chem I fupplement
edited by: MICHAEL R. SLABAUGH HELENJ. JAMES Weber State College Ogden. Utah 84408
Geochemical Exploration of the Moon lsidore Adler University of Maryland, College Park, MD 20742 The most important kevs to understandine the formation of the solar system and its subsequent evolkion lie in the chemical composition of the planetaw surfaces and interiors. the meteorite;, the comets, and the asteroids. An exhaustive study of the geochemical exdoration of the planets would require a ver;lengthy text. 'fhis paper is based on examples from my own experience in the Apollo proeram. These examples representonly a small part of the iin&ngs of the space program. The abundance of certain elements relative to cosmic ahundances, their mode of condensation, and condensation temoeratures are imoortant clues. For exam~le. . the evolutionary processes that have occurred in our solar system can he studied by examining the abundances of potassium and thallium relative to uranium. These elements are indicators of the abundance of volatile elements relative to refractory elements. Such studies also provide important information about the me- and oost-accretionam in the evolution " staees " of the solar system.Since we have vet to penetrate even the Earth's mantle. it is obvious that in-forma&onabout planetary interiors can be ohtained only bv inference. On the other hand. for surface investigations one can employ classical methods such as geological sampling, mapping, and laboratory analysis. Space-age technology using remote analytical techniques, mainly spectroscopic and photographic, are also used. An example of the latter involved the use of photogeologic methods in the Landsat program. We are presently constrained to the use of remote techniques when dealing with celestial bodies other than our moon. Study of our own moon is unique in that we have been able, because of the samples collected, to employ a largely classical approach under decidedly new circumstances. Remote methods have also been used.
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Questlons about the Moon Prior to space travel, our knowledge of the moon rested on earth-based telescopic studies. With the advent of space exploration, numerous programs carried out by the United States and the Soviet Union have provided a wealth of new information. Among the various programs were the Raneer fly hyiand impacre&, Surveyor lahders, Lunas, Orbiters. &d he Apollo and l.uankhod tliehts. Thf: first tntls remarkahle in-sit11chemical analysis of the lunar surface resulted from the Survryor missions ( 1 1. Surveyor 5 Innded ar Mare'l'ranThis feature presents relevam applications of chemistry to everyday life. informationpresented might be used directly in class, posted on bulletin boards, or otherwise used to stimulate student involvement in activities related lo chemistry. Contributions should be sent lo lhe feature editors. The
36
Journal of Chemical Education
quilitatis, Surveyor 6 a t Sinus Medii, and Surveyor 7 on the rim of the very large crater Tycho. The instrument deployed to the lunarsurface used the principle of alpha-parGcle hack-scattering developed decades earlier by Rutherford and ~--others (.2.) . This orovided chemical information ahout the landing sites. The first reasonablv successful attemot at chemical analvsis from orbit was during the flight of ~ u n 10 a (3)which orbged the moon and carried a eamma-rav snedrometer. In 1969 the Apollo 11 mission land& men on the moon for the first time; the site. Mare Tranauilitatis. This was the first instance in human history that d'wum(.ntrd samples rrom another body in the sular svsrenl hits been ohtained. Bv the end or the A~ollu 17 mission, astronauts had returned about 400 kg of soiiand rocks. The study of these samples employed a great variety of the most sophisticated analytical methods. The samples from the large lunar basins were essentially hasalts, while those from the highlands were essentially feldspathic anorthosites. The highlands were found to he the most ancient of the lunar features, somewhat over four billion years old. The basalts were found to range between three and four hillion years old. They apparently came to fill the large basins after their formation, which likely occurred as a result of the impact of enormous meteorites and planetesimals some four billion years ago. The data mentioned above was ohtained from analvsis of the samples collected a t a few chosen sites on the moon. Our view of the whole moon is an extrapolation from these facts. Then, are many reasons, however, for serking ~hemicalinformntion from other sites (manv of whirh will be hevund the possibility of manned landingsfor years to come?. Global com~ositionalmaps could be of meat value in understandine the muon.'l'hus a series of orbital experiments was proposed which bwan with the Apollo 15 mission. These experiments have helped considerably in answering a number oiquestions relating to the early history and origin of the moon. ~
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Orbltal Remote Sensing How does one plan experiments to give compositional information about a celestial body which can he conducted from an orbiting satellite? There are only a limited number of observational phenomena from which one can infer unique elemental identifications and concentrations. A survey of the electromagnetic and particulate spectrum yields only a few possibilities. Certainly one cannot expect to obtain results which are precise by laboratory standards. Nevertheless, such techniques as gamma-ray or X-ray spectroscopy can provide much useful information. A summarv of the radiation environment at the lunar surface is shown-in Figure 1.Based on this view of the lunar ambient radiation, a series of orbital experiments were desiened to supply chemical information. As we can see, both natural
Figure 1. The radiation environment at the lunar surface. Particularly important is the natural radioactivity, the solar X-rays, the gamma rays produced by the Cosmic rays and perhaps the alpha particle emission.
and induced sources of radiation are present. Among the principle naturally occurring radioactive constituents are the long-lived nuclides 40K, 2S2Th,and 238U and their decay products. These sources emit alpha, beta, and gamma radiation of various energies. Importantly, the gamma radiation is characteristic and can he used for identification of the elements that emit them. Special processes such as radon and thoron diffusion may also occur (4). Bombardment of the lunar surface by cosmic rays and energetic protons will also produce a variety of short-lived nuclides and, as a consequence, induced radioactivity. Cosmic ray interactions with the lunar surface will also ~ r o d u c ea . nromDt . emission of neutrons and protons. Even under "auiet" conditions. the sun is a conious emitter of soft X-rays. Because of the filtering nature bf our atmosnhere. i t has been possible to determine the s ~ e c t r adistril bution'of these x-rays only by flights above the atmosphere. These solar X-ravs are absorbed bv the lunar surface and produce secondary (fluorescent) X-rays which again are characteristic of some of the elements making UD the lunar surface. One can then predict the yield of the-secondary Xrays, which depends on the intensity and spectral character of the solar X-ray flux and the abundance of the elements in the lunar surface. Finally, the radon and thoron which are diffusing through the lunar surface would he expected to produce alpha particles having energies characteristic of the various decay processes. Lunar X-Ray Experiment We see that the "quiet sun" is energetically capable of producing measureable amounts of characteristic X-rays from all the abundant elements with atomic numbers of approximately 14 (Si) or smaller (Fig. 2). During brief periods of heightened solar activity, one can expect characteristic X-rays from hieher atomic number elements. As foithe design of the X-ray experiment, calculations had shown that the moon's X-ray brightness was low, and it was therefore necessary to design a highly efficient system for detecting the X-rays from the lunar surface. Not onlv was the orhitingk-rayspeitrometer required to have a high eitirienry. l that n d d I)+: hut it also had t,, provide s ~ e c t r a iniormation reduced to chemiral ql~antirirs.Therefure, an asieml)ly was built which consisted uf three larne prol)ortiunnl countt.rs with thin windows of beryllium. One detector was operated "hare" and the other two had selected X-ray filters consisting of a magnesium foil and an aluminum foil, respectively. Further, the X-ray output from the detectors was energy-analyzed by eight energy discriminator channels which covered in equal intervals 0.5 to 2.75 keV X-rays. In this manner, three differential X-ray spectra were obtained from the lunar surface which, by simple mathematics, could be reduced to intensity
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Figure 2. The "quiet' sun X-ray output.
CALIBRATION SOURCES
Ar-90%
cq-9 5% He 0.5%
Figure 3.An exploded view ol the X-ray spectrometer,
ratios of AlISi, MgISi, and MgIA1. The detectors viewed the moon through mechanical collimators so that the field of view was 60°. This meant that a t orbital altitudes of 100 km, resolution on the lunar surface permitted analysis of features about 10 km across. A view of the X-ray spectrometer is shown in Figure 3. Gamma-Ray Experiment From the very beginning of the lunar program it was assumed that K, Th, and U would be key elements in the unVolume 61
Number 1 January 1984
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APOLLO 17 .
0.01
lo2
i o3
10"
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I Figure 5. The alpha-particle experiment.
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lo5
K in ppm
Figure 4. Potassium versus uranium in various materials. derstanding of lunar evolution. In terrestrial processes the radioactive decav of these elements is considered to be a maior s~mrce~ , the f energy lrnding to \wlcanism and magmatic differentiation. Further, the nhundilnce of thew elrmenti hecomes an indicator of the extent of chen~isnldiffrrrntintion since K,'l'h, and U tend toroncentrate in the late stage crvstalline rocks such as granites. By the Apollo 15 flight, excellent determinations of K, Th, and U had been made in various returned lunar samples. As an example, K versus U values have been plotted for various lunar materials (Fie. 4). As a compnrism, a linr ii drmvn for tt:rrestrial materials ranging from basnlts to granites. The rel;ttive puiir~onof the achondritic stony meteorites is also s h o w n . ~ h ediagonals on the graph represent KIU ratios. Note that the lunar materials have lower KIU ratios than terrestrial magmatic rocks. This has been traced to high U values and low K values. A similar situation exists with regard to KITh ratios. Furthermore, it was found that there was a distinct variation from one collection site to another. Because K. Th. and U are such important indiratori of chemi(:,ll processes, the attempt to map the elobal distrihution oi these elements around the moon was considered of very great importance. The gamma-rav assemblv flown on Apollo 15 and 16 was composed of three major subassemhlies:~hescintillation detector (a thallium-activated sodium iodide crvstal). the electronics, and the thermal shield. In flight the g&ma-ray spectrometer was deployed at the end of a 7.5 m boom in order tb remove it as far i s possible from the radioactivity, both natural and cosmic ray induced, in the spacecraft. While the X-ray experiment described was useful only for the areas illuminated by the sun, the gamma-ray experiment supplied data for the entire flight path. Alpha-Particle Experiment There an!a n u m l w of pnsiible sources for alpha-particles from the lunar surface. The nrincioal ones arc aloha radioactive decay of radon and thbron ('and their daug'hter products) which, if circumstances are suitable, would have diffused out of the first few meters of lunar soil. As on the Earth, this is a reflection of subsurface activity. If one assumes thermal velocities for the escaping radon and thoron, then nearly all the molecules would be t r a o ~ e din the moon's gravitational field. Radon with its 3.8-daybalf-life should travel a considerahle distance before undergoing radioactive transformation. Thoron on the other hand, w i t h a 55-sec half-life, would he
38
Journal of Chemical Education
Figure 6. The Science Instrument Module (SIM).
expected to decay near its source. The necessary equipment for alpha particle measurements is shown in Figure 5. The detectors are solid-state surface harrier types. The flight instrument which was contained in the same housing as the X-ray experiment consisted of a series of detectors and alpha-processing electronics. The Apollo Science Instrument Module (SIM) Figure 6 shows the Apollo SIM hay. The SIM bay was part of the Anollo Command-Service Module (CSM). Beeinnine with the-b pol lo 15 flight, the CSM was used for the first time to carrv an ambitious arrav of instruments for orbital survevs of a vaiiety of lunar chara&ristics of which surface chemist& was a prominent part. The instrumental complement consisted of an X-ray fluorescence spectrometer, an alpha-particle spectrometer. a mass s~ectrometer,a panoramic camera, a laser altimeter, and a s;hsatellite. he latter was launched into an orbit of its own for the purpose of m a k i -. e geophwical . . measurements.) The X-ray, gamma-ray, and alpha-particle s~ectrometerswere used for ohtainine chemical information about the moon's surface. The massspectrometer was employed in a study of the moon's tenuous atmosphere at orbital altitudes. I t is noteworthy that the X-ray and gamma-ray instruments were also used to obtain astronomical data during the trans-Earth coast. Results The orhital experiments describrd thus far prnved t o he ~uccessfuland inihnarivr. Rrfor(: d i 4 l i n r the X-rav resuks. that entered it is important rg, describe wmt: of the co~~strdiuts into the internrru~tiunuf thedata. Since the sun is not a stahlt: X-ray source; it was necessary to devise methods of correcting for solar flux variation. Instrumentally, a small proportional counter was pointed towards the sun while the large detectors were pointed towards the lunar surface. In this manner, the sun's X-ray output was monitored during surface measurements. Other sources of variation included the sample matrix effects, the solar illumination angle, and surface roughness. These factors were minimized by the use of intensity ratios (5).In view of the near constapcyof the silicon abundance, the
Figure 7. AllSi intensity ratio along the Apollo 15 trajectory.
other elements, such as Mg and Al, were referenced to the silicon. There were other boundary conditions such as the depth of the surface samples (in the X-ray case about 10 pm), the resolution of surface sampling was about 10 km and, of course, the observations were only from the sunlit portions of the moon. The compositional profile in terms of AlISi along the Apollo 15 trajectory is shown in Figure 7. The ratios for various analyzed luna; materials are ihown on the right-hand axis. A number of conclusions have been drawn based on such results: the AIISi ratios are greater in the highlands (characteristic of feldspathic materials) and considerably lower in the mare areas (typical of hasalts); there is a general tendency for AliSi values to increase from the western maria to the eastern limb hiehlands: there was ecmd meement between the A1701values dekrmined by classical techniques obtained from &~lysisof lunar samwles . ..eathered durine the Avollo 17 mission and from that inferred from the S-my measurements. This latter fact shc,wt!d us that the N-ray nleasuremvnti werr indeed a reliahle guide to at least this aspect of the lunar chemistry. The X-rav results clearlv demonstrated that the moon's crust is chemically differencated. On a global scale, one could define the extent of differentiation and clearly the mare basalts from the highland anorthosites. The X-ray results have also been correlated with optical albedos (the way in which the lunar surface reflects light). I t was clearly demonstrated that the reflectivity was a good clue to the chemical composition. High reflectivity was positively correlated to materials of high AlISi. The gamma-ray experiment performed in two areas provided both a survey of natural radioactivity as well as considerable information about the chemistry of the surface. Data was gathered from a belt around the moon. In the X-ray experiment, samples were taken at a depth of about 10 pm. The garnma-ray experiment probed to depths of the order of tenths
of centimeters. The data gathered by such experiments on the Apollo 15 and 16 missions yielded information about the relative intensities of the natural radiation of the moon. For example, the regions within and bounding the western maria (Oceanus Procellarum) show higher levels of radioactivity than any others on the lunar surface. There is, in fact, a striking contrast between this reeion and the rest of the moon, pnrticularly the eastvrn maria. h r t h e r , thrrp is a detailed structure in the distrll~utionor radiuartivitv within the hich radioactive regions. The highlands showeda relative radbactivitv excent on the borders of the western maria where l a t e r ~ mixing l is a good possihilitv. One interesting result is the observed ratio of KlTh. (I'otassium is rrpresentative of \datile elements while the thorium is rrprc,sentative of the refrart8,ry ones.) These rneasurwnent.; have bomr out the fact that the moon is deplcted oivdatile elcmtmts relative to the refractor\, onrs, an nnportant fact which must he considered in any &ory of the koon's formation and evolution. Alpha-particle data taken during the Apollo 15 and 16 flights showed that the observed alpha activity was quite small, on the order of 10-3 countslcm2-sec. The nature of the experiment was to seek three types of signals: alpha-particles having energies consistent with the decay of 222Rnand its daughter products, alpha-particles from 220Rn and the daughter products, and finally, alpha-particles from 210Po. The first two can be associated only with current activity (because of their short half-lives) and the last with events havine occurred davs to vears aeo. P P o comes from the decayif 210~bwith half-iife of 27 ye&.) Two features were observed in the Apollo 15 data. There appeared to be a general but small increase in count rate over Oceanus Procellarum and Mare Imbrium. The trend appears to parallel the U and T h observed by the gamma-ray experiment. One of the most dramatic features was the increase in 210Poa t the edge of some of the maria. An additional correlation was the finding of heightened activity in the Aristarchus region where transient lunar phenomena had been observed. This last observation is particularly interesting because these transient glows which have been observed telescopically in the past have been attributed by some investigators to solar excitation of escaping . - . gases.
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Summary The results obtained from the remote analysis of the moon by spectroscopic techniques have contributed substantially to a unified view of the chemistry of the lunar surface. Further imnortant orinciwles have been established for future vlanet& studies. The iunar investigations give us reason to ielieve that such studies can be done on other surfaces many millions of miles from Earth. Literature Cited
(2) Rutherford, E., Chsdwiek, J., and Ellis C. D.. '"RadiationFrom R a d i o s e t i ~subsumc~s,). Cambridge University Press, 1930. I31 Vinogradov,A. P.,Surkov,I. A.,Chernov,G.M.,andKimow.F. F.,"Mes~uremenu of Gamma Radiation of the Moon'a Surface by Lhe Cosmic Station Luna 10; GRIchemistry No: 8, Vernadsky Institute of Geochemistry and Analytical Chemistry
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Number 1
January 1984
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