AAQ Solar System Section

Lunar Observing Programme

It is worthwhile reviewing the following basic facts on the Moon before commencing an introduction to lunar geology.

• The Moon orbits at a distance of 356,400 – 406,700 km from Earth with a sideral period of 27.32 days and a synodic period (the time between consecutive new moons) of 29.53 days.

• Tidal resonance between the Earth and the Moon has gravitationally locked the Moon into a captured or synchronous rotation (rotation period equals the orbital period) so that one side, referred to as the “near side”, always faces the Earth.

• The process of libration (lunar oscillations in longitude and latitude) brings 59% of the lunar surface into view from Earth.

• A low mean density of 3340 kg/m3 (compared to the Earth’s density of 5517 kg/m3) combined with a smaller diameter of 3,476 km results in the Moon having a surface gravity of one sixth of Earth’s gravity.

• The low surface gravity results in the Moon possessing a very tenuous atmosphere that is 14 orders of magnitude less dense than Earth’s atmosphere and contains only traces of hydrogen, helium, neon and argon.

• Two consequences of possessing negligible atmosphere are a large surface temperature range of approximately –230°C during the night to +120°C during the day, and the lunar surface is impacted by small meteoroids that would normally be destroyed in a denser atmosphere.

• There are two main terrain types are visible on the lunar near side (see Figure 1); rough, mountainous, intensely cratered, pale grey coloured highlands and relatively smooth, flat, less cratered, dark grey and often circular lowlands called maria or seas. In contrast the lunar far side is dominated by highlands with only a few small maria being present.

Figure 1: Full Moon Photograph courtesy of Russell Croman, © 2005 Russell Croman, www.rc-astro.com

In order to understand the geology of the Moon’s surface it is necessary to consider how the Moon formed and the major processes that shaped it since formation. The most recent theory on lunar formation involves the impact of a Mars sized body with the Proto-Earth approximately 4.6 billion years ago. At the time of impact both the Impactor and Proto-Earth had undergone some differentiation (chemical segregation) with the denser metals sinking to form a core and the lighter silicate minerals rising to form a mantle surrounding the core. During impact, the metallic core of the Impactor was absorbed into the Proto-Earth and part of the silicate mantle of the Impactor and Proto-Earth was ejected into orbit where it coalesced to form the Moon. This complex process explains differences in the chemical composition between the Earth and Moon, the lower density of the Moon due to a probable lack of a metallic core and the angular momentum of the Earth – Moon system.

After formation, the Moon continued to undergo differentiation into concentric shells of differing chemical composition due to differences in density. Seismic studies undertaken during the Apollo missions suggest the internal structure of the Moon consists of three shells:

• A central primitive unmelted core in the order of 2,750 km diameter that is composed of undifferentiated silicate minerals (containing silicon, oxygen, aluminium, calcium, iron, potassium, sodium etc) derived from the Impactor and Proto-Earth mantles.

• Overlying the core is an approximately 300 km thick mantle that initially consisted of a magma ocean of melted gabbroic rock that was the source of the mare lavas. The mantle rocks are silicate minerals rich in iron, magnesium and titanium with the dominant mineral being pyroxene.

• An approximately 60 km thick crust of silicate minerals rich in calcium and aluminium overlays the mantle. The crust can be further subdivided into a basal layer of KREEP rocks containing silicate minerals rich in potassium (K), rare earth elements (REE) and phosphorous (P) and an upper layer of anorthosite, a silicate rock dominated by the mineral plagioclase (sodium – calcium feldspar). Magnesium rich plutons (large subsurface igneous bodies) composed of the minerals olivine and plagioclase have intruded into portions of the crust.

Apart from a few isolated areas, the crustal bedrock was not exposed at the spacecraft landing sites. Rather the surface is covered by a layer of fine grained particulate material called the regolith that can be thought of as the lunar soil. The regolith is produced by and being continually reworked by meteoroid bombardment. It is typically 4 – 5 m thick over the maria and 10 – 15 m thick over the highlands. The regolith is underlain by the large scale ejecta blankets from the major impact basins to a depth of possibly 2 km. Crustal bedrock is present below these major ejecta blankets. Seismic studies suggest the crust is structurally disturbed to a depth of approximately 10 km due to subsurface displacement along faults. These faults were produced by impact, thermal stress and tectonic activity generated by tidal forces and isostatic adjustments as the crust moves to accommodate mass transfer (e.g. lava filling the impact basins to produce the mare). Extensive fracturing, without large displacements, extends up to 25 km into the crust. The rock below this depth is believed to be intact.

Planetary geologists have assembled a stratigraphy or chronology of events that shaped the lunar surface. This chronology is based on visual observations using the “Principle of Superposition” (older rocks are overlain by, cut by or intruded by younger rocks) to establish the relative ages of the surface features and radiometric dating of lunar rock samples to determine absolute ages at specific locations to calibrate the Superposition based chronology. An excellent description of the application of Superposition and radiometric dating to determining the ages of Craters Archimedes, Autolycus and Aristillus is given in The Lunar Notebook article by Charles Wood, Sky & Telescope, January 2001 issue.

Lunar history has been divided into five epochs or periods named after a prominent lunar feature whose time of formation marks the boundary between two periods. The following table summarises the ages, characteristics and some events that occurred during each period. Some publications give the Pre-Imbrian Period, which is a combination of the Nectarian and Pre-Nectarian Periods, as the oldest epoch. The Pre-Nectarian through to the Imbrian Periods are characterised by intense meteoroid bombardment as debris left over from the formation of the solar system and the Moon itself impacts the lunar surface. Most of the multi-ring impact basins and many large craters formed during the Pre-Nectarian and Nectarian Periods. Some volcanism also occurred during the Pre-Nectarian and Nectarian Periods. The intensity of the meteoroid bombardment greatly reduces during the Imbrian Period as most of the solar system debris has now been swept up by the planets and moons. This epoch is primarily characterised by intense volcanism with the flooding of the impact basins with basaltic lavas to form the maria. Oceanus Procellarum marks the end of the massive lava flooding epoch and hence the Imbrian Period. Some minor volcanism extends into the early Eratosthenian Period and abruptly ceases 3 billion years ago. Craters produced during the this period are slightly to moderately degraded and are generally devoid of ejecta rays. The final and current epoch is the Copernican Period that commenced 1 billion years ago. The Copernican Period is characterised by no volcanic activity and a greatly diminished impact rate. Craters formed during this period are less than 100 km in diameter, have sharp rims, well preserved ejecta blankets and prominent ray systems.

Lunar Chronology

Suggested further reading:

Heiken, G., Vaniman, D. and French, B.M. (1991) “Lunar Sourcebook - A User’s Guide to the Moon”, Cambridge University Press.

Taylor, S.R. (1982) “Planetary Science: A Lunar Perspective”, Lunar and Planetary Institute

Guest, J., Butterworth, P., Murray, J. and O’Donnell, W. (1979) “Planetary Geology”, David & Charles.

Rukl, A. (1996) “Atlas of the Moon”, Kalmbach Books.

Wood, C.A. (2001) “Lunar Notebook – Archimedes, Autolycus, Aristillus and Time”, Sky & Telescope January 2001.

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© 2006 Anthony Dutton, Astronomical Association of Queensland

Astronomical Association of Queensland 2006. www.aaq.org.au