He specializes in studies of the moons of the outer planets, particularly the four large "Galilean" satellites of Jupiter, and other small outer solar system bodies, using theoretical models, Earth-based telescopes, close-up spacecraft observations, and the Hubble Space Telescope. He was responsible for temperature mapping of Jupiter's moons with the Photopolarimeter- Radiometer (PPR) instrument on the Jupiter-orbiting Galileo spacecraft, and worked on the mapping of temperatures on Saturn's moons using the Composite Infrared Spectrometer (CIRS) on the Cassini Saturn orbiter. He is particularly interested in the active volcanos and atmosphere of Jupiter's moon Io, and in the active ice eruptions of Saturn's moon Enceladus. He has also published research on Mars, asteroids, Pluto, and Neptune's moon Triton.