Reset your interplanetary watches because data from NASA's Cassini probe has finally answered the question of how long a day on Saturn is. Using the planet's giant rings as a natural seismograph, a team led by UC Santa Cruz astronomy and astrophysics graduate student Christopher Mankovich calculates that Saturn rotates once every 10 hours, 33 minutes and 38 seconds.

Finding out how long a day is on a particular planet may seem like a simple job, but it's often more difficult than you might think. This is especially true with a gas giant like Saturn, which is composed mostly of hydrogen and helium, has no solid surface, and whose only visible part is a swirling layer of clouds rushing around the planet like endless hurricanes.

Up until the first unmanned probes were sent to Saturn, its day was roughly estimated by earthbound telescope observations. Then when the Voyager spacecraft flew by in 1980 and 1981, radio signals were used to measure the planet's magnetic field, which is almost perfectly aligned with its rotational axis, making it a very good indicator of the planet's day, which was measured at 10 hours, 39 minutes, and 23 seconds.

According to Mankovich, the rings are a better way of measuring Saturn's rotation because the variations in the gravitational field caused by vibrations in the planet's interior are reflected in the rings. By creating computer models, the team was able to match the interior vibrations against the planet's day length. The idea has been around since the 1980s, but it was only with the arrival of the Cassini spacecraft that it became feasible.

"Particles throughout the rings can't help but feel these oscillations in the gravity field," says Mankovich. "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."

Source: NASA