有關(guān)彗星的GRE閱讀真題
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 Nature.Its 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 Division. 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 collisions.It 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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