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Making Craters in the Classroom

Introduction

The surface of the Moon and many planetary satellites are covered in craters. These were formed by a large number of impacts by meteorites.

　The process by which impact craters are formed will be demonstrated here by means of a simple classroom experiment.
　In this experiment, an iron ball, representing a meteorite, is dropped from various heights onto a bucket of white sand, representing the surface of the Moon, to simulate a planetary impact and the formation of a crater. The results of this are then compared with craters observed on the Moon.

Experimental Method

Things to Prepare

　You will require a bucket of sand, an iron sphere (ideally have several, with a range of sizes), tape measure, ruler and set of callipers

Preparation of the Target

　Pour the sand into the bucket and ensure the top surface is smooth. This will act as the planetary surface where the crater forms.

Making Craters

　Simply drop the iron sphere onto the sand, and look at the appearance of craters formed in the sand. Use Apollo photos of the lunar surface to compare your craters with those on planetary surfaces.

Studying the Relationship between Crater Diameter and Impact Energy

　When dropping iron sphere, record the weight of the iron sphere, the height it is dropped above the target, and the diameter of crater formed. With these parameters, investigate the relationship between the energy of the impacting sphere and the crater diameter.

Experimental Results

Were impact craters produced in the sand?

The crater formed in sand from the impact of a 5cm diameter iron sphere dropped from a height of 2.5m The crater is 20cm across	An Apollo image of pair of craters on the lunar surface, in the Mare Crisium regionThe craters are 12km (left) and 5km (right) in diameter.

　The photographs above show the crater produced by dropping an iron sphere onto sand (left), and an Apollo photograph of craters on the lunar surface (right). The similarity between the craters is obvious.

Is there a relationship between the size and drop height of the impactor and the size of the crater?

　From the measurements of the mass and drop height of the iron sphere and the diameter of the impact crater formed, we can determine the relationship between the impact energy and crater size. The impact energy is defined as being the Kinetic energy of the iron sphere immediately before impact. Assuming the initial speed of the impactor to be 0, and ignoring aerodynamic drag, we can obtain the iron sphere's kinetic energy from its mass and its drop height. We are using the results from a very simple experiment, together with a basic calculation of the impact energy, but as will be demonstrated below, a graph of crater diameter versus impact energy displays an excellent correlation.

Finding the relationship between crater diameter and impact energy

　Obtain five iron spheres of 1.0, 1.5, 2.0, 3.0 and 4.0cm diameter, which have density 7800 kg/m3 (these can easily be acquired commercially), and drop them onto sand from heights in range 0.1-3.2m.
Then use this data to construct a graph with the x-axis being the impact energy and the y-axis being crater diameter. Your graph should look similar to the one shown on the right. From your graph, you should be able to observe that as the impact energy increases, then so does crater diameter.

　In this experiment, iron spheres were dropped onto three different types of sand: dry sand (density 1600 kg/m3), dry clay tuff (density 900kg/m3), and wet sand (density 2000 kg/m3), to look for the effect on the relationship between impact energy and crater diameter. Looking at the graph, the relationship is similar for all three types of sand.