Origins:The Evolution of Earth

From the Birth of the Earth to the Origins of Life

Earth Science

The Solar System

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.

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.

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

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

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

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

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

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

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

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.

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. 

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

Earth’s Layered Structure

Upper mantle

Lower mantle

Outer Core

Inner core

Based on earthquake studies, the study of meteorites, and models of Earth’s density, geologists are able to construct the inner structure of the Earth

Crust

Continental Crust

Oceanic Crust

Composition of Earth’s Interior

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

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)

……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

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

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

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

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.

The Origin of Oxygen

Oldest FossilsThe 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.

A New Type of CellEukaryotes, 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.

Making OxygenAs 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

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

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.

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.

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

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

Timeline of Earth’s Evolution

Boundary based upon explosion of life

Boundary based upon mass extinction

Boundary based upon mass extinction

The Geologic Time Scale

Origins:The Evolution of Earth

From the Birth of the Earth to the Origins of Life

This Concludes