Step 5: Bringing Water

Water is the ingredient that turns a rock into a world. It carves valleys, moves heat around the globe, softens rock so plates can slide, and is the solvent every known form of life depends on. Yet Earth formed in the hot inner part of the solar system, inside the frost line, where water ice could not survive. So how did we end up with oceans?

Three possible delivery routes

Scientists compare the chemistry of Earth's water with water from other sources. A key clue is the ratio of normal hydrogen to its heavier form, deuterium — a kind of chemical fingerprint.

• Asteroids. Some meteorites, called carbonaceous chondrites, contain water locked inside minerals, and their hydrogen fingerprint is a close match to Earth's oceans. Many researchers think water-rich asteroids from the outer asteroid belt delivered a large share of Earth's water, possibly scattered inward by Jupiter's gravity.
• Comets. Comets are icy and seem like an obvious source. But measurements of most comets — including Comet 67P, studied closely by ESA's Rosetta mission — show a fingerprint that does not match Earth's oceans well. Comets probably contributed some water, but likely not most of it.
• From the start. Some studies suggest Earth's building blocks were not as dry as once thought, and that some water may have been there from the beginning, locked in rock or produced by hydrogen reacting with the magma ocean.

The honest answer today is that Earth's water probably came from a mix of sources, and the exact proportions are still being worked out.

How early were the oceans?

Tiny zircon crystals from the Jack Hills in Western Australia are about 4.4 billion years old. Their chemistry suggests they formed in contact with liquid water, which means Earth may have had oceans within roughly 150 million years of its formation — much earlier than scientists once believed.

How much water is enough?

Oceans cover about 71% of Earth's surface, but they are a thin film on a large ball: all of Earth's water amounts to only about 0.02% of the planet's mass. Builders have options:

• Desert world: little water, mostly land. Easier to lose heat at night, larger temperature swings.
• Earth-like: oceans and continents together. Land weathering helps regulate carbon dioxide.
• Ocean world: water covers everything, possibly hundreds of kilometres deep. Some exoplanets may be like this, and so are the hidden oceans under the ice of moons such as Europa and Enceladus.

In the Design Your Planet tool, this is the “land ratio” slider.

Water builds a climate

Once there is liquid water, a planet gets a water cycle: evaporation, clouds, rain, rivers. Oceans store huge amounts of heat and carry it from the equator towards the poles, smoothing out temperature differences.

Two more things shape the climate:

• Axial tilt. Earth is tilted about 23.4° relative to its orbit. That tilt — not our distance from the Sun — is what gives us seasons.
• Reflectivity. Ice and clouds reflect sunlight; dark ocean absorbs it. More ice means more reflection and more cooling, which can feed on itself. Geological evidence suggests Earth froze over almost completely at least twice, between roughly 720 and 635 million years ago — the “Snowball Earth” episodes.

Earth's built-in thermostat

Over millions of years, Earth's temperature is held in check by the carbon–silicate cycle:

1. Volcanoes release carbon dioxide, which warms the planet.
2. Rain dissolves some of that carbon dioxide and slowly weathers rock, washing carbon into the oceans, where it ends up locked in sediments.
3. When the planet is warmer, there is more rain and faster weathering, so more carbon dioxide is removed and the planet cools.
4. When it is colder, weathering slows down and carbon dioxide builds up again.

Plate tectonics eventually carries those sediments back into the mantle, and volcanoes release the carbon again. This feedback works on timescales of hundreds of thousands to millions of years — far too slowly to offset rapid changes, but powerful enough to have kept Earth habitable for billions of years.

Builder's checklist for step 5

• Deliver water from water-rich asteroids, comets or the building blocks themselves
• Keep the planet in the habitable zone so it stays liquid
• Choose a land-to-ocean balance
• A moderate axial tilt for gentle seasons
• Volcanoes, rain and moving plates to run the carbon thermostat

Water is flowing. Next: what it takes to make a planet truly livable — Step 6: Making it habitable.