STRATIGRAPHIC MAPPING

In April 1957 the National Academy of Sciences awarded Kuiper the funding to start work on a new lunar atlas, and supplementary money was provided later in the year by the Air Force. The resulting Photographic Lunar Atlas was published in I960. The best available photographs were printed on a scale at which the lunar disk spanned 2.5 metres. It formed a striking contrast to the similarly sized map based on visual observations that was published in 1959 by H. P. Wilkins of the Lunar Section of the British Astronomical Association. Although very different in presentation, the two maps were comparable near the centre of the Moon’s disk but even in the best pictures the limb regions were marred by ‘seeing’, and it was in these areas that the visual observers had the advantage. However, the pictures were able to be projected onto a white globe and rephotographed to eliminate foreshortening and thereby gain a new perspective of the limb regions. This Rectified Lunar Atlas was issued in 1963 as a supplement to the 1960 atlas.

In 1959 the Air Force Chart and Information Center in St Louis, Missouri, began to use airbrushing to represent topography on a scale of 1:1,000,000 for a series of Lunar Astronautical Charts. Meanwhile, the Army Map Service issued ‘photomaps’. The US Geological Survey wished to map the Moon geologically. The first step was to identify the various distinct geological units in terms of their textures, delineate their outlines on a ‘base map’, and use the principle of superposition (as defined by Nicolas Steno in 1669) to determine the order of their deposition. The objective was to obtain insight into the history of the lunar surface. In 1960 Robert Hackman of the Photogeology Branch of the Survey in Washington DC demonstrated that it was possible to apply stratigraphic analysis to the Moon. When issued in 1961, his map of what he referred to as pre-maria, maria and post-maria units marked a significant departure from the astronomers’ means of mapping. The superposition relationships suggested to Hackman that the maria were volcanic, not splashes of impact melt. He drew attention to a patch of light-toned material between the Apennine mountains and the crater Archimedes. There was ejecta from Archimedes on this patch, and the dark mare had encroached upon the ejecta. The sequence was clear: the light-toned material was the floor of the cavity created by the Imbrium impact, this had been hit by Archimedes some time later, and the mare had appeared after that. Since the light patch was sufficiently elevated not to be overrun, he named it the Apennine Bench. A factor of two difference in the cratering densities of the bench and the adjacent mare was evidence that a significant interval had elapsed between the Imbrium impact and the appearance of the mare within the cavity.

Meanwhile, Gene Shoemaker had independently made a stratigraphic study of a section of the Moon to demonstrate the technique. Visiting a bookstore shortly after being shown the prototype Lunar Astronautical Chart of the Copernicus area, he had happened across a picture of this area taken by Francis Pease in 1919 while testing the 100-inch telescope at Mount Wilson. It was of sufficient clarity to show craters down to 1 km in diameter, so Shoemaker had it enlarged and set to work. Whereas Hackman had used only pre-maria, maria and post-maria units, Shoemaker mapped seven units, which he named the pre-Imbrian, Imbrian, Procellarian, Eratosthenian

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and Copernican systems. In essence the Eratosthenian and Copernican corresponded to Hackman’s post-maria, but Shoemaker distinguished the Eratosthenian from the Copernican because rays from Copernicus were superimposed on the Eratosthenes ejecta – in effect, the difference was whether a post-mare crater’s rays were fresh, or faded. On 17 March I960 Shoemaker presented a paper showing that whereas much of the material excavated by Copernicus had been ‘hinged’ to produce the rim and adjacent blanket of ejecta, some of the material was hurled ballistically and fell further out, where its impact made distinctive chains of small secondary craters. The secondary craters were less energetic because, to have fallen back at all, the ejecta could not have exceeded the escape velocity – which is an order of magnitude lower than the typical cosmic velocity of material arriving from space. This study not only established Copernicus to be an impact crater, it also refuted the assertion by the advocates of the volcanic origin of craters that the chains of small craters marked eruptions along fractures in the crust.

At the International Astronomical Union Symposium in December 1960, which was a major event for astronomers, Shoemaker and Hackman presented a joint paper entitled Stratigraphic Basis for a Lunar Time Scale. This laid the foundation for how geological units could be recognised on an extraterrestrial surface and placed into a stratigraphic sequence. In the case of Earth the units were identified by studies in the field, but for the Moon they would have to be inferred from overhead imagery – at least until expeditions were made to the lunar surface.

Having established that the maria were formed after the Imbrium impact, it was expected that all maria would be able to be assigned to the Procellarian system, but in late 1963, when patches of mare were found to be stratigraphically younger than craters attributed to the Eratosthenian system, the Procellarian system was dismissed and each mare unit was assigned to the system implied by its particular stratigraphy.

Also in late 1963, the scheme was refined by the introduction of formation names for the geological units. The reason for the change was that a formation name was objective, and did not imply a specific physical process. Also, because a formation defined a terrain type by its texture, it did not require to be contiguous. This was the case for the hummocky material peripheral to Imbrium. It had just been mapped by Richard Eggleton, who had transferred to Shoemaker’s team from the Engineering Geology Branch. It was labelled the Fra Mauro Formation, after a prominent crater within it. Although there was little doubt that it was Imbrium ejecta, to have labelled it as such would have been subjective and would have set a poor precedent.

In September 1961 Gerard Kuiper convinced the Air Force Chart and Information Center to exploit visual observations in compiling the Funar Astronautical Charts, since in moments of good ‘seeing’ the eye can resolve finer detail than is able to be recorded during a photographic exposure. The pictures were to provide the basis for mapping and the visual observations would provide the detail. On joining the team, each ‘astrogeologist’ was assigned a quadrangle to map geologically, in addition to his principal task. As one of the first such recruits, Eggleton provided training for those who followed. Observing time was allotted when the terminator was near the assigned area, to emphasise subtle topography. Those in Arizona used the 24-inch refractor of the Fowell Observatory in Flagstaff,

and those in Menlo Park used the 36-inch refractor of the Lick Observatory on Mount Hamilton near San Jose.

In February I960 the University of Arizona in Tucson established the Lunar and Planetary Laboratory, and made Gerard Kuiper its head. When William Hartmann joined in mid-1961, he assisted the team which was producing the Rectified Lunar Atlas. A major finding was the existence of systems of concentric rings. These had not been recognised from Earth owing to foreshortening, but when viewed from an ‘overhead’ perspective they stood out clearly. The most spectacular case surrounded a small dark patch which was itself only glimpsed at times of favourable libration and had been named Mare Orientale for the reason that it was on the eastern limb – a rationale rendered obsolete by the decision of the International Astronomical Union in August 1961 to switch the east and west limbs! On realising that the multiple-ring structures were a distinct class of geological feature, Hartmann introduced the term ‘basin’. He wrote up the discovery with Kuiper and published in-house on 20 June 1962 in the paper Concentric Structures Surrounding Lunar Basins. Soon, similar patterns were identified in degraded states around a dozen ‘circular maria’. This insight revealed the true violence of a basin-forming impact. Namely, a vast impact excavated a cavity, forming one or more concentric rings of mountains composed of individually faulted blocks with their steep ‘fronts’ facing inwards, whilst also piling up material in blankets immediately beyond and etching sculpture as ballistic ejecta fell further out – all of which occurred literally in an instant. Some time later, and perhaps after a considerable interval, lava rose through deep fractures in the cavity of the basin to flood it, often to a depth sufficient to submerge the inner rings. As a result, a basin consisted not only of the cavity, but also the concentric rings, the inner blankets of ejecta and the outer sculpture. The clear fact that a basin was distinct from the mare that formed later was highlighted by the discovery of concentric rings around large craters which had not been fill with mare. Since multiple-ring structures were not of volcanic origin, this lent support to the case for smaller craters also being of impact origin. In fact, although it was recognised early on that sculpture was gouged by the fall of material thrown out on shallow-angle trajectories, it was a while before it was realised that a lot of basin ejecta must have struck at a high angle and, consequently, many well-known sizeable craters are probably not primary impacts but secondaries from basin-forming events. By 1963, photogeologists were working to determine the order in which the dozen or so recognised basins were formed.

In just a few years, therefore, an examination of the Moon by geologists applying standard mapping methods had provided insights into the history of the lunar surface which had eluded astronomers for centuries.