Every day, hundreds of thousands of objects from space collide with Earth or its atmosphere, to be precise. Almost all the objects are so small that their effects are unnoticed by us. They simply burn up in the upper atmosphere – a phenomenon known as the shooting star.
However, occasionally something bigger comes along – big enough to not completely burn up in the atmosphere and make it down to the surface. Even these rarely have an impact on us. Several people have been struck by small meteorites, such as http://news.nationalgeographic.com/news/2013/02/130220-russia-meteorite-ann-hodges-science-space-hit however, no one is known to have died as a direct result. In 1992, the Peekskill Meteor was seen travelling over much of the east coast of America, before landing on a car, damaging its bodywork http://apod.nasa.gov/apod/ap061119.html .
Very rarely, a massive meteorite will hit Earth with enough speed to form an impact crater. These craters can tell us a lot about the size and speed of the meteor, and any effects it may have had on the planet. For example, the Tunguska event occurred in Russia in 1908, flattening 2000 square kilometres of forest – the most likely cause of the massive explosion was a meteorite.
Image: One of the few images of the Tunguska Event
In the grand scheme of things, this was a relatively small impact. Craters can be dated back and linked to the timeline of events in the planet’s history, to show that meteorites may have been responsible for many previously unexplained timeline changes.
The extinction of the dinosaurs has been attributed to a huge meteorite strike that caused the Chicxulub crater under the Yucatan Peninsula in Mexico. Deposits in the crater have been aged to the same time as the last dinosaur fossils. The huge impact that resulted in the crater, which is 180 km in diameter and 20 km deep, would have triggered global tsunamis, earthquakes and volcanic eruptions. The dust and gas released may have also have blacked out the Sun, plunging the world into a mini ice age.
As Earth has an active atmosphere with weather, these craters can easily become lost because of weathering. Geological activity can also hide them, for example if the surface is re-covered with lava. Solar System bodies without atmospheres or geological activity have many more visible craters than Earth. Smaller objects do not burn up or slow down on bodies without atmospheres; hence, they may cause many more small impact craters than on Earth.
Image: The cratered face of the moon
Many craters can be seen on this image of the far side of the moon. (Note: there are more craters visible on the far side than the visible side of the moon. This is thought to be because of increased volcanic activity on the Earth side covering up any large craters with fresh lava.)
The term ‘meteorite’ is given to anything that falls from space and makes it down to Earth. Asteroids and comets can both fall as meteors, causing craters on Earth if they land. Asteroids are made of rock and metal; most are found orbiting the Sun between Mars and Jupiter. Comets are made of rock fragments held together by ice. They have very elliptical (oval shaped) orbits, coming from outside Neptune towards the Sun and then going back again.
Animation: the formation of the Chicxulub crater (Credit – University of Arizona, Space Imagery Center).
Craters are formed by the impacting body imparting its energy to the Earth, causing massive shockwaves. Their formation can be separated into three stages - The Compression Stage – This is the initial transfer of energy between the meteorite and the Earth. The front of the meteor hits the Earth, compressing the ground below it, and the back moves towards the front, compressing the entire body at an immense pressure. - The Excavation Stage – The energy vapourises (melts then boils) the meteorite and parts of the ground below it, sending rock vapour miles into the atmosphere. Molten and solid rock from around the impact site can be thrown hundreds of miles by the shock wave. Material thrown out is called Ejecta. A lot of material builds up around the crater, forming the classic ring. - The Final Modification Stage – some Ejecta falls into the crater, partially filling it.
The pressure of the shockwave can change the structure of the rocks around the impact crater. The intense heat can melt rock, sand, and soil nearby, which can form ‘impact glass’, a dark coloured glass as it cools rapidly.
Larger impacts can form much more complex craters, such as the Chicxulub crater (shown in the above animation). The shockwaves can rebound, causing uplifts within the crater and other complex patterns. Over time, weathering will change the shape of the crater further.