 The
Big Splash that created
the Moon (NASA) 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!
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