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托福阅读经典材料：土星时间缩短

2017-09-26 12:12:38来源：网络

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　　Saturn's 'day' shorter by five minutes

　　An analysis of Saturn's atmosphere has resulted in the definition of the planet's 'day' becoming somewhat shorter.

　　According to a study in the journal Nature， the time it takes the ringed behemoth to complete a spin on its axis is 10 hours， 34 minutes and 13 seconds， more than five minutes shorter than previous estimates.

　　Unlike a rocky planet， Saturn has no visual landmarks. Instead it is covered in clouds of gas driven by layers of jetstreams， making it hard to measure the planet's rotation.

　　As a result， astronomers have traditionally based their calculations on Saturn's magnetic field. But this signal can fluctuate and does not accurately measure how fast the planet's deep interior is rotating.

　　Dr Andrew Prentice of Monash University in Melbourne says the problem with using Saturn's magnetic field is that it changes over time.

　　"It does not give a proper measure of Saturn's internal rotation since the magnetic field is slipping relative to the planet，" he says.

　　"As a result the period seems to have lengthened by seven to eight minutes since the time of the Voyager 1 and 2 missions in the mid-1980s."

　　Mapping the winds

　　An international team led by scientists from Oxford University and the University of Louisville， Kentucky， used a different technique based on infrared images taken by the US spacecraft Cassini orbiting Saturn.

　　"We realised that we could combine information on what was visible on the surface of Saturn with Cassini's infrared data about the planet's deep interior and build a three-dimensional map of Saturn's winds，" says Oxford professor Dr Peter Read.

　　"With this map， we were able to track how large waves and eddies develop in the atmosphere and from this come up with a new estimate for the underlying rotation of the planet."

　　Read says the fact that a Saturn day has been shortened by five minutes is a bigger deal than one might think.

　　"It implies that some of our previous estimates of wind speeds may be out by more than 160 miles (250 kilometres) per hour，" he says.

　　"It also means that the weather patterns on Saturn are much more like those we observe on Jupiter， suggesting that， despite their differences， these two giant planets have more in common than previously thought."

　　Prentice agrees， saying the result helps provide a consistent story across the solar system's gas giants.

　　"The bottom line is that Jupiter and Saturn are very similar in physical structure， origin and evolution."

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