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, this stage is not very important, since the low gravity on the comet will probably only cause some small amount of collapse near the rim. There will not be the uplift that can be seen in larger craters, so there will be no central peak. (From source 4)
The Barringer Crater
50,000 years ago, a giant fireball streaked across the North American sky. At its core was a meteorite – a chunk of nickel iron about 150 feet (50 meters) wide. The meteorite weighed 300,000 tons and travelled at a speed of 26,000 miles per hour (12 kilometers per second).
The crater measured three-quarters of a mile (about 1 kilometer) wide and 750 feet deep. A lake formed in the bottom of the crater, and sediments accumulated until the bowl was only 550 feet deep. Millions of tons of limestone and sandstone were blasted out of the crater, covering the ground for a mile in every direction. (From source 5)
What Determines the Size of an Impact Crater?
The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor, the larger the crater. Typically, materials from space hit Earth at about 20 kilometers (slightly more than 12 miles) per second. Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object. Smaller planets have less gravitational "pull" than large planets; impactors will strike at lower speeds. The greater the mass of the impactor, the greater the size of crater. (From source 6)
The energy from the impact of an object such as a meteorite or asteroid is transferred to the surface that it strikes. The energy from the impact forces the surface it strikes to move. The size, mass, speed, and angle of the falling object determine the size, shape, and complexity of the resulting crater. Small, slow-moving objects have low impact energy and cause small craters. Large, fast-moving objects release a lot of energy and form large, complex craters. (From source 7)
The Energy Changes That Happen When Impact Craters are Formed
Wherever the comet or asteroid lands, a vast amount of energy is produced, causing the meteorite to vaporise, and shock waves to move rapidly away from the impact site. Geologists believe that an impact such as that which formed the Chicxulub crater would be seismically equivalent to a magnitude 10 or 11 earthquake, and that the energy released would be at least 10 billion times greater than that produced by a nuclear bomb. (From source 8)
The greater something’s mass and the faster it’s going, the bigger its kinetic energy will be. The kinetic energy of something depends on both its mass and its speed. When something falls, its gravitational potential energy is converted into kinetic energy, so the further it falls, the faster it goes. But when a falling object reaches terminal speed it can’t go any faster so its kinetic energy doesn’t increase. Instead, the gravitational potential energy is transferred to internal energy of the object or it’s not used heating up the air particles through friction – so it’s turned into thermal energy. (From source 9)
A video of a meteoroid hitting Russia: . From this video I can tell that heat energy and light energy is produced.
- GCSE Physics: Exam Board: OCR Gateway: The Revision Guide Higher Level