Ejecta

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Ejecta, as it is used most commonly in astronomy, is the material expelled into the atmosphere or space around a body that has been impacted by another body. Usually this occurs when a meteorite impacts the surface of a larger body. The ejecta from an impact can be large enough to form secondary craters and create even more ejecta.

Ejecta usually returns to the surface in all directions and will cover an area that is referred to as the ejecta blanket by astrogeologists and other scientists. The ejecta blanket can be defined as ”a generally
symmetrical apron of ejecta that surrounds a crater”. The ejecta blanket will be heavy at the crater’s rim and thin to discontinuous at the blanket’s outer edge. Approximately half the volume of ejecta falls
within 1 crater radius of the rim, or 2 radii from the center of the crater. Over 90% of the debris falls within approximately 5 radii of the center of the crater. Ejecta which falls within that area is considered
proximal ejecta. Beyond 5 radii, the discontinuous debris is considered distal ejecta. These are general rules for how ejecta will behave on Earth.

On other terrestrial bodies, ejecta will behave according to surface gravity, atmospheric pressure, mass of the impacting body, and other conditions. For instance, ejecta blankets on Mars seem to have
consisted of heavier, mud-like material and flowed across the ground instead of arching into the air. On Mercury, ejecta shot high into the air, but did not fall very far from the original impact site. On Venus,
the outer edges of ejecta blankets resemble lobes or flower petals. This suggests a ground hugging flow, but of material lighter than that seen on Mars.

Some impacting bodies are so large that the ejecta remains in the atmosphere for a very long time. This is one proposed theory for the mass extinction event here on Earth known as the K-T event. According to the theory, an asteroid 10 km in diameter impacted in the Yucatan Peninsula about 65 million years ago. The debris thrown into the air blocked out the sunlight long enough to contribute to the extinction of 70% of the life on Earth in a relatively short time.

Ejecta can be used to approximate the size of an impactor, determine the heat after impact, and judge the speed the impactor was traveling upon impact. That is a lot to be learned from simple dust, isn’t it?

There is a little more research on ejecta here and here and an interesting related story here on Universe Today.

Sources:
http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=25
http://www.britannica.com/EBchecked/topic/181581/ejecta
http://www.daviddarling.info/encyclopedia/E/ejecta_blanket.html
http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html

Tagged as: ejecta, ejecta blanket