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Headlines Published on 25 January 2008

Title A day in the life of Saturn

Though we might not realise it, the Earth is moving through space at astronomical speeds. While the Earth zooms around the Sun at speeds of around 107 000 kilometres per hour, it is also spinning around itself, much like a spinning top, at the speed of 1 670 kilometres per hour (at the equator). The time it takes to make one complete revolution is what determines the length of one day. Now, a team of European scientists are close to determining the speed of Saturn's rotation, and thereby, the length of a Saturn day.

The planet Saturn
The planet Saturn

For years, astronomers have been trying to gain a better understanding of the planets in our solar neighbourhood. Until now, trying to determine the length of a day on one of the giant gas planets has been fraught with difficulties. In the case of Saturn, the interior of the planet is masked from normal observation as clouds in the upper atmosphere conceal important information. In order to measure the internal rotation of the planet, scientists need a property that is associated with the interior and yet is observable from space. Scientists now believe that radio emissions may be the answer they have been looking for.

In results recently published in Nature, a team of European scientists have utilised radio waves, which are then picked up by the Radio and Plasma Wave Science instrument on the Cassini Satellite to create an "audio image" of Saturn. The variation in the received intensity is due to the planet's rotation and the variation in the solar wind speed near Saturn.

Electrically charged particles trapped in the planet's magnetic field release radio waves with frequencies of around 100 kilohertz (kHz). 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.

Although radio waves are invisible, sound waves can still be picked up by Cassini. With these waves, it is possible to create an "audio image" of Saturn's radio waves and their regular variation. The latter is not random, but follows a saw-tooth pattern, first building up in speed and then suddenly slowing down. This causes the apparent rotation period fluctuations.

The team is now looking to remove the effects of the solar wind and deduce the true rotation rate of Saturn, a key 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. Ultimately, the speed of rotation of the planet is linked to the way material is distributed inside the vast globe, and so is a clue to the formation of the planet.

'If we can find the true value for Saturn's rotation then we have it for once and for all,' said Philippe Zarka, Centre National de la Recherche Scientifique (National Center for Scientific Research or CNRS), Observatoire de Paris, France, who led the research.

More information:

  • European Space Agency
  • Scientists find that Saturn's rotation period is a puzzle

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