Lunar Science Objectives:

The Rationale for Apollo Landing Site Selection

Following is a summary of material presented to the Apollo Site Selection Board at its meeting on July 10, 1969, intended to show how the Site Selection Subgroup of the Group for Lunar Exploration Planning arrived at their recommended list of primary and alternate sites (presented at the June meeting of the Board). It relates scientific experiments to the "15 Questions" tabulated above.

Age Dating.

Determination of the absolute age of lunar surface materials by radioactive-decay methods was of prime importance. The method dates the time at which a given sample became a closed system for a particular element; for example, the potassium-40-argon-40 method determines the time at which a rock containing radioactive potassium-40 cooled sufficiently to retain the gaseous argon-40 decay product. The age thus determined may reflect a period of volcanism, melting by meteoric impact, or original accretion of lunar material.

Ages of primary interest to lunar scientists were:

  1. the age of the moon's formation or of its oldest crust; this age might make it possible to distinguish among various theories of lunar origin. One key site for locating such material was the Fra Mauro Formation. This widespread blanket of debris is considered to consist of debris from the "Imbrian event," the cataclysmic occurrence that produced Mare Imbrium, the enormous circular mare in the northwestern quadrant of the Earth-facing side of the moon. The Fra Mauro Formation was given high priority on all lists of potential landing sites.
  2. The time at which the maria became filled with the relatively smooth material, probably once-molten rock, that characterizes them. This date would be relatively easy to establish, since mission planners preferred smooth, level landing sites, most often found in the maria.
  3. The time of significant post-mare events, such as the impacts that created the craters Copernicus and Tycho. Equally interesting was the origin of the sinuous rilles, which appeared to have been formed largely in this later period of lunar history.

Lunar Composition.

A primary interest of geochemists was to find "primitive" solar system material to deduce the conditions under which planets and satellites condensed from the solar nebula. Data from analysis of terrestrial and meteoritic samples, along with solar and stellar spectral studies and theoretical physics, comprised the prime source of such information, but both the earth and meteorites have been modified by subsequent heating and weathering, which have obscured the original composition. The moon's small size (possibly resulting in a smaller flow of heat from the interior) and its lack of an atmosphere suggest that it might still have original material on the surface. It was generally accepted that this material could most likely be found in the lunar highlands, which appeared to represent the oldest lunar material.

Second in importance was establishing the bulk composition of the moon. The abundance of the major elements was expected to be important in establishing the moon's origin; if it proved to be totally unlike the earth in composition it could hardly have split off from the earth. The abundance of radioactive elements would indicate how much energy had been available for heat-induced chemical changes in the moon since its formation. To determine the bulk composition of the moon it was necessary to sample as many different geologic units as possible. Particularly important were sites showing evidence of differentiation or the presence on the surface of deep-seated material, such as would be present in and around impact excavations or explosive craters and in blankets of material ejected from craters. Radioactivity could be measured by instruments in lunar orbit, hence the importance of flying experiments in the service module.

Finally, analysis of any present or past lunar atmosphere would give clues to lunar origin and evolution. Gas detectors operating over a long period of time might possibly detect transient events, such as had been reported in the crater Aristarchus. The sinuous rilles (e.g., Rima Prinz) were also likely sites for detecting any emission of gases from the lunar interior.

Major Geomorphic Processes.

The study of the processes by which lunar landforms have been created and destroyed was important mainly to second- and third-order questions about the moon, but was essential in holding the first-order questions (above) together. Knowledge of dominant processes would provide the basis for selection of samples and determining their place of origin, as well as providing major clues to past energy expenditure on the moon. Regions of particular interest in this regard were the sinuous rilles and areas of volcanic cratering. Since photography covered a large fraction of the lunar surface, data from a few landing sites would enable geomorphologists to draw conclusions about most of the moon.

Lunar Geophysics.

The only source of information on the moon's internal structure was the emplaced ALSEP experiments, which included a seismometer and a heat flow instrument. (Later ALSEPs would include different geophysical instruments.)

Seismic data were expected to yield information on layering in the moon, the rate of release of internal strain, and the number and energy of meteorite impacts. It would be very useful to the seismologists to produce an impact of known energy at a known location, such as by causing a spent S-IVB stage or a lunar module ascent stage to crash on the lunar surface. It was important to have at least four seismic instruments active at one time; they should be about 1,000 km apart with as much angular separation as possible. Thus sites at high latitudes, such as Tycho, were quite important.

Data on heat flow were difficult to interpret but could assist in determining whether the moon had originally been hot or cold.

Lunar gravity and geodesy would determine the extent of the "mascons" (mass concentrations) and whether the moon was in hydrostatic equilibrium. These were well suited to study by orbiting instruments.

The laser retroreflectors were expected to make it possible to determine earth-moon distances within a few centimeters, enabling scientists to measure the librations of the moon with previously unattainable accuracy. Best results would be obtained with widely separated reflectors.

Characteristics of the Recommended Landing Sites.

The short list of landing sites for the first 10 lunar exploration missions should include:

  1. two types of mare material, "older" or eastern and "younger" or western;
  2. regional stratigraphic units, such as blanket (ejecta) deposits around mare basins;
  3. various types and sizes of impact craters in the maria and in the highlands;
  4. morphological manifestations of volcanism in the maria and in the highlands; and
  5. areas that may give clues to the nature and extent of processes other than impact and volcanism, which may have acted on the lunar surface.
Following is a description of the 10 prime sites chosen by the Group for Lunar Exploration Planning. The sites were selected in order of their preferred execution on one "G," four "H," and five "J" missions.
(1) Landing Site 2 ("older" or eastern mare).
This site is entirely within relatively old mare (Imbrian) material. It includes many large subdued craters 200 to 600 meters in diameter but comparatively few in the size range 50-200 meters, a distribution common to many apparently old surfaces. Determination of the age and nature of this Imbrian mare material was a primary object of landing at this site.
(2) Landing Site 5 ("younger" or western mare).
Landing Site 5 is located within relatively young (Eratosthenian) mare material and displays many craters 50 to 200 meters in diameter and relatively fewer of the larger (200-600 meter) craters. It is surrounded by well developed rays from Kepler, making it likely that it contains material derived from considerable depth. The chief goal of a landing at Site 5 was to determine the age and composition of Eratosthenian mare material.
(3) Fra Mauro Formation.
This extensive geologic unit covers large portions of the surface around Mare Imbrium and is thought to be material ejected when Imbrium was formed. Samples from the Fra Mauro Formation would help to understand its nature, composition, and formation and its relation to the "Imbrian event."
(4) Rima Bode II.
Rima Bode II, a single linear rille running close to a fresh, elongate crater and a crater chain, was of interest because both the rille and the crater were possible sources of several dark geologic units most probably of volcanic origin. The site was selected as an example of a region where material of deep-seated origin was expected. An alternative site was Hyginus Rille, similar in characteristics but apparently less fresh. Another site, Littrow, would meet part of the objectives of a mission to Rima Bode H.
(5) Censorinus
Censorinus is a 3.8-km crater located within and near the edge of a highland block south-southeast of Mare Tranqillitatis. It offered the opportunity, early in the exploration program, to sample both highland material and features associated with a fresh impact crater. The proposed site was within the ejecta blanket about 1 km north of the crater rim and allowed investigation of the crater on foot, without mobility aids. If Censorinus presented operational difficulties, Littrow could be considered as an alternative site for this mission.
(6) Copernicus (peak).
This bright crater, 95 km in diameter, is the source of visible rays of ejected material extending for several hundred kilometers. Its walls expose a 4-km vertical section of the lunar crust. The floor, some 60 km across, contains multiple peaks with a maximum height of 800 meters. A mission to the central peaks would be mainly a sampling mission with the objective of bringing back material that once lay at considerable depth.
(7) Marius Hills.
Marius Hills, a group of domes and cones near the center of Oceanus Procellarum west-northwest of the crater Marius, are part of a ridge system stretching some 1,900 km through Oceanus Procellarum. The variety of features in this area and their similarity to terrestrial volcanic structures strongly suggests intensive and prolonged volcanic activity.
(8) Tycho (rim).
Tycho, like Censorinus a fresh impact crater, is in the southern highlands. It is much larger than Censorinus and offers an opportunity to study many features common to large, fresh impact events, including associated volcanism. The proposed landing site was near the Surveyor VII spacecraft, offering the option of returning some Surveyor parts. In that area are several generations of flows, a pond or pool, ejected blocks (probably from Tycho), and other ejecta features and structures.
(9) Rima Prinz I,
Rima Prinz I, in the Harbinger Mountains northeast of the Marius Hills, is a double sinuous rille - a small meandering rille enclosed within a larger sinuous rille. The origin of the rilles is of great interest because they resemble channels carved by a flowing fluid. A landing near the mouth of Rima Prinz I, selected because of the freshness of its details, would allow examination of the lower part of the eroded valley, sampling the materials and studying the exposed structures. An alternative, Schroter's V alley , displays similar characteristics but appears older than Rima Prinz I.
(10) Descartes.
The area of the southern highlands north of the crater Descartes and west of Mare Nectaris is characterized by hilly, groovy, and furrowed deposits reminiscent of terrestrial volcanoes. A mission to a region of intensive and prolonged volcanism within the lunar terrae was considered most important, from both the geological and geochemical viewpoints. An alternative to this site was Abulfeda, just to the southwest.
The Group for Lunar Exploration Planning recommended this list of sites after considering the expected evolution in capability as well as the constraints imposed by operations. Those selected for "J" missions, for example, were picked on account of the additional time on the surface that would be available and the increased mobility that would be provided by a powered vehicle. Tycho, accessible only in the early part of the year because of operational limitations, was switched from the third to the second "J" mission for that reason, although it was better than the Marius Hills site for exploration on foot.

The Apollo Site Selection Board accepted this list for planning purposes at its meeting on July 10, 1969. As was to be expected, the list underwent considerable revision during the next three years, both as to the choice of sites and the order in which they would be explored, as mission planning became more detailed and operational capability improved.

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