The large disk of our solar nebula swirled around a developing Sun. Within this disk, countless small objects collided and stuck together, gradually building larger and larger bodies, including Earth. ©NASA

Seventy percent of the Earth’s surface is covered with water. Where did it come from? How did it make its way from the deep Earth to the surface of the planet? And most importantly, why has liquid water persisted on Earth when it doesn’t appear to have done so elsewhere in the solar system? The full story takes us back to the formation of our “water planet.”

The birth of the solar system

Everything in our solar system — the Sun and the planets — formed about 4.55 billion years ago from a rotating disk of gas, dust, and ice called the solar nebula. Matter in the nebula came together to form planetary bodies of different sizes and composition, with the Sun at the center. Rocky planets formed closest to the Sun, the gas giants farther out, and icy bodies like Pluto farther still. This is because the nebula cloud was hotter toward the center, just like the solar system today.

This temperature gradient influenced the composition of these planetary bodies, including the concentration of water. Lighter elements and compounds known as volatiles (things like water, carbon dioxide, and methane, which are stable as gas in our atmosphere) accumulated further away from the Sun, concentrating in bodies like the gas planets. The rocky planets, like Venus, Earth, and Mars, formed where it may have been too hot for the water and other volatiles that form our hydrosphere and atmosphere to be present in high concentrations.

Earth takes shape

Earth formed as dust, rocks, and planetesimals (kilometer-sized boulders) collided and combined to create something just a bit smaller than the present planet. These collisions, combined with the decay of radioactive elements, created an extremely hot planet. Geologists believe that soon after Earth formed, its surface started to melt, forming an ocean of molten rock approximately 400 kilometers deep. Earth was too hot to sustain water on its surface, which boiled and evaporated into the blistering atmosphere.

Earth changed its structure over time, becoming a differentiated body with a crust, a mantle, and a core. ©AMNH

That’s when a process called differentiation began. Heavier elements sank to form the Earth’s core, which is rich in iron and nickel. The remaining elements went into forming the rocky mantle that makes up most of the planet’s volume (84%). An outer crust, like the skin of an onion, composed of light elements, was extracted from the mantle as volcanic rocks. Our atmosphere (the blanket of gas that surrounds the Earth and is kept in place by gravitational attraction) is composed of the lightest elements of all.

This differentiation — the separation of the planet into layers that are less dense as they extend outward from the core — happened very quickly by geological standards: within the first 100 million years after the Earth’s formation. This process is ongoing and is what makes the planet so dynamic. Plate tectonics, for example, occurs because the planet continues to get rid of its heat.

The Moon's gravity creates a tidal bulge on the Earth on the side closest to the Moon. Another bulge, due to the Earth's motion around the Earth-Moon center of gravity (inertia), forms on the opposite side of the planet. ©AMNH

The role of the Moon

The Moon probably formed some 50 million years after the Earth, which is when scientists think a Mars-sized body collided with our planet. This theory holds that some mass was added to the Earth, but the impact caused most of the material (including a lot of material from Earth) to fly off and get trapped in the Earth’s gravitational field. This stuff then coalesced to form the Moon in only a few hundred years. The Moon’s gravitational pull on the Earth causes the Earth to expand and contract slightly, and causes the oceans to move in tidal rhythm.

Where did Earth’s water come from?

There are two dominant theories:

• The inside-out model proposes that the Earth formed with trace amounts of water structurally bonded to the minerals in the mantle. This water makes its way to the Earth’s surface through volcanic processes.
• The outside-in model proposes that the Earth formed without water, which came with other volatiles from the meteorites or comets that bombarded the young planet. This water was probably mixed into the upper layers of the Earth and was later brought to the surface through volcanism.

Neither model is completely satisfactory, but most scientists support the first. Comets may have given the Earth a little bit of its water, and possibly as much as 20%, but nowhere near enough to fill the oceans. Meteorites, on the other hand, appear to be the building blocks of our planet because they’ve been found to have a composition similar to that of the early Earth.

Studies of meteorites allow scientists to estimate that as much as 0.5% of the weight of the Earth is made up of water. That may not sound like a lot, but considering how big the planet is, it’s more than enough to fill the world’s oceans. The Earth’s mantle is estimated to contain between three to six times as much water as in the oceans, so it’s perfectly feasible that our surface water came from inside the Earth.

A tour of our planetary neighbors shows great variation: Earth's atmosphere consists mainly of nitrogen and oxygen, with trace amounts of other gases. The thick carbon dioxide atmosphere on the surface of Venus is 91 times denser than ours. Meanwhile, the surface density of the thin carbon dioxide atmosphere of Mars is a scant 1/150th of Earth's. ©NASA

How did the hydrosphere and atmosphere form?

Earth scientists believe that during the first few hundred million years after the solar system formed, while the Earth was differentiating into a core, mantle, and crust, the planet started degassing (“burping” volatiles) through volcanic activity. Volatiles that were trapped deep in Earth were released when the rocks that contained them melted and were erupted from volcanoes. Carbon dioxide, methane, sulfur, and other volatiles stayed in their gaseous state to make up the early atmosphere. Erupted water vapor, on the other hand, largely condensed to form the early ocean. Today, volcanism continues to supply the atmosphere and hydrosphere with many of the same gases — think of the white plumes that steam from active volcanoes — but at a much lower level than when the early Earth was releasing so much heat.

The new atmosphere that formed was steamy and sizzling, holding in heat like a greenhouse and obscuring any direct view of the Sun. It would have been something like Venus today, where the dense atmosphere keeps the planet at a scorching 400° C (752° F). While our Earth’s ocean and atmosphere evolved into a life-nurturing system, our neighboring planets fared less well. When they formed, Mars and Venus probably contained concentrations of water similar to Earth’s, but they didn’t hold onto it. Because Venus is so close to the Sun and has a thick, insulating atmosphere, most of its water boiled off. Small in mass, Mars lacked the gravity to hold onto its volatiles, including liquid water. Furthermore, a thin atmosphere and a significant distance from the Sun combine to make the planet cold. The poles of Mars are believed to contain frozen water and carbon dioxide ice caps that extend underground, and recent Rover expeditions yielded evidence that liquid water once ran on its red surface.

Things cool off

Over millions of years, Earth’s thick steam atmosphere slowly cooled to the point where water was stable as liquid. Clouds formed and the atmosphere rained on the oceans — probably one very shallow ocean covering the planet’s surface, since the continents likely did not yet exist. (Since no geologic record exists for the first 150 million years of the Earth’s history, we can only speculate.) By 4.2 billion years ago, the age of some of the oldest rocks, we know there was liquid water on the Earth’s surface, and that the atmopshere was never so hot that it turned to steam nor so cold that it all froze.

Among the first known rocks in the geologic record, this specimen of Acasta gneiss from Ontario, Canada dates back to approximately 4.0 billion years ago. ©AMNH

Why has water persisted on Earth?

Earth may be considered a “Goldilocks” planet: “just right” for liquid water, and thus for life, which requires it. Many factors play a part:

• Earth is just the right distance from the Sun. The atmosphere and the ocean regulate the planet’s temperature, keeping it relatively constant and in a range where liquid water is stable.
• Earth is big enough for its gravity to hold the atmosphere in place.
• Both the atmosphere and the Earth’s magnetic field (see Essay 2.2: “How does the ocean floor get its shape?”) protect the Earth’s surface from harmful UV and solar radiation from the Sun.

Is the volume of water on Earth constant? Trace amounts of water are lost from the atmosphere into space. However, trace amounts are also added from the meteorites that are constantly bombarding the atmosphere. More water is added than lost.

All these things contribute to an exquisite dynamic system that brought the planet’s oceans into being and protects the organisms that inhabit them.

Online Resources:

• NASA: Solar System Exploration – Earth’s Moon
  The importance of the gravity relationship between the Earth and the Moon.
  http://solarsystem.nasa.gov/planets/profile.cfm?Object=Moon
  
  
• AMNH: OLogy – If Rocks Could Talk
  Dr. Rondi Davies ‘interviews’ one of the oldest rocks on Earth.
  http://www.amnh.org/ology/features/ifrockscouldtalk/
  
  
• StarDate: Planet Formation
  Steps to the formation of the planets and the factors that determine the distribution of elements and water. http://stardate.org/resources/ssguide/planet_form.html
  
  
• Wiley: Dynamic Earth – Nebular Theory
  An animation detailing the formation of the solar system. http://higheredbcs.wiley.com/legacy/college/skinner/0471152285/animations/animations/mod_1/nebular_theory.html
  
  
• Wiley: Dynamic Earth – Differentiation of the Earth
  An in-depth look at Earth’s layers during the planet’s formation. http://higheredbcs.wiley.com/legacy/college/skinner/0471152285/animations/animations/mod_1/earth_diff.html
  
  
• AMNH: Creating the Hall of Planet Earth
  The Museum’s Indonesia Expedition to collect sulfur samples as anexample of volcanic degassing. http://www.amnh.org/exhibitions/permanent-exhibitions/rose-center-for-earth-and-space/david-s.-and-ruth-l.-gottesman-hall-of-planet-earth/creating-hall-of-planet-earth