PASADENA, California -- NASA's Cassini space probe has already aided scientists to make a second discovery about Saturn, even though the craft is still a day away from beginning its main mission.
Signals detected by the probe show that Saturn's natural radio emissions are more like the sun's than the Earth's, and that a Saturnian day is not as short as once thought.
Earlier in the month, images and measurements taken by the $3.3 billion Cassini spacecraft confirmed scientists' suspicions that Saturn's dark moon, Phoebe, is a planetary building block left over from the birth of the solar system.
NASA announced the radio discovery on Monday, saying that it was based on data returned from the probe over the past year. The data showed that Saturn's radio rotational period -- a measurement often used to determine the length of a day on a planet -- was nearly six minutes longer than when measured by the Voyager 1 and Voyager 2 spacecraft in 1980 and 1981, respectively.
Scientists say this means it's highly likely that Saturn's radio emissions change depending on when and where they are measured, like those of the sun. By contrast, the period of Earth's radio emissions are fixed. They have ruled out other options, like malfunctioning equipment on the Voyager probes, or the possibility that Saturn's physical rotation has slowed over the years, they said.
"Although Saturn's radio period has clearly shifted substantially since the Voyager measurements, I don't think any of us could conceive of any process that would cause the rotation of the entire planet to actually slow down," said University of Iowa space physicist Don Gurnett, principal investigator for the Cassini Radio and Plasma Wave Science instrument.
The discovery has called into question the length of a day on Saturn. According to the new radio measurements, Saturn rotates once every 10 hours, 45 minutes and 45 seconds (give or take 36 seconds). But since that figure is based on variable radio emissions, it is likely to vary from the time required for the planet's physical rotation around its axis.
Unfortunately for astronomers, measuring the physical rotation of a gaseous planet like Saturn isn't easy. Because the clouds that envelope Saturn are constantly moving, it's not possible to pick a visible point on the planet and measure its movement, as one might do to determine the length of a day on Mars.
NASA hopes it can come up with a different solution to the problem, and uncover the mystery of what is causing the radio emissions to change, sometime during the next four years as Cassini continues to spy on Saturn and its moons. "We will be able to unravel the puzzle, but it's going to take some time," said Gurnett.
Project scientists have made the radio signals available to the public on the Jet Propulsion Laboratory's website and on Gurnett's site. The signals have been adjusted to be audible to the human ear and compressed so that one second of sound corresponds to one full rotation of Saturn.
Cassini has been hurtling through space on its way toward Saturn for the past six and a half years. On Wednesday evening, mission controllers at the Jet Propulsion Laboratory will fire one of Cassini's engines, allowing the spacecraft to slip through the space between two of Saturn's rings. The maneuver will put Cassini in orbit around Saturn, where it will remain for the next four years, studying the planet, its rings and its moons.
As of Tuesday morning, scientists at JPL said they expected Wednesday's maneuver to go off without a hitch. "We have no indications of any problems with the spacecraft that would have an adverse effect on SOI (Saturn orbit insertion)," said Cassini program manager Robert Mitchell during a press briefing. "It's looking very solid."
