Why does Earth have a moon?

Lunar exploration and research in the 20th and 21st centuries have expanded our knowledge of our natural satellite and nearest celestial neighbour, the moon.

The moon is so close (some 240,000 miles away) that, with the help of ordinary binoculars, it is easy to see a profusion of detail on the near side of the moon that is always facing the earth.

The moon is airless because its gravity is too weak to capture and retain atmosphere; hence, there is no weather—and no life. It is also waterless. The moon’s dark plains, once believed to be stretches of water and still known as maria (“seas”), are really vast flat basins, created by congealed volcanic lava.

The surface—despite the radiance of a full moon— is made up of dark grey rock that reflects only a tiny fraction of the sunlight that hits it.

Lunar exploration has stripped away some of the moon’s mysteries. In 1959, the Soviet probe Luna 3 revealed the far side that is hidden from the earth’s view. In 1969, Apollo 11 astronauts on the first manned flight to the moon brought back rock samples some 3,700 million years old—older than any rocks on earth. Samples from later missions indicate the moon may have been formed at about the same time as the rest of the solar system—4.6 billion years ago.

Analysis of the moon rocks suggests an end of intensive meteor bombardment of the surface about 4 billion years ago and an end of volcanic eruption about a billion years ago. The moon appears cold and dead—yet it still may be active. Observers report the flickering glow of what could be eruptions at the edges of the maria and in certain craters.

Read more: 10 Health hazards in outer space

A fundamental question remains unanswered: What is the origin of the moon? Before the advent of the lunar probes, scientists had proposed three theories.

The first claimed the moon was a fragment spun off from a rapidly rotating early earth. This theory, advanced by the 19th-century British astronomer Sir George H. Darwin, was dismissed in 1930.

A second origin theory posited that the moon and the earth formed side by side from the condensation of primordial gases and particles.

A third theory said the earth and the moon originated at widely separate points in the solar system but, as they came into close proximity, the moon was pulled into orbit around the earth.

Eventually, both the side-by-side and capture theories fell from favour because supporting evidence proved inconclusive.

In 1975, after studying moon rocks and close-up photographs of the lunar surface, scientists proposed another, more probable theory: “planetesimal impact.” According to this theory, a giant object—known as a planetesimal—struck the earth more than 4 billion years ago.

At first, scientists estimated the object was about the size of the planet Mars. But, in 1997, a US computer simulation showed it must have been at least two-and-a-half to three times Martian size. The cataclysmic impact, the theory goes, propelled portions of the earth and this giant object into space, where the debris eventually coalesced to create the moon.

By the 1990s, this theory had become the most widely accepted explanation of the moon’s origin. But it still poses a major problem: Where on the earth’s surface is the crater created by the planetesimal’s impact?

Some scientists believe the crater may have been erased because of a major meltdown throughout the earth after the impact. But the geochemistry of the earth shows no such radical melting. So, even though a mighty impact is probable, the theory still contains holes.

Read more: Interview with astronaut Tim Peake

The chances of life evolving in the universe are unimaginably small, so small that scientists have begun to wonder what is so unique about the earth that made it suitable for life to flourish.

Apart from the obvious—proper temperature, atmosphere, and water—it is now believed that the moon has played an important role in the evolution of life on earth.

When the earth first formed, a full day was only 10 hours long and sunlight lasted only 5 hours. Since then, the tidal drag of the moon slowed the earth’s rotation and created the 24-hour day.

How has this affected us? For one, sunlight plays a critical role in ecology: A longer day exposes plant life to more sunlight, encouraging photosynthesis and generating more CO2. The moon also acts as an asteroid shield, saving the earth from the impact of many asteroids.

Evidence for this can be seen by comparing the ratio of craters on the moon’s craggy surface to those found on the earth. With the moon in place as a shield, life was able to evolve mostly undisturbed by possible cataclysmic impacts from space. Accordingly, some astronomers believe that the existence of a large, close moon is required for life to evolve. This has led to a search for planets that have large moons, in the hope of discovering life on other planets in the universe.