Andrews-Hanna Jeffrey C, Zuber Maria T, Banerdt W Bruce
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Nature. 2008 Jun 26;453(7199):1212-5. doi: 10.1038/nature07011.
The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The isostatic compensation of these distinct provinces and the ancient population of impact craters buried beneath the young lowlands surface suggest that the dichotomy is one of the most ancient features on the planet. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact or mantle convection/overturn. Here we use the gravity and topography of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500 km, centred at 67 degrees N, 208 degrees E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.
火星表面最显著的特征是南部高地和北部低地之间近乎半球状的二分法。这种二分法的根源是地壳厚度沿着一条明显不规则的边界发生变化，这条边界可以绕着行星追踪，除了大概埋在塔尔西斯火山隆起之下的地方。这些不同区域的均衡补偿以及年轻低地表面之下掩埋的古老撞击坑表明，这种二分法是火星上最古老的特征之一。然而，这种二分法的起源仍然不确定，几乎没有证据来区分所提出的成因：巨大撞击或地幔对流/翻转。在这里，我们利用火星的重力和地形来确定塔尔西斯之下二分法边界的位置，利用塔尔西斯和二分法不同的补偿模式来区分它们的影响。我们发现，绕行星一周的二分法边界的整个路径可以被一个长约10600公里、宽约8500公里的椭圆精确拟合，该椭圆的中心位于北纬67度、东经208度。我们认为，地壳二分法的椭圆性质最简单的解释是一次巨大撞击，这代表了迄今为止在太阳系中发现的最大的此类结构。
