Giant impact hypothesis facts for kids
The giant impact hypothesis is that the Moon was created out of the debris from a collision between the young Earth and a Mars-sized protoplanet. This is the favoured scientific hypothesis for the formation of the Moon.
Evidence that supports this hypothesis include:
- The Moon's orbit has a similar orientation to Earth's rotation, both of which are at a similar angle to the ecliptic plane of the Solar System.
- The stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.
- The Earth–Moon system contains an anomalously high angular momentum, meaning the momentum contained in Earth's rotation, the Moon's rotation and the Moon revolving around Earth is significantly higher than the other terrestrial planets. A giant impact might have supplied this excess momentum.
- Moon samples indicate that the Moon was once molten to a substantial, but unknown, depth. This might have required much more energy than predicted to be available from the accretion of a celestial body of the Moon's size and mass. An extremely energetic process, such as a giant impact, could provide this energy.
- The Moon has a relatively small iron core, which gives it a much lower density than Earth. Computer models of a giant impact of a Mars-sized body with Earth indicate the impactor's core would likely penetrate deep into Earth and fuse with its own core. This would leave the Moon, which was formed from the ejecta of lighter crust and mantle fragments that went beyond the Roche limit and were not pulled back by gravity to re-fuse with Earth, with less remaining metallic iron than other planetary bodies.
- The Moon is depleted in volatile elements compared to Earth. Vaporizing at comparably lower temperatures, they could be lost in a high-energy event, with the Moon's smaller gravity unable to recapture them while Earth did.
- There is evidence in other star systems of similar collisions, resulting in debris discs.
- Giant collisions are consistent with the leading theory of the formation of the Solar System.
The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.
There are several unanswered issues with this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth, 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 (chemical elements which bond with iron), and there is no evidence that the Earth ever had the magma ocean implied by the hypothesis.
Context
The Earth's relatively large natural satellite, the Moon, is unique. During the Apollo program, rocks from the Moon's surface were brought to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. New evidence suggests the Moon formed even later, 4.48±0.02 Ga, or 70–110 Ma after the start of the Solar System. Another notable feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, which other terrestrial bodies in the solar system have. The Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis: the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet.
The impactor, sometimes called Theia, is thought to have been a little smaller than the planet Mars. Theia collided with Earth about 4.533 Ga. Models reveal that when an impactor this size struck the proto-Earth at a low angle and relatively low speed (8–20 km/s or 5.0–12.4 mi/s), much material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon.
However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.
The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy elements untouched.
Consequences
The impact had some important consequences for the young Earth. It released an enormous amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. The planet's first atmosphere must have been completely blown away by the enormous amount of energy released. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.
Recent evidence
Analysis of lunar rock brought back by Apollo astronauts appears to show traces of Theia. The researchers claim this confirms the theory that the Moon was created by a cataclysmic collision. Some scientists are surprised that the difference between the Theian material found in the Moon rock and the Earth is so small.
Comparison of the zinc isotopic composition of lunar samples with that of Earth and Mars rocks provides further evidence for the impact hypothesis. Zinc is strongly fractionated when volatilised in planetary rocks, but not during normal igneous processes, so zinc abundance and isotopic composition can distinguish the two geological processes. Moon rocks contain more heavy isotopes of zinc, and overall less zinc, than corresponding igneous Earth or Mars rocks, which is consistent with zinc being depleted from the Moon through evaporation, as expected for the giant impact origin.
Collisions between ejecta escaping Earth's gravity and asteroids would have left impact heating signatures in stony meteorites; analysis based on assuming the existence of this effect has been used to date the impact event to 4.47 billion years ago, in agreement with the date obtained by other means.
On 1 November 2023, scientists reported that, according to computer simulations, remnants of Theia could be still visible inside the Earth as two giant anomalies of the Earth's mantle.
Images for kids
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Simplistic representation of the giant-impact hypothesis.
See also
In Spanish: Teoría del gran impacto para niños
