thermophysical properties of lunar material returned by apollo missions

T H E R M O P H Y S I C A L P R O P E R T I E S O F L U N A R M A T E R I A L

R E T U R N E D B Y A P O L L O M I S S I O N S

K I - I T I H O R A I *

The Lunar Science Institute, Houston, Tex., U.S.A.

and

N A O Y U K I F U J I I

Geophysical Institute, Faculty of Science, University of Tokyo, Tokyo, Japan

Abstract. Data on thermophysical properties measured on lunar material returned by Apollo missions are reviewed. In particular, the effects of temperature and interstitial gaseous pressure on thermal conductivity and diffusivity have been studied. For crystalline rocks, breccias and fines, the thermal conductivity and diffusivity decrease as the interstitial gaseous pressure decreases from 1 atm to 10-4T. Below 10-4T, these properties become insensitive to the pressure. At a pressure of 10-4T or below, the thermal conductivity of fines is more temperature dependent than that of crystalline rocks and breccias. The bulk density also affects the thermal conductivity of the fines. An empirical relationship between thermal conductivity, bulk density and temperature derived from the study of terrestrial material is shown to be consistent with the data on lunar samples. Measurement of specific heat shows that, regardless of the differences in mineral composition, crystalline rocks and fines have almost identical specific heat in the temperature range between 100 and 340K. The thermal parameter calculated from thermal conductivity, density and specific heat shows that the thermal properties estimated by earth-based observations are those characteristic only of lunar fines and not of crystalline rocks and breccias. The rate of radioactive heat generation calculated from the content of K, Th and U in lunar samples indicates that the surface layer of the lunar highland is more heat-producing than the lunar maria. This may suggest fundamental differences between the two regions.

1. Introduction

Our knowledge of the rmal proper t ies o f lunar surface mater ia l has been substant ia l ly

increased since the samples o f lunar surface mater ia l , collected and re turned to the

Ear th by Apo l lo missions, became avai lable for direct exper imenta l measurement in

the l abora to ry .

So far, four missions (Apo l lo 11, 12, 14 and 15) have been successfully comple ted

and a to ta l o f 176 kg of lunar mate r ia l has been obta ined. On July 20, 1969, the Apo l lo

11 lunar module (LM) landed in the southwestern pa r t o f M a r e Tranqui l l i ta t i s at

0 .67°N and 23.49°E. A tota l of 21.5 kg o f lunar mater ia l was collected dur ing the

ext ravehicular act ivi ty (EVA) and was re tu rned to the Ear th . The lunar mater ia ls con-

sist o f rocks and soil. The Luna r Sample p re l iminary Examina t i on Team (LSPET,

1969), which u n d e r t o o k the pre l iminary examina t ion o f the lunar mater ia l , made a

p re l iminary classif icat ion of the samples. Accord ing to this classification, type A refers

to the f ine-grained vesicular igneous rocks, type B to the med ium-gra ined vuggy igneous

* Now at Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York, U.S.A.

Communication presented at the Conference on Lunar Geophysics, held between October 18-21, 1971, at the Lunar Science Institute in Houston, Texas, U.S.A.

448 K I - I T I H O R A I A N D N A O Y U K I F U J I I

rocks, type C to the breccia, and type D to the fines which are, by definition, the fragments less than 1 cm in diameter.

Two drive tube core samples will deserve a somewhat detailed description. They are the samples of lunar regolith obtained by driving an aluminum tube, 31.75 cm in length and 1.95 cm in diameter, into the lunar surface layer. Core No. 1 (sample 10005-0), 10 cm long, contains 22.39 g of material. Core No. 2 (sample 10004-0), 13.5 cm long, weighs 26.73 g. The bulk densities are 1.70___0.04 g/cm 3 and 1.58_+ 0.04 g/cm 3, respectively, (Fryxell et al., 1970a, b). The core material is loose, weakly cohesive, and consists of single grains except for minor aggregates of glass. Texturally, the core consists of a silty fine sand with an average grain size of about 0.11 ram. Ad- mixed with the sandy matrix are fragments of rock, glass spherules, and aggregates of glass with the maximum size of 3 mm or more. The largest glass aggregate contained in core No. 2 is 1.2 cm in diameter. The Apollo 11 lunar fines are characterized by high glass content about 50~.

The Apollo 12 LM landed on the northwest rim of the 'Surveyor' crater, south- southwest of Copernicus in Oceanus Procellarum, at 23.4 °W and 3.2 °S on November 19, 1969 (LSPET, 1970). A total of 34.3 kg of lunar materials was collected by this mission. They contain 4.5 kg of fines (materials less than 1 cm in size), less than 1.0 kg of chips (materials between 1 cm and 4 cm in size), 29.0 kg of rocks (materials larger than 4 cm in size) and two core tubes, 19 cm (sample 12026) and 40 cm (sample 12028) in length.

Reflecting the different geological environments of the landing sites, the Apollo 12 materials are contrasted with the Apollo 11 materials in various ways. For example, the Apollo 12 rock samples are predominantly crystalline as opposed to the Apollo 11 rocks about half of which are microbreccia. The modal mineralogy and the primary texture of Apollo 12 crystalline rocks show much wider varieties than those of Apollo 11 crystalline rocks. The lunar regolith, which is 3 to 6 m in thickness at the Apollo 11 landing site, is about one-half as thick at the Apollo 12 landing site. Apollo 12 fines show a chemical composition which is similar to that of Apollo 12 breccias but is different from that of Apollo 12 crystalline rocks. This feature is not pronounced in the Apollo 11 samples. Glass content in the Apollo 12 fines is about 20~o as contrasted with 50~ in the Apollo 11 fines. Both of the Apollo 12 core samples show stratigraphy which is indicated by the abrupt change in the content of coarser rock fragments and glass spherules. In sample 12028, at least 10 layers or horizons can be recognized. The most distinctive one is a 2 cm layer at the depth of 12.5 cm consisting of angular rock fragments, minerals and glass with a median grain size of about 5 mm. Overall, the mean grain size increases with depth. Bulk densities of Apollo 12 core samples are: 1.74 g/cm 3 for sample 12026, 1.98 g/cm 3 for upper portion of 12028, and 1.96 g/cm 3 for lower portion of 12028 (Scott et aI., 1971 ; Carrier et al., 1971).

Dark mare regions and bright highlands are the fundamental physiographic units of the lunar surface. The earlier Apollo landing sites were in the mare regions, whereas the Apollo 14 landing site (3°40'19"S, 17°27'46"W) was in the highland area, 1230 km south of the center of the Mare Imbrium basin, where LM arrived on February 5, 1971

THERMOPHYSICAL PROPERTIES OF LUNAR MATERIAL 449

(LSPET, 1971). The Apollo 14 landing site is situated on the Fra Mauro formation, which is part of the ejecta derived from the Mare Imbrium basin when it was excavated by meteoritic impact preceding the formation of the Mare Imbrium. Therefore, the Fra Mauro Formation is thought to represent pre-mare material.

A total of 42.9 kg of rock and soil samples were collected from 19 sampling sites along the traverses made during two EVA's, the first one being a short trip to the west with the span of 250 m and the second one a round trip to the east up to the south rim of Cone Crater about 1.3 km from the LM (Swann et al., 1971).

Reflecting the geology of the area, most of the Apollo 14 rock samples are fragmental. Some of them show a resemblance to the Apollo 11 breccias. Others are characterized by a variety of clasts embedded in matrices. The size of lithic clasts ranges from a few micrometers to 16 cm. Glass and mineral clasts are 1 mm or smaller in size. The fragmental rocks are classified into three groups: (1) those which have friable matrices, (2) those which show moderately coherent matrices, and (3) those with re- crystallized matrices. The homogeneous crystalline rocks are divided into two groups, those with basaltic textures and those with very fine-grained granulitic textures.

One double core (samples 14210 and 14211, 39.5 cm, 209.2 g) and two single cores (sample 14220, 16.5 cm, 80.7 g; sample 14230, 12.5 cm, 76.0 g) were obtained. Sample recovery and quality of Apollo 14 core tubes were not as satisfactory as those of Apollo 12, due mainly to the difficulty in driving the tubes into the resistant ground consisting of coarser soil. Stratification, layer thicknesses ranging from 1 to 13 cm, and a gradational change in mean grain size with depth are observed. Maximum size of the rock fragments contained in the core is more then 5 mm. The bulk density determined on the lower half of the double core tube (sample 14210) is 1.75 g/cm 3 (Mitchell et al.,

1971). The glass content in Apollo 14 soil samples varies considerably, from less than 10~ in sample 14141 to 40 to 75~ in samples 14163, 14148 and 14149. The regolith at the Fra Mauro site is more than 5 m deep.

The Hadley region (26°04 ' N, 3o39 ' E) where the Apollo 15 LM landed on July 31, 1971 is a transitional area between mare and highland (ALGIT, 1972). Dur- ing three lunar surface excursions (LSE), a total of 77 kg of rock, soil and core tube samples were collected from 10 sampling sites (LSPET, 1972). These sampling sites are divided into two distinct areas; the mare plain and the foot of a 3000 meter mountain called Hadley Delta. The rocks from the mare plains are extrusive and hyperbyssal basaltic rocks and glass-coated and glass-cemented breccias. The rock samples from the foot of Hadley Delta are a variety of rocks ranging from breccias to metaigneous rocks. Thirteen soil samples, 4.5 kg in total, were collected from the mare plain, and 18 soil samples, 8.5 kg in total, were from the mountain front. The chemical composition of the basalt samples is similar to those of the Apollo 11 and 12 mare basalts. The texture of basalt samples shows a wide variety ranging from massive, dense and fine-grained to highly vesicular and scoriaceous with a void space of more than 50~. The rocks from the mountain front show signs of lithification after being shock-meta- morphosed and brecciated. It is not known whether the shock-metamorphism and brecciation occured at the time of the Imbrian impact which formed the Hadley Delta

450 KI-ITI HORAI AND NAOYUKI FUJII

mountain front. Most of the breccias contain glass in the matrix as well as in the clasts. In some of the breccia samples, the glass content is more than 50~.

A rotary percussive drill was used for the first time on the Apollo 15 mission, and a core sample (samples 15001 to 15006) to a depth of 2.4 m was obtained at the site between the Hadley Delta mountain front and the LM. The total mass of the core is 1.33 kg. Fifty-eight layers, each of which show a significant difference in grain size and density, are identified. The density of the lowest section (sample 15001) is 2.15 g/cm 3 which is the highest density of any of the soil samples returned from the lunar surface. In addition to the deep drill, three drive tube core samples (two double cores and one single core) were obtained. One double core (samples 15008 and 15007) is 65 cm long and weighs 1.28 kg. The bulk density is 1.36 + 0.05 g/cm 3 for the upper half (sample 15008) and 1.66 _ 0.02 g/cm 3 for the lower half (sample 15007). Another double core, 66 cm long and weighing 1.4 kg, has bulk densities 1.69 + 0.03 g/cm 3 in the upper half (sample 15011) and 1.85 +_ 0.06 g/cm 3 in the lower half (sample 15010). A single core (sample 15009), 36.2 cm long and weighing 622 cm, has a bulk density of 1.35 g/cm 3.

The lunar specimens were distributed to the selected investigators for the detailed study of petrology, mineralogy, chemistry and physical properties. Some of the samples were used for the study of thermophysical properties. Thermal conductivity, thermal diffusivity, thermal expansion, and specific heat have been measured and their significance and implications discussed. A substantial amount of data have been pub- lished for Apollo 11 samples, and less abundant data for Apollo 12 samples. Study of Apollo 14 and 15 samples is currently under way.

It is also possible to calculate the rate of heat generation of lunar samples from the contents of radioactive elements K, Th, and U. These data, together with the informa- tion concerning the geology and morphology of the landing sites, will enable other investigators to discuss near-surface thermal processes and internal structure of the Moon.

2. Thermophysical Properties

A. THERMAL DIFFUSIVITY

Thermal diffusivity of four Apollo 11 lunar samples (10020, 10046, 10057, and 10065) was measured over the temperature range 150 to 440 K by the modified Angstrom method (Horai et al. , 1970a,b). Figure 1 summarizes the result of measurements of the four samples used for the study. Samples 10020 and 10057 are type A with densities 2.99 g/cm 3 and 2.88 g/cm 3. Samples 10046 and 10065 are type C with densities 2.21 g/cm 3 and 2.36 g/cm 3. The temperature variation of thermal diffusivity is almost identical for type A samples and type C samples, but is quite distinctive for each of these two rock types.

Kanamori et al. (1968) demonstrated experimentally that, in the temperature range in which radiation through the solid is not a dominant mechanism of thermal conduction, the thermal diffusivity is an inverse linear function of temperature.

I c - I = A + B T (1)


Fig. 1.

~ 1 0 (/)

N~

(J

0

100

-2_00 15

T ("C) -100 0 100 200

l I I I

10020 (A) A 10057 (A)

\ ,c, ° I ~oo~ 'CLo \

Z

I I ] 20O ~00 40O 500

T ( K )

Thermal diffusivity/c versus temperature Tfor Apollo 11 type A (10020 and 10057) and type C (10046 and 10065) samples under interstitial atmospheric pressure of 1 atm.

(After Horai et al., 1970a,b.)

TABLE I

Coefficients A and B of equation for thermal diffusivity x as a function of temperature T (to-1 = A + BT) .

(After Horai et al. (1970))

Sample A B ( 102 s/cm 2) (s/cm 2 K)

Type A 0.314 -L 0.159 0.378 ± 0.051 (10020, 10057)

Type C 0.545 -E 0.207 0.648 ± 0.068 (10046, 10065)


where tc is the thermal diffusivity and T the temperature. Coefficients, A and B, in Equation (1), determined from the data are indicated in Table I. Values of thermal diffusivity smoothed by this linear relationship are listed in Table III.

The thermal diffusivity of type C material is lower and less temperature-dependent than type A material. The difference may be explained in terms of texture and mineral composition of the samples. Porosity of sample 10057 (type A), obtained from the point count of cavities on the surfaces of the sample was 0.174 (Kanamori et al., 1970 a,b). Densities of sample 10046 and 10065 are smaller than those of samples 10020 and 10057, suggesting that type C material is even more porous than type A. General examination of Apollo 11 rock samples shows that closely spaced microfractures predominate in type C material. Type C material also shows various degrees of vitrification. The presence of glass, as well as more abundant pores and microcracks in type C material, is probably the cause of the lower thermal diffusivity and smaller temperature depen- dency.

The data on thermal diffusivity presented in Figure 1 were obtained in air at atmospheric pressure .As has been reviewed by Wechsler and Glaser (1965), thermal conductivity or diffusivity of porous material strongly depends on interstitial gaseous pressure. Recently Fujii and Osako (1972) measured thermal diffusivity of Apollo 11 type A samples 10049 and 10069 in the temperature range between 85 and 650 K with

Fig. 2a.

15

.?, 0

v

i I I I ' I J I ' I ' I '

~ ~ 10049

• I arm

0 10 -41orr

I I ' I , I i I J I J I i

O0 200 400 600 T (K)

Thermal diffusivity x versus temperature T for Apollo 11 type sample 10049 under interstitial atmospheric pressure of 1 arm and 10 4T. Porosity of sample is 5.65 %.

(After Fujii and Osako, 1972.)


15

"-d

eel 0 o

o

i I I l i l I i I i I I

10069

• I otm

0 10 -4 tor t

I I I I ~ I I J I I i I i I 200 400 600

T ( K )

Fig. 2b. Thermal diffusivity K versus temperature T for Apollo 11 type A sample 10069 under interstitial atmospheric pressure of 1 atm and 10-4T. Porosity of sample is 11.0%. (After Fujii

and Osako, 1972.)

the in ters t i t ia l a i r a t a t m o s p h e r i c p ressu re a n d 10 - 4 T. T h e i r resul ts a re r e p r o d u c e d in

F igu res 2a,b. Fuj i i a n d O s a k o f o u n d a r e l a t i onsh ip o f the type

= A + B / T + C T 3 (2)

fits m o r e sa t i s fac tor i ly to the da ta . T h e e x p e r i m e n t a l l y d e t e r m i n e d va lues o f the co-

efficients a re i nd i ca t ed in T a b l e II . U n d e r a p re s su re o f 1 a tm, b o t h samples exhib i t

TABLE II

Coefficients A, B and C of equation for thermal diffusivity K as a function of temperature T Oc = A 4-B/T4- CTZ). (After Fujii and Osako (1972))

Sample A B C (Bulk density Q in g/cm ~, (10 -~ cm2/s) (cm2 K/s) (1012cm2/s K 3) porosity p in %)

10049 (0 = 3.07, p = 5.56) (1 atm) 3.22 :k 0.20 0.97 4, 0.03 -- 1.82 4, 1.93 (10 -4 T) 3.05 ± 0.10 0.40 _+ 0.02 1.26 4- 0.53

10069 (0 = 2.90,p = 11.0) (1 atm) 3.51 =I=0.28 0.87 4-0.04 --3.45 4-3.20 (10 -a T) 1.30 -b 0.07 0.27 4, 0.02 4.38 4, 0.36


TABLE 1II

Thermal diffusivity of Apollo 11 type A and type C samples as a function of temperature

Temperature Thermal diffusivity, K, (10 -8 cm2/s) T (K) Type A ~ Type C a Type A b

(10020, 10057) (10046, 10065) (10049) (10069)

(1 atm) (1 atm) (1 atm) (10 -4 T) (1 atrn) (10 -4 T)

100 12.92 7.05 12.21 4.04 120 11.30 6.39 10.75 3.56 140 11.86 6.89 10.14 5.91 9.72 3.24 160 10.88 6.32 9.28 5.56 8.93 3.01 180 10.06 5.84 8.60 5.28 8.32 2.83 200 9.35 5.43 8.06 5.06 7.83 2.69 220 8.73 5.08 7.61 4.88 7.43 2.57 240 8.12 4.76 7.24 4.73 7.09 2.49 260 7.71 4.49 6.92 4.61 6.80 2.42 280 7.29 4.24 6.64 4.51 6.54 2.36 300 6.91 4.02 6.40 4.42 6.32 2.32 320 6.56 3.82 6.19 4.34 6.12 2.29 340 6.25 3.64 6.00 4.28 5.93 2.27 360 5.97 3.48 5.83 4.22 5.77 2.25 380 5.71 3.33 5.67 4.17 5.61 2.25 400 5.48 3.19 5.53 4.13 5.46 2.26 420 5.26 3.06 5.40 4.10 5.33 2.27 440 5.06 2.94 5.27 4.07 5.19 2.29 460 5.15 4.04 5.07 2.31 480 5.04 4.02 4.94 2.35 500 4.93 4.01 4.82 2.39

a After Horai et al. (1970). b After Fujii and Osako (1972).

a lmost identical thermal diffusivity at the lower pressure, however, the diffusivity

decreases substant ia l ly and becomes less t empera ture dependent . The effect o f decreas-

ing pressure on thermal diffnsivity is more p ronounced for sample 10069. The

poros i ty o f sample 10069, es t imated f rom the bu lk and the intr insic densities o f the

sample, is 0.110. Sample 10049 is less porous with a poros i ty of 0.0556. Convect ive

transfer of heat by air in the pores and the microcracks of the samples, becomes

less impor t an t with decreasing interst i t ial a tmospher ic pressure, and is the cause of

lower diffusivity under vacuum condi t ion.

F igure 3 i l lustrates the var ia t ion of thermal diffusivity with pressure for sample

10069 at the t empera tu re o f 350 K. Mos t o f the change in thermal diffusivity takes

place in the pressure range f rom 1 a t m ( = 760 T) to 10-3 T. Below 10 . 4 T, the thermal

diffusivity becomes insensitive to changes o f pressure. Since the lunar a tmospher ic

pressure is of the order o f 10-12 T, this exper iment shows tha t da t a on thermal con-

duct ivi ty or diffusivity must be taken under the pressure below 10-3 T to s imulate the

lunar surface condi t ion.


Fig. 3.

A

U

I / )

E u

rt)

' 0

5

I I I I I I I I I

1 0 0 6 9

T = :550 K / 0 "

p = 1 1 . 0 % / / /

/ /

/ /

. - - 0 6 3 / _O_ _ - c ¢ 1 . . . .

0 I I I I I I I I I

i 0 -4 i0 -2 i 0 ° I0 2

P ( tor r ) Variation of thermal diffusivity ~c with decreasing interstitial atmospheric pressure P for

Apollo 11 type A sample 10069. Temperature T--350K. Porosity of sample p - 11.0 ~ . (After Fujii and Osako, 1972.)

B. SPECIFIC HEAT

Specific heat, or heat capacity, was measured by Robie et al. (1970a,b; 1971) in the temperature range between 95 and 350 K on three Apollo 11 and one Apollo 12 samples. An adiabatic calorimeter, specially designed and constructed for the study of lunar samples, was used for the measurements. Figures 4a, b, c show the data for Apollo 11 samples 10057 (type A, fine grained vesicular basalt), 10021 (type C, breccia) and 10084 (type D, soil). The values of specific heat smoothed by fitting orthogonal polynomials are indicated in Table IV. The specific heats of these samples are almost identical, regardless of the different mineralogy of these samples. The data on the Apollo 12 crystalline rock (sample 12018, olivine dolerite) shown in Figure 5 are quite similar to those of Apollo 11 samples. It may be concluded that the specific heat of different lunar samples is essentially an identical function of temperature.

Robie and his colleagues' data on specific heat obtained in the temperature range between 90 and 360 ° K, which roughly corresponds to the range of variation of equato- rial lunar surface temperature, are very useful for the discussion of near surface thermal processes of the Moon. it is noted, however, that the trend of temperature variation of specific heat suggested by Figures 4 and 5 is not consistent with the Debye T 3 law of specific heat. If the specific heat is extrapolated to lower temperature, it is considerably higher than the value suggested by Debye's law.

This situation was best exemplified in the experiment by Morrison and Norton (1970) who measured the specific heat of Apollo 11 sample 10017 (type A) and 10046 (type C) at liquid helium temperatures. The calorimeter used for the measurements consists of


0 . 2 0

Fig. 4b.

0 . 2 0 I I t I I I I I I I I I I ~

J • 1 0 0 5 7

0 . 1 5

O

0 . 1 0

0.05 I t t = ~ I v i p ~ I ~ I I 0 0 2 0 0 5 0 0

T ( K )

Specific heat C~ versus temperature T for Apollo 11 type A sample 10057. (After Robie et al., 1970a,b.)

I = J = ¢ I = = = = I = _ ~ , / ,

I r • I 0

0.15

G o

O.lO / ' [ t t

IOO

Fig. 4a.

0 . 0 5 I i I t i r J I i 2 0 0 3 0 0

Specific heat C~ versus temperature T for Apollo 11 type C sample 10021. (After Robie and Hemingway, 1971.)


Fig. 4c.

0 , 2 0

0. I

l

0 . I

0 . 0 ~ I r I t I I I T I I I I I I 0 0 2 0 0 300

T{ K)

Specific heat Cpversus temperature T for Apollo 11 type Dsample 10084.(A~er Robie et al., 1970b.)

TABLE IV

Specific heat of lunar material. (After Robie et al., 1970a, b, 1971.)

Temperature Specific heat, Cp (cal/gK) T (K) Apollo 11 material Apollo 12 material

Type A Type C TypeD Olivine dolerite 10057 10021 10084 12018

90 (0.0571) (0.0539) (0.0615) (0.0498) 100 0.0633 0.0631 0.0665 0.0591 120 0.0771 0.0798 0.0802 0.0766 140 0.0922 0.0947 0.0955 0.0921 160 0.1075 0.1158 0.1108 0.1064 180 0.1217 0.1235 0.1235 0.1193 200 0.1343 0.1348 0.1348 0.1317 220 0.1451 0.1450 0.1446 0.1438 240 0.1546 0.1545 0.1534 0.1549 260 0.1632 0.1632 0.1617 0.1647 280 0.1711 0.1714 0.1696 0.1738 300 0.1786 0.1778 0.1771 0.1823 320 0.1853 0.1864 0.1845 0.1903 340 0.1917 0.1934 0.1916 (0.1977) 350 (0.1969) (0.2014) 360 (0.1983) (0.1970)


Fig. 5.

O.ZO

0 . 1 5

o &

" x

v

O.IC

0.05

I 1 I I I I I I I 1 I I I

• 12018

100 2 0 0 5 0 0 T ( K )

Specific heat C~ versus temperature T for Apollo 12 olivine dolerite 12018. (After Robie and Hemingway, 1971.)

a light circular copper tray about 2 cm in diameter attached to a nitrogen-filled germa- nium thermometer and an electric heater. The specific heat of sample 10017 increases monotonically from 4.62 x 10 .3 cal /gK to 6.7 x 10 .3 cal /gK in the temperature range between 2.34 K and 4.97 K. The specific heat of sample 10046, measured on the temperature range between 3.08 and 4.05 K, ranges from 2.4 x 10- 3 cal/g K to 4.5 x x 10 .3 cal/g K with a maximum at 3.54 K. These values of specific heats are two orders of magnitude larger than those expected from elastic properties of these samples. According to the Debye theory of solids, the specific heat at constant volume, Cv, at low temperature is given by

C~ 16rc5 k5 ( ~ 2 ) T 3 = 15 ~h - ~ + ~ , (3)

where 0 is the density of the solid, k and h are Boltzmann's and Planck's constants, vp and v s are the compressional and the shear wave velocities, and Tis the temperature in degrees Kelvin. For vp = 7 km/s and vs = 4 km/s of the glass spherules contained in both of the specimens 10017 and 10046, the coefficients Cv/T 3 obtained from the experimental data are of the order of 10-5 cal/g K. A likely cause of the anomalously large specific heat at low temperature is, as Morrison and Norton suggested, the exis- tence of an anomaly in the low frequency range of lattice vibrational modes. It is not


likely, however, that this feature is characteristic of lunar material. Morrison and Norton (1971) extended their experiment to Palisade diabase which has a chemical composition similar to sample 10017 and found that the specific heat of this sample measured at temperatures from 1.5 to 5.0 K (C,/T 3 = 0.38 to 7.96 x 10 .5 cal/g K 4) is 102 times higher than that predicted by Debye theory (C,/T 3 = 2.10 x 10 .7 cal/g K4). The anomalous vibrational modes seem to be common to some rock-forming minerals of complex crystal structure.

C. T H E R M A L E X P A N S I O N

The thermal expansion coefficient of three Apollo 11 samples (10020 (type A), 10046 (type C), 10057 (type A)) and one Apollo 12 sample (12022) was measured by Bal- dridge and Simmons (1971) over the temperature range between 150 and 450 K using a Brinkmann model TD IX dilatometer. A prism of single crystal quartz cut parallel to the c-axis was used for the calibration of apparatus. Figure 6 illustrates the data on relative thermal expansion, AL/Lo. For each of the four samples, no appreciable va-

o

_1

iil

10020 0 0 0 o ° ° 1 o

O

10057 0 C~)

10046 0 . , ~ 0 0 0 / ~ v

^ ° - - / °

o

I i I ~ I , I ~ I

I O0 200 300 400 500 T ( K )

Fig. 6. Variation of relative linear dimension AL/Lo with temperature T for Apollo 11 type A (10020 and 10057), type C (10046) and Apollo 12 olivine dolerite (12022). (After Baldridge and

Simmons, 1971.)


riation of AL/L o is observed below 300 K. Above 300 K, it increases monotonically with temperature. The volume expansion coefficient

1 AL ~v = 3 - - - - (4)

A T L o

is related to the change of density 0 with temperature T by

~v = - ( 5 )

The data presented in Figure 6 show that the volume thermal expansion of breccia (sample 10046) is slightly higher than that of crystalline rocks (sampels 10020, 10057 and 12022). The values of 7 are, in the unit of 10-6 ( K ) - 1 15 for breccia and 5 to 8 for crystalline rocks in the temperature range below 300K, and 22 for breccia and 14 to 16 for crystalline rocks above 300 K. The higher content of glass with large thermal expansion (c~, = 29 × 10 -6 (K) -~) is a likely cause of the larger thermal expansion of breccia. The values of c~ for the lunar samples is lower than the estimated thermal expansion coefficient calculated for a non-porous aggregate from the mineral composition (22 x 10-6(K)- 1). As is the case of thermal diffusivity, the presence of microcracks and pores is thought to be the cause of the lower thermal expansion of lunar rocks.

D. THERMAL CONDUCTIVITY OF SOLID ROCK

Since thermal conductivity k is related to thermal diffusivity tc by the relation

k = KOCp, (6)

the data on thermal diffusivity, density and specific heat can be used to estimate thermal conductivity as a function of temperature. In Figure 7, the thermal conductivity of Apollo 11 type A and type C materials is calculated from the smoothed values of thermal diffusivity (Table II1) and specific heat (Table IV). Densities are assumed to be constant over the temperature range concerned, since, as reviewed in the previous section, the temperature variation of density is negligibly small in comparison with those of thermal diffusivity and specific heat.

For reasons mentioned in the section on thermal diffusivity, the thermal conductivity of type C material is lower than that of type A. Even the type A material exhibits lower thermal conductivity than corresponding terrestrial basalt of non-porous texture. I f the bulk density of Apollo 11 type A sample 10057, 2.88 g/cm 3, is corrected for the porosity, an intrinsic density of 3.38 g/cm 3 is obtained. Likewise, if the thermal conductivity of sample 10057 at 300 K, 2.56 x 10 -3 cal/cm s K (Figure 7), is corrected for the effect of porosity by Maxwell's formula

2 (1 - p) U - - - k, (7)

p + 2

where k and k' are the corrected and uncorrected thermal conductivities and p the porosity, k = 4.68 x 10 -a cal/cm s K is obtained. This value is not unreasonable for


4.0

:3.0

,%. "G 2.0

'o

1.0

I I I I

,oo2o/// .,00%//

:2;/ / J " " /

/ /

/

10049//~ ~ ~ ~ *I004610065 t l '~" ' ' " ~ " ' " ~ ' ' " ~ " ' ' "

1 0 0 6 9 /

I I I I I 0 0 2 0 0 3 0 0 400 5 0 0

T ( K )

Fig. 7. Thermal conductivity k as a function of temperature T, estimated from data on thermal diffusivity, specific heat and density for Apollo 11 type A and type C materials. Solid and dashed

lines represent data obtained under interstitial atmospheric pressure of 1 atm and 10 4T, respectively.

the thermal conductivity of non-porous basalt with high titanium content. For samples 10049 and 10069, thermal conductivities at 300 K are 3.51 x 10 .3 cal/cm s K and 3.27 x 10 .3 cal/cm s K under atmospheric pressure of 1 atm. I f correction is applied for the effect of porosity according to formula (7), the corrected thermal conductivities are 3.82 x 10 .3 cal/cm s K for sample 10049 and 3.88 x 10 .3 cal/cm s K for sample 10069.3-he intrinsic densities of these samples are 3.25 g/cm 3 for sample 10049

and 3.26 g/cm a for sample 10069. For both of the Apollo 11 type A and type C materials, the temperature variation

of thermal conductivity is relatively small at temperatures above 200 K. Below 200 K, the thermal conductivity decreases monotonically towards zero. This observation

is consistent with what is predicted by solid state theory. The experimental study by Morrison and Norton (1970) also shows that the thermal conductivity becomes sub- stantially smaller at lower temperatures. In the course of their measurements of specific heat, Morrison and Nor ton made an estimate of thermal conductivity of two Apollo 11 samples (10017 (type A) and 10046 (type C )) at liquid heliun temperatures. The thermal conductivity was calculated from the time required for the samples to at- tain thermal equilibrium. (Supposedly, thermal diffusivity was calculated f rom the


time constant and it was converted to thermal conductivity using the data on specific heat and density.) The result for sample 10046 was 2.5 _+0.5 x 10 .6 cal/cm s K at 4 K. The conductivity of sample 10017 was roughly estimated as 10 to 100 times larger than that of sample 10046.

Figure 7 also contains the estimated thermal conductivity of two Apollo 11 samples (10049 and 10069) under vacuum condition. The thermal conductivity increases monotonically with temperature in the entire range of temperature considered. This suggests that the radiation mechanism of heat transfer becomes progressively more important under high vacuum. With an interstitial atmospheric pressure of 10- * T, the magnitude and the temperature dependence of thermal conductivity differ greatly between samples 10049 and 10069, while at one atmosphere they possess simular values of thermal conductivity. It is possible that the contribution of the radiative component to the total thermal conductivity is strongly controlled by the shape and distribution of void spaces through which the radiative thermal energy is transmitted. The differences in thermal conductivity can be attributed to the differences in micro-textures in these two samples.

E. T H E R M A L C O N D U C T I V I T Y OF FINES

It is expected that the lunar soil with much higher porosity should exhibit lower thermal conductivity than solid rock. The porosities of the Apollo 11 drive-tube core samples are 0.465 (sample 10005-0) and 0.501 (10004-0), respectively. If the grain density of 3.10 g/cm 3 is assumed, the porosities of the Apollo 12 core samples are 0.439 for sample 12026, 0.361 for sample 12028 (upper portion), and 0.368 for sample 12028 (lower portion). Besides the effect of porosity, contact resistance between grains plays an important role in thermal conduction in particulate materials. Therefore, the thermal conductivity of lunar soil depends not only on temperature and interstitial gas pressure, but also on those factors which control the contact resistance like bulk density (porosity), grain size distribution and compressive load imposed on the sample.

An intensive study of thermal conductivity of particulate material was made by Fountain and West (1970). The material used for the experiment is a terrestrial analog of lunar basalt, a basalt from the Columbia River, Oregon, crushed to sizes ranging from 37 to 62/~. The measurement was made using a line heat source technique at temperatures ranging from 140 to 370 K under an atmospheric pressure of 10 - 6 T. It was demonstrated by this experiment that, under the state of free compressive load, the thermal conductivity k of particulate Oregon basalt as a function of temperature Tconforms to Watson's (1964) equation

]C = O~ d- f i T 3 • ( 8 )

The second term on the right-hand side of Equation (8) represents the contribution from radiative transfer which becomes dominant in highly porous particulate materials under high vacuum. The constants, e and fl, in Equation (8) depends on the bulk density of sample. In Table V, the experimentally determined values of c~ and fl are listed as a function of density. As has been reviewed in the first part of this paper, the bulk


TABLE V

Coefficients ~ and p of Watson's equation for thermal conductivity of particulate material as a function of temperature (k - -~+f lT~) . Oregon basalt (grain size, 37 to 62/~). Interstitial atmospheric pressure, 10 -8 T.

(After Fountain and West, 1970.)

Density Coefficients 0 (g/cm a) ~ P

(10 -a W/mK) (10 -1° W/mK 4)

0.79 0.509 (0.122) a 0.169 (0.404) b 0.88 0.650 (0.155) 0.167 (0.399) 0.98 0.595 (0.142) 0.172 (0.411) 1.13 0.887 (0.212) 0.190 (0.454) 1.30 1.237 (0.296) 0.234 (0.581) 1.50 1.642 (0.392) 0.343 (0.820)

Values in parentheses are in 10 ~ cal/cm sK. b Values in parentheses are in 10 -la cal/cm sK 4.

density of lunar regolith varies f rom 1.35 to 2.15 g/cm 3, the most representative value

being a round 1.7 g/cm 3.

Da ta on thermal conduct ivi ty of lunar soil (fines) are available. Using a line heat

source technique, the thermal conduct ivi ty of Apollo 11 sample 10084 was measured

at various temperatures ranging from 203 to 405 K under the interstitial atmospheric

pressure varying from 10 -2 T to 10 -7 T. The bulk density of the sample was kept

1.265 g/cm 3 dur ing the measurement (Birkebak et al., 1970; Cremers et al., 1970).

Using the same technique, Apollo 12 sample 12001 with a bulk density of 1.300 g/cm 3

was measured in the temperature range from 168 to 429 K at a pressure of 10 -6 T

TABLE VI

Coefficients ct and ]? of Watson's equation for thermal conductivity of lunar fines. Interstitial atmospheric pressure, 10 -2 to 10 7 T. (After Cremers et al., 1970; Cremers

and Birkebak, 1971.)

Sample Coefficients

B (10 -z W/mK) (10 10 W/mK 4)

Apollo 11 10084 (Q = 1.265 g/cm 3)

Apollo 12 12001 (Q = 1.300 g/cm z)

1.42 (0.339) ~ 0.173 (0.413) b

0.922 (0.237) 0.319 (0.762)

Values in parenthesis are in 10 -5 cal/cm sK. b Values in parenthesis are in 10 -13 cal/cm sK.


(Cremers and Birkebak, 1971). Within the range of experimental uncertainty, they agree with Watson's equation. In Table VI, the coefficients of Watson's equation determined from the data are indicated. It is noted that the radiative component, indicated by the magnitude of fl, is more pronounced in Apollo 11 than that of terrestrial basalt at a density of 1.30 g/cm 3 (Table V). The value of c~ for terrestrial basalt is intermediate between that of Apollo 11 and 12 soils. The grain size of the particulate Oregon basalt used for the study is confined in the range between 36 and 62p, whereas that of the lunar samples ranges from 100p to less than 0.1p, the majori ty of the grains being smaller than 0.3/~ in diameter. Analyses of grain size distribution of lunar fines show, however, that no appreciable differences exist between the Apollo 11 and Apollo 12 fines (Lindsay, 1971). The differences in glass content, 50~ for Apollo 11 fines and 20~o for Apollo 12, is one of the possible causes of the preferential radiative effect.

F. THERMAL PARAMETER

The thermal parameter (the reciprocal of thermal inertia) defined by

= (k~Cp)- l/Z, (9)

to

~'6

u

2 4

I I I I

• I 0084 2 /

• 12001 7 / 3

/ /

f I , I I I i

00 200 300 400 500 T ( K )

Fig. 8. Thermal conductivity k versus temperature Tfor Apollo 11 (10084) and Apollo 12 (12001) type D materials (fines). Bulk density e=l.265g/cm a (sample 10084) and e=l.300g/cm a (sample 12001). Curves are Watson's equations fitted to the data (curve 1, 10084; curve 2, 12001 ; curve 3, particulate Oregon basalt with bulk density o=l.30g/cm 8 (Fountain and West, 1970)). (After

Cremers and Birkebak, 1971.)

90

80

70

60

v

50

"E Z 4 0

3C

2C

IC

250C

i i i i

10069,

10046 ~ +10065~ "~

,oo,,~\ \\\ \ \\\ ,oo69\ \~\',\ ,oo~9~\ \ \ \\

,oo~o \ \ ' \ "---2~ .,oo,~ \ \ \

\ \ \ ",, -- . .

O ( i i i i I 00 200 300 4 0 0 500

T ( K )

Fig. 9. Thermal parameter 7 as a function of temperature T, estimated from data on thermal diffusivity, specific heat and density for Apollo 11 type A and type C materials. Solid and dashed lines represent data obtained under interstitial atmospheric pressure of 1 atm and 10-4T, respectively.

200C iiii\ 1500

u

~E o 1000

500

I I I I

T H E R M O P H Y S I C A L PROPERTIES OF L U N A R MATERIAL 465

0 I I I I 0 I O0 200 300 400 500

T(K)

Fig. 10. Thermal parameter 7 as a function of temperature T, estimated from data on thermal conductivity, specific heat and density for Apollo 11 (sample 10084) and 12 (sample 12001) type D

materials (fines).

466 K I - I T I H O R A I A N D N A O Y U K I lzUJII

where k is the thermal conductivity, 0 the density, and Cp the specific heat, is an important quantity which controls the variation of surface temperature of the Moon. It is interesting to calculate the thermal parameters from the thermophysical quantities appearing in the righthand side of Equation (9) for lunar materials because the thermal parameter of lunar surface material has been estimated from infrared and passive microwave observations from Earth-based stations. In Figure 9, the thermal parameters of Apollo 11 type A and type C materials are calculated. The composite effect of temperature on thermal conductivity and specific heat determines the temperature characteristics of thermal parameter. In the temperature range between 100 and 500 K, the thermal parameter is a monotonically decreasing function of temperature. Under vacuum conditions, the thermal parameter becomes more temperature sensitive. At 300 K the thermal parameter is 23.3 cm 2 s K/cal (type A, 1 atm), 38.9 cm 2 s K/cal (type C, 1 atm) and 40.1 cm z s K/cal (sample 10069, 10-4T), respectively. An even higher thermal parameter would likely be exhibited by type C material under vacuum conditions. Even so, these values are definitely smaller than the estimate, 750 to 1250 cm z s 1/z K/cal, made by the infrared and passive microwave observations (Winter and Saari, 1969).

Figure 10 is the thermal parameter of Apollo 11 and 12 fines calculated from the thermal conductivity, density and the specific heat. The thermal parameter of lunar soil is more than an order of magnitude larger than that of lunar crystalline rock and breccia, and agrees satisfactorily with the result obtained by the infraled and passive microwave observations. Considering that the maximum penetration depth of micro- waves is of the order of 100 cm and that the regolith of a thickness of a few to several meters, consisting mostly of fine particles like lunar soil, is the commonest feature of the lunar surface, it is not surprising that the physical parameters of the lunar surface material determined by the infrared and passive microwave observations are mostly those of lunar regolith.

3. Rate of Heat Generation of Lunar Material

One of the most important thermal properties of lunar meterial, the rate of heat generation, can be calculated from the content of radioactive nuclides, 4°K, 232Th, 235U and 238U, in the samples. The concentrations of potassium, thorium and uranium in the lunar samples returned by Apollo missions were determined by LSPET (1969, 1970, 1971, 1972) by use of the low-background gamma-ray spectrometer. Tables VII-A, B, C, D summarize the results. The rate of heat generation of each nuclide is: 0.765 × 10 -8 cal/g-s for 4°K, 0.634 × 10 -s cal/g-s for 23ZTh, 0.136 × 10 -6 cal/g-s for 235U and 0.225 × 10 -7 cal/g-s for 238U. If the terrestrial isotopic abundances are assumed, the rate of heat generation of potassium, thorium and uranium is 0.856 × × 10 -12 cal/g-s, 0.634 × 10 -8 cal/g-s and 0.231 × 10 -7 cal/g-s, respectively. In Table

VII, the rate of heat generation of each lunar sample is calculated from the concentrations and the rates of heat generation of the radioactive elements. For comparison, the concentration of radioactive elements and the rate of heat generation for typical terrestrial rocks and meteorites are shown in Table VIII.

THERMOPHYSICAL PROPERTIES OF LUNAR MATERIAL

TABLE VII-A

Contents of radioactive elements and rate of heat generation for Apollo 11 rock samples. (Data from LSPET (1969))

467

Sample number

Weight K Th U Rate of heat (g) ( ~ ) (ppm) (ppm) generation

(10 -13 cal/g s)

Type A 10057 10072

Type B 10003 10017

Type C 10018 10019 10021

TypeD 10002

897 0.242£:0.036 3.4-k0.7 0 . 7 8 ~ 0 . 1 6 0 .417±0 .058 399 0 .232~0 .035 2 . 9 ± 0 . 4 0 .75±0 .11 0 .377±0 .036

213 0 .050±0 .008 0.95:E0.14 0 .20±0 .03 0.111 ±0.011 971 0 . 2 2 7 i 0 . 0 3 4 2 . 9 ± 0 . 4 0 . 7 0 ~ 0 . 1 0 0 .365±0 .034

213 0 .144±0 .022 2 . 3 ~ 0 . 3 0 . 6 0 ± 0 . 0 9 0 .297+0 .028 245 0.12 4-0.02 1 . 9 ~ 0 . 3 0 . 4 3 ± 0 . 0 6 0 .230±0 .024 216 0 .120±0 .018 1 . 8 ± 0 . 3 0.39+-0.06 0 .215±0 .024

302 0.11 ± 0 . 0 2 1.6___0.3 0 . 4 6 i 0 . 1 0 0 .217±0.030

TABLE VII-B


Sample number

Weight K Th U Rate of heat (g) ( ~ ) (ppm) (ppm) generation

(10 -18 cal/g s)

Crystalline rocks 12002 1530 12004 502 12039 255 12053 879 12054 687 12062 730 12064 1205

Breccia 12034 154 12073 405

Fines 12070 354

0.044 ± 0.004 0.96 ~ 0.1 0.24 i 0.033 0.120 ± 0.010 0.048 ± 0.004 0.88 ± 0.09 0.25 ~- 0.033 0.118 -k 0.010 0.060 -- 0.005 1.20 ± 0.12 0.31 ± 0.040 0.153 ± 0.012 0.051 -~ 0.004 0.89 ~- 0.09 0.25 :k 0.033 0.119 ± 0.010 0.052 ~_ 0.004 0.77 ± 0.08 0.21 ± 0.030 0.102 -k 0.009 0.052 ± 0.004 0.81 + 0.08 0.21 -E 0.030 0.104 ± 0.009 0.053 zk 0.004 0.88 ± 0.09 0.24 ± 0.035 0.116 ± 0.010

0.44 ± 0.035 13.2 ± 1.3 3.4 ± 0.4 1.66 :~ 0.12 0.278 ± 0.022 8.2 ~ 0.8 2.0 zk 0.3 1.01 ± 0.09

0.206 ~ 0.016 6.0 ± 0.6 1.5 ± 0.2 0.745 -E 0.060

Feldspathic differentiate 12013 80 2.02 ~_ 0.016 34.3 ± 3.4 10.7 ± 1.6 4.82 i 0.43

468 KI-IT1 HORAI AND NAOYUKI FUJII

TABLE VII-C


Sample Weight K Th U Rate of heat number (g) ( ~ ) (ppm) (ppm) generation

(10 -lz cal/g s)

Clastic rocks 14045 65 0.36 4- 0.04 13.8 4- 1.4 3.7 4- 0.5 1.76 4- 0.15 14066 510 0.69 ± 0.07 15.3 ± 1.5 4.1 4- 0.6 1.98 ± 0.17 14082 63 0.18 -- 0.02 4.6 4- 0.5 1.4 4- 0.2 0.63 ± 0.06 14301 1361 0.55 ± 0.05 12.8 4- 1.3 3.6 ± 0.5 1.69 ± 0 14 14302 381 0.55 4- 0.05 14.3 4- 1.4 3.8 ± 0.6 1.83 ± 0.17 14315 115 0.30 4- 0.03 9.1 ± 0.9 2.5 4- 0.4 1.18 ± 0.11 14318 600 0.49 4- 0.05 12.8 ± 1.3 3.3 4- 0.5 1.62 ± 0.14

Crystalline rocks 14053 251 0.088 4- 0,009 2.24 4- 0.22 0.64 4- 0.10 0.30 4- 0.03 14310 3425 0.49 4- 0.06 13.7 4- 1.7 3.7 4- 0.6 1.77 4- 0.18

Fines (< 1 mm) 14163 491 0.48 4- 0.05 13.9 4- 1.4 3.9 4- 0.6 1.82 4- 0.17 14259 495 0.42 ± 0.04 13.4 4- 1.3 3.8 4- 0.6 1.76 ± 0.16 14148 70 0.41 ~ 0.04 14.9 ± 1.5 4.1 4- 0.6 1.93 ± 0.17 14156 136 0.40 ± 0.04 14.5 4- 1.4 3.9 i 0.5 1.86 4- 0.17 14149 85 0.44 ± 0.04 14.8 4- 1.5 3.9 4- 0.6 1.88 4- 0.17--

TABLE VII-D

Contents of radioactive elements and rate of heat generation for Apollo 15 rock samples. (Date from LSPET (1972))

Sample Weight K Th U Rate of heat number (g) ( ~ ) (ppm) (ppm) generation

(10 -1~ cal/g s)

Soils (< 1 mm) 15431 145.4 0.19 ± 0.02 4.8 ~ 0.7 1.1 4- 0.2 0.58 ± 0.06 15021 132 0.16 ± 0.02 5.1 ± 0.7 1.3 ± 0.2 0.64 ± 0.06 15271 527.9 0.16 q- 0.03 4.2 ± 0.8 1.2 ± 0.2 0.56 4- 0.07 15211 104.2 0.15 4- 0.03 3.8 ± 0.8 0.96 ± 0.20 0.48 i 0.07 15301 557.2 0.12 4- 0.02 3.2 4- 0.5 0.88 4- 0.15 0.42 4- 0.05

Breccias and metaigneous rocks 15206 92.0 0.45 4- 0.06 11 i 2 3.0 ± 0.6 1.43 + 0.12 15265 314.2 0.19 ± 0.03 5.1 4- 1.0 ] .3 4- 0.2 0.64 i 0.08 15558 1333.3 0.17 ± 0.02 3.4 4- 0.4 1.0 ± 0.1 0.46 _ 0.03 15466 118.0 0.15 ± 0.03 3.5 4- 0.7 0.93 4- 0.20 0.45 4- 0.06 15086 172.1 0.14 4- 0.03 3.2 ± 0.5 0.76 ± 0.11 0.39 ± 0.04 15455 881.1 0.090 ± 0.020 1.9 4- 0.4 0.50 4- 0.10 0.24 4- 0.03 15426 125.7 0.082 ± 0.01 1.9 4- 0.4 0.43 4- 0.10 0.23 4- 0,03 15418 1140.7 0.0086 ~- 0.0010 0.13 -- 0.04 0.04 4- 0 .01 0.018 4- 0.003

Crystalline rocks 15085 471.3 0.041 ± 0,005 0.51 4- 0.10 0.13 ± 0.03 0.066 ± 0,009 15256 201.0 0,034 ± 0.004 0.46 4- 0.10 0.15 ± 0.02 0.067 ± 0,008 15415 269.4 0.012 ± 0.002 0.007 4- 0.030 0.0024 ± 0.007 0.0020 4- 0.0030

THERMOPHYSICAL PROPERTIES OF LUNAR MATERIAL

Table VII-D (Continued)

469

Sample Depth K Th U number (cm) (~) (ppm) (ppm)

Rate of heat generation (10 13 cal/g s)

Drill stems 15006-3 11- 21 0.19 ! 0.03 4.7 4- 1.0 1.3 ~z 0.3 0.62 150042 106-117 0.17 ± 0.03 4.3 ± 1.0 1.1 ± 0.3 0.54 15001-2 234-245 0.19 ~_ 0.03 3.7 ± 1.0 1.1 =k 0.3 0.51

+ 0.09 ± 0.09 :k 0.09

TABLE VIII

Contents of radioactive elements and rate of heat generation for typical terrestrial rocks and meteorites.

(After Kaula (1968), p. 110)

Rock type K Th U Rate of heat (~) (ppm) ( p p m ) generation

(10 -13 cal/g s)

Granite 3.79 18.5 4.75 2.58 Diorite 1.80 7.4 2.0 1.07 Basalt 0.84 2.7 0.6 0.38 Peridotite 0.0012 0.06 0.016 0.0076 Dunite 0.0010 0.004 0.001 0.0003 Eclogite (1) 0.036 0.18 0.048 0.023 Eclogite (2) 0.26 0.45 0.25 0.106 Chondrite 0.0845 0.04 0.012 0.012

The rate of heat generation of Apollo 11 crystalline rocks (types A and B), with the

exception of sample 10003, is not significantly different from that of typical terrestrial

basalt. Breccias and fines (types C and D) show slightly lower rates of heat generation

than crystalline rocks. The average rate of heat generation for Apollo 11 crystalline rock, exclusive of sample 10003, is (0.386 _+ 0.027) x 10 -13 cal/g-s. The average for

breccias and fines is (0.240 -I- 0.039) x 10 -a3 cal/g-s.

Seven Apollo 12 crystalline rock samples show remarkably constant rates of heat

generation with the average of (0.119 __ 0.017) x 10 -aa cal/g-s which is considerably

smaller than that of Apollo 11 crystalline rocks. The Apollo 12 breccias and fines show

higher rates of heat generation which are comparable to those of terrestrial intermediate rocks. The unusually high rate of heat generation of sample 12013 may imply that

this sample is the latest product of magmatic differentiation.

It is noteworthy that the rates of heat generation of Apollo 14 samples are generally high and uniform. If samples 14053 and 14082 are excluded, the rate of heat generation ranges from 1.18 to 1.98 x 10- 3 cal/g-s with an average of (1.76 + 0.22) x 10 - a cal/g-s

that is comparable to the rate of heat generation of typical terrestrial granite.

The significantly higher average rate of heat generation for Apollo 14 material than those of Apollo 11 and 12 may reflect the fundamental differences in the geology of


0.3

0.2

0.1

0.5

0.4

0.3

3~

0.2

0.1

I I I I I I I I I

o TYPE A /

• TYPE B

K/U = 0.34 xlO 4

I I I I I I I I I

0,1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

U (ppm)

Fig. 1 l a. Potassium versus uranium in Apollo 11 samples. Correlation coefficient is 0.97. K/U=(0.34~-0.03) × 104. (Data from LSPET, 1969.)

I I i I I I I

J ~' CRYSTALLINE ROCK

I I I I I I I

I 2 3

U (ppm)

Fig. 1 lb. Potassium versus uranium in Apollo 12 samples. Correlation coefficient is 0.999. K/U=(0.12~:0.002) × 104. (Data from LSPET, 1970.)

10

T H E R M O P H Y S I C A L PROPERTIES OF L U N A R MATERIAL 471

0.7

0.6

0.5

o.4

~d

0.3

0.2

0,1

0.6

0.5

0.4

A

~ o . 3

Y

0.2

0.I

0 0

i i 1 i

o CLASTIC ROCKS o

zx CRYSTALLINE ROCKS

• FI NES o o /

O o •

0

K / U = 0.12 x I0

I I I I

I 2 3 4

U (ppm)

Fig. 11c. Potassium versus uranium in Apollo 14 samples. Correlation coefficient is 0.85. K/U=(0.12±0.02) × 104. (Data from LSPET, 1971.)

0

A

o I i

i I i I i I

SOILS (<Imm) /

BRECCIAS AND METAIGNEOUS ROCKS

o m j

K / U = 0.14- x 10 4

2 3 U (ppm)

Fig. l ld . Potassium versus uranium in Apollo 15 samples. Correlation coefficient is 0.99. K/U=(0.14±0.0I) × 104. (Data from LSPET, 1972.)


landing sites. It is noted that the Apollo 11 and 12 landing sites are in lunar maria whereas the Apollo 14 landing site is situated on lunar highland material. The mineralogy and chemistry of the lunar samples also indiate that the material from Apollo 14 is distinctive from that of Apollo 11 and 12. In general, the Apollo 14 material is richer in lighter elements and lighter colored minerals than the materials from Apollo 11 and 12. It is likely that, analogous to the earth, the surface layer of lunar highland consists of lighter material and is more enriched in radioactive elements than that of lunar mare.

The radioactive heat generations of Apollo 15 samples also show a remarkable correlation with geology. Apollo 15 soil and core tube samples show a remarkably uniform rate of heat generation, the average of 8 samples listed in Table VIID being (0.544 + 0.073) x 10 -13 cal/g-s. This is slightly higher than the average for breccias and metaigneous rock samples which is, exclusive of samples 15206 and 15418, equal to (0.402 _+ 0.154) x 10 .23 cal/g-s. The unusually high and low rates of heat generation for samples 15206 and 15418 are attributed to the peculiar history of these rocks: sample 15418, supposedly a shock melted igneous rock, is rich in anorthite and has an anorthositic chemical composition with a much lower FeO/MgO ratio than other Apollo 15 rocks, indicating that this sample is an early product of magmatic crystallization. Sample 15206 is collected from a block of breccia found at the Hadley Delta mountain front. Undoubtedly, this sample represents highland material.

Apollo 15 crystalline rock samples show a rate of heat generation an order of magnitude smaller than other Apollo 15 samples. It is even smaller than the rate of heat generation of Apollo 12 crystalline rocks. One of the Apollo 15 crystalline rock samples, sample 15415, a fragment of extremely pure anorthite, is not of mare origin. The unusually low rate of heat generation suggests that this rock sample was precipitated in an early stage of differential crystallization. Of the three Apollo 15 crystalline rock samples listed in Table VIID, only sample 15085 is truly representative of mare basalt. Even so, it cannot be denied that a tendency exists for the lunar highland material to be more radioactive than the lunar mare material.

Figures 1 la, b, c, d show the plots of potassium versus uranium contents for the Apollo lunar samples listed in Table VII. Likewise, the plots of thorium versus uranium contents are shown in Figures 12a, b, c, d. Except for the data on potassium and uranium from Apollo 14 which show a considerable scatter, correlations are generally high, indicating that the radioactive elements behave systematically in the course of lunar geochemical processes.

The K/U ratios are: (0.337 ___ 0.034) x 104 for Apollo 11, (0.124 __ 0.002) x 104 for Apollo 12, (0.123 + 0.022) x 104 for Apollo 14, and (0.142 __ 0.006) x 104 for Apollo 15. These are considerably lower than the ratio for chondrite, 8 x 104 or the average for terrestrial rocks, 1 x 104. This feature, as has been noticed and discussed by many authors, implies that potassium is generally depleted in source regions from where the lunar surface material has been derived. The enrichment of U relative to K in lunar material means that a large part of the heat generated by lunar surface material is essentially the heat of disintegration of uranium nuclides. The Th/U ratios of the lunar samples,4.10 _+ 0.38 for Apollo 11, 3.96 _+0.04 for Apollo 12, 3.81 _+0.10 for Apollo


& 2

I I I I I i I I I

o TYPE A o /

/ • TYPE B

TYPE C •

• TYPE D

~ / Th/U = 4.1

I I I I I I I I I

0.1 0.2 Q3 0.4 0.5 0.6 0.7 0.8 0.9 U (ppm)

Fig. 12a. Thorium versus uranium in Apollo 11 samples. Correlation coefficient is 0.97. Th/U=4.14-0.4 . (Data from LSPET, 1969.)

1.0

10

-g &

#

I J f I i I I

~' CRYSTALLINE ROCK / / &

o BRECCIA

• FINES

~ ~ Th/U = 4.0

I I I I 1 I i

I 2 3

u (ppm)

Fig. 12b. Thorium versus uranium in Apollo 12 samples. Correlation coefficient is 0.999. Th /U=4 .0±0 .04 . (Data from LSPET, 1970.)


15

O .

O . v

e~ Q .

15

IO

o

z~

[ ]

I0

I I I I

o CLASTIC ROCKS ~°~!/e--'e-

-'~ CRYSTALLINE ROCKS ° / /

• FINES /

Th/U = :5.8

/ i I I I I 2 3 4

U (ppm) Fig. 12c. Thorium versus uranium in Apollo 14 samples. Correlation

coefficient is 0.999. Th/U=3.8±0.1 . (Data from LSPET, 1971.)

0 0

I

@

o :;'/" /

i I

I

I t I ' I i

SOILS (<lmm)

BRECCIAS AND METAIGNEOUS ROCKS

@ ~

Th/U -- 3.7

i I r I w

2 3

U (ppm)

Fig. 12d. Thorium versus uranium in Apollo 15 samples. Correlation coefficient is 0.99. Th/U=3.7d:0.1. (Data from LSPET, 1972.)


14, and 3.72 +_ 0.10 for Apo l lo 15, are not significantly different f rom chondr i t ic and

terres t r ia l averages.

Acknowledgements

Ki-i t i Hora i acknowledges the jo in t suppor t of the Univers i t ies Space Research Asso-

c ia t ion and the Na t iona l Aeronau t i c s and Space Admin i s t r a t i on M a n n e d Spacecraf t

Center under Con t rac t No. N S R 09-051-001. This pape r was p repa red when he was

s taying at The L u n a r Science Inst i tute. N a o y u k i Fuj i i grateful ly acknowledges the

oppor tun i ty to test lunar samples Nos 10049 and 10069 p rov ided by U.S. Na t iona l

Aeronau t i c s and Space Admin i s t ra t ion . W. Mande l l read the manuscr ip t and gave

valuable suggestions.

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thermophysical properties of lunar material returned by apollo missions

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