Scientific History of the Moon's Formation
Prepared by Gonzalo Tancredi, President of the Division F Planetary Systems and Astrobiology - International Astronomical Union, and collaborating members of the SC
The Apollo program has been crucial for the understanding of the origin of the Earth-Moon system.
Prior to the Apollo program our knowledge about the Moon was limited to the remote observations of its surface and the gravitational effects that produces on the Earth. By that time we knew that:
The geocentric orbit of the Moon has a non-negligible angle with respect to the Equator (the plane perpendicular to the Earth's rotation axis).
The ratio between the mass of the satellite to the mass of the corresponding planet was the highest among the planets of the Solar System.
The density of the Moon was smaller than the density of the Earth. It was considered that the Moon lacks a large iron-nickel core, like the one at the center of the Earth.
The total angular momentum of the Earth-Moon system (sum of the rotational momentum of the Earth and the Moon and the translational momentum of the Moon around the Earth) is very high. This value is close, but slightly less, than the angular momentum of a parent body, with a mass equivalent to the sum of the mass of the Earth and the Moon, that undergoes rotational fission.
Based on this information, three alternative hypothesis were proposed to explain the origin of the Moon:
Capture: The Moon was formed in an heliocentric orbit around the Sun; and in a close encounter with the Earth, it was captured in a geocentric orbit.
Fission: Break-up of a proto-Earth during early period of fast rotation, expelling a disk of material, that lately re-accreted to form the Moon.
Co-Accretion: The Earth and the Moon formed together and simultaneously, as a binary system, from the material present in the protoplanetary disk.
The Apollo program provided two key sets of data: hundreds of kilograms of igneous rocks, cumulates, and breccias from the different landing sites were brought to the Earth for detailed laboratory studies; and data from seismometers at different locations over the Moon's surface were collected. Based on this information, it was possible to reach the following conclusions:
The collected material from the surface of the Moon had acquired large enough temperatures to be melted.
By comparing the isotopic ratios of several chemical species present in lunar and terrestrial rocks, it was possible to conclude that lunar material was very similar to the material from the Earth mantle.
The mass of the iron core was estimated to be on the order of a few percent.
A very low water abundance, suggesting a bone-dry body.
The hypotheses listed above were in conflict with this new data. The capture hypothesis fails to explain the similarity in composition between the Earth and the Moon; and the transition from a temporary to a permanent capture requires the presence of dissipative forces like an extended primitive atmosphere. Co-accretion cannot explain the lack of iron and water. Nor can it explain the high angular momentum. The fission hypothesis satisfies the geochemistry by expelling material from the Earth mantle into orbit; but it lacked a mechanism to acquire such fast rotational state. In addition, it is not clear what other mechanism have subsequently reduced the angular momentum; and why the Moon’s orbit is not in the Earth’s equatorial plane.
A new idea came to the rescue: the giant-impact hypothesis. At a late stage of the formation, the proto-Earth suffered the collision of a planetary embryo the size of Mars; and the Moon was formed out of the debris left over from the catastrophic event. The event should have occurred a few tens up to a hundred million years after the formation of the Solar System. A Mars-sized body colliding in an off-axis direction should produce the large angular momentum of the present Earth-Moon system and tilt the proto-Earth.
By the time of the collision, both the impactor and the proto-Earth should have already been differentiated, with iron cores and silicates in the mantles. The collision produced global shock waves affecting the impactor and the target, and a plume of vaporized silicate was ejected. The cores of the impactor and the proto-Earth largely merged; while a much larger fraction of the silicate mantles of both bodies were ejected and partially inserted into a geocentric orbit. Numerical simulations show that most of the material re-accreted to form the Moon came from the mantle of the original impactor.
More recently, the isotopic ratios of several chemical species (e.g. Oxygen, Titanium, Chromium, Tungsten and Potassium) were precisely measured on samples of the Apollo’s lunar rocks. It was found that lunar rocks were indistinguishable from those of the Earth; raising the criticism that the Moon is too similar to the Earth in its composition to be substantially contaminated by another planetary embryo. In order to overcome theisotopic crisis, several variations of the original giant impact hypothesis have been proposed. Nevertheless, all are based around a catastrophic collision of planetary scale.
The multiplicity of craters on the surface of the Moon, the Maria (large depressions filled with lava flows associated with impact basins) covering 1/6 of the total Moon’s surface, and the formation of the Moon itself, have shown us that the late stages of planet formation were very violent. And we cannot say that the danger is completely over; because, although the impact rates have been drastically reduced, there are still objects that can impact our Earth-Moon system with catastrophic consequences for life on our planet.
The question about the origin of the Moon is still open; and there are other several relevant issues that require in situ studies on the surface of our satellite, like the existence of important reservoirs of water ice in the shadowed crater at the Moon's poles. In the coming decades several space agencies will come back to the Moon in a new step of planetary exploration.