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


5.3.6 History of the Moon

When combined with isotopically dated samples from known areas, the Apollo landing sites for instance, it has been possible to estimate an absolute age for an area from crater counting using the kind of size–frequency distribution that you saw in the previous step. This way, it is possible to estimate ages for areas for which samples are unavailable and a full history of the Moon can be inferred.

The history starts with the giant impact, the most commonly accepted current hypothesis for lunar origin. As you have seen, after accretion, the Moon was in a state called a magma ocean, which means that a significant portion of the Moon’s rock was molten.

Dating of these two lithologies in the samples gives a highland crust formation that would have started 4.56 Ga ago (where 1 Ga is 1 billion years). The youngest basalts associated with the main crust-forming events have an age of around 3.8 Ga.

Lunar mare volcanism took place after the formation of the highland crust. The volcanic eruption filled basins in the crust where it was made thinner; filling impact basins for instance. The oldest mare basalt dated isotopically is 4.23 Ga old while the youngest is 2.7 Ga old. On the basis of remote-sensing data stratigraphy and crater density, a younger age of 1.5 Ga has been suggested for the eruption of mare basalts in Oceanus Procellarum.

Thus mare volcanism overlapped with the end of crust formation and may have continued up to 1.5 Ga ago, which marked the end of volcanic activity on the Moon but not the end of its story.

The ages of polymict breccias, which are related to impacts on the Moon, cluster noticeably around 3.9 Ga. This means that prior to this time or at this time only, the impact rate was far higher than it is today. Such a clustering of ages is also observed on other planets or asteroids in the Solar System, most notably on Mars and the asteroid Vesta. This event, the Late Heavy Bombardment, was spread across the Solar System and not just localised on the Moon. The cratering rate decreased rapidly after this bombardment, by an order of 10 between 3.9 and 3.1 Ga ago. Impactors have continued to shape the surface of the Moon in more recent times, forming younger craters. Among the notable younger craters are Copernicus (1.0 Ga) and Tycho (0.3 Ga).

Figure 28 Summary of the chronology of the formation of the lunar crust. Ga (Giga-annum) is 1 billion years.

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