Scientists are finding it a real drag trying to measure the length of a day on Saturn – and tonight they revealed why. One tiny moon’s cosmic burps are weighing down the ringed planet’s magnetic field so much that standard methods do not work.

Erupting geysers on the rocky satellite, Enceladus, are making the force field around Saturn rotate more slowly than the planet itself.

The result is that normal radio techniques used to measure the length of a day on giant planets become virtually impossible.

Scientists have discovered the nature of the problem thanks to new data from Nasa’s Cassini spaceprobe in orbit around Saturn. The planet spins remarkably quickly, in less than 11 hours. The problem has come in finding an exact figure.

Cassini’s data showed that Saturn’s magnetic field lines are being forced to slip by the weight of electrically charged particles from geysers spewing steam and ice on Enceladus. That interferes with a standard method of checking the planet’s pulse by observing the rhythm of radio waves that Saturn beams out naturally.

Cassini scientist Professor Michele Dougherty, of Imperial College London, said: “The direct link between radio, magnetic field and deep planetary rotation has been taken for granted up to now. Saturn is showing we need to think further.”

Dr Don Gurnett, of the University of Iowa, said: “No one could have predicted that the little moon Enceladus would have such an influence on the radio technique that has been used for years to determine the length of the Saturn day.”

An added complication is that the length of a day on Saturn seems to be slowly changing over months to years. The day measured by Cassini is some six minutes longer than the day recorded by Nasa’s two Voyager spacecraft in the early 1980s – a change of nearly one per cent.

Scientists now want to find out whether this is because the geysers on Enceladus are more active now or if it suggests there are be seasonal variations as Saturn orbits the sun once every 29 years.

The Nasa picture is a colour-coded image of an eruption from Enceladus on March 24, 2006, that sent particles streaming hundreds of miles into space.

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