Giant Impact Hypothesis


The giant impact hypothesis proposes that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. This is the favoured scientific hypothesis for the formation of the Moon. Evidence for this hypothesis includes Moon samples which indicate the surface of the Moon was once molten, the Moon’s apparently relatively small iron core and a lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disks). The colliding body is sometimes called Theia (or Euryphaessa) for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.

There remain several unanswered issues surrounding this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth’s, with no evidence of a contribution from another solar body. Also, lunar samples do not have expected ratios of volatile elements, iron oxide, or siderophilic elements, and there is no evidence to suggest that the Earth ever had the magma ocean implied by this hypothesis.

Origins

In 1898, George Howard Darwin made an early suggestion that the Earth and Moon had once been one body. Darwin’s hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation. Using Newtonian mechanics, he calculated that the Moon had actually orbited much closer in the past and was drifting away from the Earth. This drifting was later confirmed by American and Soviet experiments using laser ranging targets placed on the Moon.

However, Darwin’s calculations could not resolve the mechanics required to trace the Moon backwards to the surface of the Earth. In 1946, Reginald Aldworth Daly of Harvard University challenged Darwin’s explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces. Little attention was paid to Professor Daly’s challenge until a conference on satellites in 1974 where it was reintroduced. It was then republished in Icarus in 1975 by Drs. William K. Hartmann and Donald R. Davis. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects may have collided with the Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. This collision could help explain the unique geological properties of the Moon.

A similar approach was taken by Alfred G. W. Cameron and William Ward, who suggested that the Moon was formed by the tangential impact of a body the size of Mars. The outer silicates of the colliding body would mostly be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision would probably escape the Solar System, whereas silicates would tend to coalesce.

Theia

The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, who gave birth to the Moon goddess Selene. According to the giant impact hypothesis, Theia formed alongside the other planet size bodies in the Solar System about 4.6 Ga (4.6 billion years ago), and was approximately the size of Mars.

One formation theory is that Theia materialized at the L4 or L5 Lagrangian points relative to Earth (in about the same orbit and about 60° ahead or behind), similar to a trojan asteroid. The stability of Theia’s orbit was affected when its growing mass exceeded a threshold of about 10% of the Earth’s mass. Gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth caused the two bodies to collide.

Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the rest of the Solar System formed. However, evidence presented in 2008 suggests that the collision may have occurred later, at about 4.48 Ga.

Impact

In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this “late-impact” scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s. Theia’s iron core sank into the young Earth’s core, as most of Theia’s mantle and a significant portion of the Earth’s mantle and crust were ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulations of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. The Earth would have gained significant amounts of angular momentum and mass from such a collision. Regardless of the rotation and inclination the Earth had before the impact, it would have had a day some five hours long after the impact, and the Earth’s equator would have shifted closer to the plane of the Moon’s orbit.

It has been suggested that other significant objects may have been created by the impact, which could have remained in orbit between the Earth and Moon, stuck in Lagrangian points. Such objects may have stayed within the Earth-Moon system for up to 100 million years, until the gravitational tugs of other planets destabilized the system enough to free the objects.

Evidence

Indirect evidence for this impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios identical to those of Earth. The highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, gave rise to the idea that a large portion of the Moon was once molten, and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean. Several lines of evidence show that if the Moon has an iron-rich core, it must be small. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the radius of the core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. Impact conditions can be found that give rise to a Moon that formed mostly from the mantles of the Earth and impactor, with the core of the impactor accreting to the Earth, and which satisfy the angular momentum constraints of the Earth-Moon system.

Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/sec) impacts between rocky bodies has been detected around the nearby (29 pc distant) young (~12 My old) Beta Pic Moving Group star HD172555 by the Spitzer Space Telescope.A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades cluster appears similar to the predicted results of Theia’s collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other. This is similar to another belt of warm dust detected around the star BD +20°307 (HIP 8920, SAO 75016).

Difficulties

This lunar origin hypothesis has some difficulties which have yet to be resolved. These difficulties include:

  • The ratios of the Moon’s volatile elements are not explained by the giant impact hypothesis. If the giant impact hypothesis is correct, they must be due to some other cause.
  • There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis), and it is likely there exists material which has never been processed by a magma ocean.
  • The iron oxide (FeO) content (13%) of the Moon, which is intermediate between Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from the Earth’s mantle.
  • If the bulk of the proto-lunar material had come from the impactor, the Moon should be enriched in siderophilic elements, when it is actually deficient in those.
  • The presence of volatiles such as water trapped in lunar basalts is more difficult to explain if the impact caused a catastrophic heating event.
  • The Moon’s oxygen isotopic ratios are essentially identical to those of Earth. Oxygen isotopic ratios, which can be measured very precisely, yield a unique and distinct signature for each solar system body. If Theia had been a separate proto-planet, it would probably have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.

Alternate hypotheses

Other mechanisms which have been suggested at various times for the Moon’s origin are that the Moon was spun off of the Earth’s molten, blobular surface by centrifugal force,that it was formed elsewhere and later captured by the Earth’s gravitational field, and that the Moon formed at the same time and place as the Earth from the same accretion disk. Each of these hypotheses is claimed to lack a mechanism to account for the high angular momentum of the Earth–Moon system.

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