The Moon is the only natural satellite of Earth, yet it is a planetary-mass object that defies simple categorization. It orbits at an average distance of approximately 384,400 kilometers, a span roughly 30 times the width of Earth itself. This celestial body completes a synodic cycle, known as the lunar month, every 29.5 days, synchronizing its rotation with its orbit to always present the same face to our planet. While it appears as a static backdrop to human history, the Moon is a dynamic world with a differentiated interior, a solid iron-rich inner core, and a fluid outer core. Its surface gravity is one-sixth that of Earth, allowing astronauts to leap with ease, yet it is strong enough to hold a tenuous exosphere and influence the tides of our oceans. The Moon is the fifth-largest moon in the Solar System, but it is the largest relative to its parent planet, making it a unique partner in a double-planet system that has shaped the history of life on Earth.
The Giant Impact
The prevailing theory for the Moon's origin involves a catastrophic collision between the proto-Earth and a Mars-sized body named Theia approximately 4.51 billion years ago. This giant impact blasted material into orbit around Earth, which then accreted to form the Moon just beyond the Earth's Roche limit. Computer simulations suggest that most of the Moon derived from the impactor rather than the proto-Earth, though models from 2007 and later indicate a larger fraction came from Earth. The impact released enough energy to liquefy both the ejecta and Earth's crust, forming a magma ocean that would eventually crystallize into the Moon's current structure. Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System, and the isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of vaporized material. On the 1st of November 2023, scientists reported that remnants of Theia could still be present inside the Earth, hidden within the deep mantle.
Volcanic Seas
The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters to form the dark plains of basalt called maria. These maria, which mean 'seas' in Latin, cover 31% of the surface of the near side compared with only 2% of the far side. The majority of these lava deposits erupted during the Imbrian period, between 3.3 and 3.7 billion years ago, though some are as young as 1.2 billion years and some as old as 4.2 billion years. The distribution of the mare basalts is uneven, with the basalts predominantly appearing on the Moon's near-side hemisphere, likely due to the relative thinness of the crust on that side. In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that appeared to be only 2 million years old, suggesting recent lunar activity. Evidence has been found for 2 to 10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin, raising the possibility of a much warmer lunar mantle than previously believed.
The far side of the Moon is on average about 2 kilometers higher than that of the near side, creating a stark topographic contrast. The most extensive topographic feature is the giant far-side South Pole-Aitken basin, some 2,500 kilometers in diameter, which is the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At 8.2 kilometers deep, its floor is the lowest point on the surface of the Moon, reaching 9 kilometers below the mean radius in a crater within Antoniadi crater. The highest elevations of the Moon's surface, with the so-called Selenean summit at 10.7 kilometers, are located directly to the northeast. The far side is also home to the South Pole-Aitken Terrane, which is marked as the largest circle on maps of the lunar surface. This region is distinct from the KREEP rich magma terrane found on the near side, and the concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side.
The Dusty Surface
Blanketed on top of the Moon's crust is a highly comminuted and impact gardened mostly gray surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces, varying in thickness from 3 to 5 meters in the highlands and 1 to 2 meters in the maria. Lunar dust is highly abrasive and can cause damage to human lungs and nervous and cardiovascular systems. Ionizing radiation from cosmic rays, their resulting neutron radiation, and the Sun results in an average radiation level of 1.369 millisieverts per day during lunar daytime, which is about 2.6 times more than the level on the International Space Station. A permanent Moon dust cloud exists around the Moon, generated by small particles from comets, with 5 tons of comet particles estimated to strike the Moon's surface every 24 hours.
The Water Ice
Liquid water cannot persist on the lunar surface, but since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks and hydrogen from solar wind. In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl in reflected sunlight, providing evidence of large quantities of water ice on the lunar surface. In 2009, LCROSS sent an impactor into a permanently shadowed polar crater and detected at least 100 kilograms of water in a plume of ejected material. Analysis of the findings of the Moon Mineralogy Mapper revealed in August 2018 for the first time definitive evidence for water-ice on the lunar surface, with ice deposits found on the North and South poles, although it is more abundant in the South.
The Human Footprint
The first spaceflights to an extraterrestrial body were to the Moon, starting in 1959 with the flyby of Luna 1, sent by the Soviet Union, and the intentional impact of Luna 2. In 1966, the first soft landing was achieved by Luna 9, followed by orbital insertion by Luna 10. Humans first arrived in orbit on the 24th of December 1968, with Apollo 8, sent by the United States, and then on the surface on the 20th of July 1969, with Apollo 11. By 1972, six Apollo missions had landed twelve humans on the Moon and stayed up to three days. Astronaut John Young was seen jumping on the Moon, illustrating that the gravitational pull of the Moon is approximately one-sixth of Earth's. The lunar surface is exposed to temperature differences ranging from minus 173 degrees Celsius to plus 127 degrees Celsius depending on the solar irradiance. Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed, with the Lunar Reconnaissance Orbiter measuring the lowest summer temperatures in craters at the southern pole at minus 238 degrees Celsius.
The Future Orbit
In 5 billion years the Moon will have wandered 40% further away from Earth than it is now, but at a similar time the Sun will have become a red giant. Assuming the Sun envelopes the Earth-Moon system, the consequent drag from the Sun's atmosphere may cause the orbital distance between the Earth and the Moon to decay to the point where the Moon comes within the Earth's Roche limit, leading it to disintegrate. Measurements from laser reflectors left during the Apollo missions have found that the Moon's distance increases by 3.8 centimeters per year, roughly the rate at which human fingernails grow. Atomic clocks show that Earth's Day lengthens by about 17 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds. The Moon's orbit is slightly elliptical, with an orbital eccentricity of 0.055, and the semi-major axis of the geocentric lunar orbit is approximately 400,000 kilometers. The Moon makes a complete orbit around Earth with respect to the fixed stars, its sidereal period, about once every 27.3 days.