Moon Size, Structure, Formation, Anomalies, and Future Missions

The Moon is the fifth‑largest moon in the solar system and measures about 3,470 km in diameter, roughly a quarter of Earth’s diameter. It orbits at an average distance of 380,000 km, making it appear about the size of a small pill held at arm’s length. The “Moon Illusion” occurs because the brain treats the sky as a flattened bowl rather than a perfect hemisphere; objects on the horizon are perceived as farther away, so the brain interprets the Moon as physically larger even though its angular size is unchanged.

Internal and Surface Structure

Inside the Moon lies a solid iron inner core about 350 km in radius, surrounded by a liquid outer core, a thick mantle, and a crust. The near side is split between heavily cratered highlands older than four billion years and smoother, darker maria that solidified 3–3.5 billion years ago. The far side, first photographed by Luna 3 in 1959, lacks extensive maria and possesses a thicker crust, a difference linked to early thermal processes.

Formation and Evolution

The Giant Impact hypothesis posits that a Mars‑sized body named Theia struck Earth in a grazing collision roughly 4.5 billion years ago, about 50 million years after the solar system formed. Debris from both Earth and Theia coalesced into the Moon, which was initially molten and quickly became tidally locked to Earth. Heat from the impact likely vaporized material on the near side, allowing it to condense on the cooler far side and create the observed crustal thickness disparity. Later, during the Late Heavy Bombardment, intense comet and asteroid impacts carved the highland craters we see today.

Surface Features and Resources

Lunar craters appear in several forms: simple bowl shapes, central‑peak craters formed by splash‑back, double craters from binary asteroids, and crater chains. Bright rays, such as those extending 1,500 km from Tycho crater, are ejecta that traveled great distances. Volcanic remnants include sinuous rilles—ancient lava channels—lava tubes, and skylights that hint at subsurface voids. Polar craters that remain in permanent shadow may hold over a billion tons of water ice, a resource of great interest for future exploration.

Future Prospects

International agencies, including NASA, are planning renewed human missions to the Moon. The presence of water ice in permanently shadowed regions makes the lunar surface a potential staging ground for deeper space travel. As the Moon continues to be studied, its unique geological history and accessible resources keep it at the forefront of scientific and exploratory priorities.