Impact jetting as the origin of chondrules

摘要

Chondrules are the millimetre-scale, previously molten, spherules found in most meteorites. Before chondrules formed, large differentiating planetesimals had already accreted2. Volatile-rich olivine reveals that chondrules formed in extremely solid-rich environments, more like impact plumes than the solar nebula. The unique chondrules in CB chondrites probably formed in a vapour-melt plume produced by a hypervelocity impact with an impact velocity greater than 10 kilometres per second. An acceptable formation model for the overwhelming majority of chondrules, however, has not been established. Here we report that impacts can produce enough chondrules during the first five million years of planetary accretion to explain their observed abundance. Building on a previous study of impact jetting, we simulate protoplanetary impacts, finding that material is melted and ejected at high speed when the impact velocity exceeds 2.5 kilometres per second. Using a Monte Carlo accretion code, we estimate the location, timing, sizes, and velocities of chondrule-forming impacts. Ejecta size estimates indicate that jetted melt will form millimetre-scale droplets. Our radiative transfer models show that these droplets experience the expected cooling rates of ten to a thousand kelvin per hour. An impact origin for chondrules implies that meteorites are a byproduct of planet formation rather than leftover building material.%陨石球粒(多数陨石中所见的毫米大小的、曾经熔化的液滴）的起源是陨石学领域一个长期未解之谜。通过利用一个hydrocode模型和一个动态吸积模型来模拟原始行星撞击，Brandon Johnson等人发现，大规模吸积撞击在行星吸积的前500万年能产生大量毫米大小的熔化液滴，其数量大到足以解释所观察到的陨石球粒丰度。这一发现支持关于陨石球粒起源的撞击假说，也说明陨石是行星形成的一种副产品，而不是该过程中剩下来的建筑材料。