Credit: NASA / STSI

“Has Saturn swallowed his children?” wondered Galileo Galilei. He was alluding to the Roman god Saturn who devoured his newborn children, fearing they would overthrow him. The year was 1612 and Galileo was confused. Two years earlier, when he had viewed Saturn for the first time through his astronomical telescopes, he saw prominent 'ears' on both sides of the planet. These features were gone in his 1612 observations, but then, even more confusingly, they were there again in 1613.

Galileo, unknowingly, had seen the first evidence of Saturn's extensive ring system, and of a crossing of the ring plane — the opening of the rings is seen in these images taken by the Hubble Space Telescope between 1996 and 2000. The position of Saturn's rings relative to the Sun and Earth influences the perceived brightness and colour of the Saturn system, and, in the journal Icarus, Richard Schmude presents a comprehensive model that describes how Saturn's appearance changes as the planet orbits the Sun (Icarus doi:10.1016/j.icarus.2010.09.018; 2010).

Saturn's rings are essentially two-dimensional structures — they are thought to be only a few tens of metres in thickness — and mainly composed of water ice. Relative to the planet itself, the rings are responsible for the larger part of the visible light reflected by the system, and hence determine the appearance of Saturn when viewed from Earth. But the angle between the Sun, Saturn and a terrestrial observer, called the phase angle, changes as Saturn revolves around the Sun — one round trip takes almost 29.5 years — as do the ring-opening angle and the Saturn–Sun and Saturn–Earth distances.

Schmude's model takes these quantities into account, and integrates brightness measurements made at different wavelengths between 1963 and 1965, as well as more recent data compiled between 1991 and 2009. He finds that the brightness of the Saturn system increases as the solar phase angle decreases; at phase angles of less than two degrees, there is a further nonlinear increase in brightness, an effect known as opposition surge. Also, the colour of the Saturn system changes moderately within one orbital period. Schmude expects his photometric model should be helpful not only in future studies of the Saturn system but also of exoplanets that have bright rings.