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How are Moons Made?

Updated on June 3, 2013
A selection of our solar system's natural satellites are shown here to scale compared to the Earth and its moon.
A selection of our solar system's natural satellites are shown here to scale compared to the Earth and its moon. | Source

What Are Moons?

There are more than 300 moons in our solar system orbiting six different planets. With such huge numbers, moons show a much wider range of environments than found on the planets of the solar system. From the potato shaped moons of Mars, to the crushing atmosphere of Titan, to the ice volcanoes of Enceladus; the moons of our solar system are far from boring, inactive chunks of rock. Moons are the sites of the biggest volcanic eruptions and largest, deepest oceans in the solar system.

But what are moons? All moons are natural satellites held in place by gravity. This is the scientific way of saying they are chunks of rock that orbit planets. These can be small and irregular such as Phobos and Deimos, the moons of Mars, or massive worlds such as Ganymede, a moon orbiting Jupiter which is larger than the planet Mercury. Far from passive observers, these worlds actively shape, mould and maintain the solar system.

There are three distinct ways in which moons can be formed:

  • Accretion
  • Impact Events
  • Capture Events

Accretion Theory

Many moons form in the same way as planets do - Accretion.

After the formation of a star, large amounts of dust and gas are left over that slowly start to form clumps under the influence of their own gravity. These clumps start to bump into each other making larger and larger clumps - like a snowball rolling down a hill - until all of the dust in the area is used up and the moon stops growing.

Impact Theory

Some moons, such as our own, are thought to have been created as the result of a massive impact between two celestial bodies. After such a monumental collision, huge amounts of debris are ejected into space. Given time, these debris clouds coalesce into moons in a similar process to accretion.

The most widely accepted theory as to the formation of our own moon is the giant impact theory. This holds that a mars-sized proto planet called Thea smashed into the young Earth. Most of Thea fused with Earth, but huge amounts of debris were ejected into space. This coalesced into a ball and formed our Moon. As you can see from the video, the early moon-rises would have been far more spectacular than they are today.

How do we know this process formed our own Moon? We have all the evidence we need from rocks brought back from Apollo 11. After analysis of the rock samples, scientists found that they were identical in composition to rocks found in the Earth's crust. Not only that, but the rocks showed distinct evidence of being superheated. A massive impact event explains both of these facts.

It is unclear, however, whether the Moon was formed from an impact into Earth, or if several smaller moons orbiting the Earth impacted each other to form the Moon we know today.

Neptune and Triton, the largest captured moon in the solar system.
Neptune and Triton, the largest captured moon in the solar system. | Source
Triton as it would look on an approach to Neptune. Triton also has a thin atmosphere that can cause winds on the surface of the moon.
Triton as it would look on an approach to Neptune. Triton also has a thin atmosphere that can cause winds on the surface of the moon. | Source

Capture Theory

Some moons did not form near their parent planet at all, but instead formed elsewhere in the solar system. As these bodies wandered through the solar system, they strayed too close to a planet and got caught up in their gravity. These wandering objects could be asteroids (like Mars' moons Deimos and Phobos), comets, or even entire planets (such as Neptune's moon Triton).

Capturing a celestial body is a delicate balance; too much gravity and the object will smash into the planet, too little gravity and the object will escape the clutches of the planet and continue wandering the cosmos.

Of all the captured bodies in the solar system, Triton is by far the largest. This moon is 2700km in diameter and orbits Neptune. Triton is a very strange object - it orbits Neptune in the opposite direction to the direction of Neptune's spin, the only moon in the solar system to orbit in this way. This is evidence that Triton could not have formed by accretion from the debris left over from the birth of Neptune.

Triton will not last forever. Neptune is slowly dragging this captured dwarf planet closer and closer. One day, the immense gravity of the gas giant will destroy Triton, possibly forming a brand new ring system around the planet.

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    • Marcy Goodfleisch profile image

      Marcy Goodfleisch 4 years ago from Planet Earth

      I had no idea how moons were formed, or that there could be multiple ways a planet could acquire them! This is an amazing and interesting hub, and it's not only packed with information, it's very engaging to read.

      After reading the impact theory for Earth's moon, I can see why the periodic focus we hear on the risk of asteroids or meteors hitting the Earth is a concern (not that I didn't think it was an issue). It's very eerie to think an impact could be huge enough to shear off a chunk of our planet as large as the Moon and knock it that far away from Earth.

      Are there theories on what may have happened to the huge planet that collided with Earth to do that damage? I guess it's not all damage, though, when you think of the role the Moon plays in tides and other cycles on Earth.

      Many votes up! And shared!

    • unknown spy profile image

      IAmForbidden 4 years ago from Neverland - where children never grow up.

      I always love to read about topics like these..they always fascinate me, the mystery that covers them always makes me want to read more. Fantastic hub!

    • chamilj profile image

      chamilj 4 years ago from Sri Lanka

      Interesting hub. I always love to learn more on entire Universe. Voted up and shared!

    • Cre8tor profile image

      Dan Robbins 4 years ago from Ohio

      Great stuff TF!

    • colombostock profile image

      colombostock 4 years ago from Colombo Sri Lanka

      Very informative article. Voted up!

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