Collisions are at the core of solar system formation (Birth of Worlds), and continue to be one of the most important processes throughout our solar system.
Those impact scars, and the materials that make up the objects themselves, tell the story of our solar system's formation -- and how planets and their moons continued to change since those early days. This topic will explore the evidence of impacts and the stories told.
Today, Earth encounters many bits of ice and rock, and most burn up in the atmosphere. Some chunks make it to Earth's surface as meteorites. Impacts by larger objects, capable of damage to people or cities, are extremely rare. Currently, no known asteroids are predicted to collide with our world.
In the past, larger collisions occurred, with profound effects. Did you know that the Earth-Moon system most likely formed by the impact of the early Earth with another planet half Earth's size? The huge northern lowlands of Mars, with its very thin crust, are also likely the result of impacts -- and possibly one incredibly large impact! Impacts resurfaced planets and moons, put water on the Moon and Mercury, delivered materials needed by life to Earth and elsewhere, and caused at least one mass extinction -- and consequently the rise of mammals -- on Earth.
NASA scientists have created their own impacts to study this important process -- on the Moon during the LCROSS mission and on comet Tempel 1 during the Deep Impact mission.
Resources in this topic will let you explore impacts and the results of impacts, safely. And what about those objects that crash into large planetary bodies -- comets and asteroids? You can learn more about them in the Small Bodies, Big Impacts and Asteroids YSS topics.
Impact cratering is the excavation of a planet's surface when it is struck by a meteoroid. Impacts are instantaneous events. They leave very characteristic features.
What are craters?
Craters are roughly circular, excavated holes made by impact events. The circular shape is due to material flying out in all directions as a result of the explosion upon impact, not a result of the impactor having a circular shape (almost no impactors are spherical). Craters are the most common surface features on many solid planets and moons -- Mercury and our Moon are covered with craters.
What happens when an impactor hits?
When an impactor strikes the solid surface of a planet, a shock wave spreads out from the site of the impact. The shock wave fractures the rock and excavates a large cavity (much larger than the impactor). The impact sprays material -- ejecta -- out in all directions. The impactor is shattered into small pieces and may melt or vaporize. Sometimes the force of the impact is great enough to melt some of the local rock. If an impactor is large enough, some of the material pushed toward the edges of the crater will slump back toward the center and the rock beneath the crater will rebound, or push back up, creating a central peak in the crater. The edges of these larger craters also may slump, creating terraces that step down into the crater.
What are the major parts of a crater?
What are the different kinds of craters?
Simple craters are small bowl-shaped, smooth-walled craters (the maximum size limit depends on the planet).
This image shows a simple crater on Mars that has no central peak or terraces around its edges. The crater is 2 km (about 1 mile) wide. An extensive blanket of ejecta covers the area around the rim.
Complex craters are large craters with complicated features. Larger craters can have terraces, central peaks and multiple rings.
This next image shows a complex crater in the northern region of Mars. This crater is about 20 km (12 miles) across and has a large central peak and terraces around its rim. The ejecta blanket has lobes, which may indicate wet material was ejected, suggesting that subsurface water or melted ice was mixed into the debris.
Impact basins are very large impact structures that are more than 300 km (185 miles) in diameter. The largest impact basin on the Moon is 2,500 km (1,550 miles) in diameter and more than 12 km (7 miles) deep. Large impact basins are also found on other planets, including Mars and Mercury.
The large circular dark areas in the image are impact basins, created as huge impactors struck the Moon. Lava later flowed across the low floors of the basins, giving them a darker, smoother appearance than the surrounding, brighter highlands. The dark basins can be seen by the naked eye.
Scientists describe other types of craters as well:
How are large craters different than small ones?
Small craters often are simple bowl-shaped depressions. The structure of large craters is more complex because they collapse, forming terraces, central peaks, central pits, or multiple rings.
The largest impact craters are called impact basins.
What influences the size and shape of a crater?
The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor is, the larger the crater becomes. Typically, materials from space hit Earth at about 20 km (slightly more than 12 miles) per second. Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object. Smaller planets have less gravitational "pull" than large planets; impactors will strike at lower speeds. The greater the mass of the impactor, the greater the size of the crater.
Craters most often are circular. More elongate craters can be produced if an impactor strikes the surface at a very low angle -- less than 20 degrees.
How can craters be used to determine the age of a planet or moon?
Scientists record the size and number of impact craters -- and how eroded they are -- to determine the ages and histories of different planetary surfaces. Early in the formation of our solar system (before 3.9 billion years ago) there was lots of large debris striking the surfaces of the young planets and moons; these older impact basins are larger than the more recent craters. As a rule of thumb, older surfaces have been exposed to impacting bodies (meteoroids, asteroids and comets) for a longer period of time than younger surfaces. Therefore, older surfaces have more impact craters. Mercury and the Moon are covered with impact craters; their surfaces are very old. Venus has fewer craters; its surface has been covered recently (in the last 500 million years!) by lava flows that obscured the older craters. Much of Earth's surface is recycled through plate tectonic activity (and erosion), so Earth also has few craters.
Why does the Moon have so many craters while Earth has so few?
On Earth, impact craters are harder to recognize because of weathering and erosion of its surface. The Moon lacks water, an atmosphere and tectonic activity, three forces that erode Earth's surface and erase all but the most recent impacts. Approximately 80% of Earth's surface is less than 200 million years old, while over 99% of the Moon's surface is more than 3 billion years old. Essentially, the Moon's surface has not been modified since early in its history, so most of its craters are still visible.
What are some of Earth's famous impact craters?
Barringer Crater (Meteor Crater) in Arizona, United States, is a simple crater created when a 50-m-wide (160-foot-wide) iron-rich meteroid struck Earth's surface about 50,000 years ago -- a very recent event to a geologist. The crater is about 1.2 km (a little more than 0.5 miles) across and 200 m (650 feet) deep. Its features, such as the ejecta blanket beyond its rim, are well preserved because of the crater's youth; it has not experienced extensive erosion. Fragments of the Canyon Diablo meteorite were found inside the crater.
The Vredefort impact crater, about 100 km (60 miles) from Johannesburg, South Africa, was formed just a little over 2 billion years ago. It is the oldest and largest impact crater recognized on Earth's surface. The crater has been extensively eroded, but is believed to originally have been as much as 300 km (185 miles) across.
The Chicxulub crater in the Yucatan peninsula, Mexico, is not visible at the surface of the seafloor. Scientists rely on geophysical images for information about its size and shape. This crater is believed to have formed when an asteroid struck Earth 65 million years ago. 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.
How many objects from space impact Earth each year?
Earth and the other planets are constantly bombarded by tiny debris from space, much of which burns up in the atmosphere. Meteors -- incorrectly called shooting stars -- are the streaks of light created as particles of dust and ice vaporize in our atmosphere. Sometimes lots of particles strike at one time, creating meteor showers. Some of this tiny debris makes it to Earth's surface and is mixed with soil and ocean sediment.
Early in the formation of the solar system, frequent and large impacts were common for all of the planets and moons. This "period of heavy bombardment" ended by about 3.9 billion years ago. However, impacts still occur across our solar system, but at a reduced rate. Meteor Crater formed only 50,000 years ago. Earth continues to be a target -- and contrary to popular opinion, the Moon does not act as a meteoroid deflector (it is too small and too distant!). Scientists estimate that Earth and the other terrestrial planets are struck by, on average, five asteroids less than 2 km (a little over 1 mile) across every million years. Larger impacts also still occur, but these are much more rare.
These sites provide even more information:
This site includes information on asteroid and comet impacts, and on the odds of Earth being struck in the future.
Origin of the Earth and Moon
The giant impact hypothesis suggests that the Moon formed as the result of a colossal impact onto the accumulating Earth, heating it and flinging the raw ingredients for the Moon into orbit around the Earth. The hypothesis is consistent with our ideas for how planets were assembled and explains some important features of the Earth-Moon system.
Impact Craters in the Solar System
This brief article explains the different shapes of impact craters and different places in the solar system they can be found.
Cosmic Earth Impacts
The NASA History Office has provided a very short description of well-known large impacts on Earth.
Asteroid and Comet Impact Hazards
FAQs about the hazard of impacts between Near Earth Objects (NEO) and Earth.
Near Earth Object Program
This site has information about the discoveries of Near Earth Objects (NEOs), tracking of NEOs, their orbits, and more.
NASA study shows asteroid impacts may have accelerated life on Earth.
This article describes the relationship between early impact bombardment on Earth and early life.
Solar System's Biggest Impact Scar Discovered
This article describes research on Mars' northern lowlands and research on their possible formation by impacts.
See also: YSS topic Comets and YSS topic Asteroids.
NASA missions to the rocky inner planets and to the moons of the outer planets are exploring evidence of impacts. A few missions to comets and asteroids are trying to uncover critical evidence about the make-up of the objects that are impacting planets and moons.
The NASA MESSENGER mission has been returning amazing images and data of Mercury for over three years and is mapping out its surface. Scientists are using the numbers of craters for various regions to help date how recent the volcanic activity on Mercury occurred.
Lunar Reconnaissance Orbiter
This mission is orbiting the Moon and mapping its surface in great detail. The number and ages of craters on the Moon are being used to date the ages of other planets and moons throughout the solar system.
This mission orbiting Saturn has been studying its many icy moons, with their varied craters. Mimas is known for its colossal crater, formed in an impact which may have come close to shattering this moon.