Giant-impact hypothesis

• 1. Historical Evolution Of Hypothesis

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  In 1898, George Darwin proposed that Earth and the Moon were once a single body. He suggested a molten Moon had been spun from Earth due to centrifugal forces. This idea became the dominant academic explanation for decades. Using Newtonian mechanics, he calculated that the Moon orbited much more closely in the past. It was drifting away from Earth over time. American and Soviet experiments later confirmed this drift using laser ranging targets placed on the Moon. However, Darwin's calculations could not resolve the mechanics required to trace the Moon back to Earth's surface.

  Reginald Aldworth Daly of Harvard University challenged Darwin's explanation in 1946. He postulated that an impact caused the Moon's creation rather than centrifugal forces. Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974. The idea was reintroduced there and published in Icarus in 1975 by William K. Hartmann and Donald R. Davis. Their models suggested satellite-sized bodies formed at the end of planet formation. These objects could collide with planets or be captured. They proposed one object might have collided with Earth. This collision would eject refractory, volatile-poor dust to coalesce into the Moon.

  Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor issued a challenge. They told fellow lunar scientists they had eighteen months to use Apollo data and computers to make up their minds. They stated no one should attend the conference unless they had something to say about the Moon's birth. At the 1984 conference held in Kona, Hawaii, the giant-impact hypothesis emerged as the most favored hypothesis.

• 2. Theia Protoplanet Dynamics

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  The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia. She gave birth to the Moon goddess Selene. English geochemist Alex N. Halliday initially proposed this designation in 2000. It has since become accepted in the scientific community. Modern theories suggest Theia was part of a population of Mars-sized bodies existing 4.5 billion years ago. Theia orbited in the L4 or L5 configuration presented by the Earth-Sun system. It tended to remain there until gravitational perturbations caused it to depart.

  Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia coalesced at the Lagrangian point relative to Earth. This orbit placed it about 60 degrees ahead or behind Earth. Two-dimensional computer models suggested stability would be affected when its growing mass exceeded approximately 10 percent of Earth's mass. Gravitational perturbations by planetesimals caused Theia to leave its stable location. Subsequent interactions with proto-Earth led to a collision between the two bodies.

  Evidence presented in 2008 suggests the collision occurred later than the accepted value of 4.53 Gya. It happened at approximately 4.48 Gya. A 2014 comparison of computer simulations with elemental abundance measurements indicated the collision occurred roughly 95 million years after Solar System formation. One possible explanation is that Theia originated in the outer Solar System, bringing molybdenum isotopic composition from there.

• 3. Collision Mechanics And Simulations

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  Astronomers think the collision between Earth and Theia happened around 4.4 to 4.45 billion years ago. This was about 0.1 billion years after the Solar System began to form. The impact would have been of moderate velocity in astronomical terms. Theia struck Earth at an oblique angle when Earth was nearly fully formed. Computer simulations suggest an initial impactor velocity below escape velocity at infinity. Speed increased as it approached to over 10 kilometers per second at impact. An impact angle of about 45 degrees was modeled.

  Oxygen isotope abundance in lunar rock suggests vigorous mixing of Theia and Earth. This indicates a steep impact angle rather than a glancing blow. Theia's iron core sank into the young Earth's core. Most of Theia's mantle accreted onto Earth's mantle. However, significant portions of mantle material from both bodies were ejected into orbit. Material ejected with velocities between orbital and escape velocity went into individual orbits around the Sun if higher velocities were reached.

  Modelling hypothesized that material in orbit accreted to form the Moon in three consecutive phases. Accretion first occurred from bodies initially present outside Earth's Roche limit. The inner disk slowly spread back out to Earth's Roche limit via resonant interactions. After several tens of years, the disk spread beyond the limit. It started producing new objects that continued Moon growth until the inner disk depleted mass after several hundreds of years. Estimates based on computer simulations suggest some twenty percent of Theia's original mass ended up as an orbiting ring of debris. About half of this matter coalesced into the Moon.

• 4. Isotopic Evidence And Composition

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  In 2001, a team at the Carnegie Institution of Washington reported Apollo program rocks carried an isotopic signature identical to Earth rocks. These samples differed from almost all other bodies in the Solar System. In 2014, a team in Germany reported Apollo samples had a slightly different isotopic signature from Earth rocks. The difference was slight but statistically significant. One possible explanation is that Theia formed near Earth.

  This empirical data showing close similarity can be explained only by the standard giant-impact hypothesis. It is extremely unlikely two bodies prior to collision had such similar composition. In 2007, researchers from the California Institute of Technology showed the likelihood of Theia having an identical isotopic signature as Earth was very small. They proposed less than one percent probability. They suggested Earth and proto-lunar disc were connected by a common silicate vapor atmosphere while molten and vaporized. The Earth-Moon system became homogenized by convective stirring while existing as a continuous fluid.

  For this equilibration scenario to be viable, the proto-lunar disc would have to endure for about 100 years. Work continues to determine if this duration is possible. Comparison of zinc isotopic composition between lunar samples and Earth or Mars rocks provides further evidence. Moon rocks contain more heavy isotopes of zinc and overall less zinc. This depletion through evaporation matches expectations for the giant impact origin.

• 5. Alternative Formation Models

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  Other mechanisms suggested at various times include the Moon spinning off from Earth's molten surface by centrifugal force. Some theories propose it formed elsewhere and was subsequently captured by Earth's gravitational field. Another model suggests Earth and Moon formed simultaneously from the same accretion disk. None of these hypotheses can account for the high angular momentum of the Earth-Moon system.

  Another hypothesis attributes formation to the impact of a large asteroid with Earth much later than previously thought. It creates the satellite primarily from debris from Earth. In this scenario, formation occurs 60 to 140 million years after Solar System formation. The asteroid impact would create a magma ocean on both bodies sharing a common plasma metal vapor atmosphere. A shared metal vapor bridge allowed material exchange into a more common composition.

  Yet another hypothesis proposes Earth and Moon formed together not from collision of once-distant bodies. Robin M. Canup published this model in 2012. It suggests Moon and Earth formed from a massive collision of two planetary bodies each larger than Mars. They then re-collided to form what is now called Earth. After re-collision, Earth was surrounded by a disk of material which accreted to form the Moon.

• 6. Current Scientific Challenges

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  This lunar origin hypothesis has some difficulties that have yet to be resolved. For example, the giant-impact hypothesis implies a surface magma ocean would have formed following the impact. Yet there is no evidence that Earth ever had such a magma ocean. It is likely there exists material that has never been processed in a magma ocean.

  A number of compositional inconsistencies need to be addressed. The ratios of the Moon's volatile elements are not explained by the giant-impact hypothesis. If correct, these ratios must be due to some other cause. The presence of volatiles such as water trapped in lunar basalts and carbon emissions from the lunar surface is difficult to explain if caused by high-temperature impact. The iron oxide content of the Moon at 13 percent rules out most source material from Earth's mantle.

  If bulk proto-lunar material came from an impactor, the Moon should be enriched in siderophilic elements. In fact, it is deficient in them. The Moon's oxygen isotopic ratios are essentially identical to those of Earth. Oxygen isotopic ratios yield a unique signature for each Solar System body. If a separate proto-planet Theia existed, it probably would have had a different oxygen isotopic signature than Earth. The Moon's titanium isotope ratio appears so close to Earth's within four parts per million that little colliding body mass could likely have been part of the Moon.