The Origin of the Moon

by Roberto Bugiolacchi PhD

 

 

Introduction and background.

 

Until Apollo’s 11 380,000 km long journey the geological history of the Moon was totally based on conjectures and observational records. Overall the Apollo mission recovered 382 kilograms of moon material from six sites and established an array of seismic stations around the landing areas.

G. Jeffrey Taylor and William K. Harman in 1984 organised a conference to discuss the first findings on the new lunar data. By the time the conference was over, a general consensus was reached about the most plausible origin of the Moon: the ‘giant impact theory’.

This theory was not entirely new: Baldwin and Wilhelm had suggested in 1946 that the Moon formed as a result of the glancing impact of a  planet-size object against the Earth.

Even before that Lord Kelvin had asserted that “when two great masses come into collision in space, it is certain that a large part of each is melted” (1908).

In 1975 Davis and Hartmann had been investigating the population of planetesimals close to the early Earth’s orbit and realised that numerous large bodies, possibly of the size or even more massive than Mars, might have been wandering near our young planet. Because of one or more of such collisions, some debris would have been ejected into orbit around the planet. Within as little as 100 years this debris could have coalesced into a planetary body.

To date this theory satisfied more than any other the various astronomical and geo-chemical constraints required by the observed facts about the Moon.

Since then an interdisciplinary research effort has been directed towards putting this theory into a rigorous scientific framework. Notably Cameron, Ward, Benz, and many other researchers have tried to tackle various problems in this model especially the problem posed by the angular momentum budget.

Making use of increasing computer power and more accurate software models, scientists have started simulating the various dynamic scenarios of a number of collision hypotheses  answering some questions but also posing some challenging new ones.

The notion of the Earth-Moon system being special and unique considering its almost double-planet nature [1] may be born out of lack of suitable comparisons.

a)    Mercury and Venus were probably affected in their formation by solar tides.

b)   The rock and ice component of Jupiter to the ratio of its satellite system mass is not much less than the Earth and Moon ratio.

Before taking a detailed look into the alternative models of lunar origin, it is worth summarising the latest observational data on our satellite.

 

Bulk Chemistry

 

It is very likely that the earliest history of the Moon included the formation of a magma ocean and the subsequent development of anorthositic cumulates. This anorthositic crust was then intruded by mafic magmas which crystallised to form the lunar highlands magnesian suite [2] .

·      The upper mantle of the Moon is probably chemically uniform and may be composed of pyroxenite containing 0.5 - 2 mol. % of free silica. [3]

 

·      As we will see in more detail later, the bulk silicate composition (mantle + crust) is significantly enriched in FeO, SiO2 and refractory elements (Ca, Al) compared to chondrites.

 

·      The lunar heat flow is consistent with similar mean abundances of U and Th existing in the bulk Moon and the Earth’s mantle.

·      About ‘half of the Moon’ and the ‘Earth’s mantle’ is (by weight) composed of oxygen (16O, 17O, 18O) [4] . The O isotope composition of both lunar and terrestrial basalts are identical. In contrast the O in most classes of meteorites possesses distinctly different proportions of the 16O-rich component.

·      The most striking geo-chemical difference between the Earth and the Moon is the depletion of iron in the latter. This fact has been suspected from gravitational data for a long time before the direct sampling of lunar material. The Fe/Si atomic ratio is equal to 0.22 as a whole (crust + mantle + core), the lowest known Fe/Si ratio of any object in the solar system2.

For a body the size and density [5] of the Moon, the inferred mass should be at least 10% less than one of a similar sized sphere formed from the assumed cosmic Mg/Fe and Mg/Si ratios. Even varying the ratio of combined oxygen and taking into account the different forms of iron, i.e. metal, oxide or substituting for Mg in silicates, it is still apparent that the Moon was formed either in an anaemic environment [6] or directly from material already depleted in iron.

·      Paleointensity data concerning the ancient lunar magnetic field, the centre of gravity centre of figure offset and other physical signs indicate a core formation about 4.1 Ga ago. Unlike the Earth, the Moon’s core formed some Ma after its origin and this represents a very significant boundary condition for theories of the Moon’s origin [7] .

·      Lithner and Marti (1974) and Leich and Niemeyer (1975) found that some lunar rocks have trapped Xenon possessing a terrestrial isotopic composition. If proven indigenous and not originally from terrestrial contamination (this gas is only released at temperatures exceeding 1000ºC), this would also indicate a close genesis of our two planetary bodies.

 

·      The current model of the formation of the Moon at a time when the Earth’s core had already formed, would be reinforced if the age of Moon’s rocks would be younger than some differentiated bodies such as chondrites. Indeed the single stage lead-isotope growth curve both for the Moon and the Earth mantle give a model age of 4.4 - 4.45 ´ 109 years against the age obtained from some meteorites of around 4.55 ´ 108 years.

 

·      Our satellite completely lacks any water-bearing minerals.

 

·      There is a depletion of heavy REE [8] in the lunar crustal rocks (usually due to fractionation caused by the crystallisation of garnet at very high pressures below the liquidus).

On Earth the pressure at depth of 200-300 km in the primitive magma ocean (at 70-100 kbar) would have caused garnet to co-precipitate with olivine causing depletion of heavy REE.

REE relative enrichment [9] in the maria indicates that their magma formation originated under highly reducing conditions and that plagioclase was absent from their source regions. The prior removal of a plagioclase component from the source region of maria basalts explains why the REE europium is instead seriously depleted [10] . Differences in the composition of the lower mantle of the Moon indicate a higher FeO and SiO2 contents than the Earth’s mantle. The source regions of mare basalt may also have a higher proportion of MnO and Cr2O3

 

Possible explanations:

On the Earth the early formation through differentiation of the core caused strong convection in the mantle and its melting to a depth of 200-400 km forming a deep ultramafic magma ocean.

Several impacts by planetesimals before extensive crystallisation (hence soon after the formation of the Earth’s core) would have evaporated and ejected material over the Roche [11] limit to form moonlets and ultimately the material constituting the highlands system (following fractionation).

Consequently this terrestrial ultramafic magma ocean  would have been dominated by fractional crystallisation resulting in the separation of olivine; this would have led to a composition richer in FeO, SiO2, MnO and Cr2O3 and significantly poorer (by half) in Ni. A large impact at this stage would have led to the formation of a significant moon nucleus with a similar inferred composition of the source regions of mare basalts. This large nucleus would than attract and consolidate the other earlier moonlets to form the lunar upper mantle. Then it would have taken around 100 years for the upper mantle to melt and differentiate, leading to the formation of the lunar crust.

Volatile Depletion

The Earth’s upper mantle is depleted in volatiles in comparison to the primordial composition of the disc-like nebula of dust and gas. Nevertheless the Moon’s depletion pattern extends to many volatile elements in relation to C1 chondrites, ordinary chondrites and the Earth’s mantle.

Take for instance the relative abundances of selected volatile and refractory elements in terrestrial and lunar basalts [12] (Table 2):

                                        Table 2.

Element

RATIO OF ABOUND-ANCES mOON-eARTH

refractory (re)

volatile

(vo)

barium (Ba)

6.0

Re >1300 K

uranium (U)

5.0

Re >1300 K

thorium (Th)

4.2

Re >1300 K

titanium (Ti)

2.0

Re >1300 K

iridium (Ir)

1.1 ´ 10-1

Re >1300 K

sulphur (S)

4.4

Vo 1300-600 K

gallium (Ga)

3.0 ´ 10-1

Vo 1300-600 K

copper (Cu)

1.1 ´ 10-1

Vo 1300-600 K

sodium (Na)

8.0 ´ 10-2

Vo 1300-600 K

germanium (Ge)

6.9 ´ 10-2

Vo 1300-600 K

potassium (K)

6.5 ´ 10-2

Vo 1300-600 K

rubidium (Rb)

3.5 ´ 10-2

Vo 1300-600 K

zinc (Zn)

8.5 ´ 10-3

Vo 1300-600 K

bismuth (Bi)

11.5 ´ 10-3

Vo <600 K

lead (Pb)

9.0 ´ 10-2

Vo <600 K

indium (In)

3.8 ´ 10-2

Vo <600 K

What appears quite clearly is that generally  the Moon is depleted in volatiles and enriched in refractory elements.

Any hypothesis that can stand up to scrutiny on the origin of the satellite requires the Moon to have selectively recondensed from the original material in circumstances under which volatiles were lost.

The size of the Moon and its limited gravitational pull have to be discounted as reasons for this process in view of newly acquired geo-chemical knowledge of similar sized planetary bodies (i.e. Io has a substantial volatile component [13] ).

The depletion of some elements such as Sb and Ge in the Moon can be modelled by the absence of metallic iron during the volatilisation-recondensation phase.

Current research and newly acquired data indicate that the estimates of the degree of volatile depletion might be incorrect due to the possibility of volatiles enriching the deep lunar interior.

Moreover estimates based on the picritic glasses infer a higher Li/Be ratio for the bulk Moon than estimated from the lunar basalts. This would indicate that the bulk Moon is less refractory than previously calculated from Li/Be data and approaches the bulk composition of the Earth [14] .

In the whole both highlands and maria rocks have similar ratios of volatile/involatile elements (such as K/Zr).

 

Siderophile  elements [15] .

The abundance of siderophile elements Fe, Ni, Co, W, and P are similar (within a factor of about two) in low-Ti mare basalts, parental (PLC) magma in the lunar crust and in terrestrial oceanic tholeiites. Cu, Ga, S and Se where also found to share the same similarities in these three different environments (after having taken into account depletion caused by volatility).

Depletion of W and P in the Earth’s mantle, and of most of the other siderophile elements in comparison to the PSN, can be explained by their preferential entry into a metallic iron phase which segregated to form the core.

However, several siderophile elements - Ni, Co, Cu, Au, Ir and Re - are present in the mantle in much higher concentrations than expected [16] (according to the model which advocates partition under equilibrium conditions and low pressures into a Fe-rich metallic phase).

These discrepancies may be the product of a number of processes, the two most important ones being the changes in metallic/silicate partition coefficients due to the presence of an element of low atomic weight (possibly oxygen or sulfur) within the segregating core and the high pressures deep within the Earth [17] . This process is difficult to apply to the Moon in view of its iron depletion; the core of the Moon, if present at all, is only 2% of the lunar mass against 32.5% on the Earth. Moreover the required pressure fields on the Moon are only 47 kbar as compared to 3900 kbar within our planet.

At this point the similarities of siderophile ratios both in the Earth and the Moon become quite significant in helping to understand the origin of the latter.

There is not a workable model which envisages the partitions and fractionations of these elements within the primordial nebula. We  must conclude that these type of relative abundances of siderophile elements are typically terrestrial in origin.



[1] The lunar mass is approx.  one eightieth of the Earth’s mass.

[2] C. R. Neal, A. N. Halliday, G. A. Snyder and L. A. Taylor, 1995

[3] O. L. Kuskov, 1997

[4] two different origins of variations in the ratios: a) primary isotopic inhomogeneities in the O in various regions of the solar nebula prior to accretion, due to the non-uniform distribution of a nuclear component rich in 16O, possibly injected via a supernova explosion; b) chemical isotopic fractionations caused, for example, by differing temperatures at which the solar matter which accreted to form planets and meteoritic parent bodies equilibrated and then became separated from  the gases in the parental solar nebula, PSN).

[5] 3.344 ± 0.002 g cm-3

[6] There is no direct evidence that the Moon has an iron core at all,  but almost certainly, if existing, it is at most ~400 in radius.

[7] S .K. Runcorn, 1996

[8] Rare Earth Elements

[9] In comparison to Carbonaceous Chondritic 1 (CC1).

[10] Europium behaves anomalously in that it can take a divalent form (Eu2+) which is the right size to substitute for Ca in the crystalline lattice of feldspar (unlike the other REE (exp. Cerium) which are trivalent).

[11] The Roche limit is the orbital distance at which a satellite with no tensile strength (a ‘liquid’ satellite) will begin to be tidally torn apart by the body it is orbiting. A real satellite can pass well within its Roche limit before being torn apart (at 2.89 Earth radii).

[12] Open University, S267 course  1994

[13] D. J. Stevenson, 1987

[14] C. K. Shearer, G. D. Layne, J. J. Papike, 1994

[15] O’Keefe, (1972); O’Keefe and Urey (1977); Ringwood and Kesson (1977); Rammensee and Wänke (1978) et al.

[16] Between 10 to > 100 times higher.

[17] Ringwood (1979).