Data from orbiting Cassini craft may help resolve debate
JPL-Caltech/NASA, Space Science Institute
PASADENA, Calif. — Saturn’s rings have maintained a youthful look, while still possibly being almost as old as the solar system itself. The dazzling belts of ice continue to keep their age a secret, but researchers hope to get answers from a spacecraft orbiting the ringed planet.
Data from the Cassini spacecraft, in orbit since 2004, may help resolve a decades-long debate over the age of Saturn’s rings, wide belts of shiny ice chunks orbiting the planet. They may be primordial, dating back to roughly 4.6 billion years ago, or a recent addition in the last 100 million years or so. Evidence for both scenarios was presented October 16 at a meeting of the American Astronomical Society’s Division for Planetary Sciences.
There’s not enough pollution in the rings for them to have been around for a long time, argues planetary scientist Paul Estrada of the SETI Institute in Mountain View, Calif. Cassini data show that about 25 times as much debris — mostly from the Kuiper belt beyond Neptune — rains down on the rings than previously thought. All that interplanetary rain should not just darken the rings, but each impact should redistribute material as well. Sharp contrasts in composition seen at the inner edge of the main ring can’t have been sustained for more than a few hundred million years, Estrada says.
The trouble with making rings so recently is how to do it. “It’s hard to make rings in the last 100 million years,” says Larry Esposito, a planetary scientist at the University of Colorado Boulder. “This is not an exciting time.” Saturn’s rings were probably created after a moon or some passing icy body got torn apart in a collision or by wandering too close to the planet. But there hasn’t been much stuff flying around Saturn or the solar system in the last several billion years.
Esposito argues that despite some youthful appearances, the rings are ancient and recycle material lurking beneath their top layers, keeping the pollution levels lower than expected. Also, some lightweight rings could have formed quite recently and still look pristine, he says, while the most massive part of the rings endured for billions of years.
Part of the solution to the age question is knowing how massive the rings are. Observations from Cassini suggest that the rings are relatively hefty — possibly comparable to Saturn’s moon Mimas — though that’s not well known. “It’s a lot easier to make a massive ring if you make it early,” says Glen Stewart, also at Colorado Boulder. Billions of years ago more material was available to make a heavy ring than in recent times.
A lightweight ring could be formed more recently, however. Computer simulations suggest that the orbits of moons around Saturn could have changed a lot in the last several hundred million years or so, Estrada says. Those shifting orbits could lead to several different scenarios in which moons destroy one another, creating icy debris that spreads out and forms the rings.
In the coming year, this debate could be moot. Leading up to the end of Cassini’s mission in September, the spacecraft is going to start some daring maneuvers to try and measure how much mass is in the rings. Cassini will dive between the planet and the rings several times, skimming Saturn’s atmosphere. “This is kamikaze stuff,” Stewart says. “For the first few orbits, they’re putting the antenna forward as an impact shield.”
By getting inside the rings, researchers can measure the gravitational tug on the spacecraft from the planet and compare that with earlier orbits where both the planet and the rings tugged on the probe. And more than scientific curiosity is on the line. “On the Cassini mission, there’s a betting pool about the mass of the rings,” Esposito says.
Figuring out the mass and age of Saturn’s rings isn’t just about solving one mystery about one aspect of one planet. Understanding how Saturn became bejeweled might offer insight into why other planets differ from one another, which in turn could reveal more details about their origins. “Why does Saturn have big rings but Jupiter doesn’t?” asks Esposito. “Is it just a matter of luck, or a matter of time?”
And rings aren’t unique to planets; they form around stars as well. Belts of ice and dust encircling young stars are thought to be where planets form throughout the universe. The disk that formed our solar system is long gone, says Esposito, but the physics underlying both that disk and the one around Saturn are largely the same. Understanding one can help researchers understand the other.
P.R. Estrada, R.H. Durisen and J.N. Cuzzi. The evolution of Saturn's rings under the influence of the Edgeworth-Kuiper belt micrometeoroid flux: tightening the constraints on ring age. 48th meeting of the American Astronomical Society’s Division for Planetary Sciences, Pasadena, Calif., October 17, 2016.
L.W. Esposito. The case for massive and ancient rings of Saturn. 48th meeting of the American Astronomical Society’s Division for Planetary Sciences, Pasadena, Calif., October 17, 2016.
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