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刊登在《科学》上有关彗星尘埃分析报告的大意

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发表于 2007-1-4 18:48:19 | 显示全部楼层 |阅读模式
在“星尘”的论坛里发布的论文概要


第一部分    81P“怀尔特2”彗星元素成分
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Hi Everyone,
嗨、大家好,

Good news! We just released the findings from our preliminary analysis on the cometary side. The results were so spectacular that we’ve hijacked a special edition of Science magazine! I figured that you’ll be interested even though this is the other side of the collector because, frankly, comets are cool too.  
好消息!我们刚刚发表了来自彗星尘埃初步分析的发现。取得的成果是如此引人入胜,以至于我们趁机抢劫了一期《科学》的特刊!我觉得你们会对此感兴趣的,即使它在收集器的背面,坦白的讲彗星也是非常棒的。

There’s a lot of material – almost two hundred scientists contributed to the preliminary analysis, so the papers published in Science are a synopsis of the last year’s work (and in many cases more than a year) for a large number of scientists. I’d like to give you a synopsis of what we found in a form that is accessible to those of you who don’t speak “scientist.” So, I figured I’d just cover one paper every couple days. That’ll give me time to get some other work done too... These are going to be very much an overview. If you want the hard numbers, and nitty gritty details, you’ll have to go get your own copy of Science. My hope is to write this in such a way that you get the broad brushstrokes in an enjoyable read.
材料太多了,差不多有200位科学家投入到这次初步分析中,所以在《科学》上发表的论文都是去年(许多情况下都超过了一年)大量科学家工作的大纲。我愿意为你们用“非科学的语言”建立一个容易理解的大体框架。所以每篇论文我会用两天的时间来完成,总要给我一点时间做些别的事...浏览一遍的工作量也是很大的呀。如果你有毅力打算啃细节,那你得出去买一本《科学》了。希望我以这种方式所写的能使你获得一次比较愉快的阅读就能抓住主要的线条。

The first article I’ll cover is the one I participated in: Elemental Compositions of Comet 81P/Wild 2 Samples Collected by Stardust
我要报到的第一篇文章是我参与过的:“星尘”号采集到的81p(“怀尔特2”)彗星样品的元素成分。

By elements we mean iron, carbon, magnesium – the standard elements you find on a periodic table. By elemental composition we mean “how much of each element is present in the comet?” and “how does this compare to meteorites and other extraterrestrial objects?”
我们所指的元素是铁、碳、镁——这些元素周期表上的标准元素。我们所说的元素成分指“每种元素在彗星中存在多少”以及“同陨石和其他地球外的物体的对比情况”。

We want to know the elemental composition of comet Wild 2 because these measurements can be used by theorists to determine how the comets formed and changed over time. As an example, you find gold veins on Earth due to geologic processes. As water journeys down toward the magma (molten rock) under the Earth, it begins to dissolve gold atoms from the nearby rocks. On it’s way back up, it then carries the gold atoms along with it and when the high pressures and temperatures decrease, the gold condenses into veins of pure metal. This is a natural process, but the simple fact that gold is separated tells us something about how the land formed. The same is true for the formation of the solar system: the concentration of elements in any solar system object, comets included, tells a story.
我们之所以想知道“怀尔特2”彗星的元素成分是因为这样的测定可以让理论学家决定彗星是怎样形成的以及随时间如何变化。举个例子,你在地上发现的金矿脉是地质过程的结果。在水流经地下的岩浆(熔化的岩石)时会从附近的岩石中溶解金原子。当流水返回时便携带着金原子,一旦高温、高压开始减弱金就会以纯金属的方式浓缩在矿脉里。这是一个自然的过程,但黄金是分散的这一事实告诉我们一些有关大陆形成的事情。对太阳系的形成同样是这个道理:浓缩在太阳系里任何物体,包括彗星在内,中的元素都有一段故事。

We compared what we found in Wild 2 to that found in a specific class of meteorites called “CI chondrites.” The CI chondrites are thought to have similar proportions of elements to those of the entire solar system. This is because many elements in CI meteorites are present in the same proportions as the solar photosphere – or the outer layer of the sun that you see through a telescope. It’s not clear why this is, and in fact it is somewhat remarkable, since CI meteorites are the most changed by water of any class of meteorites. As you saw from the gold example, water tends to move elements about and create artificial concentrations of them that don’t represent the average. It is a long-standing mystery, therefore, why CI chondrites have such similar proportions to the sun. Regardless, in our investigation of Wild 2 particles, we found a few things that were surprising:
我们把在“怀尔特2”中找到的东西同在一类称为“CI 球粒陨石”的特殊陨石中发现的相比较。CI 球粒陨石被认为是与整个太阳系有着相似的元素比例。这是因为CI 陨石中的许多元素呈现出与太阳光球层——或者说当你通过望远镜观察太阳时看到的太阳外层有相同的比例。现在还不清楚为什么会是如此,既然 CI 球粒陨石是陨石中被流水改变最大的一类,实际上多少有些不寻常。如同你在黄金例子中看到的,流水往往会搬走元素并制造出原先所未表现出的虚假集中。因此,为什么 CI 球粒陨石有着与太阳相近的(成分)比例,是个长久的谜团。先不管这些,在我们对“怀尔特2”颗粒的调查中我们发现了个别令人吃惊的事情:

1) The concentrations of elements in the comet varies tremendously between particles. We expected them to be more similar to one another. This means that the comet really is an aggregate of very different stuff, and hasn’t undergone any real geologic change to shake a stick at. It also means that the comet is not made of small, pristine interstellar materials either. If it were, the particles would have been much more similar to each other than they are.
1) 元素在彗星颗粒间的集中有着极大的差别。我们预期它们彼此间会比较相近。这意味着彗星确实是差异巨大物质的集合体,并且未曾遭受任何真正意义上的地质扰动。这同样也表示彗星不是由少量、原始星际物质构成的。否则,这些颗粒间会很相似而不是象现在这样。

2) The average composition for our sample is different in some ways from the CI chondrite meteorites: namely we found more zinc, gallium, and copper, and less sulfur. It turns out that zinc, gallium, and copper are very difficult to measure precisely in the solar photosphere so our measurements of the sun may not be quite right. They are also so-called semivolatile elements – that is, they evaporate at relatively modest temperatures. So there might be more of them in the Kuiper Belt (where comets come from) as compared with the inner solar system near the sun. If so, the consequences for solar system formation could be interesting. Or it may mean that there are more of these elements in the solar system than we thought. The sulfur mystery is so far unresolved – we’re busy scratching our heads about that one.
2) 我们样品的平均成分水平在某些方面与 CI 球粒陨石不同:即我们发现有较多的锌、镓和铜及较少的硫。其结果是在太阳光球层中的锌、镓、铜恰好是很难测定的,那么我们对太阳的测量方法可能不是很正确。它们同样也被称作“半挥发”元素,就是说在相应的适当温度下挥发掉。所以或许相比于太阳系内部,它们在“柯伊伯带”(彗星形成之地)比较多。如果是真这样,有关太阳系形成的推论就变得有趣了。或许这意味着这些元素在太阳系中比我们想的要多。硫的秘密至今还未解决——对此我们一直都在抓头。

3) Some people have reported seeing more semivolatile elements in a class of interplanetary dust particles (IDPs) called “chondritic porous” IDPs. If you look over the element ratios we measured, you might conclude that our comet particles look more like chondritic porous IDPs than CI chondrite meteorites. And in some ways this looks consistent with the mineralogy/petrology studies that have been done on the cometary Stardust samples – I’ll cover that paper too. On the other hand we also found minerals that seem to be less consistent with CI chondrites and IDPs, and more consistent with yet other classes of meteorites. The answer may be that we have a completely new kind of sample that we’ve never seen before. Wouldn’t that be neat!
3) 曾有人报告在一类被称作“多孔球粒陨石”的行星尘埃颗粒(IDPs)中见到了较多的半挥发性元素。如果你查看我们测定的元素比率,你将得出结论——我们的彗星颗粒看上去更接近多孔球粒陨石 IDPs 而不是 CI 球粒陨石。从某种意义上说这看起来与对彗星尘埃进行的矿物/岩石学研究相一致,我会报到那篇论文的。另一方面我们也发现与 CI 球粒陨石及 IDPs 似乎不怎么相符,倒是与其它类型的陨石更相合的矿物。答案或许是我们得到了一个从未见过的全新样品。

So… stay tuned! We like a good puzzle!
那么、继续收听吧!我们喜欢好的拼图!

[ Last edited by Rojer on 2007-1-5 at 13:28 ]

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