Some 7 billion miles from the Sun lies the heliopause, which marks the
boundary between the Sun’s domain and interstellar space.About 20 billion miles
from the Sun is a region of gas and dust, possibly remnants of the original solar
nebula. A ring of comets that make up the Kuiper belt lies on the ecliptic in this
region. Several trillion miles from the sun is a shell of comets called the Oort
cloud that formed from the leftover gas and ice of the original solar nebula.THE PROTOEARTH
Around 4.6 billion years ago, the primal Earth emerged from a spinning, tur-
bulent cloud of gas, dust, and planetoids surrounding an infant star. During the
next 700 million years, the cloud settled into a more tranquil solar system, and
the Sun’s third planet began to take shape. The molten Earth continued to
grow by accumulating planetesimals, most of which were hot with tempera-
tures exceeding 1,000 degrees Celsius.
Due to drag forces created by leftover gases in interplanetary space,
Earth’s orbit began to decay. The formative planet slowly spiraled inward
toward the Sun, sweeping up additional planetesimals along the way like a cos-
mic vacuum cleaner. Eventually, Earth’s path around the Sun was swept clean
of interplanetary material, leaving a gap in the disk of planetesimals, and its
orbit stabilized near its present position.
The core and mantle (Fig. 4) segregated possibly within the first 100
million years during a time when Earth was in a molten state caused by
radioactive isotopes and impact friction from planetesimals. The presence of
magnetic rocks as old as 3.5 billion years suggests that Earth had a molten
outer core and a solid inner core comparable to their present sizes at an early
age.The core attracted siderophilic, or iron-loving, materials such as gold, plat-
inum,and certain other elements originating from meteorite bombardments
during the planet’s early formation.
Earth’s interior was hotter, less viscous, and more vigorous, with a highly
active convective flow. Heavy turbulence in the mantle with a heat flow three
times greater than that today produced violent agitation on Earth’s surface.
This turmoil created a sea of molten and semimolten rock broken up by giant
fissures, from which fountains of lava spewed skyward.
During the first half-billion years, Earth’s surface was scorching hot. Heat
of compression from the primordial atmosphere with pressures a hundred
times greater than that today resulted in surface temperatures hot enough to
melt rocks.When the Sun ignited, the strong solar wind blew away the lighter
components of Earth’s atmosphere. Meanwhile,a massive bombardment of
meteorites blasted the remaining gases into space, leaving the planet in a near
vacuum much like the Moon today.Historical Geology