The Moon Guide
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The phase of the Moon right now

Phase
 
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Big Splash
The Big Splash that created the Moon (NASA)
Lunar geology

Like most older people who have a lot of good stories to tell about their lives, so does the Moon. Of course, so do all the other planets and satellites in the Solar System, which were all born around 4.6 billion years ago, but the focus of this story is the life and times of our next door neighbour, the seemingly grey and lifeless Moon that looks down on us every day.

Life in the early Solar System, where our story begins, was very chaotic. The planets and moons had just been built but many left-over planetary building blocks were still whizzing around our local neighbourhood, slamming into one another, as well as into the newly formed planets. Indeed, current theory suggests that shortly after the Earth had formed, a Mars-sized object slammed into our planet, spewing large amounts of material into space (the ‘Big Splash’). Over a period of just a few days to weeks this material began to stick together, eventually forming the familiar Moon that we see in our skies today. There are other ideas of how the Moon was born: click here to find out more. 

The newly born Moon was a fiery cauldron of molten liquid, and from this giant magma ocean, several different layers separated out into a crust, mantle and core. (See ‘What is the moon made of?’ to learn about how the different rock types came to be...] But the story doesn’t end there. The grey lifeless world that we know of today was once subject to a much more violent past.

A large portion of the Moon’s early evolution was dominated by a period of intense bombardment by asteroids and comets impacting the surface at speeds of several hundred thousand kilometres per hour, which created many of the features that we can see with our naked eye from the Earth. This period of collisions is known as the Heavy Bombardment Era: the formation of impact craters between 4.5 and 3.8 billion years ago was significantly higher than it has been in the 3.8 billion years since then.

Why was the early Solar System such a dangerous place? There are two main ideas as to how the Heavy Bombardment Era came about:  either the impactors came from left-over bits of rock that weren’t used for making planets, or movements of the giant planets forced huge amounts of rocky debris from the outer Solar System in towards the Earth and Moon. 

The consequences for the Earth would have been even greater because its larger diameter and greater gravity made it an easier target to hit. The reason why Earth is no longer a mass of craters like the Moon is today is that our planet is very active, so many ancient impact craters have long been wiped out by erosion and plate tectonics.
 
After this period of heavy bombardment there were just random and sporadic impact events. The bombardment still continues to a much lesser extent. If you’re extremely lucky, and are watching the right part of the Moon through a telescope at just the right time, you may see a flash coming from the surface, a tell-tale sign that a small piece of rock or other debris has just collided with the surface of the Moon!


 
lunar explosion
A lunar impact recorded by a NASA camera in 2008.

 
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Geologist on the Moon

Lunar geology

Moon rocks on Earth

Moon quakes

How the Moon was formed

Geology of the Moon's features

What is the Moon made of?

Lunar gardening

About impacts

Ray craters

Ageing wrinkles

Meandering channels

 
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