FOURTH SILVER JUBILEE LECTURE 20 TH NOVEMBER 1974
FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS OF THE SOLAR SYSTEM
BY D. LAL Physical Research Laboratory, Navrangpura, Ahmedabad-380009
Published by BIRBAL SAHNI INSTITUTE OF PALAEOBOTANY LUCKNOW
ISSUED 1976 FRUITS OF EXPLORATION OF MÖON ANÖ NEIGHBOURING PLANETS OF THE SOLAR SYSTEM
BY D. LAL ; Physical Research Laboratory, Navrangpura, Ahmedabad-380009
Published by BIRBAL SAHNI INSTITUTE OF PALAEOBOTANY LUCKNOW
ISSUED 1976 FOURTH SILVER JUBILEE LECTURE
FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS OF THE SOLAR SYSTEM
by D. LAL
IASED on results of recent explorations subsequently subjected to extremes of pres- of the Moon and other planets, it has sure or temperature. The inscription is now been demonstrated that one can made by particles which are omnipresent in derive a lot of quantitative information about space. We call these particles by different the palaeontology of the solar system. So names depending on where they originate; similar are some of the modern techniques e.g. solar wind, solar cosmic radiation or that have been used to those employed by galactic cosmic radiation. In each case we the great palaeobotanist, Professor Birbal are dealing with fast particles of atomic Sahni, that I would assert that, had nuclei; they are accelerated by stars like we been fortunate enough to have him our Sun. The interactions between the amongst us today, he would have been most particles and the rock are the basis for writ- delighted to learn of these new palaeonto- ing of the history of the conditions under logical methods used and of the advances which a rock evolved. made by space scientists. In the case of the earth, the fossil evidence The star of our show today is going to be is very rich indeed. But this is due to the a piece of rock; a mere rock. I would first fact that earth has an atmosphere and it has like to state, and subsequently qualify it, water. Further, since over long periods of that a rock contains usually, not only the time the earth has been the abode of a great geological records of the planet it derives variety of life, the record is very rich. We from, but also very valuable information now know that Moon has no atmosphere, it about the early birth and subsequent has no water and so far no evidence for life evolutionary history of the Sun and its has been found on the Moon. Considering planets. these facts, one may be tempted to say that Today my main attempt is to try to show the fossil record on the Mbon could hot be you that in the absence of an atmosphere on very interesting. However, this conclusion the planet, any rock which one derives from is erroneous. Recently, we have learnt that its surface contains not merely the fossil the information contained in lunar rocks, record of the time when the rock cooled ' provided one is ingenious enough to read it, down and about the average magnetic field is tremendous; what we have been able to which was present at thai, time but, in addi- leárn so far is probably only a small part of tion, it will also record a greater variety of the total information which is contained in physical processes which have occurred a rock. throughout the entire history of the existence As mentioned, in the case of the earth the of the rock itself. This record is, curiously fossil evidence owes its existence, largely to enough, written continuously in the rocies the fact that earth has an atmosphere, water and can be deciphered unless the rock was and other things (e.g. magnetic field) which FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS makes the terrestrial record very rich. In Let us begin with some discussions on the the case of the Moon however, we have a nature of lunar samples. The intellectual great variety of particles coming to it from man, even before acquisition of the lunar remote regions of the universe and these samples, has always been on the look out for particles alter the lunar rocks in many diffe- samples from extra-terrestrial environments. rent ways. These records are not found in This it, not just a curiosity. It is a very im- earth rocks and hence earth and moon rocks portant step in learning about the universe provide complementary information. The because we want to know about its composi- reason why the particle record is not seen in tion, we want to know about its evolutionary earth rocks is due to the fact that the earth history. How old is the Universe, has the has an atmosphere which acts as a shield; Universe existed for an infinity of time, are the large variety of particles which are inci- among an infinity of questions man has dent on the Moon are also incident on the asked of himself. What is the relevance earth but they can not reach the surface due of these questions to the study of lunar to energy dissipation or fragmentation in the rocks ? earth's atmosphere. Based on a superficial probing, one may Essentially, on the earth, we are leading not see how these cosmic questions could a very shielded life. We are not only shield- possibly be answered by examining a rock ed by the ceilings of our houses but we are from the solar system. But as we will soon also shielded by the atmosphere, which pro- see, a rock does contain plenty of useful in- tects us in many ways. We shall see how formation, and that is why scientists have important this shielding is, today. been trying to get " material" samples from Moon has no atmosphere. The total outside the earth. Whenever we see star- amount of lunar air, or whatever you call it, light, we see electromagnetic radiations is less than lö"la mm of mercury. Today which are indicative of the matter present it has essentially a zero dipolar magnetic in the Sun, stars and all other types of field. So even the lowest energy particles heavenly objects. When air from very high from Sun can directly reach the surface of altitudes is filtered, one finds dust in it. the Moon without any hinderance at all and Scientists have been trying to find out it is for this reason that the moon rocks whether some of this dast derived from cos- have been so important. For some reasons, mos. Meteorites constitute substantial sam- this is rather a technically involved issue. ples of matter which conies to the earth from These things are never brought to light in outside. Being th?. largest samples of cosmic general discussions which are published in matter easily available for scrutiny, they technical magazines or newspapers because have been studied extensively using a variety the news writers find these points too com- of techniques. But, not unexpectedly, only plicated and involved. ' a casual importance has been given to meteo- With all this realization, considering on rites in the pre-Apollo epoch, since whenever the one hand the Leveral recent advances in a thing becomes available in plenty, one loses space physics and the realization that rocks sight of its importance. The field of meteo- are good record keepers of the geological ritics is more than a century old and there history of a planet and considering on the are more than 1500 different known meteo- other hand that palaeobotany is indeed and rites in museums. The lunar samples, on has been the main research area óf this insti- the other hand are clearly recognized to be tute, I embarked on the difficult task of pre- very valuable because of the costs involved senting this talk as one of the annual Silver in reaching the Moon and bringing them back Jubilee Lectures of the Institute, to the earth. In our laboratory, we were FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS prepared to undertake an extensive detailed by a hypervelocity impact of a fast particle and painstaking study of lunar samples even from that due to the emission of hot volcanic if *ve had been given only a few milligrams. material. Next time you see photographs Everywhere, prior to the happening of the of the surface of the Moon, mercury or mars, successful Apollo 11 mission which brought examine them carefully and contrast them back tens of kilograms of lunar rocks/soil with the volcanic craters we see on the earth. samples, scientists were attempting to deve- Also, a keen observer will immediately ask lop completely new technologies so that some- the question why one sees so many craters thing very exciting could be learned even if on the Moon or Mars, but so few on the only very small amounts of lunar samples earth. were returned in the manned lunar missions. In a distant picture of the Moon, as in And this had very interesting consequences. PI. 1, one can clearly see something like 15 Because, ignoring for the time being, what one large craters. Such photographs of the Moon has learned from studies of moon samples, were taken long before the Apollo landings. the technologies that have been developed But, in fact, the entire face of the Moon is have opened up new scientific frontiers. pocked with craters. This becomes clear For example, from an analysis of about 1 when one takes a look at photographs of the mg of rock sample one can. determine the surface of the Moon taken at shorter and chemical composition of something like ten shorter distances. As one comes closer to elements; more than half a gram of sample the Moon, one continues to see a large num- will be needed to determine the concentra- ber of craters; the number of clear cut craters tion of all the 92 elements. On the basis of in a given photograph actually increases analysis of one speck of lunar dust, weighing somewhat in photographs taken from close one microgram or less, one can tell with fair quarters. The Apollo missions have pro- certainty whether the sample belongs to the vided a large number of such pictures (see lunar surface. None of this was possible for example Pis. 2, 3, 4, 5). The remark- before. This is indeed an exciting develop- able persistence of craters continues all the ment which has taken place, thanks to lunar way down to the smallest size craters which explorations. can be seen in lunar rocks and grains, using Let us now consider, by way of illustra- the highest resolution microscopes. The rea- tions, a few topics where lunar studies have son for the fact that not only is the lunar proved valuable towards understanding the surface itself cratered down to the smallest evolutionary history of the Moon and meteo- scale size, but even small lanar grains have rites and how we decode this information craters on them, is of course due to the fact from analysis of the lunar rocks and meteo- that the voids of space are continuously rites. traversed by high speed particles, often at 1 We will first consider the phenomena of velocities exceeding 30 km sec" . Masses of some of these particles are estimated to cratering in space due to impact of high speed 15 particles. Small particles produce small be as small as 10' grams; craters of about craters; large ones larger craters. In some 10 millionths of a centimeter have been seen respects, a very similar phenomenon takes in lunar samples. From the frequency of place when a volcanic eruption occurs. In the craters in different size regions, the flux a volcanic erruption huge amounts of material of micrometeorites, or the cosmic dust par- are thrown off and the'result, called a crater, ticles as they are called, can be deduced for is a large mound with a hole in the centre. different size intervals; the number increases Unless examined in great detail, one can not rapidly as one goes to smaller sizes, as a tell the difference between a crater produced power law. FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS
Effects of particle bombardment on the trial weathering processes due to combined lunar surface are not restricted to ".ratering. action of wind and water lead to a continuous Surface rocks rupture and develop, angular change of the morphology of the surface fea- features; for example see Pis. 3 and 4. tures so that the lifetime of craters on the Large scale cratering on the moon leads to earth is limited. extensive mixing of soil and excavation of What we term as a " shooting star " in deep seated material. Different sites on the our mythology is not a starlundergoing an Moon, mare or highland regions (Table 1), explosion.or burn-up in space; the pheno- provide ample proof for the occurrence of mena of shooting stars arises due to small recent and old craters and the influence of particles of dust which, in going through the high speed impacts on the chemical composi- atmosphere, burn and volatalize due to frica- tion of lunar materials and the morphology tion with atmospheric molecules. A very of the lunar surface (see Pis. 4 and 5). brilliant display results just from a particle Lunar craters are mostly named in of jJ5 mm size. honour of great scientists who have made In the state of Arizona, USA, there is a everlasting contributions to science and meteorite crater, about 1«4 km across, caused humanities. Six of the craters are named by an impact of an iron meteorite, named after Indian scientists: Bhabha (57°S, 165°N), Canyon Diablo. The largest crater on the Bose (54°S, 169°N), Haldane (1-7°S, 84°E)i earth is ~11 km. This crater is about (20- Raman (27°N, 55-2°W), Saha (2°S, 103°E), 30) X 103yrsold. ThereisalakeinBuldana and Sarabhai (24-7°N, 21-0°E). district in Maharashtra State, the Lonar Earth, being an object in space, is sub- Lake. This lake,was long suspected to be jected to similar continuous bombardment a meteorite crater and now it is an established by micrometeorites and larger particles as fact. Based on observations of lunar rocks, Mars, Moon, Venus, or Mercury are. Why whose composition is quite similar to Deccan then is the sculptoring of the earth's surface basalts in many respects, one has been' able no different from that of the Moon ? Why to confirm the origin of the Lonar Lake. In do we not continually have to safeguard our- turn Lonar Lake studies are useful for under- selves against the energetic impacts of these standing lunar cratering processes; The particles ? These questions find their ans- Lonar Lake event is dated to be about 20,000 wers in two facts: (1) earth has an atmosphere years old. The Arizona crater is older. The which effectively shields the surface of the main point which I wish to make, here is that earth from the numerous tiny particles which these craters do not survive for long periods are incident on the top of the atmosphere in due to denudation processes. Thus large relatively large numbers and (2) the terres- craters are formed on the earth occasionally
TABLE 1 —SAMPLE RETURNING LUNAR LANDING MISSIONS
MISSION DATE OF LANDING LANSING SHE (Latitude: Longitude) REGION
Apollo 11 20 July, 1969 Sea of Tranquility (0-67°N: 23°49'E) Mare Apollo 12 19 Nov., 1969 Ocean of Storms (3°2'S: 23°4'N) Mare Luna 16 20 Sept., 1970 Mare Fecunditatis (0-40'S: 56-3°E) Mare Apollo 14 5 Feb., 1971 Fra Mauro 3°40'S: 17°23'E) ' Highland Apollo IS 31 July. 1971 Hadley-Apenine Front (Sö^ö'N: 3°39'E) Mare Luna 20 21 Feb., 1972 Shore of Mare Fecunditatis (3°32'N: 56"33'e) Highland Apollo 16 21 April, 1972 Descartesr (8°59'S: 15°3O'E) Highland Apollo 17 11 Dec, 1972 Valley of Taurus-Littrow (20=10^: 30°46'E) Mare FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS due to impact of large meteorites and what rocks are just as interesting; of course one wu see today on the earth are those events does not make such fine thin sections of the which occurred less than 1 m.y. ago. terrestrial rocks. I would now like io go into another aspect When thin sections of lunar rocks or meteo- of changes induced by high speed particles — rites are chemically treated, they reveal solid but confining ourselves to nuclear sized parti- state damage trials due to fast particle tra- cles. The rock continually undergoes chan- versals. In PJs. 7 and 8, we can see trails of ges due to traversal of fast nuclear particles the cosmic ray nuclei arriving on the Moon which are present in space. Rock matrix from our Sun, and our galaxy,- respectively. is thus not oblivious to external influences. Sun produced lower energy particles com- It is well known that most of the rocks are pared to those produces in Supernova; the made up of crystals. The crystalline mate- fast nuclear particles accelerated by the Sun rial, in contrast to amorphous material, is a and other stars are historically called " cos- well ordered substance. Any change in it, mic rays ", although clearly solar particles e.g. due to traversal of a fast nuclear particle should not be so designated. Thus, rocks through it, is going to be appreciable in some serve as particle detectors and it turns out form. The alteration or solid state damage that on the basis of observations of the cos- will be different depending upon the velocity mic ray tracks one can identify the nucleus and the size of the particle. Besides such responsible for the track with a fairly high changes, alterations in the chemical com- resolution, in some cases with higher resolu- position of the rock can also occur due to tion than available with the most sophisti- nuclear interactions which also result in pro- cated man-made particle detectors comprised duction of several radioactive isotopes not of " artificial" solid state devices. originally present in the rock. These are the A particular advantage of moon-meteorite reasons why one can use a rock as a book. studies-lies in the possibility of studying very Essentially one can call a rock a Purana. rare elements in cosmic rays, e.g. transuranic Many rocks make up a whole collection of particles. Their intensity is so low that for Puranas and one can use them to learn about the case of contemporary studies based on the history of the solar system. balloon flights, one would have to employ Recks, when grinded dov/n, become trans- detectors of large cross-sections, say 5x5 parent ; see PI. 6, Basically, an igneous rock metres. The meteorite or moon samples, is composed of an enormous number of crys- on the jther hand, are ideally suited for such tals which individually, are transparent to studies since they have been irradiated in light. Metal particles and some minerals space for millions of years. See, for example, like ilmenite are of course not transparent the large number of tracks due to heavy to light, but by and large, rock matrix is nuclei in a meteorite crystal in PI. 7. composed of transparent grains. Crystals Let us now discuss some aspects related have the unique property of rotating the to the chemistry of the planet. If you take plane of polarization of light and when seve- a rock from a terrestrial field and another ral crystals are put together at random, as rock say, one foot away from the first and in a rock, not much light is going to be trans- then measure the chemical composition of mitted. This is the reason why a rock looks these two rocks they will most probably not black as a whole. But if you take a rock be the same. Scientists conjectured that and polish it down it becomes transparent. since the Moon is probably very old,, and it PI. 6 shows photomicrographs of thin sec- did not undergo any appreciable volcanic tions of moon rocks only, but I would like activity in the past, it may be full of rocks to assert here that sections of the terrestrial of one type. On the earth repeated volcanic FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS eruptions, melting, cooling etc. have pro- either the Moon did not originate from the duced a great variety of rocks which do not earth (as a result of some fissioning event), have the same chemical composition. We or this separation was associated with some also have lest in this way old terrestrial rocks, very high temperature condensation/volatili- and that is why we have lost a great deal of sation/metamorphic processes which altered information about processes which occurred appreciably the composition of the Moon. in the early days of evolution of the earth. We may now turn our attention to the On the Moon one finds a great variety of " age" of lunar rocks. Age is a very rocks, a situation quite different from that "broad" word; it means different things expected. If one compares any given rock in different contexts. Scientists are able to or dust sample from the Moon with corre- assign Rome 20 different ages to lunar sam- sponding earth samples, one finds no definite ples. A geologist refers io the age of a rock pattern. The concentrations of some ele- as the time elapsed since the rock was crys- ments in lunar samples are sometimes 100 tallized or since the rock solidified. This times higher and sometimes 100 times lower " age " estimate gives information as to how than those in the earth samples. This was long a given rock has existed as a solid body; a great puzzle at first, but we now have however, it contains no information on the begun to understand the situation, thanks time for which it remained molten. Other to the fact that we hold lunar samples in " age " which can be estimated refer to other great reverence. All the 92 elements in processes which occurred in the life time of several sampler fiom the eight sample-return- a rock. For example, one can determine ing lunar missions (Table 1) have been ana- how long a little grain on the Moon has re- lysed with great precision. Terrestrial sam- mained on the lunar surface or how long it ples were never treated with such adulation. has been there within 1 metre depth or Several important facts have emerged 10 metres depth. Such " age " determina- from detailed studies of trace element syste- tions allow one to construct a hypothetical, matics in the Moon, meteorites and terres- but fairly plausible, history of evolution of a trial samples. Firstly, we note certain lunar grain; the time it spent at different groups of elements are correlated, and the levels within the lunar regolith, since its correlation is so strong that if one measures parent rock was solidified, can certainly be the concentrations of one of them, it is pos- determined accurately, but the prediction sible to predict the concentration of other of time sequence of movement of the grain elements in the group. Elements K, Ba, may not be possible in most cases because Be, Yb, Rb, U, W, and La, for instance, of the complexity of mixing and transport belong to one group. Secondly, it turns out processes. This exciting probing into the that certain groups of meteorites have a com- past is a reality because of the variety of position of trace elements which looks quite interactions which one is able to study quan- alike but quite different from other meteo- titatively by directing a rock and treating rites. Hence, it would seem that meteorites it with chemicals and looking at it under a in our possession derive from several diffe- high power microscope. rent sources. Based on such analyses, it It is common knowledge, by virtue of seems that the Moon and the earth do not extensive publicity in newspapers, that Moon represent a homogeneous parent body, i.e. if rocks are 3-4 billion years old. Although the Moon tore off from the earth at some time this excites one's imagination, it should not in the past, the chemical composition of lunar make much difference, in a way, because as rocks should have been quite similar to the far as time goes, even one billion years is a " earth " rocks, which is not the case. Thus very long period. We can generally con- FRUITS OF EXPLORATION OF MOON AND NEIGHBOURING PLANETS ceive of one day, or at most what will happen with a few years, but man who lives for only TABLE 2 ~ APPROXIMATE CRYSTALLI- ZATION AGES* OF LUNAR ROCKS r^lOO years could hardly be expected to be SAMPLES AT VARIOUS LANDING SITES excited about 1, 2 or 4 billion year old rocks. MISSION CRYSTALLIZATION AGES But geologists, cosmo-physicists and cosmo- 10» VRS. chemists like to deal with billions of years r . .-* Mare region Highland because they are asking questions related region to the evolution of solar systems and Apollo 11 3 6-3-8 — galaxies. Apollo 12 ~3-2 — The time of crystallization makes a very Apollo 14 — ~4-0 great difference as far as the chemical compo- Apollo 15 ~ 3-3 =;4-0 sition of a rock is concerned. The reason is Apollo 16 — 3-9-4-2 Apoiio ;r ~3-7 ~4-0 that, when the Sun was formed the solar Luna 16 3-4-3-5 .—. material was fresh, as it had then just emerg- Luna 20 ~ 3-7 3-8-4-1 ed from the nuclear furnace of a big star and naturally a great variety of radioactive ele- *There are evidences that certain lunar rocks have crystaliiza.tinr. ages of 4-4, b.y. (1 b.y. = 10° ments were then present. Many of these yrs.), and jne can not exclude the presence of some radioactive elements have so short a half life material on the moon of 4-6 billion year age. that they can not be found at present in the rocks. The prominent radioactive elements today arc Uranium, Thorium and Potassium, 1 b.y. older than the oldest terrestrial rocks. these have a half life of a billion years or For some of the lunar rocks we have reasons more; radionuclides of shorter half life, which to believe that they go back to the time of do not belong to " series " headed by a long formation of the solar system. So this is lived parent, are all gone by radioactive the excitement about the moon rocks. These decay. This then explains why old rocks lunar rocks as well as meteorites hold clues are interesting to a scientist. A rock that about nucleo-S} nthetic processes and the time cooled off very quickly after the formation scales involved in the evolution of the Sun and of the solor system will contain many of the formation of solid bodies in the solar system. radioactive elements manufactured in stars. Lunar research is intricately related to If one melts such a rock, then whatever gases solar physics. We already discussed how were produced by radioactive decay can be folar flare accelerated high energy particles recovered for a scientific study. Solid state can be studied with lunar rocks. Addition- damage trails oi fission fragments can be ally, the composition of the " solar wind " studied in these rocks. Most of the terres- which signifies a continuous hypersonic flow trial rocks arc old, but not old enough to be of solar material, can be studied with lunar as interesting as lunar or meteorite samples. rocks; these particles penetrate the rock up The oldest rock on earth is about 3 billion to distances of the order of 10~5 cm and get years only. The reason that we do not find trapped within the lattice. Tue atoms can older rocks is, as discussed earlier, that vol- then be easily recovered by a mild chemical canic activity has melted and remelted old treatment or heating. Thus samples of the rocks on the earth. moon inevitably contain samples of matter In Table 2, we have listed the measured ejected from the Sun, and lunar rocks are crystallization ages of lunar rocks for the beginning to make important contributions eight sites (Table 1) from where lunar samples in tliis direction. have been sampled. The oldest lunar rocks Scientifically, therefore, lunar research is are (4-1-4-2) b.y., i.e (4100-4200) m.y., about very exciting. There already havi been 10 KfcÜITS OF EXPLORATION OF MOON ANt) NEIGHBOURING. PLANETS several important repercussions of the lunar has been very large in the last two decades. program. Allow me to recount one of these Only a few years ago the )Vebster's dictio- here. Hoyle, an astrophysicist, remarked nary said that the space ship is a hypothe- soon after the Apollo 11 flight to the.Moon .tical rocket .propelled air ship for.'planetary that one of the greatest benefits of lunar and •travel.;' .Information content,and;khowlédge space research would be that man will be- of lawsof riátüre.àre growing.rapidly;. .May- come conscious that hè lives on a planet be within another decade we may'have proof which is not that big that he can do any of the existence of some;type; of ;life"forms damn .thing he likes on it, but it is something on Mars, or elsewhere. It seems safetd. pre- which has to be treated with great care and dict that great discoveries lie ahead, andwhat reverente; otherwise it may become im- we have learned so far is just-.an indication possible for him to live on the planet itself. of the fruits to.be reaped "from detailed We have indeed come a long way from the exploration of the.;, planets of the .solar era-of life in caves. The gradient of change system. '•'''..'
EXPLANATION OF PLATES PLATE 1 —• Photograph of the near side of the Moon showing, the different Apollo and Luna landing sites; symbols are A for Apollo and L for Luna missions. The bright regions are highland regions which are ricli in high albedo rocks of anorthositic component; the dark regions are the mare .regions having mostly basaltic material. PLATE 2'—-A view of the Apollo 17 landing site: Valley of Taurus-Littrow;• This photograph which was taken from the lunar orbit shows the smooth valley regions (central area) surrounded by highlands. PLATE 3 — Close up view of the Apollo 17 site. (Astronaut Schmitt can be seen beside a huge lunar boulder). The boulder has been split into two by an impact event. The bombardment of the lunar surface by meteoritic bodies ranging in sizes ever several magnitudes is responsible for continuous alteration of the lunar surface morphology. PLATE 4— Close up view of a lunar rock at Apollo 14 landing site. The anguUrity and the sharp contact with the lunar surface indicate that the rock has been transported to this site by some recent impacting event. . Exposed rocks quickly merge with .the background due to crushing •and fragmentation by hits of meteorites, selénologically speaking. A rock òf this size will be buried by the lunar soil in about 100 million years. • •..,-- . . PLATE 5-^ These photographs, taken at the Apollo IS site, show a recent lunar crater fig. (a) and a' close up of a hill fig. (b). The recent appearance of the crater on the moon in fig- (a) is evident from the "blockyand angular debris indicative of recent excavation of the material- thrown on' the surface;. Fig (b) shows a close up of the distant highland regions seen in fig (à); the .stratification pattern (layered structure) seen is due to sequential deposition of ejecta from impact generated basaltic lava flow and/or base surges. .'•••:'.• ' . '"-,.-•. PLATE 6 — Lunar rocks when sectioned become transparent.since.[they, are composed of.',silicate crystals. Chemical etching of samples exposed in space reveals fossil tracks of heavy cosmic ray nuclei'which can be conveniently studied with an optical microscope,. Exemplary views-of an Apollo 11 .crystalline, lunar rock (10072), ä thin: ssqtioh of.an Apollo rock -.(10024) and' fossil tracks due to cosmic rays revealed in a lunar, dust grain are shown in figs, a, b. and. c respectively. PLATE 7 — In the case of grains exposed unshier»d in free space,' the aireal'density of tracks decreases very rapidly as a function of distance from the exposed surface since the energy spectra of track producing low energy solar heavy nuclei falls off steeply with increasing energy. Figure (a) shows tracks in a pyroxene grain recovered from the interior of the Kapoeta meteorite indicating that this crystal received an unshielded exposure to low energy cosmic rays before compaction into the meteorite body. Similarly, figure (b) shows a steep track density gradient in a, lunar grain from the surface. A quantitative study of the low.energy irradiation.pattern is very useful for studying the behaviour 3i particles in free space, or on the surface of an ãsteroidal or planetary body. PLATE is — Contemporary and ancient cosmic ray tracks: The figure on left shows five tracks produced by nuclei of atomic number Exceeding 30 in an olivine grain in the meteorite Patwar during its space irradiation of ~ 60 million years. The other figure shows a track due to a calcium* nucleus passing through a lexan sheet exposed in space from November, 1973 to February, 1974 on the SKY- LAB II m ission. FOURTH SILYF.irjUBII.UE LECTTRF. PLATE 1 PLATE 2 FOURTH SILVER JUBILEE LECTURE PLATE 3 FOURTH STLYER JUBILEE LECTURE PLATE 4 FOURTH SILVER JUBILEE- LECTURE FOURTH SILVER JUBILEE LECTURE PLATE 5 PLATE 6 FOURTH SILVER JUBILEE LECTURE
500/U PLATE 7 FCJVRTH SILVER JVB1LEE LECTURK
EDGE PLATE S FOURTH SILVER JUBILEE LECTURE
20 A!