Richard A. Pacer1 and William D. Ehmann University of Kentucky Lexington, 40506
The APOHO Missions and the Chemistry of the Moon
The purpose of this paper is to present a picture of the principal chemical features of the Mwn, in light of the several Apollo missions. General physical features of the Moon will also he discussed briefly. Finally, several theories of the origin of the Moon will be considered, particularly those presented at the Fifth Lunar Science Conference. General Features of the Lunar Landscape
From Earth, the most evident feature of the Moon is its division into hright and dark areas. One-third of the visible face is dark, and the rest bright. Without light of its own, the Moon shines only by reflecting the sunlight which falls on it. However, while the Earth reflects some 40% of the sunlight it receives, because of its atmospheric envelope, the Moon reflects only about 7%. Technically, one speaks of the average albedo (or reflectivity) of the Moon as 0.07 and of the Earth as 0.40 (1). The hright areas of the Moon correspond to the mountainous "highlands" while the lower lying dark areas are called the "maria." Superimposed on this two level relief structure are countless craters and associated ravs..rilles. " . . ridges. " . and scarps. Craters on the Moon are enormous compared to those on Earth. For example, Theophilus and T ~ E ~are O considerably larger than the largest of the craters so far identified on Earth, whether they he volcanic or meteoritic in origin (2). These circular, walled depressions dominate the lunar landscape, appearing in valleys, on the tops of slopes, in the maria, and inside other craters. The larger craters are characteristically surrounded by steep mountain walls and have floors generally lower than the neighboring landscape. Lines of small contiguous craters, known as chain craters, are also observed. Meteorite impacts are believed to he responsible for most of the lunar craters. A few craters may he of volcanic origin, hut proof of the existence of volcanic craters is still a moot point. The name of the dark smooth plains on the Moon, the maria (Seas), suggests the presence of water a t some time in the lunar history. However, large scale photography and direct chemical analyses of returned samples show that the maria were formed by vast lava flows. Except for Mare Orientale. all the laree " maria are on the near side. one often opening out into another. The largest of all, Oceanus Procellarum. exceeds the Mediterranean Sea in area. Much of the mare surface material is coated with glass and glass heads are an important component of lunar "soils." The glass is formed by rapid cooling of impact generated melts. The 12 major lunar oceans and seas are Mare Imhmm ISraofRnins, Mare Crii~um(Sen01 Crises Mare Fecunditatis (Sea nl Fertility,
Mare Vaporurn (Sea of Vapors) Mare Nubium (Sea of Clouds) Mare Humomm (Seaof Moisture) Mare Frimris lSea of Cold)
Domes on the Moon may be circular or elongated humps, often cratered, and many have a very high reflectivity, appearing as hright spots. They resemble terrestrial domed features that are formed by intrusive bodies which force upward overlying rock layers. Some are quite broad and flat in appearance, measuring up to 30 km across and 300 m in height. While the dark maria are concentrated on the front side of the Moon, the far side of the Moon consists almost entirely of highlands. It is believed that the thicker crust on the lunar far side has checked the flow of dark mare material (basalt) to the surface. The thinner crust on the near side permits the break through of mare material more readily. d f the small amount of highland on the near side, most of it is in the southern hemisphere. The most impressive mountain ranges are in the northern hemisphere with the Apennines, rising to 6,000 m from the floor of Mare lmbrium, most outstanding. U-Shaped depressions, often sinuous, known as rilles, meander over the surface near the edges of maria and also in the bottoms of craters. Rilles look much like channels which once carried fluid material, most probably liquid lava flows. Scarps are a common lunar landscape feature, ranging in height from a few feet to several thousand feet. A scarp is a geological fault, caused by vertical crustal movements, which may leave the surface on one side higher than the other. Bright rays are observed to radiate from some craters, especially the younger craters. Apparently, their brigbtness fades with time. Rays appear to he formed by ejecta from high velocitv. impact craters. . ~ u n a ridges i appear to resemble mountain ranges. They are low elevations, mostly of a few hundred meters. with rounded or flat tops. heir narrowness and extreme length make them peculiar to the Moon. Rilles and ridges often follow parallel courses. Ridges are principally mare features and occasionally border the mare, as in Mare Crisium. Some appear to he traces of the tops of craters huried below the maria. The Apollo Missions and Samples Recovered
The six Apollo missions which involved a manned-landing on the Moon recovered a total of 381 kg of lunar rock and soil. The samples have been studied by approximately 1000 scientists in 19 countries. The missions were Apollo 11 July 16-24, 1969 Mare Tranquillitatis November 14-24, 1969 Oceanus Procellarum Apolla 12 Awllo 14 Jan. 31-Feb. 9.1971 Fra Mauro Hills A I Apollu 16 Apolla 17
'Permanent Address: Purdue Univenity-Fort pus, Fort Wayne, Indiana 46805 350 / Journal of Chemical Education
Wayne Cam-
".
J18.y 21;-Aup 1971 April 16-LP. I972 l k c . 7 19, 1972
Apennine hluuntams
Dercsnei Hrghlands
Tauru.-l.lttrou Vallr) The samples recovered included soils, hreccias, and crystalline rocks. Nearly all lunar soils contain abundant
glass. This distinguishes lunar from terrestrial soils. Green glass is common in Apollo 15 soils, while Apollo 17 soils are rich in orange glass. The glass occurs as beautifully formed spberules and teardrops, as irregular masses splashed on rock surfaces, as a welding agent in the soils, and as a lining in zap pits-small, circular depressions which mark all rock surfaces on the moon exposed to particle bombardment from space (3). A breccia is a fragmented rock in which the individual particles are angular, rather than rounded like the pebbles of a conglomerate. Lunar breccias consist of bits of pieces of rock, glass, and soil from divene sources, compacted or welded into more or less cohesive aggregates. The breccias result from shock lithification, or impact melting. Some include as many as two generations of older microbreccias (3). The crystalline rocks are primarily hasalts. A basalt is a fine-grained igneous rock (often containing glass) which has a SiOa content less than 50%. Basalts are generally dark in color and are made up principally of the minerals plagioclase and pyroxene. Basalts containing olivine are also known. On Earth basalts form extrusive plateaus on land and underline the ocean basins. At least four types of lunar crystalline rocks have been identified to date Mare basalts-similar in appearance to the common volcanic basslts an earth, but differing in composition especially a t the minor and trace abundance levels. Anorthositic rocks-These rocks are composed almost exclusively of the mineral plagioclase, a solid solution series ranging from anorthite, CaA12Si208 to albite, NaAlSiaOs. These mcks are found commonlv in the hiehlands reeions of the moon. KREEP bosolts-KREEP stands for [group of elements which are highly enriched in these basalts: potassium, rare earth elements, and phosphorus. KREEP basalts also have a high ahundance of the naturally radioactive heavy elements. Rocks of this composition are unknown on earth. They are found chiefly in Mare lmbrium and Oceanus Procellarum. VHA boselts (VHA = uery high aluminum)-These basalts have been found in the Deseartes highlands region by the Apollo 16 mission.
Anorthosites on Earth are relatively uncommon and rarely form striking topographic features. Earth's anorthosites, such as those found in the Adirondacks, are frequently associated with titanium deposits. On the Moon this is not so. Each returned lunar sample receives a unique number indicating the mission, sampling station, type of sample, and split. Sample 75015,1, for example, is split I of a rock numbered 15 from station 5 of the Apollo 17mission. The following outline summarizes the principal types of lunar samples that have been recovered
Lunar Samples
I
Breecias
Crystalline Racks Soils (Fines) Anorthositic Rocks
Basaltic Racks
I
ire
Basalts
KR~EP Basalts
I
VHA Basalts
Earth-Moon Comparison The principal physical- and chemical differences hetween the Earth and the Moon are 1) The Moon's gravity is only one-sixth that of Earth. 2) Water (except perhaps in trace amounts) is absent on the Moon. 3) There is virtually no atmosphere around the Moon (see section on Solar Wind). 4) The Moon has a greater abundance of refractory elements
(high melting point elements, such as Ca and Ti) than the Earth, but is poorer in volatile elements (such as Na, or Pbl. 5) The Moon has much less iron than the Earth but, unlike the case for terrestrial samples, metallic iron is common in lunar samples. 6) The Moon's density is only three-fifths that of Earth. 7) The Moon apparently lacks the large outcrops of granitic mcks (rich in SiOn) and the sedimentary rocks which cover 75% of the land area of the Earth. 8) The Moon a t the present time has no magnetic field other than traces of remnant magnetism in rocks on the surface. 9) Moonquakes are fewer than earthquakes and they occur at greater depths. Lunar seismic energy is -lo-" that of' Earth. 10) Like the Earth, the Moon is layered with a crust, mantle, and core. However, the core does not resemble Earth's Fe-Ni core. The partially molten lunar cole_is thought by some investigators to be of iron sulfide (mp -200°C). 11) The Moon is rigid for the outer 60% of its radius, whereas only the outer 1% of the Earth is rigid. 12) The center of mass of the Moon is displaced away from its center of figure in the direction of Earth. 13) Surface temperatures on the Moon fluctuateover a much wider range than on Earth, from +llOeC a t lunar noon to - 180°C just before dawn. 14) Geological processes such as volcanism have been operative on the Earth over its entire existence, but apparently became inactive on the Moan approximately 3 X lo9 years ago. Both the Earth and the Moan are believed t o have formed approximately 4.6 X lo9 years ago.
Brief Historyof the Moon The followine outline is a brief history of the Moon which is generally agreed upon by most scientists. However. the detailed mechanisms underlying certain stages of activity are quite controversial 1) Solar nebula condenses into planets and moons approximately 4.6 billion years ago. 2) The outer portion of the Moon melts (due to radiogenic heating) to a depthof approximately 160 km. 3) Differentation takes place. Lighter materials rise to form a crust and heavv materials settle under the influence of eravity. 4) Celestial debris, some of large size such as the'Imbrium object, is accumulated for the first 600-700 million years after formation. 5) Lava, melted by heat generated by decay of radioactive elements, rises to fill the excavated basins (mare basalt farmation). 6) Small meteorites continue to strike the Moon, producing the smaller craters. but a t a decreasine rate.
Chemical Features of t h e ~ o o n The most common minerals on the moon are pyroxene, plagioclase, and olivine. Others of importance include ilmenite, cristohalite, and pyroxferroite. No new elements were found on the Moon, but three new minerals, all occurring in igneous rocks, were discovered. The minerals are shown in Table 1 (taken from Marvin (3)) along with the minor or trace elements that tend to concentrate in certain of these minerals. Numerous laboratories have reported major, minor, and trace elemental abundances in lunar samples. At the University of Kentucky we have analyzed samples from all the Apollo missions and the two Russian missions (Luna 16 and Luna 20) that returned lunar samples to Earth (4-7). The technique used in our laboratory is instmmental neutron activation analysis (INAA). The technique is essentially non-destructive. Other than having low levels of induced radioactivity, the samples are virtually unchanged and may be returned to N.A.S.A. for further studies. Data obtained largely in our laboratory for some major Volume 52, Number 6, June 1975 / 351
Table 1. The Lunar Mineralsa
Table 2. Major and Minor Elemental Abundance5 in Avollo Samples
Abundant
(Mg,Fe,Ca).(Sia.) (Ca,Na) (Ai,Si).Oa (Mg,Fe)a(SiO.)
Pyroxenes Plagioclase Oiivines Accessam
Ilmenite Chromite Ulv%pinel
FeTiOx FeCmO.
Spinel
MgAI?O< (Fe,Mg) (A1,Cr)zO. CaTi01 Cs,REE.TiOa
FesTiO4
Cr-Pleonaste
sample
10022,32 Basalt 12001.47 Soil 14003,13 Soil 16058.76
Perovskite Dysanalyte Rutile
Ti02
Basalt 60025.72 Anorthosite
Baddeleyite
ZrOl
Soil
Nh-REE-rutile
Zircon
Quarts Tridymite Cristabalite Potash Feldspar Apatile Whitlockite
Zirkeiite Amphibole
(Nb.Ta)(Cr,V,Ce,Le)TiOl ZrSiOa+REE.U,Th.Pb SiO? Si01 SiOz KAlSiaOs
+ Ba Csr(P0dz + REE,U,Th CaZrTiOi + Y,REE,U,Th,Ph
CadPOMF,CI)+REE,U.Th,Pb
72161.5
Table 3. Elemental Composition of the Moon Calculated According to the Model of Wanke ( 9 )
(Na,Ca,K)(Mg,Fe,Mn,Ti,Al)Sia011(F)
HTC
Fe
Iron
Nickel-Iron CoPw=
(Fe,Ni.Co)
HTC X 0.69
Chondritic Comp.
Chond. Comp. X 0.31
Moon Calculated
%
Cu FeS FeL
Troilite Cohenite Sehreibersite Corundum Geothite NW, minerals
(Fe,Ni)xP A103 HFeO?
.An addition sign (+) indicates minor or trace elements that tend to concentrate in a given species. The t h r e new minerals were all found on the ApoUo 11 mission to Mare Tranquillitatis. Armalcolite was named in honor of the Apollo 11 astronauts ARMstrong, ALdrin, and COLLins.
and minor abundance elements in a variety of lunar samples are given in Table 2. The specific Apollo mission is indicated by the first one or two digits of the sample number. How does one go from elemental abundances for samples recovered from six areas of the lunar surface to a model indicating the bulk composition for the entire moon? One approach is to look at ratios of the ahundances of certain key elements (8) and to reconcile these with a limited number of components which constitute the Moon (i.e., one assumes the Moon is composed of certain components and proceeds fmm there). Wanke (9) has proposed a model in which the Moon is composed of a mixture of a higb-temperature component, similar to the Ca, Al-rich inclusions of the Allende chondritic meteorite, and a material equivalent in elemental composition to bronzite chondrites, but with a different degree of oxidation. The bigh-temperature condensate would be responsible for the enrichment of the refractory elements in the Moon. That is, if we begin with a cooling gas of cosmic composition, the first species to condense would he the highly refractory compounds of Ca, Al, Mg, Ti, and Si. These compounds would be enriched, therefore, in the high-temperature condensate. The presence of elements in the Moon which condense at lower temperatures would he accounted for by bronzite-like material. (Bronzite is a form of iron magnesium silicate having an iron content of up to 15% FeO. I t occurs both in igneous m k s and in meteorites.) As an example, by using the observed K/La ratio in lunar samoles and the concentrations of K and La both in the high-tkmperature condensate (as analyzed in the Allende inclusions) and in normal chondrites. Wanke (9) has calculated the absolute elemental abundances of the Moon. Let A = chondritic fraction. Then (1 - A) = HTC (high-temperature condensate) fraction '
352 / Journal of ChemicalEducation
With the values R = 70, [K]HCT'1 0, [Klchond = 880 ppm, [LaIcbond = 0.35 ppm and [ L ~ ] H T= c 4.9 ppm, one calculates 69% HTC and 31% chondritic com~osition. Wanke's table (Table 3) shows the final steps in the calculation of elemental abundances for the Moon. Of course, other models may he devised if one chooses other components to mix to form the Moon. Ganapathy and Anders (lo), for example, use a seven-component model to estimate elemental abundances of the Moon and the Earth. They assume that these planets are formed by exactly the same processes as the chondrites, i.e., a sequential condensation from a cooling solar gas followed by several stages of geochemical fractionation. The seven components are: early condensate, remelted silicate, unremelted silicate, volatile-rich material, troilite (FeS), remelted metal, and unremeltedmetal. By using the known abundances of U, Fe, Mn, K, and TI in these components and the key abundance ratios observed for the Moon and the Earth-K/U, Tl/U, and FeO/MnO-elemental ahundances in the Moon and the Earth were calculated as given in Table 4. The model differs from that of Wanke in that calculations are based directlv on a condensation sequence, rather than on the experimentally measured com~ositionof Allende inclusions for the earlv- hiah - temDerature condensate. While there are a number of discrepancies for abundances of individual elements between the models of Wanke (9) and of Ganapathy and Anders (lo), the enrichment of refractory elements (Mg, Al, Si, Ca, Sr, Ba, Ti, Zr, Hf, rare earth elements) and the depletion of volatile elements (H, B, C, N, F, CI, Br, 1, alkali metals, Zn, In,
Table 4. Abundances of the Elements in the Moon and Earth Accordin. to GanaDathv and Anders (10)" Moon
Element
Model
Earth
Element
2150 1.84 25 1330 170 1.93 12.1 1030 103 4780 590
Ru Rh
940 2.40
Ra
35.87 0.48
1.28 0.162 0.87 026
0.1W 0.37 0.067 0.45 0 101 0.29 0.044
Moon Model
Earth
Pd
Aa m b Cd P P ~ In ppb
Sn Sb P P ~ Te IP P ~
'"Xe Cs ppb Hf Tappb W Reppb 0s Ir Pt A" Hgppb TI ppb ~O~Pbppb Bippb Th onb
-
Ppm nulees otherwise marked: noble gases 10-'o cc S T P h
TI, Hg, Cd, Ph, etc.) in the Moon as compared to the Earth is clearly evident. Ganapathy and Anders have also normalized the abundances in Table 4 to values for the primitive solar nebula abundances ("cosmic ahundances"), as derived largely from analyses of chondritic meteorites. When this is done, both the Earth and the Moon are shown to he depleted in the volatile elements, with respect to "cosmic ahundances." However, the level of depletion in the Moon is a t least ten times greater than in the Earth. A second important chemical feature of the Moon, in addition to the depletion of volatiles and the enrichment of refractories, is known as the "europium anomaly." This refen to the anomalously low ahundance of europium, when compared to other large ion lithophile elements, as shown in Figure 1 (taken from data of Gast (11)). In this figure the lunar abundances have been normalized to the ahundances in the chondritic meteorites. The chondritic meteorites are helieved to retain the primitive relative abundances of the solar nebula from which the solar system condensed. Therefore, if there was no fractionation of the rare earth elements on the Moon, the plot in Figure 1 would he a horizontal line. This figure illustrates that there has been a fractionation among the LlL (large ion lithophile) elements. The lithophile elements are those which have a greater free energy of oxidation, per gramatom of oxygen, than iron. They concentrate in the stony matter or slag crust as oxides and more often as oxysalts, especially silicates. The LlL elements include Sr, Ba, and the rare earths. The anomalous abundance of euronium is believed to be related to the different oxygen fug&ites a t the times of formation of terrestrial and lunar hasalts. Trace element studies on a relatively reduced group of terrestrial hasalts, i.e., the oceanic ridge hasalts, show that europium is partly reduced to EuZ+ (EuZ+/Eu3+ -0.2). Philpotts (12) an-
Figure 1. Abundances of rare earth and some large ion lithophile elaments in a lunar basalt. Abundances have been normalized to those in the chondritic meteorites. Figure taken from Gast (51.
alyzed coexisting phases in lunar hasalts and found that the Eu2+/Eu3+ ratio in the parent liquid of these hasalts was probably in excess of five. The EuZ+,having a smaller ionic charge and a larger ionic radius than the other trivalent rare earths would, therefore, he expected to exhibit different chemical properties leading to the fractionation observed. Both EuZ+ and SrZ+ have very similar ionic characteristics, so it is not surprising that the ahundance of S Z + is also anomalously low in lunar hasalts. If fractionation has depleted Sr and Eu in the lunar hasalts, where are they deposited? These elements would he expected to occur in phases enriched in large divalent ions. The only common mineral fitting this description is plagicclase. Indeed, anorthosite (Ca-rich plagioclase) samples from the hiehlands have been found to have anomalously high abundkces of Eu and Sr, as shown in Figure 2.
l
*
O
I
KREEP
Figure 2. Abundances of rare earth and same large ion l~thaphileelements in lunar anorthosites. Samples such as 15415 are believed to originate from the lunar highlands. Abundances are normalized to those in the chondritic meteorites.
Volume 52, Number 6,June 1975 / 353
Clayton et al. (13) have reported on the variations in isotopic compositions in lunar soils for several light elements: H, C, N, 0 , Si, S, and K. In particular, 0 , Si, S, and K all show enrichments of their heavy isotopes in soils relative to rocks. However, no such enrichment has been observed for lunar glasses. The magnitudes of the heavy isotope enrichments in oxygen and silicon are correlated witb each other and with various other measures of soil maturity, such as noble gas content and particle tracks observed in mineral grains generated by cosmic radiation from space. The isotopic fractionations are believed to be related to thermal volatilization of materials during micro-meteorite impacts and subsequent recondensation of heavy isotope enriched materials on the surfaces of lunar soil grains. The action of the solar wind (streams of hydrogen atoms and ions from the sun) on lunar surface materials also is believed to cause isotopic fractionation as a result of the loss of volatile compounds formed. Diffusion loss of compounds enriched in the lighter isotopes would be favored. Enrichments of the heavy isotopes "K and 34Shave also been observed in lunar soils. Corresponding materials enriched in the lighter isotopes 32S and 39K have not been found and it is probable that the lighter isotopes in the cases where isotopic fractionation has been observed have been lost from the Moon by vaporization. The Solar Wind and its Reducing Effect
The solar wind is essentiallv an ionized gas, or plasma, composed of particles that travel out from the sun; underaoing ex~ansionin space. It carries away about 106 tons of gas/s from the sun i n d blows outward through the solar system with a velocity of several hundred km/s. Principal components of the solar wind are the ionized gases, hydrogen and helium. The solar wind serves as the principal source in lunar samples, not only of hydrogen and helium, but of carbon, nitrogen, argon, and neon as well. Because the solar wind is only able to penetrate the outer surface of a substance to a depth of perhaps 1MH) A, there is a much higher concentration of these elements in lunar soils than in lunar crystalline rocks (13). With soils which are exposed at the regolith surface, and sufficiently fine-grained, saturation with these elements in a few years time becomes a possibility (14). At saturation, these elements attain a concentration only a t the ppm level. At least initially, a substantial fraction of the trapped H will again escape in the form of HzO, resulting in a depletion of 0 in the regolith. A large fraction of this water will be photoionized within a few months and more than half of that will he swept off the moon by the solar wind while less than half will be redeposited on the lunar surface. Therefore, within a few years a layer about 1OOO A thick on the outer surface of grains exposed on the top of the lunar regolith will be reduced to the point where the continuing exposure to the solar wind can no longer effectively remove 0 . Since the gardening rate (rate of soil mixing or turnover) due to micrometeorite impacts is slow on this time scale, essentially every grain of lunar surface material will have a thin "skin" which is highly reduced and nonstoichiometric with respect to 0. Miller et al. (7) determined oxygen and other major and minor elements by a direct neutron activation technique on bulk lunar samples and found a consistent 1-2% oxygen deficiency with respect to stoichiometry in lunar fines and many lunar breccias. They point out, however, that the true magnitude of any O-depletion by solar wind interaction can be finally established only by use of analytical techniques capable of surface analysis. Reducing Conditions During Differentiation of the Moon
Wanke (9) and others attribute the absence of ferric iron in the lunar hasalts as indicative of the presence of a 354 / Journal of ChemicalEducation
low partial pressure of oxygen (
