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
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
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
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.
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.
Cycles of a Living Planet
from the Hall of Planet Earth
The Earth is a dynamic planet. Geological and biological processes cause
energy and the elements necessary for life — carbon, hydrogen, nitrogen,
oxygen, sulfur, and phosphorus — to circulate through global “reservoirs.”
These reservoirs are the biosphere (the living portion of our planet),
the atmosphere, the ocean, and the solid Earth. It is only because of this
cycling that life can thrive. The cycling of elements determines the environment,
for example by regulating the composition, and thus the temperature, of
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
- 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
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
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
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