Discovering Alien Oceans: Magnetism
Jupiter's moon Europa poses exciting new possibilities for NASA's search for life beyond Earth. While there's still much to uncover about Europa, scientists have long known that liquid water exists beneath its icy shell. But how?
In this classroom activity, students will explore the relationship between magnetism and electricity to model how scientists determined that Europa has a magnetic field and a conductive liquid, subsurface ocean.
- It may save time and keep all students actively working to have groups determine the number of turns of wire and variables therein at the beginning of the lab. Counting turns of wire can be a time sink if one student is doing it while others are watching. Have each group pick a variable and create the coiled wire corresponding with the variable to be explored.
- The more turns of the wire the better. If you do not have strong magnets, a few turns will affect a compass, whereas 30-50 turns is enough to pick up paperclips. Likewise, the stronger the battery the fewer turns required.
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.
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 asor MOI or rotational inertia, which describes the sphericality of the body. The MOI gave scientists clues to the possible internal structure of the moon. Without the MOI, we would know the total mass of Europa, but not necessarily information about its internal makeup.
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 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.
- Begin by demonstrating how salt water is conductive. As shown in the animation below, place both leads of a battery into a cup of salt water and connect this circuit to a lightbulb. Explain how the salt water is an analogue for the subsurface ocean on Europa. Ask students what this demonstration indicates about the conductivity of Europa’s ocean.
- Now that students have seen how electricity can be conducted through liquids, review the connection between electricity and magnetism. Students should recall that the movement of current can produce a magnetic field.
As shown in the animation below, you can use a compass to demonstrate the magnetic field created when wire is wrapped around a nail and current is passed through the wire.
- Introduce the challenge to students by telling them that they will make an electromagnet of their own design that is capable of being detected by a “magnetometer” (a compass) or perhaps even strong enough to pick up metallic materials, such as paper clips. Students can work in groups of two to four. Each group should be given a battery, length of copper wire, iron nail or screw, lightbulb, and lightbulb holder if applicable. Consider provided wires, nails, and screws or different material and thickness so that students can explore what, if any, effect this has on their magnets.
- Have students create an electromagnet by wrapping the wire around the nail or screw as many times as they would like. Depending on the strength of battery and quality of the wire, 20-60 turns should get the desired result. Suggest or assign some groups to have a large number of wraps and others to have a small number. Be sure students are not crossing their turns around their screw or nail, however, as this will affect the results.
- Instruct students to record, using an annotated sketch, how they have wound the wire and how many turns are in their coil of wire.
- Have students connect their coiled wire electromagnet to their battery. Students should observe that the current has created a magnetic field. This will be evident due to the ability to attract a compass, and if there are enough turns of the wire, they may even be able to pick up light metal objects, such as paper clips.
- Instruct students to strengthen the magnetic field. They should create their own experiments by exploring variables such as:
- Changing the number of turns of the wire.
- Changing the radius of the coils by using a wider or narrower screw or nail.
- Changing the material of the screw or nail. For example, does brass work as well as iron?
- Students should observe and record how each change affects the strength of the magnetic field, such as by their ability to pick up more or less material with the electromagnet. Consider having student groups share out their data to expand their understanding of multiple variables.
- Knowing that Europa’s ocean is a conductor moving through Jupiter’s magnetic field, what can we conclude about the effect of this motion?
- The Galileo spacecraft flew by Europa, passing through its magnetic field. The spacecraft had a magnetometer on board to measure the strength of magnetic fields. How might a magnetometer work based on your knowledge of magnetic induction?
- Scientists initially did not know that Europa had a subsurface liquid ocean. Working backward from the magnetometer measurements of both Europa and Jupiter, what steps do you think scientists took to determine the presence of liquid water as opposed to solid ice beneath Europa’s surface? Think about what you've learned about the different conductive properties of materials in this lesson.
- Students should observe that the movement of the current through a coil induces a magnetic field, i.e., that magnetism and electricity are linked.
- Students should notice that wrapping their wire more times causes the strength of field to increase.
- It may be a bit difficult to detect, but students may also notice that increasing the diameter of the coils strengthens the magnetic field.
Frozen Formula: A 'Pi in the Sky' Math Challenge
In this illustrated math problem, students use the mathematical constant pi to calculate the volume of the alien ocean on Jupiter's moon Europa.
Time Less than 30 mins
Flying Through the Plume on Saturn's Moon Enceladus
Students learn about Saturn's scientifically intriguing moon Enceladus and investigate its fascinating features, including its ocean and plumes, using math.
Time 30-60 mins
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