Activity

Craters in the classroom

After carrying out this activity, students will understand the effect the mass, velocity and angle of an impacting object has on the resulting crater, in terms of diameter, depth and ejecta rays, and relate this information to the craters on the surfaces of Earth and the Moon.

The aim of this activity is to investigate the factors which affect the size of an impact crater on Earth. After carrying out this activity, students will be able to recognise and describe how impact craters are formed on Earth and the Moon.

Students must plan the experiment, including what variables to change and investigate, before carrying out the experiment in a controlled and scientific manner.

For this activity you will need the following apparatus:

  • Marbles of different sizes
  • Golf Balls
  • Stainless steel ball bearings
  • Saucepans/containers large enough for objects to be dropped in
  • Cocoa
  • Flour
  • Rulers
  • Newspaper (unless you are doing the activity outside)
  • Electronic measuring scales
  • Stopwatch

     

    Background Information

    Many of the features we see gracing the Moon’s surface are ‘impact craters’ formed when impactors smashed into the lunar surface. The resulting explosion and excavation of material at the impacted site creates piles of rock (called ejecta) around the circular hole as well as bright streaks of target material (called rays) thrown for great distances.

    Two common methods of craters formation in nature are:
1) impact of a projectile on the surface and 2) collapse of the top of a volcano creating a crater termed caldera.

    The factors affecting the appearance of impact craters and ejecta are the size and velocity of the impactor, and the geology of the target surface.

    By recording the number, size and extent of erosion of craters, lunar geologists can determine the ages of different surface units on the Moon and can piece together the geologic history. This technique works because older surfaces are exposed to impacting meteorites for a longer period of time than are younger surfaces.

    Impact craters are not unique to the Moon. They are found on all the terrestrial planets (Mercury, Venus, Earth and Mars) and on many moons of the outer planets.

    On Earth, impact craters are not as easily recognized because of weathering and erosion. Famous impact craters on Earth are Meteor Crater in Arizona, U.S.A.; Manicouagan in Quebec, Canada; Sudbury in Ontario, Canada; Ries Crater in Germany, and Chicxulub on the Yucatan coast in Mexico.

    Characteristic of Impact Craters

    Typical characteristics of a lunar impact crater are:

    • Floor: bowl shaped or flat, characteristically below surrounding ground level unless filled in with lava.
    • Ejecta rays: blanket of material surrounding the crater that was excavated during the impact event. Ejecta becomes thinner away from the crater. 
bright streaks starting from a crater and extending away for great distances. See Copernicus crater for another example.
    • Raised rim: rock thrown out of the crater and deposited as a ring-shaped pile of debris at the crater’s edge during the explosion and excavcation of an impact event.
    • Walls: characteristically steep and may have giant stairs called terraces.
    • Central uplifts: mountains formed because of the huge increase and rapid decrease in pressure during the impact event. They occur only in the center of craters that are larger than 40 km diameter. 

    Introduction

    In this activity, objects of differing densities and sizes (marbles, ball bearings and golf balls) will be dropped from a known height onto a surface of flour and cocoa. Once dropped, the kinetic energy of these objects will blast a crater into the surface, sending out rays (ejecta rays) around the object. Students will note the shape/extent of these rays, and once the object is removed from the crater, they can also measure its diameter. Results of this investigation can be presented graphically or verbally, and conclusions drawn regarding the nature of impact craters on Earth. Any improvements that can be made on the experiment can then be discussed. 

    Full Description

    This activity is best done in groups of at least 3 students, one to drop the impact object, one to time, and one to collect the results. Students should be encouraged to discuss what they think are the main factors affecting the sizes of impact craters, and write down their predictions for any trends in their results i.e. larger impact objects will create larger craters etc. 

    1. Lay down the newspapers and put the container in the centre.

    2. Fill the container to a depth of about 10cm with flour.

    3. Sprinkle the surface of the flour with a thin layer of cocoa powder. Make sure it is evenly spread.

    4. Measure the mass of each impact object and note its mass in kg (1g=0.001 kg) on the spreadsheet provided [Student_Table_Craters.xls].

    5. Measure the diameter of each impact object in metres. This can be done most easily by holding up two rulers either side of the marble and using a third ruler to measure the distance between them. Note the diameter and radius measurement in m.

    6. Using the formula below, calculate the density of each impact object and note your results on the spreadsheet:

    density= massvolume 
     

    Method

    1. Hold the impact object directly above the container and measure the height. Note: since the time taken for the impact object to hit the flour/cocoa is to be timed, this distance should be made as large as possible to minimise timing errors.

    2. Drop the impact object from this height and use the stopwatch to time the descent. The stopwatch must be stopped once the impact object has hit the flour/cocoa.

    3. Before removing the impact object from the container, look at the ejecta rays that have formed. Note down any comments you may have regarding their shape, extent etc.

    4. Remove the impact object and measure the crater diameter and ejecta ray diameter. Make a note of these values on your spreadsheet.

    5. Flatten the flour/cocoa surface and repeat the experiment twice more with the same impact object, adding the results to your table.

    6. Using impact objects of different size or density (choose one or the other), repeat steps 1-5, noting your results throughout the investigation.

    Results

    1. Refer to your spreadsheet. Using the two equations below, calculate the kinetic energy of each impact object as it hit the surface.

    The kinetic energy (K.E) of each impact object dropped can be found using the formula:

    K.E= 12 mv2
     

    where m = mass of the object and v = velocity of the object

    The velocity (speed in this case) of the object can be found by using:

    speed= distancetime

     

    2. Create two scatter plots to demonstrate your results: impact object density vs. crater diameter and impact object diameter vs. crater diameter.

    Evaluation

    After carrying out the above activity, hand each student a worksheet [Student_Worksheet_Craters.doc]. Ask them to answer and discuss the questions provided.

    a) how did the size of the impact object affect the size of the crater? How did it affect the ejecta rays?

    b) how did the density of the impact object affect the size of the crater? Did this affect the ejecta rays?

    c) do the bigger craters have more rays around them?

    d) how do the diameters of the craters compare to the diameters of the impact objects? Are they bigger/smaller/same size?

    e) What happened to the cocoa as the impact object was dropped?

    f) Was the flour visible at any time during the investigation i.e. in some impacts, or all impacts or none?

    g) What does this investigation tell us about craters on the surfaces of planets?

    h) How could this investigation be improved?

    i) What were the main sources of error in the investigation? How can these be minimised?

    j) Does K.E. affect the size of the craters made? If so, how?

    k) Were the results as expected? Did they match any predictions you made prior to carrying out the investigation?