The Moon has a 4.5-billion-year history shaped by a catastrophic birth, relentless bombardment, volcanic eruptions, and, most recently, human visitors. From its violent formation to the latest robotic sample returns, here’s what has happened on the Moon and what we’ve learned about it.
A Violent Birth
The Moon formed roughly 4.5 billion years ago when a Mars-sized protoplanet, often called Theia, slammed into the young Earth. The collision blasted an enormous cloud of debris into orbit, and that material eventually clumped together to form the Moon. This “giant impact” hypothesis is the leading explanation for the Moon’s origin, supported by decades of computer simulations and a critical piece of chemical evidence: Moon rocks and Earth rocks share nearly identical oxygen isotope signatures, meaning they came from the same pool of material.
The impact also explains some of the Moon’s quirks. It is unusually large compared to its host planet, and the Earth-Moon system carries an unusual amount of rotational momentum, both of which fit neatly with a giant collision scenario. Compared to Earth, however, the Moon ended up with far less iron and far fewer volatile elements, suggesting lighter surface material was preferentially launched into orbit while heavier metals stayed behind in Earth’s core.
Billions of Years of Bombardment
For its first billion years, the Moon was pummeled by asteroids and comets. The most dramatic evidence is the South Pole-Aitken basin on the lunar farside, the largest known impact crater in the solar system. It stretches more than 1,550 miles across (roughly the distance from Texas to Washington, D.C.) and averages about 6 miles deep. Scientists estimate this basin formed around 4 billion years ago, during a period of intense bombardment across the inner solar system.
The impact was so powerful it may have literally shifted the Moon on its axis. Gravity-mapping data from NASA’s GRAIL mission later revealed a mysterious mass buried beneath the basin floor, probably metal-rich debris from the impactor itself. This mass is roughly five times larger than Hawaii’s Big Island and drags the crater floor down by more than half a mile.
Because the Moon has no weather, no plate tectonics, and almost no atmosphere to erode its surface, those ancient craters are still visible today. That makes the Moon a time capsule. Scientists use its cratering record, calibrated against the absolute ages of rocks brought back by astronauts, to estimate the ages of surfaces on Mercury, Venus, and Mars.
Volcanoes and an Evolving Interior
The Moon is not the dead, unchanging rock people once assumed. Before the Apollo missions, its internal structure was a matter of speculation. We now know it has a layered interior similar to Earth’s: a crust about 25 miles thick, a mantle roughly 849 miles thick, and a core about 205 miles across, with a solid inner layer surrounded by a fluid outer layer. Some lunar rocks even preserve a remnant magnetic field, though the Moon generates no active field today.
Billions of years ago, volcanic eruptions flooded large impact basins with dark basalt lava, creating the smooth, dark patches visible from Earth that early astronomers mistakenly called “seas” (maria). These basalts resemble Earth’s oceanic crust but are far older. The lighter-colored highlands, by contrast, are made of an ancient rock type called anorthosite. The oldest Moon rock ever returned to Earth, collected by the Apollo 16 crew, is an anorthosite estimated at 4.46 billion years old, predating the oldest known Earth rocks by nearly 200 million years.
What Apollo Revealed
Between 1969 and 1972, six Apollo missions landed humans on the Moon. The scientific haul was enormous. Astronauts brought back hundreds of pounds of rock and soil, deployed seismic instruments, and conducted experiments that transformed lunar science from guesswork into hard data.
The seismic instruments detected moonquakes, confirming the Moon is geologically active on a small scale. Some quakes originate deep in the mantle, likely driven by tidal forces from Earth’s gravity. Others are shallow and can be surprisingly strong. A more subtle category, thermal moonquakes, occurs when the surface heats up at sunrise: rocks crack, regolith shifts along crater slopes, and even hardware left behind by astronauts expands and pops. At the Apollo 17 site, researchers have traced roughly 45 individual quake events to specific boulders cracking from temperature swings.
Apollo crews also left behind instruments that confirmed the Moon has an atmosphere, though calling it that is generous. The lunar exosphere is roughly a million billion times thinner than Earth’s atmosphere, made almost entirely of helium, neon, and argon, with trace amounts of hydrogen and sodium. It’s so sparse that individual molecules rarely collide with one another.
Water Ice at the Poles
For decades, the Moon was considered bone-dry. That changed in 2008 when NASA’s Moon Mineralogy Mapper, flying aboard India’s Chandrayaan-1 spacecraft, detected three distinct chemical signatures of water ice on the lunar surface. The ice sits in permanently shadowed craters near the poles, where temperatures never climb above minus 250°F. At the south pole, ice concentrates inside craters. At the north pole, it is more widely but sparsely distributed.
These deposits could be ancient, and their origin is still debated. But their presence is one of the main reasons the lunar south pole has become the prime target for future human missions. Water ice could theoretically be harvested for drinking water, oxygen, and even rocket fuel.
Extreme Conditions on the Surface
The Moon’s surface is one of the harshest environments in the solar system. Near the equator, daytime temperatures spike above 250°F (121°C), then plummet after nightfall to minus 208°F (minus 133°C). That swing of more than 450 degrees happens over a lunar day-night cycle that lasts about 29 Earth days. In deep polar craters that never see sunlight, NASA’s Lunar Reconnaissance Orbiter has recorded temperatures below minus 410°F (minus 246°C), colder than the surface of Pluto.
These wild temperature shifts drive the thermal moonquakes mentioned above and also explain some of the strange visual phenomena that observers have reported through telescopes for centuries. Cataloged as “lunar transient phenomena,” these are brief flashes, color changes, or hazes spotted on the surface. More than 1,400 such events have been recorded. Analysis suggests multiple causes: some brightenings correlate with sunrise and thermal stress, while reddish or bluish events near the crater Aristarchus correlate more strongly with solar flare activity and interactions with Earth’s magnetic tail. They remain an area of active study.
Recent Robotic Missions
Human boots haven’t touched the Moon since 1972, but robotic missions have kept the discoveries coming. The most significant recent achievement belongs to China’s Chang’e-6 mission, which on June 25, 2024, returned 1,935 grams of soil and rock from the lunar farside. These are the first samples ever collected from the Moon’s far hemisphere, giving scientists direct access to material from a region that may have a very different geological history than the near side explored by Apollo.
Early analysis of the Chang’e-6 samples is already refining models of the Moon’s impact history and internal evolution, adding data points that were impossible to obtain from orbit alone.
What Comes Next: Artemis
NASA’s Artemis program aims to return humans to the Moon for the first time in over 50 years. Artemis I flew an uncrewed test around the Moon in 2022. Artemis II, scheduled for April 2026, will carry astronauts on a lunar flyby, taking them farther from Earth than any human has traveled since the Apollo era. Artemis III, planned for later in the decade, intends to land astronauts near the lunar south pole, where they could study those water ice deposits up close and test technologies needed for longer stays and eventual missions to Mars.