Nimmo F, Hart S D, Korycansky D G, Agnor C B
Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA.
Nature. 2008 Jun 26;453(7199):1220-3. doi: 10.1038/nature07025.
The observation that one hemisphere of Mars is lower and has a thinner crust than the other (the 'martian hemispheric dichotomy') has been a puzzle for 30 years. The dichotomy may have arisen as a result of internal mechanisms such as convection. Alternatively, it may have been caused by one or several giant impacts, but quantitative tests of the impact hypothesis have not been published. Here we use a high-resolution, two-dimensional, axially symmetric hydrocode to model vertical impacts over a range of parameters appropriate to early Mars. We propose that the impact model, in addition to excavating a crustal cavity of the correct size, explains two other observations. First, crustal disruption at the impact antipode is probably responsible for the observed antipodal decline in magnetic field strength. Second, the impact-generated melt forming the northern lowlands crust is predicted to derive from a deep, depleted mantle source. This prediction is consistent with characteristics of martian shergottite meteorites and suggests a dichotomy formation time approximately 100 Myr after martian accretion, comparable to that of the Moon-forming impact on Earth.
火星的一个半球比另一个半球更低且地壳更薄（即“火星半球二分性”）这一观测结果30年来一直是个谜题。这种二分性可能是由对流等内部机制导致的。或者，它可能是由一次或几次巨大撞击造成的，但撞击假说的定量测试尚未发表。在此，我们使用一个高分辨率的二维轴对称流体动力学程序，对一系列适合早期火星的参数范围内的垂直撞击进行建模。我们提出，撞击模型除了能挖掘出尺寸合适的地壳空洞外，还能解释另外两个观测结果。其一，撞击对映点处的地壳破裂可能是观测到的对映点磁场强度下降的原因。其二，预计形成北部低地地壳的撞击产生的熔体源自深部、贫化的地幔源。这一预测与火星辉熔长无球粒陨石的特征相符，并表明二分性形成时间大约在火星吸积后1亿年，与月球形成时对地球的撞击时间相当。
