NASA’s Cassini Spacecraft Measures a Day on Saturn

Measuring the exact length of a Saturn day is turning out to be a bigger challenge than originally thought.

Over more than a decade in Saturn orbit, Cassini’s instruments have wrestled with confusing measurements to determine the planet’s precise rotation rate. The mission’s final year and unprecedented trajectory will carry Cassini to unexplored regions so near to Saturn that scientists might finally answer the question:

Just how long is a day on Saturn?

NASA’s Cassini spacecraft stared at Saturn for nearly 44 hours in April 2016 to obtain this movie showing four Saturn days. Cassini will begin a series of dives between the planet and its rings in April 2017, building toward a dramatic end of mission — a final plunge into the planet, six months later.

Michele Dougherty used to say that measuring the length of a Saturn day was like searching for a needle in a haystack. Now she thinks the old cliché falls short. “It’s more like searching for several needles that change color and shape unpredictably,” she said.

Based at Imperial College, London, Dougherty is principal investigator for the magnetometer instrument (MAG) on board Cassini, studying the planet more closely than any spacecraft before. Yet Cassini’s instruments can’t seem to nail down something fundamental about Saturn that, for Earth, is hard to miss: the length of a day. Part of the challenge stems from what a day truly is.

When a person says, “It’s been cloudy for days,” they’re conveying how long (that is, for how much time) the sky’s been cloudy. But a day is really a description of motion, not time. The sun doesn’t rise or set. Instead, the sun’s apparent motion is the result of Earth spinning on its axis. And an observer need not be on Earth to figure out the length of an Earth day.

Someone in space or on another planet in our solar system could choose a distinct surface feature on Earth, such as Madagascar, then note its position and click a stopwatch. When Madagascar returns to the position it was in when the stopwatch was started, the observer can note the elapsed time. If they measured precisely enough, they would find that Earth rotates once per 23.934 hours. That’s Earth’s rotation rate — the very definition of a day.

Using the same principle, Earthlings have learned the rotation rates of other planets. A day on Mercury lasts about two Earth months. And a Mars day lasts 24.623 Earth hours, barely longer than Earth’s. But watching surface features does not work equally well for all planets.

When the bulk of a planet swims beneath thousands of miles of atmosphere, the challenge of clocking its rotation rate is even tougher. The swirling cloud bands on a gas planet like Saturn and Jupiter move at different rates, making it impossible to use the clouds to measure the planet’s rotation rate. But even then, scientists have a couple of aces up their sleeve: the planet’s magnetic field and radio wave emissions.

In a planet’s interior, heat causes electrically conductive fluids to move, and those currents generate a magnetic field that can extend out into space many times the diameter of the planet. On Earth and Jupiter, the magnetic north pole is tilted from each planet’s rotation axis by about 10 degrees, meaning it’s not aligned with the “true north” pole on either planet. If you could see Jupiter’s or Earth’s magnetic field from space, and you sped up time, the magnetic field would appear to wobble like a hula hoop as the planet spins. Since the magnetic field is generated in a planet’s deep interior, for most planets, the field’s rotation rate tells scientists the rotation rate of the planet itself. One full wobble equals one day.

We can’t see magnetic fields, but instruments called magnetometers can, and radio antennas can detect radio emissions from a planet with patterns that repeat each time the planet rotates. In fact, almost as soon as radio antennas were invented, scientists figured out that Jupiter has a 9-hour and 55-minute day, according to Bill Kurth, a University of Iowa physicist and leader of Cassini’s Radio and Plasma Wave Science (RPWS) team. “Jupiter is like a clock. It doesn’t lose time. It doesn’t gain time,” he said.

But Saturn is like no other planet orbiting our sun. Its magnetic field seems to be offset from its rotation axis by very much less than a degree, so Saturn’s magnetic field doesn’t hula but instead appears to spin smoothly with no wobble. Scientists might then expect to observe a steady signal of magnetic strength and direction at Saturn, but they don’t.

The Cassini MAG instrument has detected a signal in Saturn’s magnetic field which looks like a wave in the data that repeats about every 10 hours and 47 minutes. Scientists call that regularly repeating signal a “periodicity.” But this periodicity has a different value whether you’re observing Saturn’s northern or southern hemisphere, and it also seems to change with the seasons.

This spectrogram and video show a changing pattern of radio waves from Saturn known as Saturn Kilometric Radiation, as detected by NASA’s Cassini spacecraft.

The RPWS instrument has also detected periodicities, and another of Cassini’s instruments, the Magnetospheric Imaging Instrument (MIMI) has observed energetic charged particles (protons, electrons, ions) being whipped around Saturn periodically by its magnetic field. “The MIMI instrument sees these blobs that move about the planet,” Kurth said. But observations of the blobs, the radio emissions, and the magnetic field don’t agree enough for scientists to feel they’re sure about Saturn’s rotation rate.

Cassini scientists didn’t think Saturn’s rotation rate was a puzzle they’d have to solve. “We thought we already knew, because Voyager measured it,” Kurth said. Voyager data had suggested a Saturn day was about 10.7 hours. But Cassini’s magnetometer measures it as a bit longer, or a bit shorter, depending on whether the spacecraft is observing Saturn’s northern or southern hemisphere.

“Saturn has stymied us,” Dougherty said. “Its rotation rate is somewhere between 10.6 and 10.8 hours, probably, but the signal we’re seeing, we’re not sure it’s linked to the interior at all. All we know is that, in our MAG data, we see oscillations that are different in the north or the south, and they change over time.”

One possible cause is that something in Saturn’s atmosphere is disrupting or canceling out the effects of the true planetary magnetic field, Dougherty said. If that’s the case, getting closer to Saturn might help.

For the final phase of Cassini’s mission, the spacecraft will perform 20 orbits just outside of Saturn’s main rings starting in November 2016, followed by 22 orbits flying through the unexplored space between Saturn’s upper atmosphere and its innermost ring starting in April 2017. There, Cassini should have a better shot at seeing Saturn’s rotation rate more clearly and resolving the mystery of Saturn’s day.

“By the end of May 2017,” Dougherty said, “we should know whether we’ll be able to solve it.”

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