Terrestrial ages of meteorites
Terrestrial ages of meteorites
复制标题
陨石的地球年龄
作者：
D. Sears
花了大约250年的时间，扫描流行杂志，写乞讨信，分发普及传单和直接贿赂，以收集1969年在世界博物馆中的大约2,000块陨石。在过去十年左右的时间里，当我们大多数人的注意力都集中在月球上聚集的岩石上时，在南极洲的几个地区发现了近4,000块陨石。南极洲的发现重新点燃了人们对确定陨石在地球上存在了多久的兴趣，即所谓的地球年龄。确定地球年龄的可靠理由经常被提出，例如，它可以使陨石碎片的一个单一的下降，以确定，但当这么多的陨石被发现在一个几千平方公里的面积似乎很自然地问他们已经积累了多久。地球年龄的测定之所以成为可能，是因为宇宙射线在陨石中引起了核反应。这种反应产生的核素通常是放射性的。如果人们知道它们在坠落时的丰度和它们的放射性衰变率，那么根据这些核素目前的丰度来计算陨石的地球年龄就微不足道了。然而，陨石坠落时的丰度很难确定，因此地球年龄的确定比陨石生命中其他几个重要年龄的确定要精确得多，例如它们的形成年龄或暴露于宇宙辐射的持续时间。通常，宇宙射线产生的核素的丰度是在最近坠落的一些陨石中测量的，
IT has taken about 250 years of scanning the popular journals, of writing begging letters, of circulating popularized leaflets and of outright bribery to amass the collection of around 2,000 meteorites that were in the world's museums in 1969. In the last decade or so, while the attention of most of us was focused on the rocks being gathered on the Moon, almost 4,000 meteorites have been discovered in several regions of Antarctica.The Antarctic discoveries have led to a rekindling of interest in determining how long the meteorites have been on Earth, their so-called terrestrial age. Prosaic reasons for determining terrestrial ages are often offered-for example, it may enable meteorites which are fragments of a single fall to be identified-but when so many meteorites are found over an area of a few thousand km2 it seems natural to ask how long they have been accumulating. Terrestrial age determination is made possible because nuclear reactions are induced in meteorites by cosmic rays. The nuclides produced by such reactions are usually radioactive. If one knows their abundance at the time of fall and their rate of radioactive decay, then it is a trivial matter to calculate the meteorite's terrestrial age from the present abundance of such nuclides. However, the abundance at the time of fall is difficult to determine, and consequently terrestrial age determinations are appreciably less accurate than those of the several other important ages in the life of a meteorite, such as their formation age or the duration of their exposure to cosmic radiation. Usually the abundance of a cosmic ray-produced nuclide is measured in a number of meteorites that have fallen recently, and their