The Moon, also known as Luna, is Earth's only natural satellite, with a diameter approximately one-fourth that of Earth and a mass about 1/81 of Earth's. It is the largest satellite in volume and mass relative to its planet and the fifth largest in the solar system.

Additionally, it is the second densest satellite in the solar system, surpassed only by Jupiter's moon Io.

In scientific terms, the Moon is Earth's sole natural satellite, orbiting around Earth tirelessly. For over 4 billion years, it has never strayed far from Earth's side, serving as Earth's steadfast companion.

The origin and evolution of the Moon have long fascinated humanity as fundamental questions in natural science. Over the past century, various hypotheses regarding the Moon's origin and evolution have emerged, yet consensus remains elusive.

Debates surrounding lunar formation theories focus on whether the Moon formed independently, akin to Earth, through the accretion and aggregation of material in the solar nebula or if it originated as material ejected from Earth, either during its early formation or via later capture.

Any hypothesis concerning the Moon's origin must account for several key facts: Earth's sole satellite, its orbit revolves around the center of mass of the Earth-Moon system, and its orbital plane does not align with Earth's equatorial plane. The Moon's mass is approximately 1/81 of Earth's, with an average density of 3.34 grams per cubic centimeter, about 60% of Earth's average density.

The Moon's composition differs significantly from Earth's, being richer in refractory elements but poorer in volatile and siderophile elements. It is also drier and more reducing than Earth.

The Moon exhibits a layered structure of core, mantle, and crust. The age of lunar surface rocks generally exceeds 3.1 billion years, indicating that lunar evolution primarily occurred within the first 1.5 billion years of its existence. Today, the Moon is a largely dormant celestial body nearing thermal equilibrium.

The prevailing hypothesis for the Moon's formation is the Giant Impact hypothesis. Approximately 4.5 billion years ago, a Mars-sized body, Theia, collided with a young Earth, ejecting material (primarily mantle) into space, eventually merging to form the Moon.

Evidence supporting the Giant Impact hypothesis includes similarities in oxygen isotopic compositions between Moon rocks and Earth's mantle, as revealed by samples returned by the Apollo missions.

Recently, researchers from ETH Zurich discovered rare gases—helium and neon—from Earth's mantle in lunar meteorites, further supporting the Giant Impact hypothesis. Their findings were published in Science Advances titled "Indigenous noble gases in the Moon's interior."

While the Giant Impact hypothesis may not be flawless, it currently stands as the most comprehensive explanation for the Moon's formation, awaiting further evidence for refinement.