Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California, USA.
Center for Integrative Planetary Science, University of California, Berkeley, Berkeley, California, USA.
Nature. 2018 Mar 29;555(7698):643-646. doi: 10.1038/nature26144. Epub 2018 Mar 19.
Widespread evidence points to the existence of an ancient Martian ocean. Most compelling are the putative ancient shorelines in the northern plains. However, these shorelines fail to follow an equipotential surface, and this has been used to challenge the notion that they formed via an early ocean and hence to question the existence of such an ocean. The shorelines' deviation from a constant elevation can be explained by true polar wander occurring after the formation of Tharsis, a volcanic province that dominates the gravity and topography of Mars. However, surface loading from the oceans can drive polar wander only if Tharsis formed far from the equator, and most evidence indicates that Tharsis formed near the equator, meaning that there is no current explanation for the shorelines' deviation from an equipotential that is consistent with our geophysical understanding of Mars. Here we show that variations in shoreline topography can be explained by deformation caused by the emplacement of Tharsis. We find that the shorelines must have formed before and during the emplacement of Tharsis, instead of afterwards, as previously assumed. Our results imply that oceans on Mars formed early, concurrent with the valley networks, and point to a close relationship between the evolution of oceans on Mars and the initiation and decline of Tharsis volcanism, with broad implications for the geology, hydrological cycle and climate of early Mars.
大量证据表明火星上曾经存在一个古老的海洋。最有说服力的是在北部平原上推测出的古代海岸线。然而，这些海岸线并没有遵循等位面，这一点被用来质疑它们是通过早期海洋形成的这一概念，从而对这样一个海洋的存在提出了质疑。这些海岸线偏离恒定海拔的现象可以通过在形成火星上占主导地位的重力和地形的塔尔西斯火山省之后发生的真正极移来解释。然而，只有在 Tharsis 形成远离赤道的情况下，海洋的表面负荷才能驱动极移，而大多数证据表明 Tharsis 形成于赤道附近，这意味着目前没有解释这些海岸线偏离等位面的说法与我们对火星的地球物理理解一致。在这里，我们表明海岸线地形的变化可以通过 Tharsis 的就位引起的变形来解释。我们发现，这些海岸线必须是在 Tharsis 就位之前和就位期间形成的，而不是像以前假设的那样是在之后形成的。我们的结果表明，火星上的海洋形成得很早，与山谷网络同时形成，并指出火星上海洋的演化与 Tharsis 火山活动的启动和衰退之间存在密切关系，这对早期火星的地质学、水文循环和气候具有广泛的影响。
