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?
Thursday, August 14, 2003