Comet Hartley 2 has the same deuterium-to-hydrogen ratio as ocean water, lending support to the idea that Earth's water was delivered by comets. John Matson reports.

哈特利2号彗星拥有和地球海水相同的氘氢比例，有力支持了地球上的水源于彗星（的观点）。来自约翰·马特森的报道。

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，它们从哪里来？

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.

一直以来人们认为海水可能来自早期坠入地球的小行星和彗星。但是针对哈雷彗星和其他五颗彗星的观测结果给这一结论大泼冷水。因为这些彗星的化学成分比例和海水并不相似。具体而言，在彗星中，重氢，或者说是氘，以及氢的含量更高。

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.

但是现在天文学家们得以观测一颗来自不同地区的彗星。它的化学成分比例和海水非常吻合。他们使用赫歇尔太空望远镜来观测哈特利2号，这颗来自柯伊伯带【2】的彗星。而哈雷彗星及其同类彗星则来自遥远的奥尔特彗星云【3】。

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，在法国举行的行星科学会议上，研究人员在一个《自然》杂志（举办的）研究会中宣布，哈特利2号具有和海水相似的氘氢比例。[Paul Hartogh等，“木星家族的103号彗星哈特利2号上有类似海水存在”]

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.

下次当你在沙滩漫步时也许会想起，说不定这些海水（真的）来自彗星。

—John Matson 约翰·马特森

译注：
1. 哈特利2号彗星起源自遥远的柯伊伯带(Kuiper Belt)，比海王星轨道还要遥远的柯伊伯带是矮行星与彗星的发源地。

2. 柯伊伯带是一种理论推测认为短周期彗星是来自离太阳50—500天文单位的一个环带。位于太阳系的尽头，其名称源于荷兰裔美籍天文学家柯伊伯（Kuiper）。
3. 奥尔特彗星云又译欧特云，是一个假设包围者太阳系的球体云团，布满着不少不活跃的彗星，距离太阳约50,000至100,000个天文单位，差不多等于一光年，即太阳与比邻星距离的四分一。