Most scientists believe the moon and Earth have been bombarded by meteorites at a constant rate for the past few billion years. New research suggests that – in the past 300 million years – it’s been happening 2 to 3 times more frequently.
The word crater was adopted from the Greek word for vessel. Galileo built his first telescope in late 1609, and turned it to the Moon for the first time on November 30, 1609. He discovered that, contrary to general opinion at that time, the Moon was not a perfect sphere, but had both mountains and cup-like depressions. These were named craters by Schroeter (1791), extending its previous use with volcanoes.
The competing theories for how the craters were formed at this time were…
- Volcanic eruptions blasting holes in the Moon
- Meteoric impact
- Welteislehre which is an idea developed in Germany between the two World Wars which suggested glacial action creating the craters.
Because of the Moon’s lack of water, atmosphere, and tectonic plates, there is little erosion, and craters are found that exceed two billion years in age. The age of large craters is determined by the number of smaller craters contained within it, older craters generally accumulating more small, contained craters.
The smallest craters found have been microscopic in size, found in rocks returned to Earth from the Moon. The largest crater called such is about 290 kilometres (181 mi) across in diameter, located near the lunar South Pole. However, it is believed that many of the lunar maria were formed by giant impacts, with the resulting depression filled by upwelling lava. Craters typically will have some or all of the following features:
- a surrounding area with materials splashed out of the ground when the crater was formed; this is typically lighter in shade than older materials due to exposure to solar radiation for a lesser time
- raised rim, consisting of materials ejected but landing very close by
- crater wall, the downward-sloping portion of the crater
- crater floor, a more or less smooth, flat area, which as it ages accumulates small craters of its own
- central peak, found only in some craters with a diameter exceeding 26 kilometres (16 mi); this is generally a splash effect caused by the kinetic energy of the impacting object being turned to heat and melting some lunar material.
We can simulate the impact of craters using a simple tray of flour with a light dusting of fine (not cheap) cocoa powder. Cake sprinkles can also be added to the flour to show the movement of ejecta more clearly. Take care of the cocoa on your white shirts!
This resource also looks at the changes for kinetic and gravitational potential energy which can be used for GCSE level students. The resource has some example results, but it is very easy to use your own set of balls or meteorites to create a new set of results.
Video and Links
Earthsky.org – How Craters Were Formed
Wikipedia – Lunar Craters