Astrogeobiology Laboratory, Department of Physics, Lund University, 221 00 Lund, Sweden.
Astrogeobiology Laboratory, Department of Physics, Lund University, 221 00 Lund, Sweden;
Proc Natl Acad Sci U S A. 2021 Jun 15;118(24). doi: 10.1073/pnas.2020977118.
The meteoritic material falling on Earth is believed to derive from large break-up or cratering events in the asteroid belt. The flux of extraterrestrial material would then vary in accordance with the timing of such asteroid family-forming events. In order to validate this, we investigated marine sediments representing 15 time-windows in the Phanerozoic for content of micrometeoritic relict chrome-spinel grains (>32 μm). We compare these data with the timing of the 15 largest break-up events involving chrome-spinel-bearing asteroids (S- and V-types). Unexpectedly, our Phanerozoic time windows show a stable flux dominated by ordinary chondrites similar to today's flux. Only in the mid-Ordovician, in connection with the break-up of the L-chondrite parent body, do we observe an anomalous micrometeorite regime with a two to three orders-of-magnitude increase in the flux of L-chondritic chrome-spinel grains to Earth. This corresponds to a one order-of-magnitude excess in the number of impact craters in the mid-Ordovician following the L-chondrite break-up, the only resolvable peak in Phanerozoic cratering rates indicative of an asteroid shower. We argue that meteorites and small (<1-km-sized) asteroids impacting Earth mainly sample a very small region of orbital space in the asteroid belt. This selectiveness has been remarkably stable over the past 500 Ma.
据信，落在地球上的陨石材料源自小行星带中大型天体分裂或撞击事件。那么，外星物质的通量将根据此类小行星族形成事件的时间而变化。为了验证这一点，我们研究了代表显生宙 15 个时间窗口的海洋沉积物，以研究其中残留的微米陨石铬尖晶石颗粒（>32μm）的含量。我们将这些数据与涉及含铬尖晶石小行星（S 型和 V 型）的 15 次最大分裂事件的时间进行了比较。出乎意料的是，我们在显生宙的时间窗口中观察到了一种稳定的通量，主要由普通球粒陨石组成，类似于今天的通量。仅在中奥陶世，与 L 型球粒陨石母体的分裂有关，我们观察到异常的微陨石状态，L 型球粒陨石铬尖晶石颗粒的通量增加了两到三个数量级，到达地球。这对应于 L 型球粒陨石分裂后中奥陶世撞击坑数量增加了一个数量级，这是显生宙撞击率中唯一可分辨的峰值，表明存在小行星雨。我们认为，撞击地球的陨石和小行星（<1 公里大小）主要从小行星带的轨道空间的一个非常小的区域进行采样。这种选择性在过去 5 亿年中一直非常稳定。
