Physics - Meteorite Craters Research

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Research – Impact Craters

The Meaning of Asteroid and Meteorite

Asteroids are rocky or metallic objects mainly found orbiting the Sun in a region called the asteroid belt between Mars and Jupiter. Some are large - the biggest is Ceres with a diameter of nearly 600 miles (950km) - and are sometimes called minor planets or planetoids. There are millions of small asteroids. It is thought that asteroids are material leftover from the time that the planets formed. (From source 1)

An asteroid is a rocky object in space that's smaller than a planet — they're sometimes called minor planets or planetoids, according to NASA. Other sources refer to them loosely as "space debris," or leftover fragments from the formation of the solar system. Asteroids have no atmosphere, but many are large enough to exert a gravitational pull — some have one or two companion moons, or they form binary systems, which is two similarly sized asteroids orbit each other.

Meteorites are usually categorized as iron or stony, iron meteorites are composed of about 90% iron. Stony meteorites are made up of oxygen, iron, silicon, magnesium and other elements. (From source 2)

Some meteoroids survive passage through Earth's atmosphere and hit the ground. These are called meteorites. (From source 3)

How Impact Craters are Formed

When an impactor strikes a target, it has a great deal of kinetic energy (proportional to the object's mass and the square of its velocity).

1. Compression Stage: During this stage, the impactor punches a (relatively) small hole in the target, and a shock wave begins to pass through the target. This is when the impactor's energy is converted into heat and kinetic energy in the target, as the pressure generated by the impact is so great that even solid material can act somewhat fluid, and flow away from the impact site. There is very little material ejected up and out of the forming crater during this stage, although a plume of impact-generated vapour rapidly expands above the crater. This stage is very quick, lasting an amount of time on the order of the impactor's diameter divided by its speed at impact (D/v). For Deep Impact, this stage will last only around (1 m / 10200 m/s) = 0.0001 seconds (100 microseconds).

2. Excavation Stage: During this stage, the shock wave begun in the compression stage continues outwards through the material. A very interesting part of this, however, is the fact that this wave spreads out from a point below the surface of the target. As a result, the wave actually spreads upwards from the impactor, and sends some of the target material up and out from the impact site. This material is referred to as the "ejecta." Initially the ejecta forms a plume of hot vapour melt droplets and fine debris. Then a cone-shaped "curtain" of material spreads upwards from the impact site. Some or all of this ejecta will land in the area surrounding the crater, forming an ejecta "blanket." The crater itself grows very large very quickly during this stage, and material at the lip of the crater folds over creating a rim. Fractures often spread down into the target from the crater site as well. This stage is longer than the compression stage, lasting an amount of time roughly equal to the square root of the diameter of the impactor divided by the acceleration due to gravity from the target. For Deep Impact, this stage will last around 300 seconds.

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3. Modification Stage: During this stage, loose debris from the impact will tend to slide down the steep crater walls. Some loosened material may slip in sheets, forming terraces along the crater sides. In some craters, a central peak may form as some of the target material splashes back upwards at the initial point of impact. This stage lasts about the same amount of time as the excavation stage, although of course the crater can be further modified by erosion, later impacts, lava flows or tectonic activity for millions of years afterwards depending upon conditions on the target. For Deep Impact, ...

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