Moon

The Moon is Earth’s only natural satellite and ranks as the fifth-largest natural satellite in the Solar System. The Moon has been studied in detail in terms of its diameter, mass and surface features, making it a significant subject of research both for its geological structure and astronomical observations. This celestial body, located at an average distance of 384,400 kilometers from Earth, has been analyzed from numerous technical perspectives including its orbital motion, surface morphology, internal structure and origin.

Physical Properties

• Diameter (Equatorial): 3,474.8 km
• Mass: 7.342 × 10²² kg (1.23% of Earth’s mass)
• Density: 3,344 kg/m³
• Surface Gravity: 1.62 m/s² (approximately 16.6% of Earth’s)
• Escape Velocity: 2.38 km/s
• Albedo (Reflectivity): Average 0.12
• Surface Temperature:
• • Daytime: Approximately 127 °C
  • Nighttime: Approximately -173 °C

The Moon possesses a very thin layer of gas, technically termed an “exosphere,” with negligible atmospheric accumulation. As a result, there is no sound transmission, and surface temperatures vary dramatically between day and night.

Orbital and Rotational Motion

The Moon orbits Earth with a sidereal period of approximately 27.3 days. However, due to the motion of the Earth-Moon system around the Sun, its synodic period (the time from one new moon to the next) is 29.5 days. The Moon is gravitationally locked to Earth (tidally locked), meaning it always presents the same face toward Earth.

• Orbital Distance (Average): 384,400 km
• Orbital Eccentricity: 0.0549
• Orbital Inclination (relative to the ecliptic): 5.145°
• Rotation Period: 27 days 7 hours 43 minutes

This gravitational locking results in Earth observing only one side of the Moon, except for minor oscillations known as librations.

Internal Structure

The Moon’s internal structure has been modeled using seismic data and samples returned by the Apollo missions. It generally consists of three layers:

• Crust: Average thickness of 50 km. Composed primarily of anorthosite.
• Mantle: Extends to a depth of approximately 1,000 km. Contains igneous rocks.
• Core: The inner core is predominantly iron with small amounts of sulfur and nickel. It is partially molten and has a diameter of approximately 350 km.

The Moon has a weak magnetic field and lacks a permanent magnetosphere. However, localized, weak permanent magnetic anomalies have been detected on its surface.

Surface Features

The lunar surface is primarily composed of two major geographical features:

• Maria (Latin for “seas”): Dark, flat plains formed by ancient basaltic lava flows. Appear as dark patches when viewed from Earth.
• Terrae (Highlands): Older, elevated regions heavily cratered and heavily modified by impacts.

Craters

The Moon’s surface is largely shaped by meteorite impacts and is covered with thousands of craters. One of the largest impact basins is the South Pole–Aitken Basin, with a diameter of approximately 2,500 km and a depth of about 13 km.

Regolith

The layer of dust and fragmented rock covering the lunar surface is called regolith. It has formed through the effects of micrometeorite impacts, solar wind, and cosmic rays. Its thickness varies locally between 2 and 20 meters.

Geological Evolution

The Moon’s geological evolution is divided into four main periods:

1. Pre-Nectarian (4.5 – 3.9 billion years ago): Formation of the earliest crust.
2. Nectarian (3.9 – 3.8 billion years ago): Period of formation of large impact basins.
3. Imbrian (3.8 – 3.2 billion years ago): Period of volcanic activity forming the maria.
4. Erastosthenian and Copernican Periods (3.2 billion years ago to present): Characterized by declining volcanism and ongoing crater formation.

Origin Theory

The most widely accepted model for the Moon’s origin is the giant impact hypothesis. According to this hypothesis, approximately 4.5 billion years ago, a Mars-sized celestial body named Theia collided with the young Earth, and material ejected from this impact coalesced to form the Moon. This model successfully explains the Moon’s low iron content and its isotopic similarities to Earth.

Alternative hypotheses include:

• Co-accretion theory: Simultaneous formation with Earth.
• Fission theory: Separation from Earth due to rapid rotation.
• Capture theory: Formation elsewhere and subsequent capture by Earth’s gravity.

However, these models do not adequately match the chemical and isotopic evidence.

Observational Characteristics

The Moon is the second-brightest object in the sky. Its apparent brightness varies with its phases and can reach as high as -12.7 magnitude during full moon. The observable phases result from changing geometry between the Sun, Moon and Earth. Lunar and solar eclipses occur during these alignment events.

Human Exploration

The Apollo missions conducted between 1969 and 1972 resulted in six crewed landings on the lunar surface. A total of 382 kilograms of rock and regolith samples were returned to Earth, and geological measurements were made on the Moon. These data have provided answers to many questions regarding the Moon’s structure, origin and evolution.