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2018年3月SAT亚太考试内容回顾阅读五

2018-04-17 11:01:00来源：网络

　　Andrews-Hannaand colleagues have made a good case, says Herbert Frey, a planetary scientistat NASA’s Goddard Space Flight Center in Greenbelt, Maryland, even though thenewly described features are surprising. The moon is not big enough to have thesame strong convective cooling process that Earth has in its interior, heexplains, and ordinarily convection is one of the main mechanisms thought tolead to large-scale rifting. So just what caused the rifting remains unclear.“It just means the moon continues to surprise us,” he says. Frey adds that aremaining mystery is why the rectangular features were found only beneathOceanus Procellarum. Even if the rifting is explained by the excess radioactiveelements, there is still no definitive explanation for why only the near sideof the moon ended up enriched.

　　The discoverycould also be a death knell for the impact theory for Oceanus Procellarum, anidea first put forth in the early 1970s. A basin there would have been thelargest on the moon—larger than the South Pole–Aitken Basin—and second in thesolar system only to the Borealis Basin on Mars, which covers the planet’sentire northern hemisphere.

　　RyosukeNakamura, a researcher at the National Institute of Advanced Industrial Scienceand Technology in Tsukuba, Japan, is still not convinced that an impact can beruled out. In 2012, he and his colleagues published a paper in NatureGeosciencethat found compositional evidence for an impactwithin Procellarum—a type of pyroxene mineral that is found in other, knownimpact basins such as South Pole–Aitken and is associated with the melting orexcavation of mantle rock from an asteroid impact.

　　In response tothe current study, Nakamura says that the features in the southwestern cornerof the Procellarum region look to be circular rather than rectangular, andstill consistent with an impact. But Frey, who has long been skeptical of theimpact theory, says that the features are as clear as day, and not what you’dexpect underneath a basin. “That looks like a rectangle to me.”

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