Passage Three
How long is a day on Saturn? According to a new study, it's 10 hours, 47 minutes and 6 seconds -- give or take 40 seconds.
This value differs from previous estimates by up to 8 minutes and is based on measurements of a mysterious magnetic field signal emanating from the planet. If confirmed, the finding could help scientists gain a better understanding of Saturn's turbulent atmosphere and its shrouded interior.
The study, led by Giacomo Giampieri of NASA's Jet Propulsion Laboratory, is detailed in the May 4 issue of the journal Nature.
A tricky art
The rotation period of rocky worlds like Earth can be calculated through simple observations of the motions of a particular spot on the planet in relation to other celestial objects. This method doesn't work, however, for the likes Jupiter and Saturn because the solid cores of these gas giants are completely obscured by thick clouds that make direct visual measurements impossible. Instead, the typically stated rotation periods for these planets are those of their magnetic fields, which scientists believe are closely tied to the rotations of their solid interiors.
But measuring the rotation period of Saturn's magnetic field is difficult because its rotational axis -- the imaginary line around which the planet rotates -- is nearly identical to the axis around which its magnetic field revolves.
In contrast, the magnetic and rotational axes of Jupiter are separated by a few degrees and scientists can use this difference to calculate its rotation period.
It's like measuring the spin rate of a CD that has a distinct label on it versus a blank one that doesn't, Giampieri explained. "It's very difficult to tell if a blank CD is rotating at all."
For Saturn, the accepted workaround had been to analyze distortions in radio signals emitted by the planet -- the signals are thought to be linked to the planet's magnetic field.
Analysis of Saturnian radio emissions by NASA's Voyager spacecrafts during the 1980s revealed a rotation period of about 10 hours, 39 minutes and 22 seconds. But more recent radio data collected by the agency's Cassini spacecraft in 2003 and 2004 got a different number: 10 hours, 45 minutes and 45 seconds.
1. What is the principle help to measure how long a day on Saturn?
A. Measurement of a magnetic field signal from the planet
B. Observations of a spot on the given planet in relation to other space objects
C. Studies of the magnetic fields of planets
D. Work on the magnetic and rotational axes of the planet
2.What is the meaning of “give or take”
A. more than
B. more often than not
C. sometimes
D. tolerable difference about
3. Why the rotation calculation method of Earth can not be applied to that of Jupiter and Saturn ?
A. Because Jupiter and Saturn are too far away to be observed from Earth.
B. Because thick clouds prevent us from seeing directly the solid cores of gas giants
C. Because the magnetic fields of Jupiter and Saturn are irrelevant to their solid cores.
D. Because there exist giant gas around Jupiter and Saturn.
4. Scientists can calculate the rotation of Jupiter with no difficulties because_______
A. the rotational axis and magnetic axis are similar.
B. the magnetic and rotational axes have a few degree difference
C. its magnetic field is close to the solid cores.
D. the rotation axis is to easy to be identified.
5. Radio signals are used to analyze Saturn’s ________.
A. gas giants
B. solid cores
C. magnetic fields
D. None of above
