In the solar system, there are many moons, so Moon is not unique in that sense. Some planets, such as Jupiter and Saturn, have dozens of moons in almost all shapes and sizes. Indeed, every moon-bearing planet has more than one … except Earth!
The fifth largest moon in the solar system, Earth’s moon is the only place beyond Earth where humans have set foot. Earth’s moon isn’t the biggest — not the longest shot. The largest planet also boasts the largest moon with the Ganymede of Jupiter having a diameter of more than 5,200 km. It’s ~67% larger than the closest planet, Neptune. The Moon is on the register of the largest satellites in Number 5 among Io and Europa (two additional moons from Jupiter).
Our Moon is alone for the terrestrial planets (Mercury, Venus, Earth and Mars). Mars has two moons, surely, but Phobos and Demos are glorified asteroids, which zip around the planet. Our Moon is circular and circles out some 27 days per orbit relatively easily. It is also the only force other than the Moon which has an impact on Earth’s processes (consider: waves, but not earthquakes or volcanic eruptions).
It’s also distinguished by the birth of our moon. A vast majority of small pieces of ice and rock that orbit other planets have likely been produced or caught by the gravitational well of the Earth. Not our Moon. Not our Moon. It is the result of an early earth-related cataclysm. A massive collision with an object with the size of Mars has thrown material away from which the Earth-Moon system has coalesced.
A comparison between the various densities of the Earth and their partner can also show the story of the Moon formation. Grind the Earth and measure the density of the material, and you get ~5.5 grams per cubic centimeter. Do the same for the Moon, which really is similar to the thickness of the upper layers of the Earth: just ~3.3 grams per cubic centimeter. It probably means that the Moon is made up of the outer parts of the proto-earth (crust and mantle) after the impact, not the hard metallic innards.
Nearly the entire Moon is covered by rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.
The Moon is in synchronous rotation with Earth, and thus always shows the same side to Earth, the near side. The near side is marked by dark volcanic maria (large, dark, basaltic plains on Earth’s Moon) that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest regularly visible celestial object in Earth’s sky. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.
In contrast, our Moon lacks a genuine atmosphere and is not covered with ice, as other big moons like Ganymeda, Titan, Europah, Callista and Triton. It is definitely far closer to the Sun, so it may have lost a great deal of ice (some of them), but it’s much less barren than the beautiful solar system planets.
We don’t know a lot about the Moon and its past, so many new moon explorations by India and other countries have taken place in the last year.