The early stages of Earth and Moon formation are theorized to have occurred around 4.5 billion years ago as part of a process known as the Giant Impact Hypothesis. According to this hypothesis, a Mars-sized object called Theia collided with the young Earth, resulting in the ejection of debris that eventually coalesced to form the Moon. While scientists do not have direct observations of this period, they have developed models based on various lines of evidence.
During the early stages of formation, both the Earth and the Moon would have been surrounded by a disk of gas and dust called the protoplanetary disk. This disk was composed of remnants from the solar nebula, the cloud of gas and dust from which the entire solar system formed. Over time, the material in the disk began to clump together due to gravity, forming larger and larger objects called planetesimals.
As these planetesimals grew, they started to undergo more collisions and mergers, ultimately leading to the formation of protoplanets. The proto-Earth emerged as one of these protoplanets, while the impact between Earth and Theia resulted in the ejection of material that eventually formed the Moon.
In the early stages, the Earth would have been a hot, molten mass, constantly bombarded by other planetesimals and undergoing intense geological activity. The surface would have been dominated by volcanic activity, with frequent eruptions shaping the landscape. The atmosphere was likely composed primarily of gases like hydrogen, helium, methane, and ammonia, but it was gradually transformed as volcanic outgassing released water vapor, carbon dioxide, and other gases that contributed to the formation of Earth's early atmosphere.
As for the Moon, it would have started as a massive cloud of debris surrounding the proto-Earth after the impact. Over time, this debris began to come together due to gravitational forces, forming a molten Moon. The Moon's surface would have been covered in lava flows and impact craters, similar to the early Earth. As the Moon gradually cooled, the lava solidified, forming the Moon's crust.
It's important to note that the details of Earth and Moon formation are still actively studied, and ongoing research continues to refine our understanding of these early stages. The information provided here represents our current understanding based on scientific theories and models.