Page 95 - How It Works - Book Of Amazing Answers To Curious Questions, Volume 05-15

Space

P l a n et s c a n a n d d o
Planets can and do
1. The big splash
Scientists believe that our Moon was formed when a small protoplanet about collide, usually either
the size of Mars – which astronomers have called Theia – slammed into the
young Earth. This collision took place less than 100 million years after
the birth of the Solar System. The impact destroyed Theia and sent when they are very
some of Earth’s mantle flying into space, which formed a ring of
young or very old
debris around our planet that coalesced into the Moon.
2. Merger
When Theia struck Earth
at high speed and an The giant impact
angle of around 45
degrees, the protoplanet
was pulverised, its iron hypothesis
core sinking into the now
molten Earth which had The main evidence for the theory that the Moon was
been heated by the shock formed by a giant impact comes from lunar rocks
of the impact to create a returned to Earth during the Apollo missions. These
global ocean of lava. rocks contain ratios of oxygen isotopes (atoms of a
given element that have a different number of
neutrons) that are almost exactly the same as those
found in Earth’s mantle. This suggests that the Moon is
made from material that came from our own planet’s
mantle. Material in those Moon rocks was also found
to have once been molten long ago, and an impact
would have provided the necessary energy to
efficiently melt rock.

Although the impact theory is now widely accepted,
a number of puzzling problems remain. For example,
some believe there should be more evidence of debris
3. Goodbye mantle material from Theia found in the Moon rocks. Also, the
While most of Theia was absorbed rocks contain so-called volatile elements (materials
by the Earth – which grew in mass that evaporate easily in relatively low temperatures)
in the process – some of Earth’s such as water, which were embedded in the rocks
mantle and crust was ripped from when they formed, yet the heat of an impact should
the planet and thrown into orbit in have evaporated them. However, these puzzles remain
molten chunks. The impact as details to be ironed out, rather than serious threats
changed the rotation of Earth, to the impact theory.
speeding it up so that a day lasted
only a few hours.

4. Molten ring
After the impact Earth actually
had a ring, but unlike Saturn’s
rings this one was glowing hot
with molten rock. Some rained
back down onto the Earth, but
most stayed in orbit and cooled.

5. Forming the Moon
Gradually, over a few thousand
years, the rubble in the ring
began to merge into larger
bodies which then combined © Thinkstock; University of Arizona/Tom Story
to finally form the Moon, made
out of the remains of Earth’s A 77g (2.7oz), golf-ball-sized piece of Moon rock that
ancient mantle, with a bit of was collected by astronaut Dave Scott during the
Theia included too. Apollo 15 mission

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