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  3. (1) As objects go, Earth is getting on in years. But it is definitely not ancient by cosmic standards. The very oldest native pl

(1) As objects go, Earth is getting on in years. But it is definitely not ancient by cosmic standards. The very oldest native pl

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问题     (1) As objects go, Earth is getting on in years. But it is definitely not ancient by cosmic standards. The very oldest native planetary material we can examine are tiny zircon crystals that formed here about 4.4 billion years ago. But the universe has been around for approximately 13. 8 billion years, and has been building stars and forging heavy elements since it was some 200 million years old.
    (2) That means that element-rich matter has been polluting interstellar space for a long, long while. Some of that matter, especially from older stars and supernova, has condensed and frozen into microscopic grains of silicate (硅酸盐) and carbon compounds. Eventually those Stardust grains might find themselves pulled into the gravity-well of new-forming stars, and planets. But the vigor and turmoil of star and planet formation can erase the evidence of the age of those building blocks.
    (3) Some of the most ancient crumbs of building material we can study are those inside certain kinds of meteorites that still conveniently fall to Earth. Stardust swept up as the solar system combined.
    (4) Now a study by Heck et al. , reported in the Proceedings of the National Academy of Sciences in the U.S. , seems to have located interstellar grains that are as old as 7. 5 billion years. Pre-dating our entire solar system by some 3 billion years.
    (5) The investigation took material from the famous Murchison meteorite that fell and scattered across parts of Australia (including the town of Murchison) in 1969. This meteorite is a carbonaceous chondrite (碳质球粒陨石), a supremely "primitive" object, rich in carbon compounds. It represents material that attached together 4.5 billion years ago out of interstellar stuff—never being processed inside pro to-planetary interiors.
    (6) By crushing a sample and dissolving it in acid, the researchers were able to isolate so-called pre-solar or interstellar grains—hardy little crumbs, barely several micrometers across.
    (7) But how do you estimate their ages? It turns out that the universe provides, in a manner, a clock. Cosmic rays are constantly zipping through space, and when these energetic particles (like protons and alpha-particles) collide with the atomic nuclei in a grain the nuclear damage leaves an isotopic (同位素的) signature. The longer the grain has been floating around, the more of those nuclear changes accumulate.
    (8) With a lot of additional detective work to account for different initial compositions from different stellar sources of elements, and all the potential traumas of heating and bashing around that the grains might have endured, it’s possible to use the cosmic ray signature to come up with an estimate of how long the grains were hanging around in interstellar space.
    (9) In this case, using the accumulation of Neon-21 in particular, most of the grains in Murchison seem to be less than a few hundred million years old (commensurate with the amount of time they were originally adrift before getting stuck in the Murchison body). But at least 8％ are astonishingly old, reaching back to approximately 7. 5 billion years, some two billion years older than any previous samples.
    (10) The range of grain ages is also consistent with the idea that some 7.5 billion years ago there was a lot more star formation going on, not just in the Milky Way but across the universe.
    (11) It’s a remarkable result, and one that demonstrates that sometimes the secrets of the cosmos can literally fall in our backyard. (本文选自 Scientific American)
We can conclude from Paras. 7—10 that________.
选项 A、we can calculate the age of the solar system by using Neon-21
B、we can deduce the ages of interstellar grains from the cosmic rays damages
C、the longer the grains are adrift in the universe, the less nuclear damage they will have
D、it has proved that star formation has already been going on 7.5 billion years ago
答案D
解析 推断题。由第十段可知，大约75亿年前，不仅在银河系，而且在整个宇宙范围内，都有更多恒星正在形成，D与文义相符，故为答案。由第九段第一句可知，利用氖-21的积聚，可以推算出默奇森陨石的大部分颗粒的年龄，而不是太阳系的年龄，故排除A；由第八段最后一句可知，通过宇宙射线能够判断出这些颗粒在星际空间漂浮的时间，而并非其年龄，故排除B；由第七段最后一句可知，颗粒在宇宙间漂浮的时间越长，其所积累的核变化印记也就越多，C表述与之相反，故排除。
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