March
2007
All in a day’s work0
Links to free teaching resources for the story on Saturn’s moon Enceladus:
Teaching resources (UK US) designed specifically for this story at www.realscience.org.uk
Story, audio files, images and podcast from NASA.
Saturn’s tiny moon Enceladus is a “highlight of the Cassini mission and should be targeted in future searches for life.”
Centauri Dreams, one of my favourite blogs, on the topic of little Enceladus. The blog is about the science of star-travelling: “Building a star-faring craft is something like building a cathedral: it will take the combined efforts of scientists and engineers through several generations to make it happen.”
Make your own Cassini presentation.
Download lesson sets for Reading, Writing and Rings, the Cassini Mission’s language arts and science program for students in grades 1-4.
Without liquid water, terrestrial life could not exist. All living organisms on Earth depend on water and its unique chemical and physical properties. In the search for life beyond Earth, scientists have focused their efforts on looking for signs of liquid water. This essay from NOVA Online explores why liquid water is considered an essential ingredient for life as we know it
Imaginative captain’s log and wonderful images from Cassini
Voyage to the mystery moon. From Nova.
Background on how long is a day on Saturn. Useful for the science methods, but the conclusion is now superseded by this latest story.
More links
NASA resources and information on Enceladus.
Nice explanation of difference between rotation period and length of day. Note that Saturn’s figures are wrong.
The story:
All in a day’s work
How long is a day? Well it depends on where you’re standing. One day is the time it takes for the sun to rotate right around the sky, and come back to the same place. Of course it’s the planet spinning that makes the sun seem to move.
Because planets spin at different rates, each has a different length of day. You might think these numbers would be well known, but not all of them are. Rocky planets are easy. Scientists simply choose a landmark on the surface and wait till it comes round again.
But four planets are made of gas not rock. So finding fixed features on Jupiter, Saturn, Uranus and Neptune is tricky.
Until now a method of measuring radio waves from deep inside these gas giants was thought to be accurate. These radio waves are linked to the planet’s magnetic field. This should rotate at the same rate as the planet.
New data from NASA’s Cassini spacecraft show that things are more complicated at Saturn. The problems are being caused by a small moon called Enceladus. At just over 300 miles in diameter this satellite of Saturn could easily fit into Arizona. It’s a small moon. But it is giving scientists a big headache.
The new data from Cassini show how Enceladus is making it almost impossible to measure the length of Saturn’s day. They demonstrate that the planet’s magnetic field is rotating at a different rate to the planet.
The reason seems to be electrically charged particles coming from huge geysers on Enceladus. These are spewing water vapour and ice.
These results are based on joint observations by two Cassini instruments, the radio and plasma wave instrument and the magnetometer. The new findings are reported online in the 22 March issue of Science
No one could have predicted that Enceladus would have such an effect on the radio technique used to measure the length of Saturn’s day, said Dr. Don Gurnett of the University of Iowa.
Gurnett is the principal investigator on the radio and plasma wave science experiment onboard NASA’s Cassini spacecraft.
What seems to be happening is this. Neutral gas particles are thrown out by geysers on Enceladus. They form a donut-shaped ring around Saturn. As these particles become electrically charged, they are captured by Saturn’s magnetic field. They then form a disk of ionised gas or plasma. This surrounds the planet near the equator.
The scientists now believe that the period Cassini has been measuring from radio emissions is not the length of the Saturn day. Instead it is the rotation of this plasma disk. At present Saturn’s cloud motion means that no technique is known that can accurately measure the planet’s internal rotation.
“We have linked the pulsing radio signal to a rotating magnetic signal,” said Dr. David Southwood, director of science at the European Space Agency. “Once each rotation of Saturn’s magnetic field, an asymmetry in the field triggers a burst of radio waves. We have then linked both signals to material that has come from Enceladus.
The direct link between radio, magnetic field and deep planetary rotation has been taken for granted, said Michele Dougherty of Imperial College London. He is principal investigator on Cassini’s magnetometer instrument.
“Saturn is showing we need to think further.”
