Our Solar System
Solar System Formation and Evolution
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.
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.
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 (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 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
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 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 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).
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.
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
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.
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 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
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
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
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
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
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
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
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.
A Mars-sized object 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.
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
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
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
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.
The lunar magma ocean cooled and crystallized, forming a crust about 40 kilometers thick. Asteroids continued to bombard the Moon, leaving impact craters.
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
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
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 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.
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, 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's Geologic History
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.
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
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
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
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
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 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, 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
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
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
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 "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 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.
While the oceans were teeming with life, the land remained essentially barren, populated only by microbial life such as bacterial mats, algae, and lichens.
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.