Search

The deficiency of siderophiles in the crust of the Earth is customarily attributed to leaching by metallic nickel-iron, which eventually sank to form the core. A similar deficiency exists in the Moon, which has, at best, a very much smaller core. Hence it is logical to consider the hypothesis that the Moon formed from the mantle of the Earth, after the siderophiles had been removed. It is shown that the non-hydrostatic figure of the Moon, and the requirement that the mascons must be supported, together with the high heat-flow, imply that the metal of the Moon is collected in the core. It probably amounts to less than 1 % of the Moon’s mass. Calculations show that if the core is in chemical equilibrium with the lunar silicates, then the nickel has been removed from the Moon as a whole to an extent which is greater than can be explained by theories of direct formation from a nebula. The only salvation for the idea of direct formation from a nebula appears to be an efficient process of extraction of the siderophiles by successive passage of small amounts of reduced metal through the silicate portion of the Moon. Since natural processes do not usually operate with the required efficiency, it can be concluded that the formation of the Moon by the fission of the Earth is geochemically plausible.

Recent evidence indicates that some of the chondritic meteorites may come from the Moon or Mars. The arguments leading to such a conclusion are reviewed. Such places of origin require that the origin of the chondrules be reconsidered. It is concluded that a collisional process is the most reasonable one consistent with a lunar origin.

Lunar fines and a chip from inside a rock pyrolyzed in helium at 700 degrees C gave methane, other gases, and aromatic hydrocarbons. Benzene/methanol extracts of fines yielded traces of high molecular weight alkanes and sulfur. Traces of glycine, alanine, ethanolamine, and urea were found in aqueous extracts. Biological controls and a terrestrial rock, dunite, subjected to exhaust from the lunar module descent engine showed a different amino acid distribution. Interpretation of the origin of the carbon compounds requires extreme care, because of possible contamination acquired during initial sample processing.

A search for diamonds in the chondrites Richardton, Forest City, Holbrook. Warrenton, Indarch, Cold Bokaveld and Cape Girardeau and in the Goalpara achondrite has been made. Diamonds were found in Goalpara only.

The moon's surface was formed by the collision of planetesimals up to approximately 100 km in radius. The time required for the addition of the last few kilometers was about 105 years. The earth's surface was formed at temperatures below the melting point of lavas. Iron-nickel objects a few kilometers in radius were embedded in the largest planetesimals that fell on the moon. It is shown that the earth may have accumulated much more rapidly than the moon and yet have not been molten. The densities of the planets calculated at zero pressure are not the same, and the chemical composition of Mars is nearly uniform throughout though it contains about 30% of iron-nickel alloy. The elements oxygen in water, nitrogen as ammonium salts or nitrides, and sulfur as iron sulfide were accumulated in condensed form at low temperatures, while carbon was retained at high temperatures as graphite or iron carbide. A two-stage process of accumulation into planetesimals is postulated, the first at low temperatures and the second at high temperatures, both followed by a period during which planetesimals accumulated to form the terrestrial planets. During the high temperature stage a chemical process for the volatilization of silicate while iron remains liquid is possible and it is proposed that these processes account for the varying densities of the planets. The primitive atmosphere of the earth contained water, ammonia, and methane as well as large quantities of hydrogen which escaped. The newly formed moon was much as it is now. The earth had nearly uniform chemical composition. At present the earth has an iron-nickel core and it is postulated that the mantle below the 20° discontinuity has about 25% by weight of iron-nickel alloy. The heat balance of the moon indicates that the interior was formed at low temperatures, < 300°C, if the equatorial bulge of Jeffreys is real. The heat balance of the earth is attempted but the results are not decisive. The gravitational energy due to the formation of the core during geological time makes it difficult but not impossible to account for a solid earth, if it is added to the radioactive energy. The constants for calculation are uncertain. A changing moment of inertia for the earth due to the continuous formation of the core explains discrepancies between the observed secular accelerations and those required by tidal theory. The mass of the inner core relative to the outer core suggests that solid nickel accumulates at the inner core. Such an accumulation could supply sufficient energy for the maintenance of the earth's magnetic field. Convection currents have been postulated by others as the mechanism for the formation of mountains. It is proposed that an important and probably decisive contribution to the energy arises from the gravitational energy liberated as the core is formed. The moon has no folded mountains and no core has formed. Mars appears to have no mountains, in the opinion of some students at least, and it apparently has no high density core.

During the last 10 years, the writer has presented evidence indicating that the Moon was captured by the Earth and that the large collisions with its surface occurred within a surprisingly short period of time. These observations have been a continuous preoccupation during the past years and some explanation that seemed physically possible and reasonably probable has been sought.

IN a recent communication1, Dr. Kohman suggests that tektites are meteorites which originate in interstellar space and collide with the solar system. I should like to direct attention again to the various facts that must be explained by any acceptable postulated history of these objects. (1) Tektites have chemical compositions remarkably similar to those of the more acid sedimentary rocks. This is true for the major and minor constituents. (I have been privileged to see analyses as yet unpublished on these minor constituents.) Such a chemical composition is not produced by any other naturally occurring chemical processes that we know of, except perhaps in very rare and special circumstances. (2) Tektites obviously have been melted, and this means that a temperature of some 1,500° C. or more was supplied by some means. In fact some appear to have been reheated to the melting point by passage through gas or by a blast of gas over the object. No terrestrial source of heat is known which could produce such high temperatures. (3) They are distributed thinly over areas of hundreds and thousands of kilometres in linear dimensions. (4) They are reported to contain aluminium-26 and beryllium-10. This observation is due to Dr. Kohman and Dr. Ehmann.

Chemical composition of moon from Surveyor results, noting resemblance to terrestrial basalt

The proper sample for the primitive elemental abundances in the solar system is the Sun itself, but the data on solar abundances are not of high precision. Analytical methods applied to the meteorites are of high precision, but it is probable that no meteorite or class of meteorites can be regarded as the correct sample of primitive solar nonvolatile elements. The abundance of iron relative to other elements in the meteorites is markedly different from that in the Sun. In particular, the Orgueil meteorite probably is a sample of the maria of the Moon and can hardly be the desired sample. The Orgueil meteorite has the structure and composition expected for the mud of a primitive lake bed before sorting of minerals by running water had occurred. The maria of the Moon have the appearance and color expected for such primitive lakes. The time that the object has been in space, i.e. some two or three million years, is consistent with a lunar origin. The origin of the moon as proposed by the writer is consistent with material of this composition being at the lunar surface.

The Rb87-Sr87 ages of many of the lunar rocks suggest that the fundamental differentiation took place 4.5 × 109 yr ago and that remelting occurred without exchange of the rubidium and strontium with the surroundings. The Apollo A rocks are an exception to this. They appear to have acquired rubidium without all of the Sr87produced during the first aeon. Also in the remelting about half of the radiogenic leads were lost to the surroundings probably the soil by a vaporization process. We interpret these results to mean that remelting occurred in a system that was nearly closed to the surroundings and that the high early concentrations of radioactive elements in highly insulating surroundings made this possible.

At the conference Professor Urey gave an outline of those ideas which he had developed during the past few years which have a bearing on the origin of the Moon. Papers showing the development of these ideas are given in the bibliography appended to this abstract.

Objects of the solar system, in addition to the Sun, can be classified into four groups — the planets, objects of lunar mass, smaller objects of variable mass and the comets.

Though the existence of cometary meteor swarms is well established, it has not been conclusively demonstrated that meteorites within one group are of related origin. Selection of similar meteorites having closely related dates of fall offers a means of possibly obtaining related members of a group. Theoretical calculations have indicated that a small swarm of meteorites would have a comparatively short lifetime before being disrupted by planetary perturbations, so that capture by the earth would have to occur shortly after the formation of the swarm if any orbital correlation were to remain. This short lifetime should be reflected in an unusually low Al26 content, since the halflife of this nuclide (0.78 × 106 years) is well suited for the determination of cosmic-ray exposure ages in the interesting range.

Kuiper [1954] has proposed that the moon accumulated at low temperature and was heated to the melting point by the decay of radioactive elements, until the interior melted, leaving a sintered ‘accreted crust’ on the outside. At this stage massive objects collided with the moon and broke through the crust, and lava from the interior flowed up into the holes thus formed. On solidification, these pools of lava formed the maria and the flooded craters (referred to as the ‘wet craters’). Later the moon solidified, and subsequent collisions produced the ‘dry craters,‘ i.e., those not having flooded interiors.

The relationship between temperature and O^(18) content relative to that for a Cretaceous belemnite of the Pee Dee formation previously reported (Epstein, Buchsbaum, Lowenstam, and Urey, 1951) has been re-determined using modified procedures for removing organic matter from shells, and is found to be t(°C) = 16.5 - 4.3δ + 0.14δ^2 where δ is the difference in per mil of the O^(18) to O^(16) ratio between the sample and reference gas. The new relationship agrees with that determined by McCrea (1950) for inorganically precipitated calcium carbonate. Carbonate-carbon dioxide exchange experiments were done to determine the direct and indirect effects of organic matter in the shell on the mass spectrometer analyses.

IN a recent communication, Brian Mason1 asks that I supply a list of chemical compositions of sedimentary rocks which parallel those of the tektites. I offer the following references for his samples of tektites labelled A1–G1. The pages in the following list refer to Pettijohn's book on sedimentary rocks2: A1, 298; B1, 364; C1 319; D1; 319; E1, F1 and G1, 306. Except for water content, fair agreements between analyses can be secured by selecting analyses from these tables. The low water content of tektites is a real puzzle. There are some small differences in analyses between those listed by Pettijohn and the tektites. It is very probable that some granites of the composition listed by Mason are fused sediments. What would be more interesting than an argument about average sedimentary rocks would be a comparison of the composition of particular groups of tektites and sedimentary or igneous rocks in the general area in which the tektites are found. If cometary collisions melted the local rock and scattered the fragments over great distances3, it should be possible to find the original material at an appropriate place. But this requires study by an expert in these matters and probably additional observational data, and these cannot be supplied by me.

Since the abundance of the O 18 isotope in calcium carbonate varies with the temperature at which it is deposited from water, the variation in abundance can be used as a thermometer. This paper discusses the following problems: (1) the magnitude of the effect expected, (2) the mass spectrometer of high sensitivity, (3) the preservation of the record during geological time, (4) the constancy of the isotopic composition of the ocean, (5) the impossibility of using skeletons of air breathing animals, and (6) the temperature of marine animals relative to their surroundings. A Jurassic belemnite is used to show that the record has been retained since Jurassic times, and belemnites of the Upper Cretaceous of the United States, England, and Denmark are used to determine the temperature of this time at these localities. The temperatures are about 15°–16°C and indicate nearly uniform temperature over this latitudinal belt. Because of the possible variation in O 18 content of the oceans and the limited number of samples, these temperatures are regarded as preliminary.

A review of the sources of analytical data for the chondritic meteorites is given, and the latest data, particularly for the minor elements, are tabulated. Though the composition of the chondrites is remarkably constant as compared with terrestrial rocks, important variations in the concentrations of certain major and minor constituents occur, which make it impossible to conclude that any one type of chondrite could have been produced from another by simple physical processes. This is true for the carbonaceous, enstatitic, and the high- and low-iron-group chondrites. The origin of these chondrites is discussed mostly with reference to the author’s suggestions about the origin of the solar system. Only very complicated processes can account for the detailed characteristics of these objects. The possibility that the carbonaceous chondrites and particularly Wiik's type 1 of this group are the primitive material from which the meteorites and planets originated is considered, and it is concluded that these objects as a group cannot be such material (a conclusion reached by the author some ten years ago). The case for the type 1 carbonaceous chondrites' being this material is better, but serious arguments against this conclusion can be advanced; it is concluded that even this material has undergone some extensive chemical processing.

In a recent paper, Mason [1960] has presented arguments maintaining that ordinary chondrites originated from carbonaceous chondrites by a simple heating process, and that the chondrites were never part of larger objects. He criticizes views that the writer has advanced in the past, namely, that the minerals were formed in primary objects which were broken up and reaccumulated into secondary objects as broken fragments. It is certainly true that complete scattering of broken fragments over great distances and complete, thorough mixing has not occurred as he states and as was stated by Urey and Mayeda [1959]. However, the writer believes that his simple outline of events cannot be correct and that much more complicated histories are required.

A relationship between temperature and relative O 18 abundance in calcium carbonate in marine shells has been determined. If the relative O 18 abundance of the water in which the shell grew is known, the temperature can be determined to an accuracy of ±1.0°C. The relative O 18 contents of marine waters increase with salinity and, in the case of surface marine waters, with salinity and temperature.