Discovering Alien Oceans: Density

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?

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.

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.

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.

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.

To take a closer look at Europa's interior makeup, NASA has launched 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.

• 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

• Facts & Figures: Galileo Mission Overview
• Facts & Figures: Europa Clipper Mission Website
• Expert Talk - Video: Exploring Europa - Ocean Worlds of the Outer Solar System
• Expert Talk - Video: Teaching Space With NASA – Exploring Europa
• Poster: Deep Dive on Jupiter's Moon Europa

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