Category

Determining Subsurface Crater Structure

Introduction

Investigation of the subsurface structure of impact craters

　There is a large number of craters on the surface of the Moon and planetary satellites. These were produced by the impact of small bodies such as meteorites.
　Apart from the obvious surface changes, will there also be changes to the subsurface material underneath the point of impact?
　In this experiment, an iron sphere is dropped into a bucket containing stratified layers of coloured sand. By seeing how the layers of sand move during the impact, it is possible to observe subsurface changes.

Experimental Method

Things to Prepare

　Dry sand; iron sphere; bucket; tape measure; ruler; callipers; quick-drying oil-based spray paint; plastic sheet; scissors; agar solution; gas cooker; acrylic board

Preparation of the Target

　Here we prepare the ground where the crater will be formed.

1. Apply colour to the sand using the spray paint
   
2. Use the plastic sheet to line the bucket
   
3. Put a deep layer of grey sand into the bottom of the bucket to ensure that the effects of the iron sphere's impact do not reach the bottom of the bucket
   
4. Insert the acrylic board into the side of the bucket. This ensures that the crater shape is preserved, and allows the agar water to extend across the whole bucket.
   
5. Add layers of coloured sand of 6-8 different colours. Each layer should be 3-4mm thick.

Top view of target	Cross-section of target

Observation of Crater Surface

1. Drop the iron sphere onto the target
   
2. Observe the distribution of crater ejecta on the surface of the target

Observation of Crater Cross-Section

1. Prepare the agar solution. The required volume will be 30% of the total volume of sand.
   
2. Pour heated agar solution onto the target between the acrylic board and the plastic sheet. Allow to dry.
   
3. Once the target has dried to a semi-solid state, remove the plastic sheet with the help of scissors.
   
4. Observe how the sand layering has been changed by the impact.

Prediction of Subsurface Crater Structure

　Try to predict what changes will occur to the target surface and interior during the iron sphere impact

Surface Prediction	Cross-Section Prediction
Write your predictions The diagrams above predict that the uppermost green layer slides off the sides of the crater as it is forming and is carried on the back of the iron sphere some distance below the surface. This movement of the surface layer exposes the yellow and black layers below around the rim of the crater.

Experimental Results

　The next step is to make observations of the changes in the target surface and cross-section.
　　Note how the coloured sand has moved in the centre and exterior of the crater.

　The photographs above show the surface (left) and cross-section (right) of the crater formed from the impact of a 4cm diameter iron sphere dropped from a height of 5m. Before impact, the layers of coloured sand were each 4mm thick.
　From these photographs you can see how the layers of coloured sand have been moved by the impact. In the region outside the crater rim, the yellow layer has become visible, the rim is completely yellow, and inside the rim is mostly black. Hence we see that at the surface the 2nd and 3rd layers of sand have replaced the top layer.
　What is the meaning of these observations?
　From this experiment we can see that the surface material closest to the centre of the crater has travelled the greatest distance during impact, so has received the most energy.
　Was the experimentally observed crater structure what you expected??