听力文摘：燃烧仅在地球可行

What is fire anyway?

火究竟是什么？

Well, fire is rapid oxidization. It's the energy released when oxygen atoms start combining with carbon and hydrogen atoms.

火，即燃烧，是快速的氧化反应。氧化反应是氧原子和碳原子或氢原子结合释放能量。

Exactly, so without oxygen in the air, nothing burns.

没错，那么如果空气中没有氧气，就不会有燃烧。

You mean there was a time before oxygen?

你是说氧气还不存在的那个时代？

Yes. By roughly a billion years ago, simple forms of plant life called "eukaryotes" were already busily converting carbon dioxide into oxygen gas, just like their descendants do today. But before this process had been in place for a while, there wasn't enough oxygen in the air for things to burn.

对，大约十亿年前，和眼下的植物一样，地球上最原始的植物祖先“真核细胞”忙碌地将二氧化碳转变成氧气。但是，在此转化之前的一段时间，空气中没有足够的氧气可以燃烧东西。

In fact, of all the planets, moons, asteroids¹, and comets in our solar system, the only place where you can actually start a fire is right here on earth. And even here, it's only been possible relatively² recently.

而事实上，整个太阳系的所有的行星、月球、小行星、彗星，你会发现唯一可以生活的地方只有地球。而就算是地球，也是不久以前才可行。

1 asteroids   

n.小行星( asteroid的名词复数 )；海盘车，海星

参考例句：

• Asteroids,also known as "minor planets",are numerous in the outer space. 小行星，亦称为“小型行星”，在外太空中不计其数。
• Most stars probably have their quota of planets, meteorids, comets, and asteroids. 多数恒星也许还拥有若干行星、流星、彗星和小行星。

2 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。