This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course


5.3.5 Crater counting in detail

Since the return of lunar samples, estimation of age has become a much more exact science. Dates for the lunar maria and highland areas have been used to calibrate the relationship between crater sizes and the number of craters in a given area of the Moon’s surface. The Moon’s surface has been heavily cratered and, unlike on the Earth, lunar craters are preserved. We even know there is a good record of the Late Heavy Bombardment, which pre-dates any remaining rocks at the Earth’s surface. The Earth certainly went through a similar rain of asteroids and comets around 4 billion years ago but the subsequent re-surfacing of the Earth by plate tectonics, weathering and erosion has erased most impacts, even the largest ones.

Figure 26 Image of the Tsiolkovskiy crater. Look carefully at the boundary between the dark mare and the rugged highlands. The area shown is about 200 km across.

The counting of craters on different surfaces can give an idea of the sequence in which different terrains formed. While it doesn’t straightforwardly give an age in years, this method is very precise for working out the order in which events happened on the Moon and other celestial bodies. When you expanded the image did you notice how few craters there were in the mare compared to the highlands? There are two main factors to be taken into account: the density (i.e. number per unit area) and the size of the craters. Within a given terrain there is a strongly reproducible relationship between the density of craters and their size. There are fewer large craters and more small craters as the size–frequency distribution plot below shows. Using this, scientists have managed to characterise the age of different terrains on the Moon’s surface and use the same calibration to determine the age of surfaces on other bodies such as Mars. While the calibration is different, the same effect is the reason why the surface of Europa, for example, has few craters and the asteroids (which have no re-surfacing) are heavily cratered.

Figure 27 Size–frequency distribution plot for three important lunar terrains: Mare Orientale, Mare Imbrium and Mare Nectaris.

At this stage you may be wondering if a geological map of the Moon has been made yet. The answer is that it has. The United States Geological Survey has prepared a map of the Moon's near-side [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has over 40 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus