Lunar Sample Science Today

Lunar Sample Science Today

The scientific rationale for lunar exploration is to establish the Moon's composition, internal structure, and history (or evolution). Before man walked on the Moon, scientists thought that the Moon was a relatively primitive (simple) object that would record the earliest history of the Solar System. Scientists also hoped that the lunar surface would contain a record of the Sun’s history and of cosmic radiation, and they wanted to know if life had ever existed beyond that on Earth.

More than 50 U.S. and U.S.S.R. spacecraft have flown near or landed on the Moon. A total of 24 U.S. astronauts have observed the Moon in close proximity (3 astronauts went twice!) and 12 U.S. astronauts have walked on the lunar surface (including one geologist!) During 80 hours of surface activities, the astronauts carefully collected 382 kg of lunar samples (see sample inventory on table 1).

In addition to the samples gathered from the six Apollo (U.S.) sites, we also have samples collected from the three Luna (U.S.S.R.) missions. Many exotic samples in the Apollo collection come from rays that extend as much as halfway around the Moon (fig. 1). Recently, numerous meteorites found in Antarctica and the Sahara desert have been shown to have originated on the Moon (Korotev 2003).

Figure 1. The Moon is divided into two basic physiographic regions: smooth maria and cratered highlands. The smooth maria fill the large circular basins and spill out onto low lying regions. They were sampled by Apollo 11, 12, 15 and 17 and Luna 16 and 24. The ejecta blankets of the large basins were sampled by Apollo 14, 15 and 17. The heavily cratered highlands were sampled by Apollo 16 and Luna 20. Note the great distances covered by rays from fresh craters. NASA photo no. 84-31673 (Lick Observatory)

The scientific objectives and results of each Apollo mission are summarized in table 2. From these missions, we learned that the Moon is not the simple, primitive object we once thought it to be, but, rather, a satellite that has been melted and differentiated twice in its history. The first melting occurred shortly after formation and apparently resulted in a Moon-wide magma ocean that cooled to form a thick crust of anorthositic material. While cooling to form a solid body, parts of the interior melted again to form lava flows on the front side. The heat for the first melting comes from the terminal accretion; the heat for the second melting comes from the radioactive decay of K, U and Th in the interior. Much of this heat is brought to the surface with the extruded magma. Carefully refined computer models are now available with which to illustrate the thermal evolution of many planetary bodies, including that of the Earth.

Table 2. Summary of Findings from Apollo Missions

From our study of lunar and meteorite samples, we have learned that different planets and moons have different chemical and isotopic signatures. For example, meteorites can be distinguished from samples of the Earth and the Moon by their high Ir and Au contents. Similarly, lunar samples can be distinguished from Earth samples by identifying their low Mn/Fe and K/U ratios. Small extraterrestrial samples have high He and D (2H) contents. Martian samples have high 15N/14N ratios such as were found by the Viking missions to Mars. Using such signatures, scientists have shown that several meteorites found on the ice in Antarctica are actually lunar samples (Bogard 1983). A clay layer found at the Cretaceous-Tertiary boundary has been identified as having an extraterrestrial cause. Tektites, once thought to have come from the Moon, have proven to be of terrestrial origin. Can chemical signatures like these be used to tell whether we have samples of other planets in meteorite collections?

We have discovered the bombardment history of the Moon by careful radiometric age dating of lunar samples (fig. 2). A surprising result was that most of the samples from the lunar highlands gave the same age; i.e., 3900 to 4000 million years (Ryder 1990; Cohen 2001). This indicates that the Moon (and perhaps other planets as well) was subjected to an intensive period of bombardment late in its evolutionary history. The lack of intense cratering on the mare surface, which has been dated to as great an age as 3800 million years, means that this intense bombardment ended suddenly (a cataclysm). However, there appears to be no record of this cataclysmic bombardment in meteorites. Can we deduce from this the age of heavily cratered terrain either on Mars or Mercury?

Figure 2. Impact breccias from the rims of large basins and the lunar highlands, all dated 3900-4000 million years. This has been termed the lunar cataclysm. Clasts of pristine lunar plutonic rocks are older. Mare basalts have ages spanning 3200-3900 million years. A few breccias are younger (e.g. sample 15405).

A search for the oldest lunar rock is still ongoing but the identification of the most ancient sample has remained elusive (see Table 5 in the section of Plutonic Rocks). Perhaps it is the anorthosite sample 60025 included in this set. This research has continued for 30 years after the last Apollo mission, because new instrumental techniques and revised procedures have had to be developed in order to date the oldest lunar rocks.

Laboratory techniques include Sm/Nd and Lu/Hf age dating, U/Th/Pb ion microprobe age dating, ³⁹Ar/ ^4OAr plateau age-dating, ferromagnetic resonance analysis (Is/FeO), and low-level neutron activation analysis for Au, Ir, and REE. These improvements have also directly benefited the study of terrestrial samples and several, well-equipped geochemical laboratories await additional extraterrestrial samples from other planetary bodies. Scientific studies of lunar samples currently in progress include; trace element partitioning between mineral phases and melt, initial isotopic ratios of lunar reservoirs as a function of time, analysis of volcanic glasses, dating of zircons and granite clasts, and modeling of Zr/Hf fractionation.

If we are clever enough to unravel the multiple processes which occur on the lunar surface, lunar samples might also reveal the history of the Sun and cosmic irradiation over hundreds of millions of years. This is because the lunar surface is directly exposed to micrometeorites, solar wind and cosmic radiation. A general conclusion which might be drawn from the study of solar-wind-implanted ions and tracks in soil grains is that the ancient Sun greatly resembled the modern Sun. The ratios of rare gases, hydrogen (H), and Fe in the Sun have remained almost constant.

Speculation and even some theory concerning the Moon's origin is discussed in Hartmann et al. 1986 and Canup and Righter 2000. Briefly, it is now thought that the Moon was formed from debris left over after a large impact occurred on the Earth. This theory is mainly supported by the fact that lunar samples are found to have the same trend of oxygen isotopes as terrestrial samples.

About 50 meteorite fragments are now recognized as having a lunar origin (surprisingly none before 1982). Some of these are paired with one another such that there are about 29 different lunar meteorites (listed in Table 1). They represent a great addition to the Apollo and Luna collections, because they provide a more statistically representative sampling of the lunar surface (Korotev 2003). One wonders whether these would have been recognized as having a lunar origin, if we did not have the Apollo and Lunar collections with which to compare!

Remote sensing of the lunar surface has continued with Earth-based telescopes, Galileo flyby (Belton et al. 1992), Clementine (see Science 266, 1994) and Lunar Prospector (Lawrence et al. 1998). These missions have discovered water in the regolith at the poles, confirmed the Th-rich region around Procellarum, mapped the Fe and Ti over the whole lunar surface and provided context with which to understand the rocks.

Lunar sample science is documented most fully in the Lunar Sourcebook (Heiken et al. 1991) and Lunar Samples (Papike et al. 1998). The books by Ross Taylor (1975, 1982) and Peter Cadogen (1981) provide great summaries of what was learned during Apollo. The web sites provided by Eric Jones and Kevin Clarke are highly recommended if one wants to revisit the excitement that was Apollo. Essays on various topics by Jeff Taylor are always worth reading and might be the best way for the student to get started (see References).

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