英语听力：自然百科 地球力场 Earth's Force Field—7

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How have Earth and Mars evolved differently such that Earth still has a field and Mars doesn't. Actually the problem at the moment is more one of explaining where the energy is coming from, that powers / the Earth's magnetic field. 

 

 

 

To find out exactly how and where our magnetic field is generated, scientists need to explore the Earth's interior, but this isn't easy. 

 

 

 

We really don't know a lot of detail about what's happening 2000 miles down because there is no way to send a probe there. 

 

 

 

Since there is no way to probe it directly, scientists study the structure of our planet using one of nature's most powerful phenomena¹, earthquakes. 

 

 

 

So, seismologists, they wait for an earthquake, as we say in Japan. And then they put an array of detectors² on the other side of the Earth, look at the sound going through the Earth, and by listening carefully, they can construct a map of what's inside the Earth.

 

 

 

 

 

Scientists measure earthquake vibrations³ called seismic⁴ waves. These waves don't travel through the planet in a straight line. Instead, they bend, changing speed and direction from they pass through different materials. By measuring these waves carefully, scientists learn/ that the Earth must be made up of distinct layers, like an onion. 

 

 

 

 

 

The surface layer, the crust, is made of solid rock just a few miles thick. Below sits the mantle⁵, it consists of denser⁶ semi-liquid rock. Eighteen hundred miles down is the liquid outer core, a churning sea of molten iron and nickel. It surrounds the inner core, a solid iron sphere around the size of our moon, but it's hot as the surface of the sun. Scientists believe that the magnetic iron core generates our magnetic field, but how?

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