). [5] A.J. Hundhausen, Direct observations of solar wind particles, Space Sci. Rev. 8 (1968) 690. [6] W.I. Axford, Observations of the interplanetary plasma, Space Sci. Rev. 8 (1968) 331. [7] J.D. Mihalov, D.S. Colburn and C.P. Sonett, Observations of magnetopause geometry and waves at the lunar distance, Planet. Space Sci. 18 (1970) 239. [8] C.W. Snyder, D.R. Clay and M. Neugebauer, The solarwind spectrometer experiment, Apollo 12 Prelim. Sci. Rept., NASA SP-235 (1970) 75. [9] D.E. Robbins, A.J. Hundhausen and S.J. Bame, Helium in the solar wind, J. Geophys. Res. 75 (1970) 1178. [10] S.J. Bame, A.J. Hundhausen, I.R. Asbridge and I.B. Strong, Solar wind ion composition, Phys. Rev. Letters 20 (1968) 393. [11] D. Heymann and A. Yaniv, 4°Ar anomaly in lunar samples from Apollo 11, Geochim. Cosmochim. Acta, Suppl. 1, 2 (1970) 1261. [12] P. Eberhardt, J. Geiss, H. Graf, N. Gr6gler, U. Kr~ihenbiihl, H. SchwaUer, J. Schwarzmiiller and A. Stettler, Trapped solar wind noble gases, exposure age and K/Atage in Apollo 11 lunar fine material, Geochim. Cosmochim. Acta, Suppl. 1, 2 (1970) 1037.
306
F. Bi~hler et al., Trapped solar wind helium and neon in Surveyor 3 material
[13] R.H. Manka and F.C. Michel, Lunar atmosphere as a source of argon-40 and other lunar surface elements, Science 169 (1970) 278. [14] W.I. Axford, The polar wind and the terrestrial helium budget, J. Geophys. Res. 73 (1968) 6855. [15] P. Dyal, C.W. Parkin and C.P. Sonett, Apollo 12 magnetometer:Measurement,of a steady magnetic field on the surface of the moon, Science 169 (1970) 762.
[16] K.O. Nielsen, The range of atomic particles with energies about 50 k,eV, in: Elect~omagnetically Enriched Isotopes and Mass Spectrometry, ed. by M.L. Smith, (Butterworths Scientific Publications, London, 1956) p. 68. [17] Lunar Sample Preliminary Examination Team, Preliminary examination of lunar samples from Apollo 12, Science 167 (1970) 1325.