Measurements of Saturn’s gravitational field show its rings are younger than the planet

Using data from the Cassini spacecraft’s final orbits, scientists report new measurements of the gravitational field around Saturn and its rings, which allowed them to constrain its internal structure, the depth of its winds, and the mass and age of its rings. The results suggest the rings did not form together with Saturn in the early Solar System, but are instead much younger than the planet. Prior to the Grand Finale phase of the Cassini mission, the spacecraft’s orbit was always outside Saturn’s rings. This meant that the gravitational effects of the rings could not be disentangled from those of the planet, which has made determining the mass of the rings – theorized to be directly linked to their age – challenging. In the final phase of the Cassini mission, however, the spacecraft dove between the planet and the rings. During six of these crossings, a radio link with Earth was monitored to precisely measure the gravitational field around the planet, using tiny Doppler shifts in the spacecraft’s radio signals. Luciano Iess and colleagues now report a new, more accurate gravity field for the planet that deviates from theoretical predictions. Iess and colleagues say this signal is explained by the gas giant’s so-called differential rotation, or the way different parts of this non-solid planet move with different rates of rotation, depending on their depth. The East-West winds seen on the surface of Saturn extend deep into the planet’s interior, to a depth of at least 9,000 kilometers, say Iess and team. The researchers also used the gravity data to determine the total mass of Saturn’s rings – and hence their age. The mass of the rings they report, compared to previous efforts, implies that the rings of Saturn are only 10- to 100-million-years-old, much younger than Saturn’s age of 4.5 billion years. The data do not indicate how the ring system formed so recently, the authors say.

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