How old are the objects within our Solar System? One method scientists use to answer this important question is counting the number of craters on their surface. This information, combined with the time it takes for craters to form on each body, gives us a strong estimate how old the object is. In this activity students will put this method into practise to calculate the age of five bodies within our Solar System.

- Learn one method scientists use to calculate the age of bodies in our Solar System, and practise this method.
- Compare results across the group and discuss why results vary, whether this happens in real research, and how this may affect scientific method and understanding.
- Understand that scientists do not deal in certainties; they also have to work with possibilities, probabilities and uncertainties.
- Understand that science is an active subject, with many discoveries left to which students could one day contribute.

- Printed Counting Craters worksheets (per student)
- Pencil or pen (per student)
- Calculator
- Computer and projector
- Counting Craters spreadsheet

By studying a variety of objects and using several techniques, scientists have determined that the Solar System is 4.6 billion years old. It is believed that, give or take a few million years, this age is true for most of the objects in the Solar System.

In this activity students will use the crater counting method to calculate their own ages for several Solar System bodies. This exercise is precisely what scientists on the New Horizons mission (NASA’s first mission to Pluto and the Kuiper Belt beyond) are doing to determine the age of the dwarf planet Pluto and its moon, Charon.

Craters are formed when objects such as comets, asteroid and meteoroids crash into other objects; the energy of the impact creates a hole in the surface. All time estimates for crater formation used in this activity come from published calculations.

- Provide each student with at least one Crater Counting worksheet. Older students should complete the worksheet for all five Solar System bodies.
- Ask student to follow the instructions on their worksheet. They will count how many craters they see (down to the smallest size visible) inside the red box.
- Each worksheet includes the length of time it takes for one crater to form in the surface region encompassed in the red box on each sheet. Students will multiply the time scale provided by the number of craters they counted. This calculation will tell them how old that surface is.
- Ask your students to write their answer in the space provided on their sheet.
- Ask each student to share the age they computed for each of their planetary bodies and enter their results into the Crater Counting Spreadsheet to find the average age calculated across the entire group. (For younger students use the ‘Beginner tab’ and simply enter the number of craters they counted, the Spreadsheet will automatically calculate the age of the body.)

By the end of this activity students should be familiar with one method for calculating the age of objects within the Solar System, and using this method they should have determined rough estimates for the age of several cosmic bodies. Students should understand that scientists must deal with uncertainties in their research and how these can be minimised.

**Authors:** Sarah Greenstreet and Sarah Eve Roberts

Results in the activity will vary, below you will find only an example of results:

Charon: 16 craters = 4 billion years

Pluto: 17 craters = 4.25 billion years

Far side of the Moon: 23 craters = 4.6 billion years

Near side of the Moon: 15 craters = 1.2 billion years

Mercury: 37 craters = 0.4 billion years

The activity is set up so that students across the group will determine a different crater count and cosmic age. Use this to introduce students to the uncertainty present in these types of calculations, and many other aspects of scientific research.

Once you have collect and compared results from each student, and calculated the average age of each body, open a discussion session on the following points:

- Did each student count the same number of craters?
- Why may results be varied?
- Does this make the result more or less accurate?
- How could this be minimised?
- Do you think results vary from one scientist to another during real research programmes?
- Do you think there are benefits to this?

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