The big craters are then filled up with city waste and covered up with a layer of soil and vegetation acclimatized to local conditions.
用城市垃圾进行填充开采坑，并覆盖一层同当地植被特征相适应的土层。
As on the Moon, the extent of large impact cratering (i.e. craters too big to have been filled in by erosion since they were formed) serves as an age indicator for the Martian surface.
与月球上相同，那些巨大陨石坑的范围（例如，那些巨大到自形成后尚未被尘埃填满的陨石坑）标志着火星表面的年龄。
Most meteorites are small. But big meteorites hit the earth sometimes. These huge meteors explode when they hit the ground, and dig out craters.
大多数陨石都很小。但是，有时，也有大陨石袭击地球。这些巨大的流星碰到地面时，发生爆炸，形成陨石坑。
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