Moon
Four point five one billion years ago, a Mars-sized body named Theia collided with the proto-Earth. This oblique impact blasted material into orbit about our planet. The ejected matter accreted and formed the Moon just beyond Earth's Roche limit of roughly 18,500 kilometers. Computer simulations show that most of the Moon derived from the impactor rather than the proto-Earth itself. Isotope dating of lunar samples suggests this event occurred around 50 million years after the origin of the Solar System. The newly formed Moon had its own magma ocean with an estimated depth ranging between 200 and 500 kilometers. This violent beginning explains why the Moon is smaller than Mercury but considerably larger than any dwarf planet in the Solar System.
The Moon was volcanically active until 1.2 billion years ago. Lava flooded ancient craters on the thinner near side to form dark plains called maria. Most mare basalts erupted during the Imbrian period between 3.3 and 3.7 billion years ago. Some lava flows are as young as 1.2 billion years while others date back to 4.2 billion years. The distribution of these basaltic deposits remains uneven across the surface. Scientists hypothesize that the relative thinness of the crust on the near side allowed lava to flow more easily onto the surface. Today the lunar surface features craters of all ages, from simple bowl-shaped interiors to complex multi-ring basins like the South Pole-Aitken basin. This giant crater spans some 2,500 kilometers in diameter and reaches depths of up to 8 kilometers at its lowest point.
Lunar dust is highly abrasive and can cause damage to human lungs and cardiovascular systems. Ionizing radiation from cosmic rays results in an average level of 1.369 millisieverts per day during lunar daytime. This exposure is about 2.6 times higher than levels found on the International Space Station. Solar radiation electrically charges this fine regolith and makes it levitate above the surface. Parts of many polar craters remain permanently shadowed with temperatures reaching minus 238 degrees Celsius. These cold traps allow water ice to persist for billions of years despite the harsh environment. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in southern polar craters at minus 173 degrees Celsius.
The Moon completes an orbit around Earth every 27.3 days relative to fixed stars. Tidal locking synchronizes this rotation period to match the orbital period exactly. As a result, only one side of the Moon faces Earth at any given time. Measurements from laser reflectors left by Apollo astronauts show that the Moon's distance increases by roughly 3.8 centimeters each year. This tidal acceleration lifts the Moon into a higher orbit while lowering its speed around our planet. The cumulative effects of stress from these forces produce moonquakes that can last up to an hour. Scientists suggest that the Moon's influence may contribute to maintaining Earth's magnetic field over geological time scales.
Some believe the oldest cave paintings from up to 40,000 BP depicted bulls and geometric shapes used to observe lunar phases. Ancient tally sticks dating back 20 to 30 thousand years tracked the waxing and waning of the Moon for timekeeping purposes. Galileo discovered satellites orbiting Jupiter in 1610 and named them moons because they orbited another planet like Earth's satellite does. The first spaceflight to an extraterrestrial body occurred in 1959 with the Soviet Union's Luna 1 flyby. Humans first arrived on the surface on the 20th of July 1969, when Apollo 11 landed. By 1972, six missions had returned twelve humans to explore the terrain directly.
The intentional impact of Luna 2 followed the 1959 flyby as part of early Soviet exploration efforts. Luna 9 achieved the first soft landing on the Moon in 1966 while Luna 10 performed orbital insertion that same year. Apollo 8 became the first human mission to reach lunar orbit on the 24th of December 1968. Twelve astronauts walked across the surface between 1969 and 1972 during six successful landings. Each crew stayed up to three days conducting experiments and collecting samples from diverse locations. Modern robotic missions like Chandrayaan-1 confirmed water ice presence near both poles using spectrometers aboard orbiters. Future plans include returning humans via the Artemis program scheduled for the late 2020s.
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Common questions
When did the Moon form after the Solar System origin?
The Moon formed approximately 50 million years after the origin of the Solar System. Isotope dating of lunar samples suggests this event occurred around that time following a collision between proto-Earth and Theia.
How deep was the magma ocean on the newly formed Moon?
The newly formed Moon had its own magma ocean with an estimated depth ranging between 200 and 500 kilometers. This violent beginning explains why the Moon is smaller than Mercury but considerably larger than any dwarf planet in the Solar System.
What are the lowest temperatures recorded in southern polar craters by the Lunar Reconnaissance Orbiter?
The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in southern polar craters at minus 173 degrees Celsius. Parts of many polar craters remain permanently shadowed with temperatures reaching minus 238 degrees Celsius to allow water ice to persist for billions of years.
On what date did humans first arrive on the surface of the Moon during Apollo 11?
Humans first arrived on the surface on the 20th of July 1969 when Apollo 11 landed. Twelve astronauts walked across the surface between 1969 and 1972 during six successful landings.
When did Luna 9 achieve the first soft landing on the Moon?
Luna 9 achieved the first soft landing on the Moon in 1966 while Luna 10 performed orbital insertion that same year. The intentional impact of Luna 2 followed the 1959 flyby as part of early Soviet exploration efforts.