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The earliest material found in the Solar System is dated to 4.5672±0.0006 bya;[166] therefore, it is inferred that the Earth must have been formed by accretion around this time. By 4.54±0.04 bya[24] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[167] The Moon formed shortly thereafter, about 4.53 bya.[168] The Moon's formation remains debated. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object dubbed Theia impacted with Earth.[169] The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth's mass,[170] it impacts with the Earth in a glancing blow,[171] and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth. |
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