According to NASA's current understanding, what is the leading hypothesis for how Earth's Moon formed?
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Earth & Space Science · Foundations
Moons
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In 30 seconds
Moons are natural satellites: solid worlds that orbit planets. Earth's Moon Earth's natural satellite, a rocky world orbiting at an average distance of about 384,400 kilometers. Full entry →, our closest neighbor at about 384,400 kilometers away, preserves the solar system's violent early history. Scientists think it formed when a Mars-sized body slammed into the young Earth, and its surface records that story in craters, Highlands Bright, heavily cratered regions of the Moon that make up its older, higher-standing crust. Full entry →, and dark lava-filled plains called Maria Dark, relatively smooth regions of the Moon that are ancient impact basins later flooded by lava. Full entry →. The Moon also shapes Earth's tides and the monthly cycle of phases. Across the solar system, moons are strikingly diverse, from volcanic Io to ocean-bearing Europa and Enceladus.
Why this matters
Studying moons trains students to read a landscape as evidence. The Moon is the one world beyond Earth whose rocks we have sampled directly, and those samples anchor our account of how planets form. Moons also widen the search for life: several icy moons are thought to hide oceans of liquid water beneath their crusts, making them prime targets for missions such as Europa Clipper. Understanding moons means understanding how small worlds record history, how gravity shapes orbits and tides, and why astronomers keep revising the catalog of moons as instruments improve.
The college version
Earth's Moon: a small world close by
Earth's Moon is small compared with its planet - about 1,740 kilometers in radius, less than a third of Earth's width - but it is close: about 384,400 kilometers on average, a distance that could fit roughly 30 Earths in a row. Because the Moon orbits Earth once every 27 days and spins at the same rate, the same hemisphere always faces us; the far side is not perpetually dark, just never visible from Earth. As the Moon travels, the sunlit half we see changes, producing the monthly cycle of phases, from new moon to full moon and back in about 29 days. The Moon's gravity also raises Earth's ocean tides; the physics of tides is treated in depth in the ocean-systems lesson. Laser reflectors left by Apollo astronauts show the Moon is slowly drifting away, roughly an inch and a half per year. Because Earth and the Moon travel around the Sun together, the Moon's phase cycle shifts a little each night, which is why the roughly 29-day lunar month became one of humanity's earliest calendars.
Formation: the giant-impact hypothesis
NASA's leading explanation for the Moon's origin is the Giant-impact hypothesis The leading explanation that Earth's Moon formed from debris flung into orbit when a Mars-sized body struck the young Earth. Full entry →. Early in solar system history, a body about the size of Mars - named Theia after a figure in Greek mythology - is thought to have crashed into the young Earth, and debris from that collision coalesced into the Moon. Older competing ideas, such as the Moon being captured by Earth's gravity or forming alongside Earth from the same material, fell out of favor after the Apollo missions. Astronauts returned about 382 kilograms (842 pounds) of lunar samples, and those rocks showed that the Moon was molten early in its history, that its composition closely resembles Earth's, including matching oxygen isotopes, and that it formed roughly 60 million years after the solar system began - all consistent with a giant impact. The hypothesis remains the best-supported account, though computer models are still being refined as new samples and data arrive.
The Moon's surface: a record of impacts
With no wind or flowing water to erase the record, the Moon preserves billions of years of impact history. The bright, heavily cratered regions are the highlands, ancient crust much of which is thought to have crystallized from a deep magma ocean. The dark, relatively smooth areas, called maria (Latin for "seas"), are not water: they are giant impact basins later flooded by lava between roughly 4.2 and 1.2 billion years ago. Impact craters of every size pock the surface, and nearly the whole Moon is blanketed by a layer of pulverized rock called regolith. The Moon's gravity is about one-sixth of Earth's, so Apollo astronauts bounced across a world that preserves, in its craters, an impact history shared by the inner solar system.
A solar system full of moons
Moons - natural satellites - come in many shapes, sizes, and types, and there are hundreds of them. NASA's catalog listed 891 confirmed moons as of March 25, 2025, and the number keeps growing as telescopes and spacecraft reveal small bodies that were previously unknown. Jupiter's four largest moons, the Galilean moons The four largest moons of Jupiter - Io, Europa, Ganymede, and Callisto - first observed by Galileo Galilei in 1610. Full entry → first seen by Galileo in 1610, show how different neighboring worlds can be: Io is the most volcanically active world in the solar system; Europa's icy crust is thought to hide a liquid-water ocean; Ganymede is the largest moon in the solar system, bigger than the planet Mercury; and Callisto is an ancient, heavily cratered world. Saturn's moons are equally surprising. Titan has a thick, nitrogen-rich atmosphere and lakes and rivers of liquid methane and ethane, while small Enceladus vents geyser-like jets of water vapor and ice from a Subsurface ocean A layer of liquid water hidden beneath a world's icy crust, as is thought to exist on Europa and Enceladus. Full entry → into space. These small worlds matter because they record solar system history and because several may hold liquid-water oceans beneath ice, making them priority targets in the search for life beyond Earth. Ocean moons are especially compelling: liquid water is essential to life on Earth, so worlds that may hold subsurface oceans are where the search for life beyond Earth now concentrates.
Eli explains
The same idea, in plain words
Explain it like I’m 10
A moon is a world that travels around a planet. Earth has one moon, and it does a lot: it lights up our night sky in a cycle that repeats about every 29 days, and its gravity helps pull the oceans into tides. The Moon is covered with craters from ancient impacts, bright highlands, and dark flat areas that are really old lava floods, not seas. Scientists think it formed when a giant object smashed into Earth long ago. Other planets have many moons - some are giant balls of ice, one is covered in volcanoes, and some hide oceans of liquid water under their frozen surfaces.
Picture it like this
Imagine a history book made of rock. Earth keeps erasing its pages with weather, moving continents, and life, but the Moon's pages stay readable for billions of years. Each crater is a note about a long-ago collision, and the dark maria record ancient floods of lava. Reading the Moon is like reading the solar system's early diary.
Where the picture stops working
The book analogy breaks down because the Moon is not a passive record: impacts and lava have reshaped it for billions of years, and it is still slowly moving away from Earth. Also, while a book only records the past, the Moon acts on Earth today, driving tides and pacing our months.
Worked example
A student watches the Moon for a month and makes two observations: the bright shape changes from a thin crescent to full and back, and the Moon rises a little later each night. The student wonders whether the Moon is changing shape. It is not. The Moon keeps the same face toward Earth because it rotates once per orbit, and the changing appearance is the phase cycle, set by how much of the sunlit half we can see as the Moon completes its 27-day orbit. Because Earth is also moving around the Sun, the full cycle from one new moon to the next takes about 29 days. The same orbit and gravity that create the phases also help raise the ocean tides, which the ocean-systems lesson explores in depth.
Key takeaway
Moons are small worlds that record and shape solar system history: Earth's Moon preserves the story of a giant impact and drives our tides, while other moons reveal volcanoes, thick atmospheres, and hidden oceans.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
Which four moons are Jupiter's Galilean moons, the large moons first seen by Galileo in 1610?
Choose an answer, then check it.
The dark, relatively smooth regions of the Moon called maria are best described as what?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define a natural satellite and distinguish moons from the planets they orbit.
- Describe the giant-impact hypothesis for the Moon's formation and the Apollo-era evidence that supports it.
- Distinguish the Moon's maria, highlands, and craters and explain what each one records.
- Explain how the Moon's gravity drives Earth's tides and how its orbit produces the monthly cycle of phases.
- Compare notable moons of the outer solar system, including the Galilean moons, Titan, and Enceladus.
- Analyze why the cataloged number of moons keeps changing and why ocean moons matter to the search for life.
Common mistakes
The dark areas of the Moon, called maria, are seas of water.
Maria means "seas" in Latin, but they are ancient impact basins flooded with lava between about 4.2 and 1.2 billion years ago.
The Moon does not rotate because we always see the same side.
The Moon rotates once per orbit - synchronous rotation - which is exactly why the same hemisphere always faces Earth.
The far side of the Moon is always dark.
The far side receives sunlight like the near side; "dark side" is a misleading name for the hemisphere we never see from Earth.
The number of moons in the solar system is a fixed, final count.
Counts are as-of-date snapshots; improved telescopes and missions keep confirming small new moons, so the catalog grows.
Easily confused
Maria vs. Highlands
Maria are dark lava-flooded impact basins, while highlands are bright, heavily cratered ancient crust.
Earth's Moon vs. Titan
The Moon is airless, while Titan is the only moon known with a thick atmosphere and liquid hydrocarbon lakes.
Io vs. Europa
Io is the most volcanically active world known; Europa's icy surface is thought to hide a liquid-water ocean.
Sidereal month (27 days) vs. Synodic month (about 29 days)
The Moon's orbit relative to the stars versus the full cycle of phases as seen from Earth.
Key vocabulary
- Moon
- Earth's natural satellite, a rocky world orbiting at an average distance of about 384,400 kilometers.
- Natural satellite
- A solid body that formed naturally and orbits a planet, dwarf planet, or other non-stellar object.
- Giant-impact hypothesis
- The leading explanation that Earth's Moon formed from debris flung into orbit when a Mars-sized body struck the young Earth.
- Maria
- Dark, relatively smooth regions of the Moon that are ancient impact basins later flooded by lava.
- Highlands
- Bright, heavily cratered regions of the Moon that make up its older, higher-standing crust.
- Crater
- A bowl-shaped depression formed when an impactor such as an asteroid or comet strikes a solid surface.
- Galilean moons
- The four largest moons of Jupiter - Io, Europa, Ganymede, and Callisto - first observed by Galileo Galilei in 1610.
- Subsurface ocean
- A layer of liquid water hidden beneath a world's icy crust, as is thought to exist on Europa and Enceladus.
Sources & references
- Earth's Moon (Overview) — NASA Science
- Moon Facts — NASA Science
- How Did the Moon Form? — NASA Science
- Moons of Our Solar System (Overview) — NASA Science
- Moons of Jupiter — NASA Science
- Titan (Saturn's Largest Moon) — NASA Science
- Enceladus (Saturn's Icy Moon) — NASA Science
- Astronomy 2e, 12.2 The Galilean Moons of Jupiter — OpenStax, Rice University
- Astronomy 2e, 12.3 Titan and Triton — OpenStax, Rice University
- What are tides? — NOAA National Ocean Service
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-21
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