Theia (planet) in the context of "Moon"

The natural history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution.

The geological time scale (GTS), as defined by international convention, depicts the large spans of time from the beginning of Earth to the present, and its divisions chronicle some definitive events of Earth history. Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While Earth was in its earliest stage (Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface.

The origin of the Moon is usually explained by a Mars-sized body, known as Theia, striking the Earth, creating a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be formed. Other proposed scenarios include captured body, fission, formed together (accretion, synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories.

The standard giant-impact hypothesis suggests that a Mars-sized body called Theia impacted the proto-Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23.5° tilted axis of the Earth, thus causing the seasons. The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature than proto-Earth, as would the ejected mixed material. Also, the Moon's titanium isotope ratio (Ti/Ti) appears so close to the Earth's (within 4 parts per million) that little if any of the colliding body's mass could have been part of the Moon.

The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Proto-Earth collided with a Mars-sized co-orbital protoplanet likely from the L₄ or L₅ Lagrange points of the Earth's orbit approximately 4.5 billion years ago in the early Hadean eon (about 20 to 100 million years after the Solar System formed), and some of the ejected debris from the impact event later re-accreted to form the Moon. The impactor planet is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies.

Early Earth, also known as Proto-Earth, is loosely defined as Earth in the first one billion years — or gigayear (10 y or Ga) — of its geological history, from its initial formation in the young Solar System at about 4.55 billion years ago (Gya), to the end of the Eoarchean era at approximately 3.5 Gya. On the geologic time scale, this comprises all of the Hadean eon and approximately one-third of the Archean eon, starting with the formation of the Earth about 4.6 Gya, and ended at the start of the Paleoarchean era 3.6 Gya.

This period of Earth's history involved the planet's formation from the solar nebula via a process known as accretion, and transition of the Earth's atmosphere from a hydrogen/helium-predominant primary atmosphere collected from the protoplanetary disk to a reductant secondary atmosphere rich in nitrogen, methane and CO₂. This time period included intense impact events as the young Proto-Earth, a protoplanet of about 0.63 Earth masses, began clearing the neighborhood, including the early Moon-forming collision with Theia — a Mars-sized co-orbital planet likely perturbed from the L₄ Lagrange point — around 0.032 Ga after formation of the Solar System, which resulted in a series of magma oceans and episodes of core formation. After formation of the core, meteorites or comets from the Outer Solar System might have delivered water and other volatile compounds to the Earth's mantle, crust and ancient atmosphere in an intense "late veneer" bombardment. As the Earth's planetary surface eventually cooled and formed a stable but evolving crust during the end-Hadean, most of the water vapor condensed out of the atmosphere and precipitated into a superocean that covered nearly all of the Earth's surface, transforming the initially lava planet Earth of the Hadean into an ocean planet at the early Archean, where the earliest known life forms appeared soon afterwards.

The Moon is the only natural satellite of Earth. It orbits around Earth at an average distance of 384,399 kilometres (238,854 mi), a distance roughly 30 times the width of Earth. It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on the sides facing each other. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month).

In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is larger and more massive than any known dwarf planet, and the fifth-largest and fifth-most massive moon, as well as the largest and most massive in relation to its parent planet. Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with only a minuscule hydrosphere, atmosphere, and magnetic field. The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters, with some younger ones featuring bright ray-like streaks. The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters, which through cooling formed the today prominently visible dark plains of basalt called maria ('seas'). The origin of the Moon is not clear, although it has been hypothesized to have formed out of material from Earth, ejected by a giant impact into Earth of a Mars-sized body named Theia 4.51 billion years ago, not long after Earth's formation.