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Interpreting the History of the Moon

Nobuyuki Hasebe
Professor, Faculty of Science and Engineering, Waseda University

The Moon is the closest celestial body to our Earth and the only one that has been visited and geologically surveyed by mankind. Because the Moon is smaller in size than the Earth, its igneous activity ended when it was comparatively young and so a great deal of information on its early stage of formation has been stored. The Earth's surface is exposed to air and water so its rocks have been eroded by natural phenomena such as wind and rain, which means there is no ancient matter left on the surface. But because the Moon has no air or water and its volcanic activity has ended, ancient matter still remains on its surface. Knowing the origins of the Moon therefore enables us to know more about our Earth. Lunar science to date has been developed steady by (1) the analysis of matter brought back from the Moon by the Apollo program and lunar meteorites found in places such as Antarctica and the Sahara Desert, and (2) remote observations by the Lunar Prospector spacecraft and more recently lunar orbiter spacecraft such as Kaguya. Here I will introduce some of the most recent progresses in lunar science about the Moon's origin and its asymmetric nature.

The origin of the Moon

The most widely-accepted explanation of the Moon's origin is the giant impact theory: A collision between proto-Earth and a Mars-sized protoplanet, illustrated in Fugure 1, led to a vast amount of debris being strewn around the Earth, which formed into orbiting debris discs that then coalesced into the Moon. The period from debris discs to the formation of the Moon is thought to have been between as short as one month and one year. Simulations of this giant impact by an ultrahigh-speed computer using state-of-the-art technology have produced an image of the birth of the Moon[1]. Upon collision, most of the heavy iron core of the impacting body would have sunk into proto-Earth's surface whereas matter from the impacting body's high temperature mantle was vaporized and scattered into orbit around the Earth. This matter cooled and its particles formed into discs, which repeatedly collided and merged with one another until they grew into the Moon. About 70% of such disc matter is thought to have come from the impacting body and 30% from proto-Earth. Computer experiments have shown that the creation of a satellite like the Moon from such a giant impact is not an unrealistic event[2].

Figure 1
Concept image of the giant impact theory. The Moon was thought to be born when a Mars-sized protoplanet collided with proto-Earth. (Image by NASA/JPL-Caltech)

Now let's look at the chemical composition of the Moon. The collision would have released a huge amount of energy Which heated and evaporated material to form the disk matter. The gravitational energy released when the Moon was formed from the accumulation of these discs in such a short time would have caused the Moon to completely melt. This would have therefore depleted the volatile elements within lunar material while elevating the refractory elements, which is consistent with the observational evidence. Next, what about the oxygen isotope ratio? Lunar samples show the same oxygen isotope ratio as the Earth[3]. In the giant impact theory it is natural to think that the impacting body was formed somewhere away from proto-Earth, like in the capture theory, and so it would be valid to assume that the impacting body and proto-Earth had different isotope ratios[4]. But when the discs that formed in the giant impact cooled down, intense convection occurred within them and their material mixed with the mantle material of the Earth, so their isotope ratios became the same[5]. It has been pointed out, however, that it would be difficult for the disc matter to reach the same isotope ratio in a short period of time. Furthermore, it is not obvious at present whether the oxygen isotopes of the impacting body really need to match those of the Earth.

The asymmetry of the Moon

It is thought that just after the formation of the Moon about 4.5 or 4.6 billion years ago, a lunar magma ocean (LMO) covered the Moon's surface. This LMO was formed because the surface of the proto-Moon was in a high-temperature state and covered in molten magma. The depth of the LMO is thought to be at least 100 kilometers. In the LMO's cooling process, olivine and pyroxene were separated from molten magma by precipitation and sedimentation to form the mantle while lighter plagioclase rose to the surface and formed the crust. The thickness of the lunar crust is several dozen kilometers, and in the highland regions it is composed of anorthosite, a rock composed purely of calcium-rich plagioclase[6]. The LMO theory is strongly supported by the fact that the crust which is typical of the highlands and the mantle presumed from ocean rock show the chemical characteristic of complementary europium anomalies found in the crustal and mantle materials[7], as well as by the formation dates of the highlands and maria[8].

The residual liquid that did not crystallize out by the end of the LMO's crystallization process remains between the anorthosite crust and the mantle. This final residual liquid called KREEP is rich in incompatible elements including potassium (K), rare earth elements (REE) and phosphorus (P). After the formation and crystallization of such an LMO, the entire Moon was thought to be composed of a spherical shell with three layers: the crust, KREEP, and mantle[9]. However, the distribution of thorium (Th, one of the incompatible elements in the liquid layer) clearly shown by gamma-ray spectrometers (GRS) on the Lunar Prospector and Kaguya negates the model obtained from the Apollo missions of the Moon having a uniform spherical structure[10]. The distribution of KREEP obtained from GRS observations shows localization only in the vicinities of Oceanus Procellarum and Mare Imbrium, proving that the Moon's structure is nowhere near that of a simple sphere. Because of thorium's characteristic of being difficult to incorporate into minerals during solidification, thorium concentrations are less in earlier-formed crust matter. According to Kaguya 's GRS measurements, thorium concentrations are low in the highland regions on the far side of the Moon (<1ppm), and the area with the lowest concentration is the equatorial region on the far side (see Fig.2) [11]. This region is where the Moon's crust is thickest, topographically it has the highest places, and it is thought to be the oldest region in terms of LMO crystallization. According to the infrared spectrograph SPI on Kaguya, this region has the highest Mg#, which means it was formed in the early period of LMO crystallization (See Fig.3) [12]. The value of Mg# [= MgO / (MgO + FeO)] is a good indicator of the degree of differentiation of matter in the crystallization of magma.

Figure 2
Map of abundance of naturally occurring radioactive thorium, an element concentrated in the liquid layer, as observed by the gamma-ray spectrometer on the lunar orbiter Kaguya [11]. The central area of the map (180尊) indicates the furthest side of the Moon and the areas at the two edges (0尊) indicate the center of the near side. The thorium concentration is lowest in the equatorial region on the far side (Zone A and Zone B) where the topological elevation is highest. The crust is also thickest in this region.

Figure 3
Distribution map of the whole Moon of the mafic minerals Mg# included in highland crust, based on visible and near-infrared spectroscopic data obtained by the spectral profiler on the lunar orbiter Kaguya [12].
highland material with high Mg# can be seen in the equatorial region on the far side. This corresponds to the regions of low concentration of the element thorium, which is concentrated in the liquid layer and which is also a heat source.

The gamma ray spectrometer on Kaguya identified with high accuracy not only naturally occurring radioactive such as thorium, uranium and potassium but also major elements like iron, titanium, calcium, silicon, aluminum and others, and created an abundance distribution map for the entire Moon of all of these. The Moon's topography, geological features and their age, rocks, and element distribution learned from Kaguya's comprehensive observations have led to significant changes in our understanding of the Moon's near side and far side, the east and west regions of the near side, the thickness of the crust, and the existence of a KREEP layer, and has taught us that the Moon is an asymmetric and heterogeneous body.

The highly accurate and fundamental lunar data obtained from the remote investigations conducted by orbiters to date will form the basic information required for lunar landings in the near future, and direct observations from those moon landings are bound to produce new discoveries. Further into the future, this will lead to the construction of a lunar base and a manned expedition to Mars [13].

  • [1] A.G.W. Cameron, 2000, in Origin of the Moon, 133.
  • [2] E. Kokubo and S. Ida, 1998, Icarus 131, 171.
  • [3] U. Weichert et al., 2001, Science 294, 345.
  • [4] H. Genda, 2010, Yuseijin, 19, 76.
  • [5] K. Pahlevan and D.J. Stevenson, 2007, EPSL 262,438.
  • [6] M. Ohtake et al., 2009, Nature 461, 236
  • [7] R.S. Taylor, 2001, Solar System Evolution: A New Perspective (2nd ed) Cambridge Univ. Press.
  • [8] Shearer, C. K. & Papike, J. J. 1999, Geochim. Cosmochim. Acta 53, 3331.
  • [9] C.K. Shearer and J.J. Papike, 1999, American Mineralogist 84, 1469-1494.
  • [10] B.L. Jolliff et al., J. Geophys. Res. 105 E2 (2000) 4197-4216.
  • [11] S. Kobayashi et al., Earth and Planetary Science Letters. 337-338, 10-16, 2012.
  • [12] M. Ohtake et al., Nature Geoscience Letter. 5, 384-388, 2012.
  • [13] N. Hasebe, K. Sakurai, 2013, The Moon - Inspiring Mankind's Dreams [Jinrui no yume wo hagukumu "tsuki"] , Koseisha-koseikaku, see Chapters 8 and 9

Nobuyuki Hasebe
Professor, Faculty of Science and Engineering, Waseda University

Professor on the Faculty of Science and Engineering at Waseda University as well as the Department of Physics in the School of Advanced Science and Engineering and the Graduate School of Advanced Science and Engineering.
He graduated from the Faculty of Science and Engineering at Waseda University in 1972, and in 1977 obtained a PhD in Physics from the Graduate School of Science and Engineering. He worked as a researcher at the Institute for Cosmic Ray Research at University of Tokyo from 1978, lecturer and assistant professor on the Faculty of General Science at Ehime University from 1979, and assistant professor and professor on the Faculty of Engineering at Ehime University from 1995 before taking up his current post in 1998.
Fields of specialization: Nuclear planetology, radiation physics, space physics, cosmic ray physics