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文章结构:证据显示火星上曾有大量的水 - 2种features分别介绍 - 科学家因此怀疑火星上很早就有水 - 观点还是controversial - 提出反对的证据 - 至少部分是有水存在的 - 所以水去哪里了?P1: Evidence suggests water once existed in great quantity on the surface of Mars.
Why? 2 types of features are seen: runoff channels and outflow channels.
Runoff channels: found in southern highlands; extensive systems of ...; .... (出题:细节题?)
P2: Outflow channels
Outflow channels: relics of ... ; appear only in ... and generally .... Instead, they are the paths taken by ... (出题:细节题?)
P3: "Mars have enjoyed an extended early period ..."
Some ...
Other researchers go even further ... (出题:even further?)
A computer-generated view .. shows the ... (出题:电脑的view作用?都谁可能是candidate?)
P4: Opposite possibilities
But, could also have been created by geological activity.
Furthermore, ...
Key Words: cold dry mars
However, ... did in fact experience long periods in the past during which water existed on the surface.
P5: Additional discussion from astronomers
Astronomers have no direct evidence for liquid water anywhere on the surface of Mars today, and ....
Water? Maybe ... (出题:最后一段的作用?)
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