D.C. E L E C T R I C A L
CONDUCTIVITY
OF L U N A R S U R F A C E R O C K S
F. C. SCHWERER, G. P. HUFFMAN, and R. M. FISHER U.S. Steel Research Center, Monroeville, Pa., U.S.A.
and TAKESI NAGATA Geophysics Research Laboratory, University of Tokyo, Japan
Electrical conductivities, o-(T), of several samples of lunar surface rocks returned by Apollo 11 and 12 missions have been measured from 300 K to 1100 K. Details of these studies have been reported by Nagata et aL, 1971 ; and Schwerer et al., 1971 and only salient features are reviewed here. Samples approximately 6 x 6 x 2 mm (current along short dimension) were cut from a coarse-grained crystalline rock (10024,22), a fine-grained rock (12053,47) and a microbreccia or fragmental rock (10048,55). Conductivities measured during primary heating cycles are represented in Figure 1 by heavy solid lines. Data are shown explicitly only for 10024,22; however, the scatter of these data about the line is representative of that found for other samples. Analytically, the lines in Figure 1 are described by two or three exponential terms of the form a ( T ) - - - ~ o -° e x p ( - E i / k T ) with conductivity prefactors and activation energies listed in Table I. This form for ~(T) is expected for thermally-activated ionic or electronic conduction processes; however, the type and source of charge carriers have not been definitely identified. Several experimental difficulties must be emphasized with respect to these data. One difficulty is illustrated in Figure 1 by the difference in conductivity measured for two samples from 12053,47. A plausible explanation is that in these studies the dimensions of the samples are such that a few large grains of a high-conductivity phase may dominate the observations. For example, specimen thickness is not much greater than the dimensions of some of the mineral phases present. A second experimental difficulty concerns large changes in the conductivity which are observed after heating to temperatures above approximately 500 °C. This thermal hysteresis was observed in all samples studied which included samples heated in vacuum (10 -6 T) and in Ar, He and He-2% H2 atmospheres and for samples heated alone in quartz capsules as well as in the conductivity holder and furnace. This hysteresis is illustrated in Figure 2 by the conductivity for a sample from 12053,47 during successive heating cycles to progressively higher maximum temperatures. The reason for this hysteresis is not known although several supplementary studies have provided addition information about the changes which occur. Magnetic measurements at 23 °C and from 5 K to 90 K for a sample from 10048,55 show that during heating to 800°C, a large fraction of the fine-particle native irons disappeared and that some redistribution of iron occurred in the pyroxene and, possibly ilmenite, Communication presented at the Conference on Lunar Geophysics, held between October 18-21, 1971, at the Lunar ScienceInstitute in Houston, Texas, U.S.A.
188
F. C. SCHWERER
ET AL.
TABLE I Conductivity prefactors cr~o a n d activation energies E~ for l u n a r surface Sample
rocks a~o (f2 cm) -1
E~ (eV)
10024, 22
7.9 3.1 8.2 1.96 2.6 8.5 5.3 1.53 3.13 1.4
0.51 1.25 0.35 0.48 3.2 0.59 2.3 0.17 0.55 1.6
10048, 55
12053, 47 (1) 12053, 47 (3)
10-2
900
× 10 -2 × 104 × 10 a × 10 .3 × 10t4 ~; 10 -~ x 103 × 10 -6 x 10 -2 × l0 s
TEMPERATURE,°C 500 200 I00
500
50
50
10-:
T
10E o i E
-~ 10b
~-10-' O zD
10-:
J
c~ 10-~
J
aa 10-9
I0- ic
I0-"
Fig. 1,
0.8
1.2
1.4
2.0 2.4 103/T (K) -L
2.8
3.2
Electrical conductivities of l u n a r surface rocks. D a t a for terrestrial olivine a n d peridotite f r o m E n g l a n d et aL (1968).
D. C. ELECTRICAL
900
CONDUCTIVITY
t
I
SURFACE
TEMPERATURE *C 300 200
500
i
OF LUNAR
I
I
I
ROCKS
I00
I
189
50 30
I
[
I
10-3
~\
10-4
(i)
~
\
MAX,MUM ,,o~c (41 4so°c
(2)2i0 C
I" -
(5) 600°C
b
> m
§
i0-5
i0-6
o
"x_
10-7 APOLLO 12 ROCK 12055,47
uJ d bd
SAMPLE 3
10 - 8
0.8
'~...... ~ ......... .. I "~-~_":...~22(I 2) 4 " ~ " ' ~ " ";" 't- / " " ~ ~
IO0°C/ hr. He--2-%Hz I
i
1.2
[
I
I
1.6
2.0 I031T
I
I
I
2.4 (K) H
[
I
2.8
I
I
I
3.2
Fig. 2. Electrical conductivity of sample (3) from Apollo lunar rock 12053, 47 during successive runs (1 through 6) to progressively higher maximum temperatures. Only the initial value is shown for run 3. Point 7 shows the final conductivity value at room temperature.
phases. Preliminary M6ssbauer studies at room and liquid nitrogen temperatures of samples from 12053, 47 show no large changes during heating. A slight, and expected (cf Hafner and Virgo, 1970), redistribution of Fe among cation sites occurs in the pyroxenes, and the formation of an extremely small amount of wustite (FeO) is indicated. Further M6ssbauer studies of these samples at temperatures below 2 K in small applied magnetic fields are in progress. Visual examination reveals darkened zones in the samples after heating. Electron microprobe observations are in progress and it is expected that the source of this thermal hysteresis in the conductivity measurements will soon be known. References England, A. W., Simmons, G., and Strangway, D. : 1968, J. Geophys. Res. 73, 3219. Hafner, S. S. and Virgo, D.: 1970, Proceedings of the Apollo 11 Lunar Science Conference, Vol. 3, Pergamon Press, p. 2183. Nagata, T., Rikitake, T., and Kono, M. : 1970, 'Proceedings of COSPAR Assembly Committee on Space Research', Space Sci. XI, Akademie-Verlag, Berlin. Schwerer, F. C., Nagata, T., and Fisher, R. M. : 1971, The Moon 2, 408.
