Professor Fran Bagenal informed a packed Wilder 104 of the science behind auroras on Jupiter in her keynote address, “Erupting Volcanoes and Dazzling Auroras: Exploring the Planets,” last Thursday at the 19th annual Wetterhahn Symposium.
Having grown up in the UK during the Apollo era, Bagenal developed an early interest in “a world out there.” “I was a young kid hanging out in a small village, who was interested in science, interested in the world around me,” she said. It was not until she arrived at MIT in 1976, however, that she began to pursue the interest.
Choosing to focus on Jupiter in her talk, Bagenal introduced the planet as a “big ball of gas” with a mass 325 times that of Earth held together by a strong gravitational field. Hydrogen at the surface of Jupiter is present in the form of gas, said Bagenal. However, at the center of Jupiterthe pressure of 50-70 million bars splits hydrogen into protons and electrons that exist as a metallic material. This liquid, spinning, conducting medium creates a magnetic field around Jupiter known as a magnetosphere.
Jupiter’s magnetosphere spans 1,100 earth radii and would fit 1,000 suns. This magnetic sphere of influence shields Jupiter and its atmosphere from the sun’s discharge. It also traps two megatons of charged particles along certain curvatures around of Jupiter that are known as radiation belts.
Earth also has a magnetosphere and radiation belts. When solar particulars disturb Earth’s magnetosphere, particles from Earth’s radiation belt bombard Earth’s atmosphere. This cascade causes an aurora on Earth. On Jupiter, however, auroras are slightly more complex and can be one of three types.
The first type of aurora results from the elliptical motion of Jupiter’s four major moons, Callisto, Europa, Io, and Ganymede, in the magnetosphere.
A second, more stable aurora occurs during the attempt to keep the charged particles in motion around Jupiter. When charged particles are close Jupiter, they are coupled with the planet. Angular momentum is transferred from the rotating planet to the ionized sulfur and oxygen, collectively known as the plasma. However, as the particles move farther away from Jupiter, the coupling begins to break down, resulting in an aurora.
Finally, a third type of aurora exists. The cause of this aurora, however, is unknown. Bagenal speculates that the cause will most likely be found at the poles of Jupiter, which have yet to be closely explored. Currently, Bagenal said, a spacecraft is being built to examine the poles. It will be launched next summer and is scheduled to land in 2016.
Bagenal also touched on the compositional differences of Jupiter’s four main moons: Io, Europa, Callisto, and Ganymede. Jupiter exerts tidal forces on its moons, which are inversely proportional to distance. Therefore, the further the distance between the planet and the moon , the weaker the tidal force on the planet. These distance-derived differential effects of tidal forces account for differences among the moons
For example, Io, the closest moon to Jupiter, is almost complete rock with a metallic center.Ganymede and Callisto are furthest from Jupiter. Both are approximately half-water and half-rock. However, Callisto has been found to have a more uniform density, while Ganymede likely has a more stratified structure. Ganymede is also the only moon with its own magnetosphere.
Second closest to Jupiter is Europa, “the most exciting moon to go explore next,” according to Bagenal. Europa is made mostly of rock with a thin layer of water at its surface. Sparsely cracked ice makes up the very top layer of this water, but beneath this layer of ice is a layer of liquid water. Moreover, in the cracks of ice, astronomers have found “brown gunk.” “Is it whale poop?” joked Bagenal. She went on to speculate that the “brown gunk” maybe living organismsor merely dirt. Regardless, it is not unreasonable to think that life could exist in the lower depths of the liquid water layer, considering the deep sea life found on Earth.
Ending the scientific portion of her talk with the mysteries of Jupiter’s auroras and possible life of Europa, Bagenal left the audience with three “little words of wisdom” about how to be a good scientist. First, we should all pursue topics of curiosity with careful thought, good questions, and persistence. Second, we should always take time for quality, deep thought. Bagenal personally found that her “best ideas come in the shower.” And finally, Bengal reminded the audience to always seek to understand science on a relatable, “human scale.”