The planetesimals collided; some grew in size, forming planets, and others fragmented.

Most of the nebula’s mass was lost in the process.

Earth formed in a million years. Earth was dominated by rock, its snow boiled away by the young sun’s heat, its surface pounded by planetesimals.

Further out, it was different.

Further out, everything was moving more slowly, and the nebula was less dense. It was cold enough for water, carbon dioxide, ammonia and methane to condense into ice. So while the inner planets were dominated by rock, the accreting planetesimals at Jupiter’s orbit and beyond swept up dirty snow.

Hundreds of millions of years after Earth, Saturn formed, gigantic, gaseous. It radiated heat as it collapsed, warming the orbiting fragments of nebula gas and dust.

Around Saturn, an accretion disc formed. Moons coalesced, from a mixture of water ice, silicates, ammonia, methane and other trace elements.

One of them was massive.

It was half rock, half ice. It heated as it collapsed, because of its huge mass; the primordial ices melted and vaporized. The rock settled to the center, because of its greater density. At last, at the core of the moon, a ball of silicate formed, overlaid by a shell of ice, six hundred miles thick.

An ocean gathered. It was a mixture of ammonia and methane, A dense atmosphere was raised over it. The new world was a cauldron, with pressures hundreds of times that of Earth’s sea level in human times, and temperatures measured in hundreds of degrees.”

The high pressure and temperature were sustained, for millions of years. And in the organic soup of the ammonia-water ocean, complex chemistry seethed.

But the new ocean and atmosphere were not stable. Ultra-violet flux from the young sun beat down; planetesimals continued to fall, blasting away swathes of the air; the atmospheric gases dissolved in the ocean.