Saturn's elusive rotation path revealed

Paris, Dec. 14: A team of European scientists has claimed that it is close to understanding what prompts the Planet Saturn to rotate.

Using data retrieved from the Radio and Plasma Wave Science instrument on the spacecraft Cassini, the scientists claimed in their report, which was published in one of the issues of Nature this week, that electrically charged particles were trapped in the planet's magnetic field and released radio waves with frequencies around 100 kilo Hertz. This enabled them to monitor the planet’s rotation rate.

They said that the magnetic field itself is generated deep inside the planet, so watching the variation of the radio emission as the magnetic field sweeps around can reveal the planet's rotation rate.

Using nine months worth of data from NASA's Voyager spacecraft, which flew past the planet during the 1980s, planetary scientists calculated Saturn's rotation period to be 10 hours 39 minutes 24 seconds, with an uncertainty of 7 seconds.

Repeating the measurement over 15 years later, the Ulysses spacecraft discovered that Saturn's period of radio emission varied and most recently, the Cassini spacecraft found the planet apparently rotating in 10 hours 45 minutes 45 seconds, with an uncertainty of 36 seconds.

According to the scientists, it is inconceivable that a planet could have slowed down by six minutes in a few decades.

As well as this long-period variation, Cassini's near continuous observations have also shown that the rotation rate seemed to vary by as much as one percent in a week.

The scientists forwarded two models to explain the variations in the radio emission period.

Firstly, that the wind of electrically charged particles given off by the Sun, the so-called solar wind, impacted the magnetic field, causing the radio emission to vary due to the variation of the solar wind speed.

Secondly, particles from the geysers on Saturn's icy moon Enceladus were affecting the magnetic field, causing it to drag around Saturn.

Now, after further careful analysis, Cassini's data strongly implicates the solar wind as the source of at least some of the radio period variation.

It shows that there is a characteristic variation in the behavior of the short-period radio emission every 25 days.

"This immediately points to the Sun because it is the rotation rate of the Sun as seen from Saturn, " says Philippe Zarka, CNRS, Observatoire de Paris, France, who led the research.

Zarka and colleagues analysed the properties of the solar wind and found that the speed variation of the wind is probably responsible.

It does not vary completely randomly but instead follows a saw-tooth pattern, first building up in speed and then suddenly slowing down.

The work is not finished yet because the long-period variation must still be explained.

The team is now seeing if they can remove the effects of the solar wind and deduce the true rotation rate of Saturn, a key piece of information to understand Saturn's atmosphere and interior.

Knowledge of the planet's true rotation rate will allow planetary scientists to compare observations taken years apart and calculate the true wind speeds on the planet. (ANI)