GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 957248T45043LBWSK

In the late 1990s, the Mars Global Surveyor spacecraft detected magnetized patches of terrain near the north pole of Mars and also in the Terra Sirenum region, which is in the planet's southern hemisphere. These magnetic features may be relics of a global magnetic field in the vicinity of several huge, ancient craters in the Terra Sirenum region, astronomers believe these craters were formed some 3.8 billion years ago from the impact of chunks of debris pelting the inner solar system, where Mars orbits the Sun. If a global magnetic field once existed on Mars, it must have vanished before these craters formed. Such large impacts beat rock to temperatures well above 600 degrees Celsius – high enough to erase any magnetic field that metallic particle within the rock strong at the time the craters formed, it would have realigned and remagnetized the articles as they cooled. The Surveyor findings thus suggest the magnetic field vanished early in the planet's 4.5 billion year existence.