Early in the formation of our solar system before 3. 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 million years! 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. Essentially, the Moon's surface has not been modified since early in its history, so most of its craters are still visible. Barringer Crater Meteor Crater in Arizona, United States, is a simple crater created when a meter-wide foot-wide iron-rich meteroid struck Earth's surface about 50, years ago — a very recent event to a geologist.
The crater is about 1. 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 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 kilometers 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 image shows the variations in the gravity field near the buried impact crater.
The image shows ring-like structures that extend to about kilometers 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.
How many objects from space impact Earth each year? Obviously, an impact in the ocean would not leave a lasting crater in the water; the ocean floor may or may not be altered, depending on the size of the impactor. If the impactor is large enough it will not be stopped by the water and reach the bottom of the ocean producing a crater.
It is estimated that for an ocean about 4 km deep this requires a rocky asteroid at least 2 km in diameter this impact event is large enough to have worldwide effects. Therefore, oceanic impacts leave behind a much smaller crater, if any, than land impacts. The material ejected in an oceanic impact is mostly water droplets and water vapor, with small amounts of dust, while in a land impact most of the material ejected is solid and melted rock. A projectile is shot through a barrel as it goes through there are appropriate traps that measure the speed of the projectile and it eventually reaches the chamber, where there is an appropriate target of a given material.
The chamber and barrel can hold different pressures, from atmospheric surface pressure down to very low pressures. The main difference between real cratering events and laboratory ones is the energy involved: the projectiles are much smaller as are the impact velocities. As a result, while crater formation in the laboratory is representative of what happens in reality on planetary surfaces, at least up to the formation of the final transient crater before the onset of crater collapse , the magnitude of the shock generated by the impact is generally lower.
It is then very hard to melt target material and even harder to vaporize it! All types of rocks are modified by impact cratering. Generally impact rocks are local rocks that have been modified by the impact event. However, their chemical composition is that of the local rock, where the impact occurred. The actual formation of shatter cones is still debated.
There are different theories on how shatter cones form. They all associated with the interaction of the shock wave generated in the impact event with the rocks. One theory suggests that shatter cones are formed as a result of the compression of the rock as the shock wave goes through.
Another suggests that the shatter cones are the result of the tension in the rock that occurs after the shock wave as gone through and the pressure in the rock goes back to normal. One thing is clear: shatter cones are formed only when the shock wave has weakened below the point of melting or completely fracturing rocks.
For example, you cannot find shatter cone at the bottom of a simple crater unless there is massive slumping of the crater rim , but you may find shatter cones in the central peak of a complex crater, because the central peak is formed by uplifting of material located below the bottom of the original transient crater, where the shock wave was low enough.
Crater erosion on Earth is much larger than on Venus, Mars, or the Moon. Earth, Venus, and Mars have atmospheres, thus some sort of climate. This causes erosion of the craters think sand blasting. All objects experienced some volcanism. The main difference is that on the Moon volcanism ended pretty soon after its formation, it ended a bit later on for Mars, and much later on Venus.
Vredefort crater in South Africa is the largest known impact crater on Earth—almost miles across! At over 2 billion years old, it is also one of the oldest.
Because of erosion over this long time period, the crater is a bit difficult to see. What Is an Impact Crater? Tycho Crater, in the moon's southern hemisphere.
Over time, weathering will change the shape of the crater further. Show the class the photo of the craters on the moon, and ask them what they could be and what could have caused them. Tell them about their origins as comets and asteroids, and discuss with them the impact they can have on Earth. Show the class the images of the Peekskill meteor and the damage it caused to the car and the injury sustained by Ann Hodges in Explain that these were only small meteorites, and that larger ones can lead to greater damage.
Show them the photo of the Tunguska event, and explain how much damage a relatively small strike caused. Discuss the Chicxulub crater and the extinction of the dinosaurs. Show them the video of Meteor Crater in Arizona:. If the class can understand the concepts, talk them through the three stages of crater formation: Compression, Excavation and the Final stage. Otherwise, explain that the crater is caused by the impact pushing material out from underneath the meteorite. Ask the students what factors they think might affect the size of the crater formed, and what effect they might have.
In the first experiment, the students will investigate how the mass of a meteor affects the size of its crater. They should:. Depending on the class, some of the above steps filling containers etc may be done by the teacher prior to the lesson. Take repeat measurements and average the data or create error bars on the graph if appropriate for the class. If the class is not comfortable data graphs, it would be possible to simply relate the idea that bigger rocks make bigger craters.
Students will investigate how the drop height affects the size of the impact crater. Emphasise the idea that the drop height is related to impact speed, so a greater drop height would be the equivalent of a more fast-moving meteorite. If the class is familiar with the equations of motion for constant acceleration SUVAT equations , it would be possible to get them to plot impact speed against crater size instead.
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