THE mystery of how Saturn鈥檚 rings were formed may be solved in 2004, when the spacecraft Cassini is due to arrive. There鈥檚 one problem though: the spacecraft won鈥檛 be allowed to fly through the denser rings in case it鈥檚 hit by a large boulder.
Fortunately, researchers in Norway have devised a way of studying the rings without endangering the spacecraft. The idea is to analyse the V-shaped wakes that boulders leave in the rings鈥� 鈥渄usty plasma鈥濃�攊onised gases that contain small particles of solid matter.
As the bigger boulders fly through the rings, they leave behind a cone-shaped wake similar to the pattern of shock waves a supersonic jet makes when it breaks the sound barrier. The width and shape of these 鈥淢ach cones鈥� will reveal the size of the dust particles, as well as their density and charges.
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Once they know how the dusty plasma reacts to disturbance, researchers hope to settle a long-standing conundrum: whether the rings were mainly formed by the influence of Saturn鈥檚 moons, or whether so-called 鈥渄usty-plasma effects鈥� also play a role. 鈥淭he motion of these small particles can change the ring鈥檚 structure, given enough time,鈥� says Ove Havnes of the University of Troms酶 in Norway. There are some structures that probably have no other explanation, says Havnes, such as the strange spoke-like streaks in the main rings.
To determine what kinds of Mach cones to look for, Havnes and his colleagues have now created a computer model of the effect they expect to see when a boulder moves through a ring. But there is a hitch: they don鈥檛 yet know if there鈥檚 enough dust in Saturn鈥檚 rings for Cassini鈥檚 instruments to detect the wakes that a boulder makes. We won鈥檛 know that until Cassini arrives, Havnes says.