Koziel's results on the librations have been rediscussed. Some serious departures from independence of the errors were detected and have been allowed for. The results are

β = 0.0006271 ± 0.0000010

γ = 0.0002362 ± 0.0000082

These do not differ much from Koziel's values but the uncertainties are larger. Both are consistent with results previously derived by comparison of results of different authors, and γ is consistent with a reinterpretation of a result of Yakovkin.

The effect of elasticity is considered. It is shown that the elastic strain does not contribute to the librations but would affect the perturbations of a satellite. Allowance for this difference reduces Michael's estimate of 0.4015 for C/Ma2 to 0.4001 ± 0.0030, which would be consistent with either uniform density or with the value I found in 1936 after allowance for compressibility and a possible thin surface layer, namely 0.3971 ± 0.0007.

Since the days of Cassini (1693), study of the position of the lunar axis relative to an inertial system of reference has been based on the assumption that “the inclination of the Moon's equator to the plane of the ecliptic is constant”. And, although the actual value of that inclination has been subject to continuous changes and modifications, reduced from the originally suggested 4½° to 1°32′4″ (Koziel, 1967), no sufficient attention has been paid to the fact that, as the lunar globe moves within the field of varying external forces, the inclination of its axis cannot remain constant.

The determination of the heights of points on the lunar surface by Earth based astronomy using the geometrical librations, although individually of low accuracy, still provides our best method of obtaining the global shape of the Moon. The intrinsic scatter of the results arises from the effects of ‘seeing’ and simple statistical analysis is required to derive valid conclusions about the shape. Baldwin's method of fitting ellipsoidal surfaces to the points on the maria and uplands, separately by the method of least squares proves to be a valuable tool.

Analyses of the ACIC points and of the Pic du Midi studies of G. A. Mills show that good first descriptions of the global shape of the Moon for both the maria and uplands are triaxial ellipsoids with their long axes within 10° of the Earth direction, the major axis of the maria being about 1.3 km smaller than that of the uplands. Of particular significance is that the ellipticity of these surfaces is about 2½ times greater than the dynamical ellipticity; thus the non-hydrostatic figure of the Moon is not simply the result of distortion from a uniform Moon during its early history. The angular variation in density within the Moon cannot be simply a phenomena within the crust but must extend to a great depth. Convection could provide an explanation.

The departures of the lunar surface from the idealised ellipsoids are also of interest. The circular maria are systematically depressed relative to the maria ellipsoid: can the mascons have adjusted isostatically since their formation? Systematic differences in height between the western and eastern southern uplands are also noted.

The geometrical shape of the Moon is determined from measurements of absolute heights of the lunar surface, while its dynamical shape is described by means of the Moon's gravity field parameters. All these data are derived from observations of the lunar artificial satellites (‘Luna-10’, ‘Orbiters 1-4’) and astronomical measurements.

In the paper differences of the lunar geometrical and dynamical figures are analysed. It is shown, that the homogeneous model of the Moon is not capable of explaining these differences. It is found, that the lunar centre of gravity situated about 0.9 km to the north, and 1.1 km nearer to the Earth, than the centre of its geometrical figure.

It is shown that large (hundred-km diam) near-surface disks are capable of accurately representing the lunar mascons with very few parameters. While this does not ‘prove’ that the mascons are excess mass in disk form, it is highly suggestive. It is pointed out that virtually every proposed mascon theory is consistent with a disk shape for the excess mass. The tentative hypothesis is advanced that the mascons are, in fact, disk shaped mass excesses. The conditions for mascon formation and preservation are reviewed in the light of this hypothesis.

Fundamental parameters of the geomorphology of a lunar surface are (i) the number and size distribution of craters, (ii) the degree of erosion of the craters and (iii) thickness and other characteristics of the regolith. These parameters are not independent of one another; as one changes through time, the other two will also change in a statistically predictable way.

In the continuous degradation sequence, the relationship between the number of craters per unit area and the degree of erosion of the craters is continuous and monotonic. This sequence occurs in areas subjected to intermediate to small impacts, and eroded mainly by the impacts themselves plus other small-scale erosional processes. In areas subjected to large impacts and mare flooding the discontinuous degradation sequence is predominant.

The relationship of the first two parameters, the number of craters and the degree of erosion, with the third parameter, the regolith, is not simple and is not yet understood. It appears, however, that the geomorphological stage is more important than the mare-versus-highland dichotomy of the lunar surface.

The solution of the function relating craters of the continuous degradation sequence with degree of erosion was defined as the geomorphic index of the area. Studies of the geomorphic index of stratigraphic surfaces show that areas covered by considerable ballistic sediments have a geomorphic index which is not a monotonic function of time. On the other hand, areas covered almost exclusively by mare flooding show an index which is a monotonic function of the age of the flooding. As each mare surface shows a considerable range in indices, it is concluded that maria are covered by surfaces formed through a considerable length of time. By using Apollo 11 and 12 radiometric ages it is suggested that the time of mare flooding lasted on the order of one billion (109) years.

The geomorphic index of highland surfaces shows a remarkable degree of order; i.e., the farther an area is inland from the mare shores, the higher will be the index. No explanation is given to this phenomenon, but is suggested that lunar erosion is not just a localized phenomenon centered on the locus of an impact, but has lateral trends of regional dimensions. Electrostatic transportation as suggested by Gold is a possible mechanism.

Rock dust appears to have been redistributed over the Moon by effects other than impact explosions. A core sample on Apollo 12 showed sharp and distinctive layers and was clearly unmixed. Surface transportation processes that deposit the dust very gently must have been at work. Orbiter pictures confirm that such surface creep has taken place on a very large scale.

The seismic evidence makes clear that there is no continuous sheet of bedrock at a shallow depth in the vicinity of the Apollo 12 site. A deep deposit of powder would match the seismic properties observed. Mascons require for their explanation a surface transportation process that tends to fill in the large impact basins after their formation.

Surface transportation of lunar dust has been demonstrated in the laboratory to occur most readily as a result of electrostatic forces produced by electron bombardment in the energy range of a few hundred volts. Such bombardment happens on the Moon predominantly when it is in the magnetic tail of the Earth, and this may be the reason why mare ground is so remarkably dominant on the hemispere facing the Earth.

The result of Lunar radar mapping observations are presented, which were made at Jodrell Bank in January and February 1970 at 162.4 MHz in the depolarized return. The Maps show that most of the depolarized return comes from the highland regions. No significant return has been detected from the Maria. Certain isolated features are particularly prominent, notably the craters Tycho and Theophilus which appear almost equally intense, also the areas around Werner and Fourier. Various other features can be identified with less reliability. These results are discussed in qualitative terms.

To set the context of our current exploration of the Moon, it is well to recall: Apollo 11 landed near the equator and was capable of affording the astronauts only a few hours on the lunar surface; Apollo 12 checked out pin-point landing which then permitted going to more difficult, nonequatorial sites; in Apollo 14 enlarged propellant tanks permitted some 42 kg of samples to be brought back. Three more Apollo missions follow Apollo 14. The Apollo missions, complemented by unmanned missions to the various planets, form a program to explore the solar system.

Captain Scherer will elaborate on the significant increase in Apollo capability in the last three missions. Of note are the lunar heat flow experiments, the lunar surface gravimeter, and, for Apollo 15, the laser ranging retroreflector which will be three times the area of and four times as efficient as retroreflectors in the previous Apollo missions. In the lunar orbital equipment will be the sub-satellite with instruments for measuring various physical properties of the Moon and its environment. Also noteworthy is the lunar sounder, for Apollo 17, which will use radar for probing a large fraction of the lunar surface to considerable depth.

Hardware changes will allow the astronauts to remain on the lunar surface for up to 66 hrs – an increase of 100%; thus the landed scientific payload may be doubled to approximately 550 kg. Increased range and efficiency of surface operations will result from improved suit mobility, an improved life support system, and a Lunar Roving Vehicle capable of up to 90 km traverses. Changes in the Command and Service Modules will allow for up to 16 days total flight duration.

We will again welcome proposals for analysis of the new samples, but now that the initial exploratory work has been done we intend to be far less duplicative in choosing proposals to support. We are particularly pleased with the growing attention being given by the scientific societies to the results of lunar research.

In the last talk of this series, Mr. Burke will describe some of the future possibilities that have been studied for exploration of the Moon and planets. We will particularly welcome suggestions as to how to improve the value of future missions.

In February 1971, the Apollo 14 mission successfully landed in the Fra Mauro region of the Moon. Astronauts deployed a sophisticated long-lived geophysical station. While on the surface they performed active seismic and magnetic experiments, deployed a solar wind collector and laser reflector, made an extensive geologic traverse, and performed television, movie, and still camera photography and other minor tasks. They selected a variety of samples of lunar material and returned to Earth over 40 kg for study. In orbit about the Moon, a variety of scientific and operational photography was accomplished.

The three remaining Apollo missions will have significantly increased scientific capabilities. These result from increased payload, more time on the surface, improved range, and more sophisticated experiments on the surface and in orbit. Landing sites for the last three missions will be carefully selected to maximize the total scientific return.

With results of past and present lunar exploration programs in hand, it is now possible to outline the next logical steps in the exploration and use of the Moon. This paper describes first, a set of mission objectives and engineering capabilities considered desirable and feasible for projects following soon after Apollo, and second, some objectives for the farther future.

Experimental studies at 1 bar and up to 30 kbar establish the crystallization sequences for basalts 12021, 12065, 12022, 12009 and 12040. Olivine is the liquidus phase at low pressures, with minor chromium spinel and pigeonitic clinopyroxene joining the olivine at lower temperatures or accompanying the olivine in the less magnesian basalts (12021, 12065). At higher pressures, subcalcic clinopyroxene becomes the liquidus phase except in the most magnesian basalt (12040) where orthopyroxene joins the olivine and becomes the liquidus phase at pressures of 25 kbar. Integration of experimental studies with observed mineralogy of natural rocks shows conclusively that the basalt compositions studied do not lie on a plagioclase + pyroxene + spinel ± olivine cotectic nor have these rocks been derived by accumulation of olivine or pyroxene into such a low temperature cotectic liquid. The Apollo 12 basalts provide clear evidence for the genesis of olivine-rich basalts in the lunar interior. The nature of the source rock is deduced to be pyroxenite or olivine-bearing pyroxenite in which orthopyroxene is probably the major phase with lesser sub-calcic or pigeonitic clinopyroxene. The 100 Mg/(Mg + Fe) ratio of the source region in the deep lunar interior is 75–80.

Ion-thinned samples of rock 12052 have been examined in the electron microscope at 100 kV. This rock contains numerous phenocrysts of pyroxene, Ca1 – P (Mg, Fe)1 + P Si2O6, and a few rounded olivine crystals (Mg, Fe)2 SiO4 set in a finer-grained groundmass of pyroxene, plagioclase (Na, Ca) (Al, Si)4O8, ilmenite, FeTiO3 and other minor minerals (Champness et al., 1971). The cores of the pyroxene phenocrysts consist of a calcium-poor pyroxene, pigeonite, with an epitaxially grown rim of a calcium-rich pyroxene, augite. Both the pigeonite and augite are zoned, and indication of non-equilibrium crystallisation (Bence et al., 1970).

The outer parts of the pigeonite cores of the phenocrysts show very finescale composition modulations on (001) (major) and (100) (minor) (Figure 1). This texture is interpreted as exsolution by a spinodal mechanism (Cahn, 1968).

The augite rim of the phenocryst exhibits a coarse exsolution of pigeonite on (001) on which is superimposed fine-scale composition modulations on (001) and (100) (Figure 2).

The two-stage exsolution process indicates that the cooling rate of the pyroxene increased suddenly, possibly when the magma was extruded on the Moon's surface.

Antiphase domains produced during the polymorphic transition from space group C2/c to P21/c have been found in all primary and precipitated pigeonites (Figure 3). This confirms Morimoto and Tokonami's (1969) suggestion that all clino-pyroxenes have the C2/c space group at high temperatures.

Cristobalite, a high-temperature polymorph of silica, occurs as very small grains in the groundmass. It contains numerous microtwins on {100} planes produced during the β→α polymorphic transition which occurs at 268°C (Figure 4a). Diffraction patterns show extensive diffuse streaks in various directions (Figure 4b) which correspond to intersections of planes in reciprocal space with the Ewald Sphere. These diffuse planes originate from the cooperative thermal vibration of Si-O chains parallel to {100} and {111}.

Experimental data show Apollo 11 and 12 lava compositions to be controlled by fractional crystallization close to the lunar surface, in a process which yields achondrite-like igneous rocks as underlying complementary crystal accumulates. Volatilization losses during eruption can account for most other chemical differences between lunar lavas and common terrestrial magmas. No specific hypotheses of the composition, mineralogy, or origin of lunar interior can be sustained until the extent of these processes is known. A terrestrial upper-mantle-type lunar interior cannot yet be excluded. The assumption that maria surface lavas are primary partial melts is unjustified and leads to a postulated lunar interior with too low Mg/Mg+Fe to serve as a source for Apollo 14 and other igneous liquids. Other workers' uncontrolled visual estimates of crystallinity in experimental charges, purporting to show that maria lavas were not modified by low pressure fractionation, are irreconcilable with the chemistry of the residual liquids developed in our ‘reversed’ equilibrium experiments. The undesirability of using glass as a starting material for this type of experiment is re-emphasized.