Moon

The Moon is Earth's only natural satellite and ranks as the fifth-largest natural satellite in the Solar System. The Moon, which has been extensively studied in terms of diameter, mass, and surface features, is a significant subject of research both geologically and astronomically. Located at an average distance of 384,400 kilometers from Earth, this celestial body has been analyzed from many technical perspectives, including its orbital motions, surface morphology, internal structure, and origin.

The Moon (NASA)

Physical Characteristics

• Equatorial Diameter: 3,474.8 km
• Mass: 7.342 × 10²² kg (1.23% of Earth's)
• 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 gaseous layer with extremely low atmospheric accumulation, technically classified as an “exosphere.” As a result, sound cannot propagate, and surface temperatures vary drastically between day and night.

Orbital and Rotational Motion

The Moon orbits Earth in approximately 27.3 days. However, due to the Earth-Moon system’s orbit around the Sun, the synodic period (time from one New Moon to the next) is 29.5 days. The Moon is gravitationally locked to Earth (tidal locking), which means it always shows the same face to Earth.

• Average Orbital Distance: 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 causes only one side of the Moon to be visible from Earth, excluding minor oscillations known as libration.

Internal Structure

The internal structure of the Moon has been modeled using seismic data and samples brought back by the Apollo missions. It consists of three main layers:

• Crust: Average thickness of 50 km, composed mainly of anorthositic materials.
• Mantle: Extends to a depth of about 1,000 km and contains rocks of magmatic origin.
• Core: The inner core is mostly composed of iron with small amounts of sulfur/nickel, partially molten, and about 350 km in diameter.

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

Surface Features

The surface of the Moon consists mainly of two primary geographic formations:

• Maria (Latin for "seas"): Dark, basaltic plains formed by ancient lava flows. These appear as dark spots when viewed from Earth.
• Terrae (Highlands): Older, elevated regions heavily cratered by impacts.

Craters

The lunar surface has been extensively shaped by meteorite impacts and is covered in thousands of craters. One of the largest impact basins is the South Pole–Aitken Basin, with a diameter of about 2,500 km and a depth of approximately 13 km.

AI-Generated Visualization of Craters on the Lunar Surface

Regolith

The Moon's surface is covered by regolith, a mixture of dust and fragmented rock. This layer formed through micrometeorite impacts, solar wind, and cosmic rays. Its thickness varies between 2 and 20 meters in different locations.

AI-Generated Visualization of Lunar Regolith

Geological Evolution

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

• Pre-Nectarian (4.5 – 3.9 billion years ago): Formation of the oldest crust.
• Nectarian (3.9 – 3.8 billion years ago): Formation of large impact basins.
• Imbrian (3.8 – 3.2 billion years ago): Volcanic activity led to the formation of Maria.
• Eratosthenian and Copernican Periods (3.2 billion years ago to present): Characterized by decreasing volcanism and the formation of new craters.

Formation Theory

The most widely accepted model of the Moon’s origin is the giant impact hypothesis. According to this theory, about 4.5 billion years ago, a Mars-sized celestial body (Theia) collided with the young Earth, and the Moon formed from the debris ejected by this impact. This model successfully explains the Moon’s low iron content and isotopic similarities to Earth.

Alternative hypotheses include:

• Co-accretion theory: Simultaneous formation with Earth.
• Fission theory: Separation from Earth.
• Capture theory: The Moon formed elsewhere and was captured by Earth.

However, these models do not align as well with chemical and isotopic evidence.

Observational Characteristics

The Moon is the second-brightest object in the sky. Its apparent brightness varies according to its phases, reaching up to -12.7 magnitude at full Moon. Observable phases result from the changing geometry between the Sun, Moon, and Earth throughout the synodic period. Lunar and solar eclipses occur during these alignments.

Human Exploration

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

Buzz Aldrin Standing Next to the U.S. Flag on the Moon (NASA)