月老贵庚: How old are you, Moon?

2014/04/03 18:35:02

月球的年龄至少在过去三十年里一直是地球化学家所关注的一个焦点。若干种计时方法曾被用来解决这个问题，但不同方法的结果却不相同，部分是由于在所谓的“模型年龄”的计算中所需的假设各不相同。Seth Jacobson等人采用了另一种方法。他们进行了大量数值模拟，其中有些基于早期月球形成事件，有些基于后来发生的事件。然后他们发现，在月球的形成年龄与自那时以来被地球吸积的物质(即所谓的Late Veneer)数量之间存在一个独立于模型的关联。在地球地幔中所观测到的“高嗜铁”元素的浓度，为月球形成的时间提供了一个约束条件，排除了一个早期月球形成事件。相反，作者通过计算认为，形成了月球的碰撞事件一定是在太阳系形成之后至少4000万年发生的。

Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact

Seth A. Jacobson, Alessandro Morbidelli, Sean N. Raymond, David P. O’Brien, Kevin J. Walsh, David C. Rubie

According to the generally accepted scenario, the last giant impact on Earth formed the Moon and initiated the final phase of core formation by melting Earth’s mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others claim a date later than 50 Myr (and possibly as late as around 100 Myr) after condensation. Here we show that a Moon-forming event at 40 Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario and the Grand Tack scenario of terrestrial planet formation, and holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth’s mantle constrains the mass of chondritic material added to Earth during Late Accretion. Using HSE abundance measurements, we determine a Moon-formation age of 95 ± 32 Myr after condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth’s mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth’s mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 Myr after condensation.