英语听力：自然百科 地球力场 Earth's Force Field—6

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Why did Mars lose it magnetic shield? And what effect did it have on the planet? 

 

To find / out, scientists need to analyze¹ Mars' rock.

Mars Global Surveyor didn't stop to take samples but Vice² has managed to get hold of something extremely rare, Martian meteorites⁴. 

We have some rocks from Mars that we're naturally transferred to Earth. These are meteorites that were blasted off from Mars by the asteroid⁵ common impact. 

 

Vice takes the meteorite³ sample into a special magnetically shielded chamber⁶. He places it in a superconducting quantum interference device or SQUID microscope for analysis. 

 

For the magnetic field, the rock is extremely weak. So, in order to do that well, we need to make these measurements in a very weak magnetic field..

 

This highly sensitive machine measures the exact strength of the rock's magnetism⁷. By analyzing⁸ different samples, Vice builds up a picture of Mars’ magnetic field at the very beginning of the planet's existence.

 

We have actually one rock from Mars that's 4.5 billion years old. And we've actually recently shown by looking the intensity⁹ of the magnetization record in that rock, that Mars had a magnetic field that was roughly the strength of the Earth/ today.

 

Vice's research reveals that in its first 50 million years, Mars had a strong magnetic shield. It's amazing that diversity of magnetic fields that we see about different samples. Some of them like this one: this is an ancient rock from Mars, / it's 4.5 billion years old. You will see tiny, little, isolated¹⁰ magnetic anomaly with huge areas of non- magnetic rock in between. 

 

Why Mars lost its shield is a mystery, but what happened to the planet as a result is not. When Mars lost its magnetic field, it lost its shield that’s protecting it from the solar wind, from cosmic radiation and that had consequences for the evolution of its atmosphere. The weak atmosphere allows solar wind to strip away Mars' delicate atmosphere. Its oceans vaporize, temperatures plummet¹¹ and any primitive¹² life forms die.

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