英语听力：探索发现 科学新发现：我们的大气层-9
时间：2014-11-05 08:31:22
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（单词翻译）
We were left with this atmosphere made of molten rock and vaporous rock. It took millions of years for that atmosphere to fall out and to be replaced by an atmosphere made up of nitrogen and carbon dioxide and water vapour. These new gases came from above and below. Volcanic1 eruptions2 spewed out fresh stocks of nitrogen and carbon dioxide from within the planet’s molten core.
While the vital element of water may have been delivered by icy comets, each impact brings in a new supply of frozen liquid. But the air still lacked one of its key ingredients. One of the things that was missing is free oxygen—molecular oxygen, O2, the stuff we breathe.
Dr. Janet Seifert of Rice University in Texas searches for the event that produced the atmosphere’s final element. We know from the rock record that for the first half of Earth’s life there was no oxygen in the atmosphere
whatsoever3. There were greenhouse gases like
methane4 and CO2, but there was no oxygen. But the rock record shows a sudden change dated to around 2.5 billion years ago.
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