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Module 2 - Planetary Formation and Differentiation

Giant impact hypothesis

The favored hypothesis for the origin of the Earth’s Moon is spectacular! Get out the popcorn…
In a previous section, we described how energy of an impacting body is transferred to its surroundings, creating light, sound, and possibly a crater and melting. But the Moon-forming impact was much, much bigger. The giant impact hypothesis posits that ~4.45 billion years ago, a Mars-sized proto-planet (called Theia) made a direct impact into proto-Earth, creating widespread melting and forming the Moon.

Supporting evidence for a Moon-forming giant impact includes:

Earth’s spin and the Moon's orbit have similar orientations
Lunar samples suggest the Moon once had a molten surface
The Moon has a relatively small core for its size.
The Moon has an overall lower density than the Earth.
The stable-isotope (oxygen) ratios (a sort of chemical fingerprint) of lunar and terrestrial rock are identical, implying a common origin. More simply stated, the Earth and Moon are made of the same basic stuff.
Giant collisions are consistent with leading theories for the formation of the solar system.

The giant impact model holds that the collision occurred roughly 30-50 million years after the formation of the Solar System. Upon collision, energetic mixing of Theia and the Earth would have been necessary for the Earth and Moon to share similar oxygen isotopic signatures. This implies very high temperatures, vaporization of silicate rock at the site of impact, and the formation of a global magma ocean. Theia’s core would have sunk into the Earth’s core, while most of Theia’s mantle was incorporated into the Earth’s mantle. A significant portion of Theia’s and the Earth’s mantle (~20% the original mass of Theia) would have been ejected into orbit. The ejected material eventually settled into an orbit and coalesced to form the Moon.

Magma Ocean Hypothesis

Both the Earth and the Moon were largely molten after the giant impact and had vigorous magma oceans. Reworking of the Earth’s surface by geologic activity has erased most evidence of this event. But the Moon still retains evidence of its early magma ocean….

Have you ever noticed that the surface of the Moon has light and dark-toned regions? The light-toned regions of the Moon make up the lunar highlands and are places where rocks have high abundances of the low-density mineral plagioclase feldspar. More specifically the lunar highland rocks contain over 80% of a calcium-rich plagioclase feldspar mineral called anorthite, and the rocks themselves are called anorthosite.

The lunar highlands are the most heavily cratered and oldest regions of the Moon. The presence of anorthosite in these regions is well explained by the lunar magma ocean hypothesis. This hypothesis suggests the Moon was largely molten soon after accretion and differentiation into its core, mantle and crust was driven by differences in density.

As the molten Moon cooled to space, magnesium and iron-rich minerals (olivine and pyroxene) began to crystallize from the magma ocean. Olivine and pyroxene were denser than the remaining magma and, thus, settled towards the core and formed the mantle.
As magnesium and iron-rich minerals were removed from the magma ocean, the composition of the remaining magma changed to become depleted in those elements.
The mineral plagioclase feldspar eventually became stable and began to crystallize. That mineral, however, was less dense than the magma and floated to the top of the magma ocean, producing a crust, seen today as the light-colored, anorthosite highlands on the lunar surface.

In honor of completing reading this module, we encourage you to go outside and take a look at the Moon!

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Lunar magma ocean hypothesis The lunar magma ocean hypothesis suggests the Moon was largely molten soon after accretion. As the Moon cooled to space, the dense magnesium and iron-rich minerals olivine and pyroxene began to crystallize from the magma ocean. Olivine and pyroxene were denser than the remaining magma and, thus, settled towards the core. As those minerals were removed from the magma ocean, the composition of the remaining magma evolved. The mineral plagioclase eventually became stable and began to crystallize. That mineral, however, was less dense than the magma and floated to the top of the magma ocean, producing the light-colored, anorthosite highlands seen on the lunar surface today. Image credit: Jennifer Rapp, LPI
Molten Moon
Lunar anorthosite
Full Moon
Giant impact hypothesis