The southern highlands of Mars are dissected by hundreds of valley networks, which are evidence that water once sculpted the surface. Characterizing the mechanisms of valley incision may constrain early Mars climate and the search for ancient life. Previous interpretations of the geological record require precipitation and surface water runoff to form the valley networks, in contradiction with climate simulations that predict a cold, icy ancient Mars. Here we present a global comparative study of valley network morphometry, using a principal-component-based analysis with physical models of fluvial, groundwater sapping and glacial and subglacial erosion. We found that valley formation involved all these processes, but that subglacial and fluvial erosion are the predominant mechanisms. This is supported by predictions from models of steady-state erosion and geomorphological comparisons to terrestrial analogues. The inference of subglacial channels among the valley networks supports the presence of ice sheets that covered the southern highlands during the time of valley network emplacement.

火星的南部高地被数百个山谷网络解剖，这证明水曾经在地表上雕刻。描述山谷切开的机制可能会限制火星早期的气候和对古代生命的寻求。先前对地质记录的解释要求降水和地表水径流形成山谷网络，这与预测寒冷，冰冷的古代火星的气候模拟相矛盾。在这里，我们对河谷网络形态进行了一项全球比较研究，该研究使用了基于主要成分的分析方法，包括河流，地下水渗漏以及冰川和冰川下侵蚀的物理模型。我们发现山谷形成涉及所有这些过程，但是冰川下和河流冲刷是主要的机制。稳态侵蚀和地貌比较模型与地面类似物的预测结果为这一点提供了支持。山谷网络中冰下通道的推断支持了在山谷网络安装时覆盖南部高地的冰盖的存在。