Skip navigation link -- Finding Impact Craters with Landsat
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Background: Impact events are Key to Earth's History.

Many people know that craters cover the surface of the moon. In fact, impact craters appear on all rocky (terrestrial) planets and many of their moons. The Earth has been shaped by these dramatic impact events no less than other planetary bodies have been, and one can see evidence on the Earth in terms of its geology, biology, and chemistry. The knowledge we can gain from studying impact craters is fundamental and interdisciplinary.

Science is an exploratory, dynamic process, and new ideas frequently come to light. Until about 40 years ago, impacts byextraterrestrial objects were not considered very significant by most geologists. Impact events are now recognized as more abundant, larger, older, more geologically complex, more economically important, and even more biologically significant than scientists had believed. Impact events have formed major ore deposits, generated large crustal disturbances, and produced huge volumes of igneous rock. We know that at least one major biological extinction event was probably triggered by the impact of an extraterrestrial object, and the coastline of Chesapeake Bay was partially shaped by an impact event. Impact events may have played a key role in the formation of the ocean basins and of the oceans themselves. Large impacts can drastically alter the chemical composition of the atmosphere.

Researchers have identified approximately 140 individual impact craters on Earth. These craters probably represent only about 25 percent of those to be found. Assuming an equal rate of impact around the globe, many more craters remain to be discovered, particularly in remote land regions of our planet and in the oceans.

NASA scientists currently study satellite images for evidence of impact events. Finding the evidence requires careful interpretation of satellite images. Wind and water have eroded away most of the evidence; various other geologic processes have concealed it; oceans and vegetation now cover much of the rest. Satellite observation technology enables us to see landforms that we can't see with our eyes alone. When impact craters are found in satellite images, interdisciplinary teams of scientists can go to the sites on the ground to learn more about them and how they have changed their surroundings.

These are some questions scientists are currently asking about impact events:

• How often do impacts occur? Does the rate of impacts vary over time? If so, does it vary regularly or randomly?
• Are asteroids or comets the more frequent impacting bodies?
• Have impact events caused more than one major biological extinction event?
• When will the next impact event take place? How big will it be, and how will it affect life?

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