Two clear plastic cups filled with different amounts of water, ice, and types of granular materials sit side by side on a table.


Scientists believe that Jupiter's icy moon, Europa, has a liquid-water ocean underneath its frozen surface. While we can’t see that ocean directly, we can use the mass and density of known substances to predict what exists underneath the surface. In this activity, students will use the fundamentals of density to propose a model for the interior structure of Europa.



  • Remind students to be careful about water displacement as they add layers to their models. This can be managed by having them write “do not fill above this line" on their jars.
  • Team sizes are best kept between two and four to ensure all members are engaged.


Scientists searching for life beyond Earth have recently set their sights on Europa, a small icy moon orbiting Jupiter. One reason why is that they've collected strong evidence from spacecraft and ground-based telescopes to suggest that a liquid water ocean is hidden beneath Europa's icy shell. But how can scientists tell there's an ocean on Europa without actually seeing it?

Europa appears yellowish white with faint cracks across its surface in a fuzzy image. Beside that, another closer up image of one area of Europa shows layers upon layers of cracks. A third image shows the northern hemisphere of Europa in the most detail with the moon appearing light blue with splatters and tangles of orange cracks.

These three views of Jupiter's moon Europa were captured by the Voyager 1, Voyager 2, and Galileo spacecraft in March 1979, June 1979, and the 1990s, respectively. Image credit: NASA/JPL-Caltech | › Full image and caption


In the 1950s and '60s, astronomers took some of the first spectral measurements of the Galilean moons – Io, Callisto, Europa, and Ganymede. By looking at the particular wavelengths of light absorbed and reflected from the surface of Europa, scientists determined the chemical makeup of its surface. The relative amounts of each color reflected from the surface indicated the presence of solid H2O (ice) at Europa’s surface. But it wasn’t until decades later, when the Galileo spacecraft entered orbit around Jupiter that further evidence was found.

Moment of Inertia Measurements

The Galileo spacecraft orbited Jupiter for nearly eight years, between 1995 and 2003, studying the gas giant and its moons and making countless discoveries.

While at Jupiter, the Galileo spacecraft measured Europa's moment of inertia, also known as MOI or rotational inertia, which describes the roundness, or sphericality of the body. Stronger gravity pulling in all directions will cause more sphericality, whereas weaker gravity allows the body to have more give as it rotates. Europa's MOI gave scientists clues as to the possible internal structure of the moon. Without the MOI, we would know the mass of Europa, but not necessarily how its gravitational and magnetic fields affect the moon's internal structure.

An interior view of Europa makes it look as if a slice has been taken out of it. As it orbits Jupiter, the moon and its interior gets stretched and compressed.

As Europa travels around Jupiter, large tides, raised by Jupiter, rise and fall. Image credit: NASA/JPL-Caltech | › Full image and caption

Gravity Measurements

During the Galileo mission, scientists used data from the Deep Space Network, which is used to carefully track and communicate with spacecraft, to monitor deviations in Galileo's trajectory as it orbited Europa. This allowed them to get a better idea of the moon's internal mass and material makeup. Tracking gravitational perturbations led to the conclusion that a 100-km-thick layer of H2O material (both ice and liquid water) exists at Europa’s surface. However, the densities of liquid and solid water are too close together for these gravity measurements to have told them apart.

Europa is shown in the foreground with the giant planet Jupiter looming behind. Lines extend from the top through the bottom of the frame and encircle Europa like a cage. The lines are yellow-green at the top of the frame until they reach Europa's northern hemisphere, where they go through the spectrum from red to blue in a wave-like pattern before turning yellow-green again below Europa.

The animation illustrates the connection between Europa’s induced magnetic field and Jupiter’s magnetic field. Image credit: NASA/JPL-Caltech | › Full image and caption

The Europa Clipper spacecraft has a box-like center with rectangular solar panels extending from either side like wings. It's shown flying above Europa with Jupiter visible in the distance.

Artist's rendering of the Europa Clipper spacecraft orbiting Europa. Image credit: NASA/JPL-Caltech | + Expand image

Magnetic Field

The final piece of evidence pointing to Europa's subsurface ocean came from the Galileo spacecraft's magnetometer measurements. Jupiter has a strong magnetic field that extends beyond Europa and induces a magnetic field on the Galilean moons. By measuring Europa's magnetic response to Jupiter’s magnetic field with the Galileo spacecraft, scientists concluded that the moon must have a conductive layer near its surface. The measured magnetic responses were consistent with that of a salty liquid water layer, which ultimately led to the conclusion that Europa must have a subsurface salty ocean.

Deeper Dive

To take a closer look at Europa's interior makeup, NASA is launching the Europa Clipper mission. The mission will return to Europa with an advanced suite of instruments capable of determining the thickness of the moon's icy shell, the depth of the ocean below, as well as the temperature and composition of the water. While Europa Clipper won’t reach Jupiter until 2030, students can begin to model the interior of the moon for themselves using similar methods as scientists do – namely, what we know about Europa’s mass and volume.

In this science talk, JPL planetary geologist Cynthia Phillips joins education specialist Brandon Rodriguez to discuss how future NASA missions will explore Jupiter's moon Europa and other ocean worlds. | Watch on YouTube


  1. Create teams of two to four students and provide each team with a jar, a ruler, ice, water, and two granular materials (representing a rocky core). Consider having students write “do not fill above this line" on their jar to keep water from spilling out of the jar.
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  3. Discuss with the students how density is measured as mass divided by volume. Discuss as a class how teams can compare their results even if the interior of their jars aren’t identical. Students should determine that they can use scales or balances to measure the mass of their empty jars and rulers to measure the inner radius and height of the empty jars. Students should record both measurements.
  4. Have teams fill their jars with at least 1 cm of granular material, then, determine the resulting mass and density of their jars.
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  6. Continue by having them add at least 1 cm of water and repeat the measurements.
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  8. Lastly, have them add a few ice cubes to their jars and complete the mass and density measurements once more.
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  10. How did the mass, volume, and density change with each layer?
  11. Now, students will predict how they could create a second jar with the same density but using different amounts of each material.
  12. Repeat steps 3-6 in a second jar with students using their predictions as a guide.
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  14. Have students measure the new density. How did it compare to the prediction from Step 7? What is different about the second jar, and how does it help us determine which jar is more closely related to the structure of Europa?
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  • What is the same between each of the jars? Students should observe that the jars all have the same overall total mass, and that the jars themselves are approximately the same radius.
  • What is different between the jars? Students should note that the materials inside of the jars differ between the teams and make the connection that knowing a planet’s entire mass is like knowing the mass of a jar that they can’t see into.
  • Discuss the implications of different density and mass distributions in determining the composition of planets.


  • Have students determine the total volume of all the materials in the jar (using V = πhr2, where h is the height of the layers and r is the radius of the jar). Students should be able to defend their calculations with the individually collected volumes of each layer, and be able to explain any discrepancies (such as through settling of the materials)
  • Have students use the individual masses and volumes from their layers to measure the average density of their jar and compare this number to the individual layers. Students should be able to explain how if the amount of one layer changed it would have an effect on the overall density of the jar.


Open-source Matlab code for constructing 1D models of icy moon interiors based on planetary properties. View on GitHub

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Lesson concept by Mara Niesyt, STEM Teacher and Researcher (STAR) fellow, University of California Santa Barbara, physics