Impact jetting as the origin of chondrules

Abstract

Chondrules are the millimetre-scale, previously molten, spherules found in most meteorites. Before chondrules formed, large differentiating planetesimals had already accreted. 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.

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

“…This implies that multiple chondrule formation mechanisms were operating in distinct regions of the protoplanetary disk. The high surface densities and high-energy environments typical of the inner protoplanetary disk may provide a regime for chondrule formation through shock-related transient heating events (55), whereas chondrule formation in the outer Solar System may occur via impacts and bow shocks (56)(57)(58). Therefore, comets are still the best candidates to provide insights into the composition of the primordial interstellar matter parental to our Solar System.…”

Chondrule Formation via Impact Jetting Triggered by Planetary Accretion

“…At that time, however, it was thought that collisions did not play an important role in forming chondrules, because the outcome of collisions is fragmentation of the colliding bodies, rather melting them. One of the new findings made by Johnson et al (2015) is that, when planetesimals collide with each other, some melted materials can be ejected from their collisional surface. This process is referred to as impact jetting and was indeed suggested by Sanders & Scott (2012).…”

“…It is currently recognized that none of them can fully reproduce the wealth of chondrules' properties. Johnson et al (2015) have recently performed a series of numerical simulations and shown that collisions among planetesimals and the resultant impact jetting may be a plausible candidate to form chondrules. It is interesting that the idea of planetesimal collisions was initially proposed in the 1960s (e.g., Urey & Craig 1953).…”

“…This process is referred to as impact jetting and was indeed suggested by Sanders & Scott (2012). Johnson et al (2015) have for the first time performed numerical simulations of impact jetting, and shown that ejected materials can reach the melting temperature of chondrules when the impact velocity of planetesimals exceeds about 2.5 km s −1 . They have thus found that impact jetting triggered by planetesimal collisions can serve as an essential process to reproduce a number of the key properties of chondrules including their abundance.…”

Tungsten isotopic constraints on the age and origin of chondrules

“…However, the origin of chondrules remains poorly understood, and a wide range of possible formation mechanisms has been proposed: (i) chondrules may have formed through collisions between protoplanetary bodies (4-6); (ii) chondrules may have formed near the Sun and were then transported outward by protostellar jets (7); and (iii) chondrules may have formed through more localized melting events of nebular dust caused by shock waves (8,9) or current sheets (10). Although many recent models argue for an impact origin of chondrules (4)(5)(6), the observation that chondrules and matrix from carbonaceous chondrites are chemically complementary lends strong support to an origin of both components from a single reservoir of nebular dust (11)(12)(13). However, the significance of this chemical chondrule-matrix complementarity and whether it can distinguish between a nebular or impact origin of chondrules is debated (6).…”