Impact Cratering
Name: _________________________ Pg: _______
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.
This portion of the Moon is covered by numerous circular holes. These are
impact craters, each of which was formed when an asteroid or comet
collided with the Moon's surface. The large number of craters in this
region indicates that this part of the Moon is quite ancient. Geologic
processes have not erased the craters with time.
Apollo 16 photograph courtesy of NASA.
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?
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Floor – The bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding
ground.
Central peaks – Peaks formed in the central area of the floor of a large crater. For larger craters
(typically a few tens of kilometers in diameter) the excavated crater becomes so great that it
collapses on itself. Collapse of the material back into the crater pushes up the mound that forms the
central peak. At the same time, the rock beneath the crater rebounds, or bounces back up to add to
the peak.
Walls – The interior sides of a crater, usually steep. They may have giant stair-like terraces that are
created by slumping of the walls due to gravity.
Rim – The edge of the crater. It is elevated above the surrounding terrain because it is composed of
material pushed up at the edge during excavation.
Ejecta – Rock material thrown out of the crater area during an impact event. It is distributed outward
from the crater's rim onto the planet's surface as debris. It can be loose materials or a blanket of
debris surrounding the crater, thinning at the outermost regions.
Rays – Bright streaks extending away from the crater sometimes for great distances, composed of
ejecta material.
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 kilometers (about 1 mile) wide. An extensive blanket of
ejecta covers the area around the rim.
Image from the Mars Global Surveyor, courtesy of the Lunar and Planetary Institute.
Complex craters are large craters with complicated features. Larger craters can have terraces, central
peaks, and multiple rings.
Copernicus is a large crater (93 kilometers or 60 miles wide) on the Moon.
The inner walls of the crater have collapsed to form a series of step-like
terraces, and a central peak is visible in the center of the image.
Apollo 17 image courtesy of NASA.
A complex crater in the northern region of Mars. This crater is about
20 kilometers (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.
Image from the Viking Orbiter, courtesy of the Lunar and Planetary
Institute.
Impact basins are very large impact structures that are
more than 300 kilometers (185 miles) in diameter. The
largest impact basin on the Moon is 2500 kilometers
(1550 miles) in diameter and more than 12 kilometers
(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.
Galileo Image (PIA00405), produced by the United
States Geological Survey, courtesy of NASA.
Scientists describe other types of craters as well:
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Multi-ring basins – A very large impact basin surrounded by as many as five or six circular rings of
mountain chains in addition to the main basin rim.
Irregular craters – Craters with irregular shapes or multiple impact craters formed at the same time.
Oblong craters can be created by impacts striking the surface at a very low angle.
Degraded craters – Craters that have become eroded due to weathering, lava flows, impacting, or
downslope movement of material.
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. Very large impact
craters greater than 300 kilometers (185 miles) across are called impact basins.
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-meter-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 kilometers (a little more than 0.5
miles) across and 200 meters (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.
Image courtesy of D. Roddy through the Lunar and Planetary Institute.
The Vredefort impact crater, about 100 kilometers (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 kilometers (185 miles) across.
Space shuttle image STS51I-33-56AA, courtesy of the Lunar and
Planetary Institute.
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 image shows the variations in
the gravity field near the buried impact crater. The image shows ring-like
structures that extend to about 280 kilometers (175 miles) from the
center.
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.
Image courtesy of V. L. Sharpton through the Lunar and Planetary
Institute.
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 kilometers (a little over 1 mile) across every million
years. Larger impacts also still occur, but these are much more rare.
This article can be found here:
http://www.lpi.usra.edu/education/explore/shaping_the_planets/impact_cratering.shtml
HOMEWORK
1. Read the article.
2. Highlight/underline and take notes in the margin for what you think is important.
3. Answer the questions on the Cratering Questioning Worksheet