Giving Saturn the time of day


A day on Saturn is surprisingly hard to measure, but astronomers have realised the answer was hidden in the rings. Alan Duffy reports.


Saturn as captured by the Cassini Orbiter.

Saturn as captured by the Cassini Orbiter.

NASA/JPL-Caltech/Space Science Institute

There’s a certain irony that Saturn, the planet named after the ancient Roman god of time, has a day that is difficult to measure.

Now we may have an answer, with new research published in The Astrophysical Journal by astronomers from the University of California Santa Cruz in the US and NASA pinning down an accurate measure for the first time.

Using the now-departed Cassini satellite, the researchers discovered how to measure time with Saturn’s rings, coming to the conclusion that a day lasts 10 hours, 33 minutes and 38 seconds.

The challenge when trying to make a measurement on a Gas Giant is that there’s no constant reference point to use to measure one rotation of the planet. On a rocky world like Earth or Mars it is easy to measure – just time how long it takes for the same surface features to appear.

With no constant reliable surface features, the astronomers needed to turn to the planet’s magnetic field.

Even this provided no easy answer, however. Typically, the magnetic field of a planet can act like a yardstick, jutting at an angle to the axis of rotation and therefore pointing towards (and then away from) passing spacecraft. Such a measurement was made for both Jupiter (a staggeringly quick 9h 55m 29.71s) and Saturn (10h 39m 23s) by NASA’s Voyager.

However, when the researchers repeated the measurement of Saturn’s day using Cassini, it had appeared to gain six minutes. Even more curiously, the apparent “days” were different for the northern and southern hemispheres, with measurements over the planet ranging over 12 minutes.

With Saturn’s magnetic field almost perfectly aligned with the axis of rotation, unlike Earth or Jupiter, the astronomers looked instead to the planet’s most famous feature – its rings.

Beneath the clouds the planet naturally vibrates, which is reflected in a changing gravitational field. That is then picked up by the rings in a similar way to seismographs on Earth responding to tremors in the ground.

As Christopher Mankovich, who led the research at UC Santa Cruz, explains, “particles throughout the rings can't help but feel these oscillations in the gravity field.

"At specific locations in the rings these oscillations catch ring particles at just the right time in their orbits to gradually build up energy, and that energy gets carried away as an observable wave."

Those observable waves in the rings were seen up close by the Cassini spacecraft, allowing the team to create models of the planetary interior that could generate these patterns.

With that interior model, they could then track the interior movements of the planet as easily as the recognisable features on a rocky planet.

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
  1. http://iopscience.iop.org/article/10.3847/1538-4357/aaf798/meta
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