Enceladus geysers mask the length of Saturn's day

March 22, 2007
Enceladus geysers mask the length of Saturn's day
Geysers on Saturn's little moon Enceladus are throwing off Saturn's internal clock, making it hard to measure the length of Saturn's day. Credits: NASA

In a David and Goliath story of Saturnian proportions, the little moon Enceladus is weighing down giant 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.

Enceladus geysers mask the length of Saturn's day
The image of Saturn's rings was taken with the Cassini wide-angle camera on 14 December 2004. It was taken at a distance of approximately 654 000 kilometres from Saturn through a filter sensitive to wavelengths of infrared light centred at 728 nanometres. The image scale is 35 kilometres per pixel. Credits: NASA/JPL/Space Science Institute

A new study of Cassini data, reported this week in the online version of the journal Science, determined that Saturn’s magnetic field lines - invisible lines originating from the interior of a magnetized planet - 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 (RPWS) and the magnetometer (MAG).

The neutral gas particles ejected from the geysers on Enceladus form a donut-like torus around Saturn. As these particles become electrically charged, they are captured by Saturn’s magnetic field, forming a disc of ionized gas, or plasma, which surrounds the planet near the equator. The particles weigh down the magnetic field so much that the rate of rotation of the plasma disc slows down slightly. This slippage causes the radio period, controlled by the plasma disc 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 period of the plasma disc. 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 one 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 Prof. 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 disc, 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 disc, and a shorter period,” said Gurnett.

"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 Michele Dougherty, principal investigator on Cassini’s magnetometer instrument, Imperial College London.

Source: ESA

Explore further: Bright radio bursts probe universe's hidden matter

Related Stories

Bright radio bursts probe universe's hidden matter

November 17, 2016

Fast radio bursts, or FRBs, are mysterious flashes of radio waves originating outside our Milky Way galaxy. A team of scientists, jointly led by Caltech postdoctoral scholar Vikram Ravi and Curtin University research fellow ...

X-ray pulsars fade as propeller effect sets in

November 18, 2016

An international team of astrophysicists including Russian scientists from the Space Research Institute of the Russian Academy of Sciences (RAS), MIPT, and Pulkovo Observatory of RAS has detected an abrupt decrease of pulsar ...

What are active galactic nuclei?

November 9, 2016

In the 1970s, astronomers became aware of a compact radio source at the center of the Milky Way Galaxy – which they named Sagittarius A. After many decades of observation and mounting evidence, it was theorized that the ...

Twin jets pinpoint the heart of an active galaxy

September 19, 2016

An international team of astronomers has measured the magnetic field in the vicinity of a supermassive black hole. A bright and compact feature of only 2 light days in size was directly observed by a world-wide ensemble of ...

Stellar ghosts reveal galactic origins

September 21, 2016

Our sky is blanketed in a sea of stellar ghosts; all potential phantoms that have been dead for millions of years and yet we don't know it yet. That is what we will be discussing today. What happens to the largest of our ...

Recommended for you

Scientists sweep stodgy stature from Saturn's C ring

December 9, 2016

As a cosmic dust magnet, Saturn's C ring gives away its youth. Once thought formed in an older, primordial era, the ring may be but a mere babe – less than 100 million years old, according to Cornell-led astronomers in ...

Japan launching 'space junk' collector

December 9, 2016

Japan will launch a cargo ship Friday bound for the International Space Station, carrying a 'space junk' collector that was made with the help of a fishnet company.

Dark matter may be smoother than expected

December 7, 2016

Analysis of a giant new galaxy survey, made with ESO's VLT Survey Telescope in Chile, suggests that dark matter may be less dense and more smoothly distributed throughout space than previously thought. An international team ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.