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How long is Saturn’s day? Search reveals an even deeper mystery

For decades, researchers have puzzled over a seemingly simple mystery: the length of a day on Saturn. Unlike the rocky planets of the inner solar system, whose rotations are measured by simply tracking objects on their spinning surfaces, the fixed interiors of planets like Jupiter and Saturn are veiled by ever-shifting flows of gas.

To get around this problem, scientists have turned to distinctive radio waves created by each planet’s magnetic field. On Jupiter, Uranus, and Neptune, the axis of the magnetic field is offset against the planet’s rotation; as the two axes wobble around each other, a predictable pattern of radio waves is generated, pegged to the start of each day.

On Saturn, however, the two axes are almost perfectly aligned, leading to inconclusive results. In the early 1980s, the Voyager spacecraft estimated Saturn’s day at 10 hours, 39 minutes; when the Cassini spacecraft arrived more than a decade ago, its estimate was 10 hours, 45 minutes—and the number kept changing.

Last year, as Cassini made its grand finale, diving between Saturn and its rings (and ultimately vaporizing into the planet), researchers hoped new measures of the planet’s magnetic field might finally resolve the mystery. Such a close-up view, they hoped, would allow them to detangle signals that stemmed from the planet’s atmosphere versus those generated by its dense interior layer of metallic hydrogen, thought to be the source of its magnetic field.

But Saturn, it seems, had other plans. In new work published today in Science, the Cassini team disclosed the finest measures yet of Saturn’s magnetic field, revealing its two axes are offset by less than a measly 0.0095°, with the exact offset still unknown. The extreme alignment has, so far, not allowed a finer measure of the planet’s day. But it also points toward a deeper mystery: Planetary dynamos, which generate magnetic fields, typically require an offset between these two axes to continue. Given this nearly perfect symmetry, then, how does Saturn have a magnetic field at all?