First author

In 1999, the Galileo spacecraft sent back its first images of Jupiter's outer rings. These confirmed that two of Jupiter's moons, Amalthea and Thebe, are the sources of the dust that makes up the planet's two outermost, gossamer rings. But the images also showed a barely visible, unexplained extension of dust beyond the most distant of the two moons, which has been dubbed the Thebe extension. On page 72, astronomers Douglas Hamilton from the University of Maryland in College Park and Harald Krüger at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany perform planetary-dust forensics to show that Jupiter's shadow is responsible for creating the gossamer extension.

What motivated you to explain the Thebe extension?

I wrote a graduate thesis on the ring systems of Jupiter and Saturn. The 1999 Galileo images of the Thebe extension puzzled the ring community and piqued my interest. In 2003, Galileo became the first spacecraft to fly through a planetary ring, and it gathered new data from Jupiter's outer ring, including dust grain size and speed, and even orbital orientation, allowing us to solve the puzzle.

Why did you suspect that Jupiter's shadow might be the culprit?

The main clue is that the ring material extends both inward and outward from the source satellite, Thebe. Simple circular orbits just can't do this. So I went searching for forces capable of producing elliptical orbits. Electromagnetic forces from Jupiter's spinning magnetic field, enhanced by grain charges varying in Jupiter's shadow, worked. The new Galileo dust data — interpreted by my co-author's indispensable efforts — verified parts of our theory.

Are these dust dynamics applicable to other planetary ring systems?

Yes. Although Saturn's main rings are composed mainly of large chunks of ice, some of the fainter rings are dust. Uranus and Neptune also have dusty systems. The shadow effect is strongest at Jupiter because it is nearest to the Sun and experiences the most sunlight of all ringed planets.

What else is left to know about planetary rings?

We can't fully answer some basic questions about rings. For example, we want to know if Saturn's rings formed at the same time as the planet or more recently. Studying rings in our Solar System lays the foundation for understanding ringed extrasolar planets when they are eventually detected.