465
Birth of a Planet
Birth of a Planet
According to the Big Bang theory, the
formation of our planet began about 13.7
billion years ago with a cataclysmic explosion that created all matter and space
(FIGURE 19.4). Initially, atomic particles
(protons, neutrons, and electrons)
formed. Later, as this debris cooled, atoms
of hydrogen and helium, the two lightest
elements, began to form. Within a few hundred million years, clouds of these gases
condensed and coalesced into stars that
compose the galactic systems we now
observe.
As these gases contracted to become
the first stars, heating triggered the process
of nuclear fusion. Within stars’ interiors,
hydrogen atoms convert to helium atoms,
releasing enormous amounts of radiant
energy (heat, light, cosmic rays). Astronomers have determined that in stars more
massive than our Sun, other thermonuclear
reactions occur that generate all the elements on the periodic table up to number
26, iron. The heaviest elements (beyond
number 26) are only created at extreme
temperatures during the explosive death of
a star perhaps 10 to 20 times more massive
than the Sun. During these cataclysmic
supernova events, exploding stars produce
all of the elements heavier than iron and
spew them into interstellar space. It is from
such debris that our Sun and solar system
formed. According to the Big Bang scenario, atoms in your body were produced
billions of years ago in the hot interior of
now defunct stars, and the formation of the
gold in your jewelry was triggered by a
supernova explosion that occurred trillions
of miles away.
From Planetesimals
to Protoplanets
Recall that the solar system, including
Earth, formed about 4.6 billion years ago
from the solar nebula, a large rotating cloud
of interstellar dust and gas (see Figure 1.23,
page 21). As the solar nebula contracted,
most of the matter collected in the center
to create the hot protosun, while the
remainder became a flattened spinning
disk. Within this spinning disk, matter
gradually coalesced into clumps that
collided and stuck together to become
asteroid-size objects called planetesimals.
The composition of each planetesimal
was determined largely by its distance
from the hot protosun.
Near the orbit of Mercury only metallic
grains condensed from the solar nebula.
Near Earth’ s orbit, metallic as well as
rocky substances condensed, and beyond
Mars ices of water, carbon dioxide,
methane, and ammonia formed. It was
from these clumps of matter that the planetesimals were made and, through repeated
collisions and accretion (sticking together),
grew into eight protoplanets and their
moons (Figure 19.4).
At some point in Earth’ s evolution
a giant impact occurred between a Marssized planetesimal and a young, semimolten Earth. This collision ejected huge
amounts of debris into space, some of
which coalesced to form the Moon
(Figure 19.4 J, K, L).
Earth’ s Early Evolution
As material continued to accumulate, the
high-velocity impact of interplanetary
debris (planetesimals) and the decay of
radioactive elements caused the
temperature of our planet to steadily
increase. During this period of intense
heating, Earth became hot enough that
iron and nickel began to melt. Melting
produced liquid blobs of heavy metal
that sank under their own weight. This
process occurred rapidly on the scale of
geologic time and produced Earth’ s dense
iron-rich core. The formation of a molten
iron core was the first of many stages of
chemical differentiation in which Earth
converted from a homogeneous body, with
roughly the same matter at all depths, to a
layered planet with material sorted by
density.
This early period of heating also
resulted in a magma ocean, perhaps
several hundred kilometers deep. Within
the magma ocean, buoyant masses of
molten rock rose toward the surface and
eventually solidified to produce a thin,
primitive crust. Earth’ s first crust was
likely basaltic in composition, similar to
modern oceanic crust.
This period of chemical differentiation
established the three major divisions of
Earth’ s interior—the iron-rich core, the thin
primitive crust, and Earth’ s thickest layer,
the mantle, located between the core and
the crust. In addition, the lightest materials,
including water vapor, carbon dioxide, and
other gases, escaped to form a primitive
atmosphere and shortly thereafter the
oceans (FIGURE 19.5).
C O N C E P T C H E C K 1 9 . 2
What two elements made up most of the
very early universe?
What is the name of the cataclysmic event
in which an exploding star produces all of
the elements heavier than iron?
Briefly describe the formation of the
planets from the solar nebula.
3
2
1
D I D Y O U K N O W ?
The names of several periods on the
geologic time scale refer to places that
have prominent strata of that age. For
example, the Cambrian period is taken
from the Roman name for Wales
(Cambria). The Permian is named for
the province of Perm in Russia, while
the Jurassic period gets its name from
the Jura Mountains located between
France and Switzerland.
Birth of a Planet
Birth of a Planet
According to the Big Bang theory, the
formation of our planet began about 13.7
billion years ago with a cataclysmic explosion that created all matter and space
(FIGURE 19.4). Initially, atomic particles
(protons, neutrons, and electrons)
formed. Later, as this debris cooled, atoms
of hydrogen and helium, the two lightest
elements, began to form. Within a few hundred million years, clouds of these gases
condensed and coalesced into stars that
compose the galactic systems we now
observe.
As these gases contracted to become
the first stars, heating triggered the process
of nuclear fusion. Within stars’ interiors,
hydrogen atoms convert to helium atoms,
releasing enormous amounts of radiant
energy (heat, light, cosmic rays). Astronomers have determined that in stars more
massive than our Sun, other thermonuclear
reactions occur that generate all the elements on the periodic table up to number
26, iron. The heaviest elements (beyond
number 26) are only created at extreme
temperatures during the explosive death of
a star perhaps 10 to 20 times more massive
than the Sun. During these cataclysmic
supernova events, exploding stars produce
all of the elements heavier than iron and
spew them into interstellar space. It is from
such debris that our Sun and solar system
formed. According to the Big Bang scenario, atoms in your body were produced
billions of years ago in the hot interior of
now defunct stars, and the formation of the
gold in your jewelry was triggered by a
supernova explosion that occurred trillions
of miles away.
From Planetesimals
to Protoplanets
Recall that the solar system, including
Earth, formed about 4.6 billion years ago
from the solar nebula, a large rotating cloud
of interstellar dust and gas (see Figure 1.23,
page 21). As the solar nebula contracted,
most of the matter collected in the center
to create the hot protosun, while the
remainder became a flattened spinning
disk. Within this spinning disk, matter
gradually coalesced into clumps that
collided and stuck together to become
asteroid-size objects called planetesimals.
The composition of each planetesimal
was determined largely by its distance
from the hot protosun.
Near the orbit of Mercury only metallic
grains condensed from the solar nebula.
Near Earth’ s orbit, metallic as well as
rocky substances condensed, and beyond
Mars ices of water, carbon dioxide,
methane, and ammonia formed. It was
from these clumps of matter that the planetesimals were made and, through repeated
collisions and accretion (sticking together),
grew into eight protoplanets and their
moons (Figure 19.4).
At some point in Earth’ s evolution
a giant impact occurred between a Marssized planetesimal and a young, semimolten Earth. This collision ejected huge
amounts of debris into space, some of
which coalesced to form the Moon
(Figure 19.4 J, K, L).
Earth’ s Early Evolution
As material continued to accumulate, the
high-velocity impact of interplanetary
debris (planetesimals) and the decay of
radioactive elements caused the
temperature of our planet to steadily
increase. During this period of intense
heating, Earth became hot enough that
iron and nickel began to melt. Melting
produced liquid blobs of heavy metal
that sank under their own weight. This
process occurred rapidly on the scale of
geologic time and produced Earth’ s dense
iron-rich core. The formation of a molten
iron core was the first of many stages of
chemical differentiation in which Earth
converted from a homogeneous body, with
roughly the same matter at all depths, to a
layered planet with material sorted by
density.
This early period of heating also
resulted in a magma ocean, perhaps
several hundred kilometers deep. Within
the magma ocean, buoyant masses of
molten rock rose toward the surface and
eventually solidified to produce a thin,
primitive crust. Earth’ s first crust was
likely basaltic in composition, similar to
modern oceanic crust.
This period of chemical differentiation
established the three major divisions of
Earth’ s interior—the iron-rich core, the thin
primitive crust, and Earth’ s thickest layer,
the mantle, located between the core and
the crust. In addition, the lightest materials,
including water vapor, carbon dioxide, and
other gases, escaped to form a primitive
atmosphere and shortly thereafter the
oceans (FIGURE 19.5).
C O N C E P T C H E C K 1 9 . 2
What two elements made up most of the
very early universe?
What is the name of the cataclysmic event
in which an exploding star produces all of
the elements heavier than iron?
Briefly describe the formation of the
planets from the solar nebula.
3
2
1
D I D Y O U K N O W ?
The names of several periods on the
geologic time scale refer to places that
have prominent strata of that age. For
example, the Cambrian period is taken
from the Roman name for Wales
(Cambria). The Permian is named for
the province of Perm in Russia, while
the Jurassic period gets its name from
the Jura Mountains located between
France and Switzerland.