Earth's surface is dominated by oceans. But where did all that water come from?

地球表面大部分都是海洋，它們從哪里來？

Asteroids and comets smashing into the early Earth have long been thought to be a promising  source. But measurements of Halley's Comet and five others threw cold water on the idea that  comets provided a large share of the oceans. Because the chemical signatures of the comets did  not match the oceans. Specifically, the ratio of heavy hydrogen, or deuterium, to regular  hydrogen was too high in the comets.

一直以來人們認為海水可能來自早期墜入地球的小行星和彗星。但是針對哈雷彗星和其他五顆彗星的觀測結果給這一結論大潑冷水。因為這些彗星的化學成分比例和海水并不相似。具體而言，在彗星中，重氫，或者說是氘以及氫的含量更高。

But now astronomers have gotten a glimpse of a comet with a different origin, and it matches the  oceans much better. They used the Herschel space telescope to examine Comet Hartley 2, which  originated in the Kuiper Belt. Halley and its ilk came from a distant swarm of comets called the Oort Cloud.

但是現在天文學家們得以觀測一顆來自不同地區的彗星。它的化學成分比例和海水非常吻合。他們使用赫歇爾太空鏡來觀測哈特利2號，這顆來自柯伊伯帶的彗星。而哈雷彗星及其同類彗星則來自遙遠的奧爾特彗星云。

At a planetary science meeting last week in France, and in a study in Nature, the researchers  announced that Comet Hartley 2 has the same deuterium-to-hydrogen ratio as ocean water. [Paul  Hartogh et al., "Oceanlike water in the Jupiter-family comet 103P/Hartley 2"]

上周，在法國舉行的行星科學會議上，研究人員在一個《自然》雜志（舉辦的）研究會中宣布，哈特利2號具有和海水相似的氘氫比例。[Paul Hartogh等，“木星家族的103號彗星哈特利2號上有類似海水存在”]

So maybe comets did play a major role in delivering Earth's oceans. Something to ponder next time you have a day at the beach.

下次當你在沙灘漫步時也許會想起，說不定這些海水（真的）來自彗星。

surface n.表面
smash v. 撞擊
chemical signatures 化學成分
originate vi.起源于
deuterium-to-hydrogen ratio 氘氫比例