The Moon is Earth’s only natural satellite and is the fifth largest moon in the solar system.
It is about one-quarter the diameter of Earth, making it unusually large compared to its planet.
The Moon’s diameter is approximately 3,470 kilometers, and it orbits Earth at an average distance of about 380,000 kilometers.
Despite its apparent size in the sky, the Moon is much smaller than it seems; it would appear about the same size as a small pill held at arm’s length.
The Moon’s internal structure is similar to Earth’s, with a solid iron inner core, a liquid outer core, a thick mantle, and a crust of lighter material. However, the Moon’s core is much smaller (about 350 km in radius) and not as hot as Earth’s. The lower mantle may be fluid, but the upper mantle is solid.
Surface Features
The Moon’s surface is divided into two main types of regions on the near side: the highlands and the maria.
Highlands: Heavily cratered, lighter areas that are over 4 billion years old, formed by countless impacts from asteroids and comets.
Maria: Darker, smoother plains formed by ancient lava flows. These are younger than the highlands, with rocks dating to about 3–3.5 billion years old.
The far side of the Moon, first photographed in 1959 by the Soviet Luna 3 probe, is very different from the near side. It has a much thicker crust and almost no maria, unlike the near side which is covered with them.
The Moon’s surface displays a variety of features:
Simple bowl-shaped craters and large, complex craters with central peaks.
Double craters, likely formed by binary asteroids.
Crater chains, possibly created by debris from large impacts.
Rays: bright streaks of ejected material radiating from large craters, such as those from Tycho, which can stretch up to 1,500 kilometers.
Long, winding rilles that resemble dry riverbeds, formed by ancient lava flows.
Lava tubes, where the surface of a lava flow solidified, creating tunnels; sometimes the roof collapses, forming skylights.
Cliffs, mounds, ancient volcanoes, and mountain chains, often formed at the rims of giant impact craters.
There is no tectonic activity on the Moon, but mountain ranges exist due to the forces from massive impacts.
Differences Between the Near and Far Sides
The near side, visible from Earth, has many maria and a thinner crust.
The far side has a much thicker crust and very few maria.
These differences are thought to be related to the Moon’s formation and the effects of Earth’s heat on the Moon’s near side after its creation.
Formation and Evolution of the Moon
The leading theory for the Moon’s origin is the Giant Impact hypothesis:
About 4.5 billion years ago, a Mars-sized object (named Theia) collided with the young Earth in a grazing impact.
The collision ejected material, mostly from Earth’s outer layers, into space. This debris quickly coalesced to form the Moon.
The Moon’s composition is similar to Earth’s crust but not identical, as some material came from Theia.
Evidence for this includes the ratios of oxygen isotopes found in lunar rocks.
After the impact, both Earth and the Moon were molten. The Moon formed close to Earth (about 20,000 km away).
Tidal forces quickly synchronized the Moon’s rotation and orbit, so one side always faces Earth.
The hot, molten Earth radiated intense heat, especially onto the Moon’s near side, causing it to be hotter and resulting in a thinner crust. The far side, being cooler, developed a thicker crust as vaporized material condensed there.
The Late Heavy Bombardment, a period of intense impacts from comets and asteroids, created most of the craters in the highlands. Later, lava flooded some of these craters, forming the maria.
The thicker crust on the far side made it harder for lava to reach the surface, explaining the lack of maria there.
Theories about the Moon’s crustal differences are still being debated, and new evidence continues to shape our understanding.
Moon Illusion and Human Perception
The Moon often appears much larger when it is near the horizon than when it is high in the sky, a phenomenon known as the Moon Illusion.
This illusion is caused by the way our brains perceive the sky: we see the horizon as farther away than the zenith, so the Moon seems larger when it is low.
In reality, the Moon’s size in the sky does not change; it is always about the same apparent size.
Water on the Moon
Water ice has been discovered in deep, permanently shadowed craters near the Moon’s poles.
These craters never receive sunlight, allowing water delivered by comet impacts to accumulate and persist as ice.
Estimates suggest there could be over a billion tons of water ice on the Moon.
This water could be a valuable resource for future lunar exploration and colonization, as transporting water from Earth is costly.
Key Terms & Definitions
Maria: Dark, flat plains on the Moon formed by ancient lava flows.
Highlands: Older, heavily cratered regions of the Moon’s surface.
Giant Impact hypothesis: The theory that the Moon formed from debris after a Mars-sized object struck Earth.
Tidal Locking: The phenomenon where one side of the Moon always faces Earth due to synchronized rotation and orbit.
Rilles: Long, winding channels on the Moon’s surface formed by ancient lava flows.
Rays: Bright streaks of ejected material radiating from large impact craters.