Moons of our Solar System
Moons of our Solar System

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

Moons of our Solar System

1.1 Inside a moon

Here’s your chance to learn what scientists think the internal structures and compositions of the Moon and Jupiter’s four Galilean moons are like.

Activity 1 Structures of the Moon and Jupiter’s four Galilean moons

Allow approximately 15 minutes.

For each diagram, try to work out which parts of each moon’s structure the empty labels are referring to.

When you’re ready, click the ‘Reveal answer’ link below each diagram to see a fully labelled version.

The Moon

The Moon is a differentiated body: it has a geochemically distinct crust, mantle and core. It also has a largely or entirely solid iron-rich core with a radius of 300 km. The lowest part of the mantle may be partially molten.

Figure 2a The Moon

Try to decide where each of these labels belongs:

  • solid silicate mantle
  • silicate crust
  • small iron core
  • partial melt.


Figure 2b The Moon


Callisto is the third-largest moon in the Solar System and it appears to lack a fully differentiated core. Callisto’s battered surface lies on top of a cold, stiff and icy lithosphere that is between 80 and 150 km thick. A salty ocean may lie beneath the crust, indicated by studies of the magnetic fields around Jupiter and its moons.

Figure 3a Callisto

Try to decide where each of these labels belongs:

  • ice-rock interior
  • icy crust
  • subsurface ocean.


Figure 3b Callisto


It is believed that Europa has an outer layer of water around 100 km thick, some as frozen-ice upper crust and some as liquid ocean underneath the ice. Below the ocean, occupying most of Europa’s volume, is a rocky interior. This probably contains metallic iron core at the centre.

Figure 4a Europa

Try to decide where each of these labels belongs:

  • silicate mantle
  • iron core
  • subsurface ocean
  • thin ice crust.


Figure 4b Europa


Ganymede is composed of approximately equal amounts of silicate rock and water-ice. It is believed to be a fully differentiated body with an iron-rich core. A saltwater ocean is believed to exist nearly 200 km below Ganymede’s surface, sandwiched between layers of ice.

Figure 5a Ganymede

Try to decide where each of these labels belongs:

  • silicate mantle
  • mostly solid iron core
  • ice crust
  • icy water.


Figure 5b Ganymede


Io is composed primarily of silicate rock and iron. Io is closer in bulk composition to the terrestrial planets than to other moons in the outer Solar System, which are mostly composed of a mix of water-ice and silicates. Models based on measurements of density and mass distribution suggest that its interior is differentiated between a silicate-rich crust and mantle, and an iron- or iron-sulfide-rich core. Io’s metallic core makes up approximately 20% of its mass.

Figure 6a Io

Try to decide where each of these labels belongs:

  • partially molten silicate mantle
  • silicate crust
  • iron core.


Figure 6b Io

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 50 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