人的記憶力會隨著歲月的流逝而衰退，寫作可以彌補(bǔ)記憶的不足，將曾經(jīng)的人生經(jīng)歷和感悟記錄下來，也便于保存一份美好的回憶。相信許多人會覺得范文很難寫？以下是小編為大家收集的優(yōu)秀范文，歡迎大家分享閱讀。
彗星閱讀題及答案篇一
彗星的pristine relics
the long-held perspective that comets are pristine remnants from the formation of the solar system has evolved from the prevailing views of 30 years ago, finds planetary scientist dr.s. alan stern in a paper published in the journal fair to say that a sea change has taken place, says stern, director of the space studies department in the swri space science and engineering spanision. we used to consider comets as wholly unchanged relics that had been stored ever since the era of solar system formation in a distant, cold, timeless deep freeze called the oort cloud. we now appreciate that a variety of processes slowly modify comets during their storage there, he says. as a result,its become clear that the oort cloud and its cousin the kuiper belt are not such perfect deep freezes.
the first evolutionary process to be recognized as affecting comets during their long storage was radiation damage, followed by the discovery that sandblasting from dust grains in the interstellar medium plays an important role. next, researchers theorized that comets in the oort cloud are heated to scientifically significant temperatures by passing stars and supernovae,says stern. more recently, researchers are finding that comets in the kuiper belt are heavily damaged by also now seems inevitable that most comets from the kuiper belt, though constructed of ancient material, cannot themselves be ancient -- instead they must be recently created chips off larger kuiper belt objects, formed as a result of violent impacts, says stern. this is truly a paradigm shift. many of the short-period comets we see arent even ancient!
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