Tiny Enceladus masks mighty Saturn’s clock

New readings from NASA's Cassini spacecraft show that Saturn's moon might be making it impossible for scientists to measure the giant planet's day<em> - News Release</em>

News release issued by the Particle Physics and Astronomy Research Council

Under strict embargo for
18.00 hours GMT
Thursday 22nd March 2007

New data from NASA's Cassini spacecraft, reported in the online version of Science today (22 March 2007), shows how the small moon, Enceladus, is weighing down Saturn's magnetic field so much that the field is rotating slower than the planet. This phenomenon makes it nearly impossible to measure the length of the Saturn day using techniques that work at the other giant planets.

"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," said Dr. Don Gurnett of the University of Iowa, Iowa City. Gurnett is the principal investigator on the radio and plasma wave science experiment onboard NASA's Cassini spacecraft. The radio technique measures the rotation of the planet by taking its radio pulse rate - the rhythm of natural radio signals from the planet.

A new study of Cassini data determined that Saturn's magnetic field lines are being forced to slip relative to the rotation of the planet by the weight of electrically charged particles originating from geysers spewing water vapor and ice from Enceladus. These results are based on joint observations by two Cassini instruments - the radio and plasma wave instrument and the magnetometer.

"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," said Professor Michele Dougherty , principal investigator on Cassini's magnetometer instrument from Imperial College London.

The neutral gas particles ejected from the geysers on Enceladus form a doughnut-like ring around Saturn. As these particles become electrically charged, they are captured by Saturn's magnetic field, forming a disk of ionized gas, or plasma, which surrounds the planet near the equator. The particles weigh down the magnetic field so much that the plasma disk slows down slightly. This slippage causes the radio period, controlled by the plasma disk rotation, to be longer than the planet's actual rotation period.

Scientists conclude the period Cassini has been measuring from radio emission is not the length of the Saturn day, but rather the rotation of the plasma disk. At present, because of Saturn's cloud motion, no technique is known that can accurately measure the planet's actual internal rotation.

Finding out the length of Saturn's day has been a challenge because the gaseous planet has no surface or fixed point to clock its rotation rate. Initially, the approach was to use periodic regular radio signals, as has been done for Jupiter, Uranus and Neptune. However, Saturn's radio period has turned out to be troubling in two ways. It seems to be a pulsed signal rather than a rotating, lighthouse-like beam. Secondly, the period 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 Voyager spacecraft in the early 1980s, a change of nearly 1 percent.

"We have linked the pulsing radio signal to a rotating magnetic signal. Once each rotation of Saturn's magnetic field, an asymmetry in the field triggers a burst of radio waves," said Professor David Southwood , co-author, Imperial College London, and director of science at the European Space Agency. "We have then linked both signals to material that has come from Enceladus."

Based on the new observations, scientists now think there are two possible reasons for the change in radio period. The first theory is that the geysers on Enceladus could be more active now than in Voyagers’ time. The second is that there may be seasonal variations as Saturn orbits the sun once every 29 years.

"One would predict that when the geysers are very active, the particles load down the magnetic field and increase the slippage of the plasma disk, thereby increasing the radio emission period even more. If the geysers are less active, there would be less of a load on the magnetic field, and therefore less slippage of the plasma disk, and a shorter period," said Gurnett.

Dr Hugo Alleyne from the University of Sheffield is a UK co-investigator on the radio and plasma wave instrument. He comments, "This is a very significant result which would not have been possible without the suite of instruments on the Cassini spacecraft making simultaneous measurements of fields, waves and plasma. It is likely to have implications in the interpretation of other astronomical and planetary measurements."

Contacts
Gill Ormrod – PPARC Press Office
Tel: 01793 442012
Email: gill.ormrod@pparc.ac.uk

Danielle Reeves – Imperial College London Press Office
Tel: 020 7594 2198
Email: Danielle.reeves@imperial.ac.uk

Lindsey Bird - University of Sheffield Press Office
Tel: 0114 2225338
Email: l.bird@sheffield.ac.uk

Carolina Martinez - Jet Propulsion Laboratory, Pasadena, Califonia, USA
Tel: +1 (818) 354-9382.
Email: carolina.martinez@jpl.nasa.gov

UK Science contacts
Professor Michele Dougherty – Principal Investigator, Magnetometer
Imperial College London
Tel: +44 20 7594 7757. Mobile: +44 7990 973761.
Email: m.dougherty@imperial.ac.uk

Dr Hugo Alleyne – Co-Investigator on Cassini's Radio and Plasma Waves Instrument
University of Sheffield
Tel: 0114 222 5630.
Email: h.alleyne@sheffield.ac.uk

UK involvement in Cassini

UK scientists involved in 6 of the instruments on Cassini (and two instruments on the Huygens probe). They include researchers and students from the Open University (previously at Kent University), Imperial College London, Queen Mary University of London (QMUL), Mullard Space Science Laboratory (MSSL), Rutherford Appleton Laboratory (RAL), the Universities of Oxford, Sheffield, Cardiff, Southampton and Leicester, and Queens University Belfast.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL.

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public engagement in four areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), and the European Space Agency. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank observatory.

PPARC is a partner in the British National Space Centre [BNSC] which coordinates the UK’s civil space activities.