3. The Origin of the Solar System
Planetary-origin theories explain the origin of planets by accretion of progressively
larger masses of dust and gas into planetesimals, planetary embryos, and finally into
planets. This theory of solar system formation is known as the Nebular Theory. Most
scientists now refer to this as The Condensation Theory.
4. The Nebular Hypothesis
4.6 billion years ago, our solar system began forming within a concentration of
interstellar dust and hydrogen gas called a molecular cloud. The cloud contracted
under its own gravity and our proto-Sun formed in the hot dense center. The
remainder of the cloud formed a swirling disk called the solar nebula.
5. Within the solar nebula, dust and ice particles embedded in the gas moved, occasionally
colliding and merging. Through this process, called “accretion,” these microscopic particles
formed larger bodies that eventually became planetesimals with sizes up to a few kilometers
across. In the inner, hotter part of the solar nebula, planetesimals were composed mostly of
silicates and metals. In the outer, cooler portion of the nebula, water ice was the dominant
component.
Formation of the Planetisimals
6. Planetesimals were massive enough that their gravity influenced motions of other
planetesimals. This increased the frequency of collisions, through which the largest bodies
grew most rapidly. Eventually, regions of the nebula were dominated by large bodies called
planetary embryos. The process of collision and accretion continued until only four large
bodies remained — Mercury, Venus, Earth, and Mars, the terrestrial planets of our inner
solar system.
The Growth of the Planets
7. Most stars forming in our galaxy, like those of the Orion Nebula, are surrounded by disks of
dust and hydrogen gas called circumstellar disks. Scientists study these disks to learn about
processes that occurred billions of years ago in our solar nebula.
The Nebular Hypothesis
8. The growing proto-Sun accumulated much of the original material from the nebula long before
planets formed. A small portion was incorporated into the planets, but the remainder was swept
away when increasing temperatures and pressures initiated nuclear reactions in our Sun's core.
The force of the reaction caused a strong solar wind to expel the outer layers of the Sun into
space beyond our solar system. A much weaker solar wind continues to flow from our Sun
today. This resulted in the outer planets most likely forming first.
Solar Winds and the Outer Planets
9. The reason that the inner planets are rocky and the outer planets are gaseous is NOT
because of density but rather because of TEMPERATURES within the nebular cloud
Comparative Planetology
10. Asteroids are rocky remnants from our early solar system. Most asteroids orbit between
the inner and outer planets. Asteroids occasionally reach Earth's surface as meteorites,
providing scientists with information about formation of our inner solar system.
Asteroids
11. Comets formed in the outer reaches of our solar system early in its development. They are
made of ice and dust, materials from the original nebula. Comets periodically pass close
enough to the Sun to heat up and release a long tail of dust and gas. Planetesimals that have
not had enough time to accrete into planets populate the Kuiper belt, which extends
beyond Neptune. Some scientists consider Pluto to be a large member of the Kuiper belt,
rather than a planet. The Oort cloud, which envelops our solar system and may extend 30
trillion kilometers away from our Sun, contains icy planetesimals. Comets come from the
Oort cloud and the Kuiper belt.
Comets
12. The Dwarf Planets
A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More
explicitly, the International Astronomical Union (IAU) defines a dwarf planet as (1) a celestial
body in direct orbit of the Sun that is (2) massive enough for its shape to be controlled
by gravitational rather than mechanical forces (that is, it has sufficient mass to overcome its
internal compressive strength and achieve hydrostatic equilibrium, and is thus an ellipsoid in
shape), but that (3) unlike a planet has not cleared its orbital region of other objects.
13. The Age of the Solar System
So far scientists have not found a way to determine the exact age of the Earth directly from Earth
rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate
tectonics. If there are any of Earth's primordial rocks left in their original state, they have not yet
been found. Nevertheless, scientists have been able to determine the probable age of the Solar
System and to calculate an age for the Earth by assuming that the Earth and the rest of the solid
bodies in the Solar System formed at the same time and are, therefore, of the same age. The
ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived
radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with
half lives of 700 million to more than 100 billion years to stable isotopes of other elements.
14. As the inner planets formed they heated up. Their interiors melted and reorganized into layers
of different densities. Melting was caused by heat from impactors striking and accreting, the
sinking of heavy materials to the center, and the decay of radioactive elements. This process
caused the rocky planets to have dense, metal-rich inner cores, less-dense mantles, and outer
crusts formed from the lightest materials.
Planetary Differentiation
17. The magnetosphere soon formed as a result of this differentiation and the Earth’s
rotation. This is important as the magnetosphere acts a a barrier that deflects the solar
winds which protects our atmosphere from being swept away. The interactions
between the magnetosphere and the solar wind creates the Auroraes.
The Magnetosphere
18. The Origin of the Moon
Several theories have been proposed to explain the origins of the moon. Four
theories have been set forth: 1) Simultaneous Formation 2) Fission Theory 3)
Capture Theory and lastly…….
(1) (2)
(3)
19. ……The Impact Theory. According to the impact theory….A Mars-sized planet collided with
Earth, vaporizing, melting, and throwing debris from the impactor and Earth's outer layer into
orbit around Earth, creating an encircling debris ring. Material in the debris ring accreted to
form our Moon, possibly within a few hundred years. Early in its formation, our Moon was closer
to Earth, orbiting once every few days.
The Origin of the Moon
20. The concept that the Moon melted substantially (possibly completely) when it
formed, nicknamed the "magma ocean concept," is a fundamental tenet of lunar
science. These three panels, from left to right, illustrate the lunar magma ocean
concept. The basic concept suggests that as the molten Moon crystallized,
lightweight minerals floated and heavy ones sank. The lighter minerals formed the
primary crust of the Moon. The real magma ocean was much more complicated,
with convection stirring the pot, crystallization taking place at both the bottom and
top, and the magma changing in composition as crystals formed.
The Lunar Magma Ocean
21. The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time
approximately 4.1 to 3.8 billion years ago (Ga) during which a large number of impact craters were formed
on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well. The LHB is "late" only in relation
to the main period of accretion, when the Earth and the other three rocky planets first formed and gained
most of their mass; in relation to Earth or Solar System history as a whole, it is still a fairly early phase.
The Late Heavy Bombardment
22. Volcanic eruptions spewed gases from Earth's interior to the atmosphere, a process called
outgassing that continues today. Most of the gas was carbon dioxide and water vapor. The
water vapor condensed to form part of Earth's oceans as the surface cooled. Comets may
also have contributed water and complex organic molecules to Earth's environments.
The Origin of the Atmosphere
23. The Origin of Life
Earliest life may have begun soon after the
asteroid impacts declined and Earth's surface
and oceans stabilized, but there is no
undisputed fossil evidence for life in the rock
record until about 3 billion years ago.
The earliest life on Earth consisted of
prokaryotes — small single-celled
organisms without nuclei. These earliest
organisms were anaerobic — they did not
require oxygen to live.
24. The Origin of Oxygen
Oldest Fossils
The oldest undisputed fossils known are stromatolites. Modern stromatolites are made of
alternating thin layers of sediment and microbes, primarily bacteria, photosynthetic bacteria,
and archaea that live in warm shallow seas. Photosynthesizing organisms ultimately changed
Earth's atmosphere by consuming its carbon dioxide and releasing oxygen.
25. A New Type of Cell
Eukaryotes, single-celled organisms with distinct nuclei, appeared. Eukaryotes today
include fungi, protists, plants, and animals. Eukaryotes have more complex DNA than
prokaryotes and can reproduce by exchanging DNA between cells, resulting in greater
diversity and more rapid evolution.
26. Making Oxygen
As photosynthesizing organisms pumped oxygen into Earth's atmosphere and ocean, the
oxygen reacted with dissolved iron in the oceans and formed massive rock deposits called
“banded iron formations.” Once the dissolved iron was used in chemical reactions, oxygen
began to increase in the atmosphere. Much of the iron used in industry today originated at
this time. Photograph of a banded iron formation outcrop located on the Upper Peninsula,
Michigan.
Banded Iron Formations
27. As oxygen, primarily from photosynthesis, became more abundant, and the
dissolved iron was depleted through chemical reactions to produce banded iron
formations, oxygen in the atmosphere increased from less than 0.1% to more than
10%. Oxygen eventually formed ozone in the upper atmosphere; ozone shields Earth
from tissue-damaging ultraviolet light.
Oxygen Increases in the Atmosphere
28. The Origin of the Ozone Layer
If it wasn't for stratospheric ozone, life as we know it now wouldn't be possible on Earth.
Ozone prevents harmful ultra-violet radiation from the Sun (light with wavelengths less
than 320 nm) reaching the ground. If allowed to reach Earth, this radiation would severely
damage the cells that plants and animals are made up of. Ozone was first formed in the
Earth's atmosphere after the release of oxygen, between 2000 and 600 million years before
the first humans appeared.
29. The Chemical Origins of Life
The Miller-Urey experiment was an experiment that simulated the conditions thought at the
time to be present on the early Earth, and tested for the occurrence of chemical origins of
life.
30. Patterns in sedimentary rocks more than a billion years old have been interpreted by some
scientists as animal tracks and burrows, suggesting very early multi-cellular life. However,
the Ediacaran Fauna of Australia provides the first direct evidence of large, complex, multi-
cellular animals. The Ediacaran organisms are interpreted to be soft-bodied creatures that
lived together on the surface of the seafloor. Although their relationship to later animals is
unclear, they may have been the ancestors to corals, jellyfish, worms, and mollusks
Timeline of Earth’s Evolution
31. Approximately 540 million years ago, at the beginning of the Cambrian Period, the fossil
record at locations across Earth is marked by the dramatic appearance of complex,
diverse, multicellular organisms with hard parts. By the close of the Cambrian Period
(490 million years ago), virtually every major animal group that exists today — excluding
bryozoans — had appeared. Some scientists think the burst in diversity was rapid,
perhaps in as little as 10 million years.
The Cambrian Explosion of Life
