Dust particles reveal the age of Saturn’s rings

Saturn’s rings are mostly water ice, but a small fraction of them (less than 2%) are polluted with non-icy stuff. Because the rings occupy an enormous surface area—10⁴ to 10⁵ times as great as a moon of equal mass—they are quite vulnerable to being hit by dust in interplanetary space. That raises the likelihood of the dust coating and darkening Saturn’s rings.

Sascha Kempf , an atmospheric and space physicist at the University of Colorado Boulder, has led a team of international researchers that figured out how quickly that dust builds up. He and his colleagues analyzed data from the late Cassini spacecraft’s Cosmic Dust Analyzer, which collected 2 million dust particles around Saturn between 2004 and 2017 and measured the grains’ velocity and pervasiveness. The team filtered out most of the measurements to leave only those of the dust that originated from outside the immediate neighborhood of Saturn’s moons. The filtering left them with just 160 particles. A substantial fraction of those particles came from the Kuiper belt, some from the Oort Cloud, and a few even from outside the solar system.

The influx of those particles yielded an interplanetary contamination rate, or mass flux, of about 3 × 10⁻⁶ kg m⁻² s⁻¹. Using that rate and the extent of the rings’ current contamination, as measured by Cassini, Kempf and his colleagues calculated the rings’ age at no more than 100 million to 400 million years old. The measurement confirms that the currently existing rings cannot be primordial—a question that has long been debated among planetary scientists. Saturn’s ring system must thus be far younger than Saturn itself and our solar system’s other planets, which were born 4.5 billion years ago. (S. Kempf et al., Sci. Adv. 9, eadf8537, 2023 .)