Heat could be retained as debris fall back into impactcraters, creating an insulating blanket.
当碎片落回撞击坑时，热量可以被保留下来，形成隔热层。
2
The Mariner spacecraft found that the surface of Mars, as well as that of its two moons, is pitted with impactcraters formed by meteoroids falling in from space.
水手号宇宙飞船发现火星表面，还有它的两个卫星表面，都有由太空坠落的流星体造成的陨石坑。
3
From the small number of impactcraters that appear on Martian lava flows, one can estimate that the planet was volcanically active as recently as a half-billion years ago—and may be active today.