The Moon has always been a source of awe and wonder for mankind. It has been worshiped and feared, and it has inspired both lovers and writers. Our Moon is the only alien world that can be seen with the naked eye. Men have walked on its surface. But what exactly is the Moon and where did it come from? A moon is defined as an object that orbits a planet. It is the planet's natural satellite. And when it comes to satellites, the Earth is not alone. Most of the other planets in our solar system also have moons. The number of moons ranges from one for the Earth, to as many as 21 for Uranus. Two of our planets, Mercury and Venus, have no moons. Jupiter has 16 known moons, Saturn has 18, and Neptune has 8. Pluto has one moon, but this moon is so large in comparison that many astronomers consider the two objects to be a double planet rather than a planet and a moon. Moons also vary greatly in size. The smallest is Mars' moon Deimos with a diameter of about 10 miles (11 km). The largest moon in the Solar System is Jupiter's moon Ganymede with a diameter of 3280 miles (5262 km). This makes Ganymede larger than the planet Mercury. By comparison, the Earth's moon is 2160 miles (3476 km) in diameter. All moons orbit their host planets as well as rotating about their own axis. Our moon takes a little over 27 days to make one complete orbit of the Earth. Some moons, including our own, orbit in what is called a synchronous rotation. In this case, the time it takes the Moon to make one orbit of the planet is the same as the time it takes the Moon to make one complete rotation on its axis. This means that the Moon always keeps the same face towards the planet.
Most moons are composed mainly of rock, ice, or a combination of both. They are much less dense than planets and do not have metal cores. A few, such as Saturn's moon Titan, even have a thick atmosphere. This atmosphere makes astronomers believe that there could be some form of life on this moon. Since most moons have no atmosphere, they have no natural protection from meteors. Most of the moons in our solar system have heavily cratered surfaces. Many of these moons also display a large number of unique surface features such as deep rift valleys, gigantic faults, and Jupiter has some of the most fascinating moons in the Solar System. The largest four, Ganymede, Io, Europa, and Callisto, are known as the Galilean moons. this is because they were studied extensively by the astronomer Galileo Galilei in the 1600s. Io is particular interest because it was the first moon to be observed with active volcanoes. The Voyager spacecraft found massive volcanic craters spewing molten sulfur hundreds of miles into space. Another moon of interest here is Europa. On the outside, it appears to be a frozen ball of ice. But astronomers believe it may have a liquid ocean below the ice. If this is true, then it could be a candidate for extraterrestrial life. It is believed that primitive life forms could have evolved near deep-water hydrothermal vents similar to those recently discovered on Earth.
Astronomers are not exactly sure how moons are formed or where they came from, but there are a number of working theories. Many of the smaller moons are believed to be captured asteroids. In the early days of our solar system, millions of cosmic boulders roamed the heavens. Most of these were formed from material left over from the Solar System's formation. Others may have been the remains of planets that were smashed to pieces by massive cosmic collisions. Some of these rocks may have been captures by the gravitational pull of the planets to become orbiting moons. The larger moons are a little more difficult to explain. Some of them may have originated in an area of the Solar System known as the Kuiper belt. This is a zone at the outer edge of the Solar System known to be populated by thousands of small, planet-like objects. In fact, many astronomers believe the planet Pluto and its moon Charon may actually be Kuiper objects rather than planets. Occasionally, gravitational forces may send one of these objects on a path through the inner solar system where is could be captured by one of the planets. The solar system's largest moons may have actually condensed from the primordial solar nebula that created the Sun and the other planets. May astronomers believe that the Moon may have broken off from the Earth billions of years ago. Rock samples brought back by the Apollo missions showed that the composition of the Moon is very similar to that of the Earth. It is believed that a glancing blow by a large planet-like object may have broken off a large chunk of the young Earth. This matter then coalesced into a sphere shaped moon and achieved a stable orbit around the planet. Although this is the most popular theory for the formation of our moon, it is not the only one. Many believe the Moon was captured by the Earth after forming elsewhere. It could even be a Kuiper belt object as many of the other planets' moons may be.
When we look up at the Moon at night it is difficult to imagine the Earth without it. But one day in the future, there may be no moon. Moons are by no means permanent. By making precise measurements with laser beams, astronomers have discovered that our moon is actually moving away from the Earth at the rate of about two inches per year. Millions of years ago, it was much closer than it is today. In fact, during the days when dinosaurs walked the planet, the Moon appeared several times larger in the sky than it does today. Astronomers believe that the Moon could one day break free of the Earth's gravity and wander off into the cosmos. Other moons face similar fates. Phobos, one of the moons of Mars, is actually getting closer to the planet. It will one day end its life in a fiery crash as it dives into the Martian atmosphere and plows into the red planet. Many other moons in the Solar System may eventually be broken up by the tidal forces of the planets they orbit. The many rings surrounding the outer planets have been found to be composed of small particles of ice and rock. They may have been formed when a moon or moons were pulverized by the planet's gravity. Over time, these particles spread out into the delicate rings we see today. Other moons near the rings keep them from falling apart. The gravity of the moon keeps the particles from escaping orbit or falling back to the planet. We refer to these as shepherd moons because they help keep the rings in line much as a shepherd does his sheep. If it weren't for these moons, the wondrous rings of Saturn would have disappeared millions of years ago.