The composition of the Moon

The mean lunar radius is 1737.1 km, which is intermediate between that of the two jovian satellites of Jupiter, Europa (r =1561 km) and Io (r =1818 km). The Moon is much smaller than the jovian satellite Ganymede (r = 2634 km), which in turn is the largest satellite in the Solar System and like the saturnian satellite Titan, is larger than Mercury. Although the jovian satellites and also Titan are comparable in mass, the Moon/Earth ratio is the largest satellite-to-parent ratio in the planetary system, a consequence of a distinctive origin. The Charon/Pluto ratio is larger, but Pluto, an icy planetesimal, is less than 20% of the mass of the Moon and is one of the largest Trans-Neptunian Objects in the Kuiper Belt, rather than being a planet. Charon is only relevant here as another example of the collisional origin of a satellite.

The mass ofthe Moon is 7.35 x 1025g, which is 1/81 ofthemass ofthe Earth. The lunar density is 3.346 g/cm3, a fact that has always excited interest on account of the Moon's proximity to the Earth, which has a much higher density of 5.54 g/cm [2]. This major difference in density posed a serious dilemma for workers seeking to

Table 2.1 The major element composition of the bulk Moon and that of the lunar highland crust (wt% oxides). Data from Note 4

Oxide

Bulk Moon

Lunar highland crust

SiO2

47.0

46.0

TiO2

0.3

0.3

Al2O3

6.0

28.0

FeO

13.0*

4.5

MgO

29.0

4.5

CaO

4.6

16.0

Na2O

0.09

0.45

K2O

0.01

0.075

I

100.0

99.8

* 2.3% Fe or FeS is allotted to a lunar core and 10.7% to a lunar mantle; Mg# [molar (Mg/Mg + Fe)] =0.80.

understand the origin of these two closely associated rocky bodies. The low density of the Moon tells us that it is depleted in metallic iron relative to the inner planets. The high value for the moment of inertia of our satellite (I/Mr2 = 0.3935) means that there is only a slight increase in density toward the center, so that the Moon has only a tiny core (350 km radius) at best.

Although the Earth contains about 28% metallic iron, the Moon has less than about 2 or 3%. However, the lunar mantle has a high FeO content (present in silicates) so that the bulk Moon iron content, expressed as FeO, is 13%, that is 50% more than for the current estimates of 8% FeO in the terrestrial mantle. Along with its depletion in metallic iron, the Moon also has a low abundance of the other siderophile elements [3].

Compositional data for the bulk Moon and the lunar highland crust are given in Tables 2.1 and 2.2 [4]. Notable differences between the Moon and the Earth are the bulk FeO content and the Al2O3 abundance that is reflected in the abundances of the other refractory elements. These are enriched in the Moon by a factor of 1.5 compared to the Earth or of about 3 compared to CI.

Figure 2.1 shows the relative compositions of the Earth and Moon, normalized to the CI abundances. This shows the two extreme features of the lunar composition that make it unique. These are the enrichment in refractory elements and the depletion in volatile elements. The depletion in the Earth relative to CI correlates with volatility. But although the Moon is more highly depleted in volatile elements than the Earth, curiously this depletion is apparently uniform relative to the Earth [5]. Clearly the Moon has a composition that cannot be made by any single-stage process from the material of the primordial solar nebula and calls for a distinctive mode of origin. Thus the cause of the enrichment in refractory elements and the

Table 2.2 The elemental composition of the bulk Moon and that of the lunar highland crust. Data from Note 4

Element Bulk Moon Lunar highland crust

Ti (ppm) 1800 1800

Y (ppm) 6.3 13.4 Zr (ppm) 17 63 Nb (ppm) 1.3 4.5 Mo (ppb) 1.4

Table 2.2 (cont.)

Element

Bulk Moon

Lunar highland crust

Eu (ppm)

0.26

1.0

Gd (ppm)

0.92

2.3

Tb (ppm)

0.17

0.35

Dy (ppm)

1.14

2.3

Ho (ppm)

0.255

0.53

Er (ppm)

0.75

1.51

Tm (ppm)

0.11

0.22

Yb (ppm)

0.74

1.4

Lu (ppm)

0.11

0.21

Hf (ppm)

0.51

1.4

W (ppm)

0.008

-

Re (ppb)

1.6

-

Os (ppb)

25

-

Ir (ppb)

23

-

Pt (ppb)

40

-

Au (ppb)

7

-

Hg (ppb)

0.7

-

Tl (ppb)

0.2

-

Pb (ppm)

0.004

1.0

Bi (ppb)

0.17

-

Th (ppb)

115

900

U (ppb)

30

240

uniform depletion of the very volatile elements may be a consequence of the condensation of the Moon from vapor following the giant impact [6].

The initial interest in the bulk lunar composition was to test the hypothesis that the Moon was derived from the silicate mantle of the Earth, a notion dating back to George Darwin, resolving the difference in density between the two bodies. Although hotly debated following the Apollo sample return, the siren-like attractions of the model have diminished, despite the similarity in oxygen isotopes between the two bodies [7]. This model has not survived the demonstration of significant differences (e.g. FeO content) and the acceptance of the single-impact hypothesis for lunar origin that derive 85% of the Moon from the mantle of the impactor (Theia), not from the Earth.

The bulk composition of the Moon can thus be understood within the general framework of the standard model for the evolution of the inner Solar System. All inner bodies in the Solar System are depleted in volatile elements relative to the composition of the primordial rocky component (CI) of the solar nebula (Chapter 1 ). Initial differences have been exacerbated by massive collisions that have produced extremes. The composition of the metal-poor Moon in contrast to metal-rich Mercury represents examples of the range in compositions that result from the

"5j E

"5j E

Elemental enrichment - depletion relative to CI

Fig. 2.1 The composition of the Moon compared with that of the Earth, both normalized to CI carbonaceous chondrites. The material in the impactor mantle (Theia), from which the Moon was derived, was inner Solar-System material already depleted in volatile elements at Tzero. The abundances in the Earth provide an analogue for its composition. The significant point is that, compared to the Earth (or Theia), the lunar depletion is uniform and not related to volatility, which would produce a much steeper depletion pattern (lower dotted line). Thus, the lunar pattern is interpreted as resulting from a single-stage condensation from vapor (>2500 K) that effectively cut-off around 1000 K [4].

Elemental enrichment - depletion relative to CI

Fig. 2.1 The composition of the Moon compared with that of the Earth, both normalized to CI carbonaceous chondrites. The material in the impactor mantle (Theia), from which the Moon was derived, was inner Solar-System material already depleted in volatile elements at Tzero. The abundances in the Earth provide an analogue for its composition. The significant point is that, compared to the Earth (or Theia), the lunar depletion is uniform and not related to volatility, which would produce a much steeper depletion pattern (lower dotted line). Thus, the lunar pattern is interpreted as resulting from a single-stage condensation from vapor (>2500 K) that effectively cut-off around 1000 K [4].

stochastic assembly of planetary-sized bodies from a hierarchy of smaller bodies. It points to the importance of such processes in the inner solar nebula that have led to such random outcomes.

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