THE EARTH SEEN from space is mostly the colours of water: water-­blue of oceans, water-­white of cloud-­swirl and ice. But early Earth was a scorched-­rock planet, fiery and forming, when it is thought to have collided with protoplanet Theia, breaking off debris that formed the moon, and burying some of the rock from that collision deep in the Earth’s mantle, as a child deposits cells in its mother. Bone marrow, brain, heart.

Bearing a very similar rock composition to Earth, the moon is our constant companion – a translucent coin in our day skies, a silvery cape on night waters. Both are 4.5 billion years old, a timescale that reduces humanity’s existence to barely a wink.