Oxide minerals in lithic fragments from Luna 20 fines

One hundred and seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvöspinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO₂-rich phase were also found.Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The $\text{Fe}\text{(Fe + Mg)}$ ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains.Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinzet al. (1973a).Sub-micron wide rods of metallic Fe occur in plagioclase grains and may have formed by sub-solidus reduction processes.     

A lunar core of Fe-Ni-S

Several workers have proposed that lunar samples were magnetized by a field created by a lunar core of molten Fe. Low abundances of siderophile elements in lunar rocks are compatible with formation of a metallic lunar core. A molten Fe core requires that the bulk of the Moon was above, or close to, the melting point, a requirement which disagrees with most models of the lunar thermal regime.A core (or perhaps a layer or pockets) of molten Fe-Ni-S, at or close to the eutectic composition would act as a lunar dynamo, and be at a temperature (approx. 1000°C) consistent with some reasonable models of lunar thermal history. The existence of a Fe-Ni-S core would also partly explain the depletion in volatile elements in lunar basalts. Such a core, occupying up to 20 per cent of the Moon's radius requires a bulk S content for the moon of only 0.3 wt per cent.     

The lunar crust: a product of heterogeneous accretion or differentiation of a homogeneous Moon?

The outer portion of the Moon including the Al-rich crust and the source regions of mare basalts was either accreted heterogeneously or was the product of widespread differentiation of an originally homogeneous source. A number of constraints make the heterogeneous accretion model unlikely; the differentiation model appears more plausible.If the differentiation model is correct, a series of cumulate rocks complimentary to the Al-rich crustal rocks must exist. The mare basalts may have been derived from such a complimentary cumulate for several reasons. For example, Philpottset al. (1973) on the basis of REE studies, suggest that Apollo 11 and 17 mare basalts were formed by partial melting of a cumulate rich in a phase(s) containing high Ti and heavy REE. The high Ti of Apollo 11 and 17 basalts is not readily explained in terms of partial melting of an undifferentiated mantle, but is consistent with partial melting of a pyroxene cumulate enriched in Fe, Ti oxides. The characteristic Fe-rich nature of mare basalts would be partly a consequence of melting of oxide cumulate minerals. It is postulated that the plagioclase-poor source region of mare basalts was enriched in an intercumulus residual liquid. During the partial melting that produced mare basalts, this material was largely incorporated into the melt, thus explaining the ancient model ages observed in most mare basalts. If the cumulate model is correct, then samples derived from the true (undifferentiated) lunar mantle have not been identified.     

Lunar physical librations and laser ranging

The analysis of lunar laser ranging data requires very accurate calculations of the lunar physical librations. Libration terms are given which arise from the additive and planetary terms in the lunar theory. The large size of the recently discovered terms due to third degree gravitational harmonics will allow some of these harmonics to be measured, in addition to and, by laser ranging to the Moon. Combining the laser ranging determinations of = 630.6 ± 0.5 × 10^–6 and = 226.4 ± 3.0 × 10^–6 with lunar orbiter measurements ofC ₂₀ andC ₂₂ givesC/MR ²=0.395 _-0.010 ^+0.006 . Numerical integration promises to be an effective method of calculating librations. Comparison of numerical integrations with analytic series indicates that the calculation of the series due to third and fourth degree harmonics is not yet as accurate as the more extensively developed second degree terms.Communication presented at the Conference on Lunar Dynamics and Observational Coordinate Systems, held January 15–17, 1973, at the Lunar Science Institute, Houston, Tex., U.S.A.     

Distances from auroral zones to the magnetic and geographic equators

Distance from auroral zone is a fundamental parameter in studies of disturbances produced in the thermosphere and ionosphere through the action of the solar wind. Calculations showing the great variation of the distances of the auroral “zones” from the magnetic equator and geographic equator are presented in diagrams. An auroral zone proximity index is proposed for use in correlative studies of upper atmosphere and of ionospheric disturbances.