The Moon revealed

SEA OF TRANQUILITY

The Air Force C-141 Starlifter carrying NASA Administrator Thomas O. Paine and the first rock box landed at Ellington Air Force Base on Friday, 25 July 1969. Awaiting it were Samuel C. Phillips, the Apollo Program Director, Robert R. Gilruth, Director of the Manned Spacecraft Center, and George M. Low, Manager of the Apollo Spacecraft Program Office in Houston. Gilruth and Low posed for photographs holding the box, before taking it to the Lunar Receiving Laboratory on the campus of the Manned Spacecraft Center. The second box arrived later that day. The next day, a member of the 50-strong Preliminary Examination Team used a vacuum chamber with a window and rubberised ‘arms’ to raise the lid of the first box, and found the interior so coated with black dust as to make it impractical to say anything definitive about the contents! When the boxes were emptied, there was found to be 48 pounds of lunar material in the form of 20 individual rocks and a pile of fragments and grains. One by one, the rocks were cleaned for inspection. At a press conference on 28 July, Persa R. Bell, Director of the Lunar Receiving Laboratory, opined that the rocks had been ‘‘beautifully selected’’. Elbert King, the curator, announced that the first rock to be examined under a microscope appeared to be a granular igneous rock. Gene Shoemaker of the US Geological Survey suggested that it represented a lava flow. But this was only a first impression. Once the material had been catalogued, small samples were issued to 150 principal investigators who had spent years developing the means to subject such material to almost every possible kind of analysis. The investigations proceeded at such a pace that on 15 September NASA was able to announce the preliminary findings and, to follow up, on 4 January 1970 the agency hosted the first of what was to become an annual Lunar Science Conference.1

To Harold C. Urey, who favoured the ‘cold’ Moon theory in which the interior was uniformly composed of ‘pristine’ material, the dark plains were the result of

These gatherings are now entitled the Lunar and Planetary Sciences Conferences.

impact melting on a vast scale. While the astronauts were out on the surface, Urey had been concerned when Armstrong reported a vesicular rock, encouraged when Armstrong changed his mind, and dismissed Armstrong’s later report of a rock he was sure was vesicular. Most of all, Urey was encouraged that they did not report finding the ‘frothy vacuum lava’ predicted by his leading rival, Gerard P. Kuiper, who favoured the ‘hot’ Moon theory in which the interior was differentiated and the dark plains were the result of upwellings of lava through fractures in the floors of major impact basins. The rocks proved to be a form of basalt rich in magnesium and iron (and therefore described as being ‘mafic’) which isotopic dating revealed to have crystallised some 3.84 to 3.57 billion years ago. In terms of texture, it was strikingly similar to terrestrial basalt. It was not impact melt. This meant that the Moon had undergone a process of thermal differentiation in which lightweight aluminous minerals had migrated up to the surface and the heavier minerals had sunk into the interior. The fact that some of this denser material had later been erupted indicated

that the interior had remained ‘hot’ for a significant period. However, when compared to terrestrial basalt, the lunar variety was enriched in titanium. The titanium-bearing mineral, which was new to mineralogists, was named ‘armalcolite’, in honour of the astronauts.[51] The lack of oxidised iron meant that the lava was created in a reducing environment (i. e. one devoid of oxygen). The most striking fact was the total absence of hydrous minerals. The lunar basalt was also deficient in volatile metals such as sodium. The low-alkali (i. e. sodium-depleted) lava would have had an extremely low viscosity, which is why it flowed so readily, and why it left so few ‘positive-relief’ features. The Sea of Tranquility was evidently accumulated by episodic volcanism over a period of several hundred million years. The presence of two types of basalt implied either that there were separate reservoirs of magma or that the single source had undergone chemical evolution over time.

As Armstrong later reflected of the lunar surface, ‘‘My impression was that we were taking a ‘snapshot’ of a steady-state process in which rocks are being worn down on the surface of the Moon with time, and other rocks are being thrown out on top as a result of new events somewhere near or far away. In other words, no matter when you had visited this spot before – 1,000 years ago or 100 years ago, or if you come back to it 1,000,000 years from now – you’d see some different things each time but the scene would generally be the same.’’ This was insightful. On the airless Moon there was little chemical erosion. Large impacts simply excavated bedrock, and this was progressively worn down by smaller impacts to produce the regolith, the majority of which was pulverised basalt. There was little meteoritic material. Many of the discrete samples proved to be regolith compacted by shock. When subjected to physical stress these ‘regolith breccias’ tended to fall apart. The ‘glassy material’ found in a small fresh-looking crater was regolith that had been heated and fused by a high-energy impact. This impact-driven weathering process was given the name ‘gardening’.

There was a small residue of the regolith that was very different in character. On the basis of his analysis of chemical data provided by Surveyor 7, which had landed near the crater Tycho in the southern highlands in 1968, Shoemaker had predicted that 4 per cent of the regolith at the Apollo 11 site would comprise minuscule fragments of light-coloured rock – and this proved to be the case. This light rock was plagioclase feldspar. Terrestrial plagioclase is rich in sodium, but the Moon is depleted in sodium and the lunar variant had calcium, making it calcic-plagioclase. Some of the fragments were sufficiently pure to justify being called anorthosite, this being the name for a rock comprising at least 90 per cent plagioclase, but most were diluted with mafic minerals and therefore were more properly called anorthositic gabbro; like the material Surveyor 7 had analysed. Shoemaker’s rationale for there being highland material in the regolith of the Sea of Tranquility was based on the manner in which the most recently formed highland craters splashed out ‘rays’ of material. Regarding the highlands, it could now be inferred that the primitive crust was composed of anorthositic rock. At the Lunar Science Conference, J. A. Wood noted that if the ‘exotic’ fragments in the Apollo 11 regolith were indeed highland rock, then their density of 2.9 grams per cubic centimetre (in comparison to the 3.4 average for the Moon) meant that the heat generated by giant impacts during the accretion of the Moon from planetesimals had created a ‘magma ocean’ which later solidified to form the crust. This was a significant insight into early lunar history.

What a difference one brief field trip had made; its ‘ground truth’ had scythed through the long-held theories without consideration for the professional standing of their proponents. Previously minor players found themselves in the limelight by virtue of having been proved right. For example, in a paper published a few weeks prior to Apollo 11, Anthony Turkevich reported a study of data from Surveyor 5, which landed in the Sea of Tranquility in 1967, near where Apollo 11 was to try to land, and he predicted the astronauts would return with titanium-enriched basalt.