The birth of the planets 4.5 billion years ago was extremely
violent. They grew to full size by absorbing rival planet embryos in a series
of titanic collisions – one of which probably gave Earth its moon. The moon’s
large size, low density, and other features suggest that it emerged from an
explosion of debris after a Mars-size protoplanet struck earth, vaporizing
itself and part of Earth’s rocky mantle. According to one recent hypothesis,
the moon had a little sister first.
A-Birth
C-Splat
Rocky debris blasted into orbit coalesces into a moon – or
maybe two – in less than a century. Most of the incoming protoplanet’s iron
sank into Earth’s core, so the moon mass is less dense than Earth.
B-Moving out
Lunar gravity raises a tidal bulge on Earth; its spinning in
turn accelerates the moon, causing it to spiral slowly outward. A sister moon,
about a third as wide, orbits at a distance.
C-Splat
Within tens of millions of years the moon reels in its
sister. Splatting onto the moon’s far side, it creates highlands there – a
striking contrast to the low plains, called maria, on the side we see.
