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Solar Nebula

Solar Nebula

Our solar system began forming in a concentration of interstellar dust and hydrogen gas. The cloud contracted under its own gravity and our proto-Sun formed in the center, surrounded by the swirling disk of the solar nebula.

Circumstellar Disks

Circumstellar Disks

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. Hubble Space Telescope image of the Orion Nebula, courtesy of C. R. O'Dell (Rice University) and NASA.

Planetesimals

Planetesimals

In the solar nebula, dust and ice particles occasionally collided and merged. Through this accretion, these tiny particles formed larger bodies that eventually became planetesimals up to a few kilometers across. In the inner, hotter part of the nebula, planetesimals were composed of silicates and metals. In the outer, cooler portion, water ice was the dominant component.

Terrestrial Planets of the Inner Solar System

Terrestrial Planets (Inner
Solar System)

Planetesimals were massive enough that their gravity influenced other planetesimals. This increased the frequency of collisions, causing the largest bodies to grow more rapidly, eventually becoming planetary embryos. Accretion continued until only four large bodies remained — Mercury, Venus, Earth, and Mars.

Gas Giant Planets of the Outer Solar System

Gas Giants (Outer Solar System)

In the cold outer solar nebula, where our Sun's gravity was weaker, much larger planetary embryos formed. The largest ones swept up other embryos, planetesimals, and nebular gas, leading to the formation of Jupiter, Saturn, Uranus, and Neptune.

Solar Nebula Disperses

Solar Nebula Disperses

The growing proto-Sun accumulated much of the nebula’s material long before planets formed. A small portion was later incorporated into the planets, but the remainder was swept away when nuclear reactions were initiated in our Sun’s core. These reactions created a strong solar wind, which expelled the Sun’s outer layers far beyond our solar system. A much weaker solar wind continues to flow today.

Asteroids

Asteroids

Asteroids are rocky remnants from our early solar system; most orbit between the inner and outer planets. Occasionally, asteroids reach Earth’s surface as meteorites, providing scientists with information about the inner solar system.

Asteroid Itokawa image courtesy of the Japan Aerospace Exploration Agency (JAXA).

Comets

Comets

Comets formed in the outer reaches of our solar system early in its development. They are made of ice and dust, materials left from the original nebula. Comets periodically pass close enough to the Sun to heat up and release a long tail of dust and gas.

Comet 67P/Churyumov-Geraimenko, courtesy of the European Space Agency (ESA).

Kuiper Belt

Kuiper Belt

Planetesimals that have not had enough time to accrete into planets populate the Kuiper belt, which extends beyond Neptune. Pluto, considered a dwarf planet, is a large member of the Kuiper belt. The Oort cloud envelops our solar system and contains icy planetesimals. Comets originate in the Oort cloud and the Kuiper belt.

Pluto image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/ Southwest Research Institute.

Planetary Layers

Planetary Layers

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 reorganization caused the rocky planets to have dense, metal-rich inner cores, less-dense mantles, and outer crusts formed from the lightest materials.

Oceans on Mars

Oceans on Mars

Exploration of Mars suggests that the planet has abundant water ice and may have had oceans in the northern lowlands early in its history. As Mars cooled, the water collected as ice beneath the surface and in the polar ice caps.

Planetary Impacts

Planetary Impacts

The last large asteroid impacts on the Moon occurredabout 3.8 billion years ago and produced impact basins up to 1000 kilometers across. Large basins on other planets — such as Mercury and Mars — are thought to have formed at the same time. Erosion, volcanism, and plate-tectonic forces erased traces of these ancient impacts on Earth. Although asteroids and comets continue to strike planets and moons throughout our solar system, the rate of impact events became less frequent after this time.

Lunar Reconnaissance Orbiter image of the Mare Orientale basin, courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Mercury Cools

Mercury Cools

Mercury is covered in curving, cliff-like scarps and wrinkle ridges. These landforms were created when the planet’s crust contracted, or shrank, buckling the surface. This shrinking reduced Mercury’s radius by as much as 7 kilometers and occurred several hundred million years after the planet formed.

Mercury global image and detail of Carnegie Rupes courtesy of NASA/Johns Hopkins University Applied Physics Laboratory.

Valles Marineris, Mars

Valles Marineris, Mars

The Valles Marineris canyon system is over 10 kilometers deep in areas and stretches 4000 kilometers from end to end, about the same distance from California to New York. This canyon began forming 3.0 billion years ago as heat from the interior caused the crust to stretch and break.

Viking Orbiter 1 image, courtesy of NASA and the U.S. Geological Survey.

Outflow Channels on Mars

Outflow Channels on Mars

Occassionally, water trapped beneath the surface of Mars catastrophically flooded across the surface, carving huge channels up to 100 kilometers wide, 1 kilometer deep, and thousands of kilometers long. The floods left long mesas and teardrop-shaped islands, such as those in Osuga Vallis. The floods emptied into the northern lowlands, possibly creating short-lived seas.

Oblique view of Osuga Vallis, courtesy of the European Space Agency (ESA).

Meteorites From Mars

Meteorites from Mars

A few meteorites that have been found on Earth actually came from Mars. These rare samples provide scientists with information about the martian environment and history. Martian meteorites are between 4.5 billion and 180 million years old. This specimen cooled from a lava flow on Mars about 1.3 billion years ago.

Photograph of the Lafayette meteorite by Chip Clark, courtesy of the Smithsonian Institution.

Olympus Mons, Mars

Olympus Mons, Mars

The age of Saturn's thin rings is not well known. Based on the rate at which the rings are spreading, they are estimated to be about 200 million years old. The rings are made of centimeter- to meter-sized particles of ice and dust. Voyager 2 image, courtesy of NASA and the Jet Propulsion Laboratory.

Lava Flows on Venus

Lava Flows on Venus

Unlike other terrestrial planets, the surface of Venus is not heavily cratered. Most of the surface has been covered by lava flows in the last billion years. Volcanos probably continue to erupt on Venus today.

Computer-generated three-dimensional perspective view of the surface of Venus courtesy of NASA/Jet Propulsion Laboratory.

Rings of Saturn

Rings of Saturn

The age of Saturn’s thin rings is not well known. Based on the rate at which the rings are spreading, they are estimated to be less than 200 million years old. The rings are made of centimeter- to meter-sized particles of ice and dust.

Cassini spacecraft images of Saturn and rings courtesy of NASA/Jet Propulsion Laboratory/ Space Science Institute.

Volcanism Ends on Mercury

Volcanism Ends on Mercury

Widespread effusive volcanism on Mercury ended relatively early in the planet’s history, about 3.5 billion years ago. However, some volcanic activity persisted until at least the last second-half of solar system history. The youngest volcanic material is found within the central-peak ring of the 290-kilometer diameter Rachmaninoff basin.

Rachmaninoff basin image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory.

Earth's Moon

Giant Impact

Giant Impact

65 million years ago an asteroid (or comet), 10 to 16 kilometers in diameter, struck Earth onwhat is now the Yucatán Peninsula of Mexico, forming a crater ~180 km wide. This impact is thought to have triggered fires and tsunamis and created a cloud of dust and water vapor that enveloped the globe in a matter of days, resulting in fluctuating global climate changes. The extreme environmental shifts caused a mass extinction of 75% of Earth’s species, including the dinosaurs.

The Moon Forms

The Moon Forms

Material in the debris ring accreted to form our Moon, possibly within a few hundred years. The young Moon was much closer to Earth, and orbited the planet once every few days.

Lunar Magma Ocean

Lunar Magma Ocean

The heat from accreting particles caused the Moon to at least partially melt, creating a lunar magma ocean.

Magma ocean graphic courtesy of NASA Goddard Space Flight Center.

Ancient Lunar Atmosphere

Ancient Lunar Atmosphere

Lava erupting onto the lunar surface ~3.5 billion years ago released gases above the surface faster than those gases could escape to space. This created a temporary atmosphere that dissipated as the frequency of volcanic eruptions decreased.

Ancient lunar atmosphere graphic courtesy of NASA Marshall Space Flight Center.

Oldest Moon Rocks

Oldest Moon Rocks

The Apollo missions returned samples of ancient lunar crustal rocks. These rocks are about 4.5 billion years old, indicating that parts of the Moon’s crust solidified soon after the Moon formed.

Photographs courtesy of NASA Johnson Space Center.

Lunar Crust

Lunar Crust

The lunar magma ocean cooled and crystallized, forming a crust about 40 kilometers thick. Asteroids continued to bombard the Moon, leaving impact craters.

Lunar Volcanism

Lunar Volcanism

Portions of the Moon’s interior remained hot enough to produce magma for more than a billion years after it formed. Molten rock flowed onto the lunar surface through cracks in the crust, spreading out and filling the low regions in the impact basins. Thelava cooled quickly, forming the fine-grained, dark rocks — basalts — sampled during the Apollo missions. The dark areas seen on the Moon are basaltic lava plains.

Apollo 17 image AS17-2444 courtesy of NASA.

Moon Becomes Geologically Inactive

Moon Becomes Geologically Inactive

Lunar volcanism decreased significantly by 3 billion years ago and ceased completely by about 1 billion years ago as the interior of this small body cooled.

Near-side image of the Moon, courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Longer Days More Distant Moon

Longer Days More Distant Moon

The length of our Earth day has increased through time. Approximately 900 million years ago, each day was about 18 hours long. By 370 million years ago, the day was 22 hours long. Today, of course, Earth experiences a 24-hour day. The drag of the tides, caused by the gravitational pull of our Moon, slows Earth’s rotation.

Lunar Reconnaissance Orbiter image showing the ringed Mare Orientale basin, courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Copernicus Crater

Copernicus Crater

Copernicus Crater formed on our Moon less than a billion years ago when an impactor, several kilometers across, struck the surface. The impact produced a circular crater nearly 100 kilometers across and blew material out in prominent rays. The Apollo 12 astronauts collected samples from one of the rays. These samples provide evidence of the timing of the impact.

Lunar Reconnaissance Orbiter image of Copernicus crater courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Closer Moon

Closer Moon

The Moon currently orbits the Earth at a distance of ~384,400 kilometers. It is estimated that 3.9 billion years ago, the Moon orbited the Earth at a distance of ~133,800kilometers. This would have caused the Moon to appear about 3 times larger in the sky.

Images of the Moon courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Tycho Crater

Tycho Crater

Tycho Crater, about 85 kilometers across, is clearly visible on our Moon’s surface. The freshness of the crater and the rays of material radiating from it suggest that this is a young crater; there has been little time to erode it.

Image of the Moon courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Earth Geologic History

Oldest Terrestrial Impact Record

Oldest Terrestrial Impact Record

The oldest impact deposits on Earth are spherules of impact melt found in 3.47 billion year-old rocks within the Barberton Greenstone Belt of South Africa and the Pilbara block of Western Australia.

Thin section of impact melt spherules, courtesy of Dr. Timmons Erickson.

Early Earth Crust

Earliest Remnants of the
Earth's Crust

Tiny zircon grains within the sedimentary rocks of the Jack Hills of Western Australia formed about 4.4 billion years ago. These zircon grains are the remnants of some of the Earth's oldest crust and have survived multiple cycles of erosion, redeposition and tectonic deformation.

Thin section image of zircon grain courtesy of Dr. Timmons Erickson.

Earth's Initial Crust

Earth's Initial Crust

The surface of the early Earth was molten, heated mostly by asteroid impacts such as the one that formed our Moon. As Earth cooled, its outer surface solidified into a crust. Until it thickened, continued asteroid bombardment broke up the crust.

Volcanism on Earth

Volcanism on Earth

The interior of the early Earth was heated primarily from decay of radioactive elements. While this heat-generating process is still important today, it was much more significant in the early Earth, causing the planet to be more volcanically active than it is now.

Hadean Earth landscape graphic courtesy of Dr. Simone Marchi and Dr. Dan Durda, Southwest Research Institute.

Earth's Early Atmosphere and Oceans

Earth's Early Atmosphere
and Oceans

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.

Earth Adds Land

Earth Adds Land

Undersea volcanos erupted lava that eventually reached the ocean surface, forming active volcanic islands. Similar processes are observed on the Hawai‘ian Islands and other volcanic island chains today.

Earth's Earliest Continental Rocks

Earth's Earliest Continental Rocks
and Oceans

The oldest rocks exposed on Earth are nearly 4.0 billion years old. These metamorphic rocks—the Acasta gneisses— are found in Canada. It is probably no coincidence that the oldest rocks found are those that formed as the rate of asteroid bombardment in our solar system slowed.

Earth's Oldest Sedimentary Rocks

Earth's Oldest Sedimentary Rocks

Earth's oldest sedimentary rocks, found in Greenland, are about 3.9 billion years old. Unusual chemical traces in these rocks may suggest that life existed when they formed. Image courtesy of Dr. Graham Ryder.

Oxygen Increases in the Atmosphere

Oxygen Increases in the Atmosphere

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.

Vredefort Impact Crater

Vredefort Impact Crater

The Vredefort crater in South Africa is the circular remnant of an impact that struck Earth about 2 billion years ago. Impact events had decreased in our solar system, but they still occurred occasionally. The original crater was probably about 140 kilometers across, but erosion and sediment cover have reduced the exposed crater to about 80 kilometers in diameter. Space shuttle image STS51I-33-56AA, courtesy of NASA.

The Oldest Rocks in the Grand Canyon

The Oldest Rocks in the Grand Canyon

Unlike Valles Marineris on Mars, the Grand Canyon was carved by river action. In the last 10 million years or so, the Colorado River has cut a 1.5-kilometer-deep channel into Earth’s crust, slicing through almost 1.5 billion years of geologic history. The oldest rocks are exposed at the bottom of the Grand Canyon, providing geologists with evidence of ancient environments and events.

Grand Canyon image courtesy of Dr. Graham Ryder.

The Oldest Rocks in the Grand Canyon - Basement Rocks

The Oldest Rocks in the Grand Canyon - Basement Rocks

The "Vishnu Basement Rocks" were originally volcanic rocks exposed at the surface covered by sediments. Over time, overlaying deposits of rock and sediment put great pressure on the original Basement Rocks, metamorphosing the rocks. Other volcanic, or igneous rocks, also intruded into the Basement Rocks.

Basement rocks image courtesy of the National Park Service.

Earth Goes into a Deep Freeze

Earth Goes into a Deep Freeze

Earth experienced several ice ages that may have almost completely enveloped it in ice. These repeated deep freezes lasted for millions of years, apparently ending with abrupt warming. The fluctuations from ice age to warm period may have nearly wiped out life, but could have ultimately driven the evolution of multicellular organisms.

Barren Land

Barren Land

While the oceans were teeming with life, the land remained essentially barren, populated only by microbial life such as bacterial mats, algae, and lichens.

Sudbury Impact

Sudbury Impact

An asteroid ~12 kilometers in diameter impacted the Canadian Shield 1.85 billion years ago. The impact left behind shatter cones and produced one of the Earth’s oldest and largest impact structures, the ~200–250 kilometer-wide Sudbury crater in Ontario. The crater hosts one of the world’s richest nickel, copper, and platinum group element deposits.

Shatter cones in the Sudbury impact complex, image courtesy of Dr. Martin Schmieder.

Life on Earth

Origin of Life

Origin of Life

Life on Earth may have begun soon after the asteroid impacts became less frequent and Earth’s surface and oceans stabilized. While there is strong evidence to support life existing as early as 4.3 billion years ago, there is no undisputed fossil evidence for life in the rock record until about 3.5 billion years ago.

Early Life

Early Life

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.

A New Type of Cell

A New Type of Cell

The first eukaryotes were single-celled organisms with distinct nuclei. Today, eukaryotes have evolved into complex organisms, including fungi, protists, plants, and animals. Prokaryotes, like bacteria, have a small amount of DNA, contained in single, round chromosomes. Eukaryotes have DNA organized in multiple chromosomes and contained in both a nucleus and mitochondria. These organisms can reproduce by exchanging DNA between cells, resulting in greater diversity and more rapid evolution.

Making Oxygen

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. The lens cap gives an idea of scale. Image courtesy of Sarah Hanson, Earth Science Department, Adrian College, Adrian, Michigan.

Multicellular marine Life

Multicellular Marine Life

The first marine life was likely a plant. Dating back more 555 million years, Chinggiskhaania bifurcata, a fossilized seaweed discovered in Mongolia, is one of the earliest known multicellular marine algae. The simple, multicellular placozoa, or Trichoplax adharens, was most likely the first marine animal.

Cambrian Explosion of Life

Cambrian Explosion of Life

Approximately 543 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 shells. 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, occurring in perhaps as little as 10 million years.

Terrestrial Plants and Insects

Terrestrial Plants and Insects

Primitive land plants, evolved from algae, provided food sources and niches for other organisms to exploit. The earliest plants were small, hugged the ground and required water to reproduce. Within about 60 million years, towering forests dominated the landscape. Insects originated during the Ordovician period, around 480 million years. They are thought to have developed at the same time as terrestrial plants.

Ocean Life Diversifies

Ocean Life Diversifies

An increasing variety and abundance of marine life populated the warm seas. While sponges, corals, and brachiopods occupied the seafloor, trilobites, cephalopods (related to squid and octopi), and jawed fishes swam in the waters above. Despite this diversity only 20% of all known species on Earth today live in water.

The first fish were jawless and belonged to the “Agnatha” superclass of fish, with characteristically defined skulls, made of either bone or cartilage. An example of this type of fish is the Haikouichthys which is now extinct but lived during the Cambrian Explosion.

In Devonian time, from about 416 to 355 million years ago, fishes of many different types swam and hunted in the seas. Lobe-finned fishes — ancestors to the amphibians — and the early sharks made their appearance by this time.

Amphibians

Amphibians

Some lobe-finned fish evolved webbed, leg-like limbs. They probably lived in shallow swampy areas where their limbs allowed them to maneuver more easily than fins. Eventually (over ~50 million years), they evolved other support systems that prevented them from drying out and allowed them to move on land as the first amphibians. Although amphibians live on land, they must return to water to lay their eggs. All extant adult amphibians are carnivorous.

Seed Plants

Seed Plants

Because seed-bearing plants do not need water to reproduce, they were able to spread into environments not open to the earliest plants. The casing keeps the seeds from drying out and protects the nutrients, allowing seeds to lie dormant through harsh conditions. The first seed plants were the Gymnosperms, which include the pine and giant redwood trees of today.

Reptiles

Reptiles

While catastrophic for some species, extinctions offer opportunities to survivors. Reptiles arose about 300 million years ago, and they replaced amphibians as the dominant land-dwelling animal following the Permian Extinction. Reptiles produce an egg that contains nutrients within a protective shell; unlike amphibians, they do not have to return to the water to reproduce. This difference allowed reptiles to move into new land environments. Early reptiles, like Euparkeria , may have been ancestors of the dinosaurs.

Permian Mass Extinction

Permian Mass Extinction

About 250 million years ago, more than 90% of all species on Earth mysteriously perished during a mass extinction, perhaps within just a few thousand years. Trilobites became extinct; coral, bryozoan, and brachiopod species diminished. Land plants and animals were also impacted; amphibian species dwindled and fungi dominated some ecosystems. The cause of the “Great Dying” is not yet known, although many scientists attribute the catastrophe to major environmental changes, perhaps involving reorganization of ocean circulation or massive volcanism.

First Dinosaurs

First Dinosaurs

Approximately 230 million years ago, during the Triassic Period, the dinosaurs appeared, evolved from the reptiles. Plateosaurus was one of the first large plant-eating dinosaurs, a relative of the much larger sauropods. It grew to about 9 meters in length. Plateosaurus probably walked on all four legs most of the time, occasionally rearing up to eat from the tops of trees.

First Mammals

First Mammals

Fossils of the earliest mammals are more than 200 million years old. These small, shrew-like animals probably lived in caves or burrows and hunted insects and small reptiles at night.

First Bird

First Bird

Archaeopteryx is the earliest undisputed bird. A weak flyer, it shared characteristics with its dinosaur ancestors. Fossils show that Archaeopteryx , like dinosaurs, had teeth, a long bony tail, and grasping claws on its wings, but also had a bird-style hip and feathers.

Reptiles Diversify

Reptiles Diversify

Reptiles conquered land, sea, and air during the Mesozoic Era between 250 and 65 million years ago. Plesiosaurs were marine reptiles that reached 13 meters in length.

Prehistoric Turtles

Prehistoric Turtles

Giant land turtles lived at the time of the dinosaurs, and looked similar to turtles today. The largest of the land turtles was about 2.5 meters long, weighed as much as 4 metric tons, and had a sturdy shell to survive predators.

First Flowering Plants

First Flowering Plants

Flowering plants — angiosperms — are also seed plants. The flowers attract insects and other organisms, leading to pollination and development of fruit-encased seeds. Much of our food supply — fruit, wheat, rice — comes from angiosperms.

Giant Impact

Giant Impact

An asteroid or comet, 10 to 16 kilometers in diameter, struck Earth in what is now the Yucatán Peninsula of Mexico. This impact is thought to have triggered fires and tsunamis and created a cloud of dust and water vapor that enveloped the globe in a matter of days, resulting in fluctuating global climate changes. The extreme environmental shifts would have caused a mass extinction of 75% of Earth's species, including the dinosaurs.

Age of Mammals

Age of Mammals

Fossils of the earliest mammals are more than 200 million years old. These small, shrew-like animals probably lived in caves or burrows and hunted insects and small reptiles at night.

Human Ancestors

Human Ancestors

The first early hominids may have been bipedal — walking upright on two legs. By about 2.5 million years ago, hominids were making tools. Humans 500,000 years ago were using fire. By 3000 years ago calendars, crop planting, and navigation were based on the stars, Moon, and Sun.

One Small Step -One Giant Leap

One Small Step -One Giant Leap

On July 20, 1969, Neil Armstrong was the first human to set foot on the lunar soil of our Moon. The adventure of exploring where we come from and where we are going continues. Apollo 11 image AS11-40-5877, courtesy of NASA.