Chondrule Formation via Impact Jetting Triggered by Planetary Accretion

Abstract

Chondrules are one of the most primitive elements that can serve as a fundamental clue to the origin of our solar system. We investigate a formation scenario of chondrules that involves planetesimal collisions and the resultant impact jetting. Planetesimal collisions are the main agent to regulate planetary accretion that leads to the formation of terrestrial planets and cores of gas giants. The key component of this scenario is that ejected materials can melt when the impact velocity between colliding planetesimals exceeds about 2.5 km s −1 . Previous simulations have shown that the process is efficient enough to reproduce the primordial abundance of chondrules. We examine this scenario carefully by performing semi-analytical calculations that are developed based on the results of direct Nbody simulations. As found in the previous work, we confirm that planetesimal collisions that occur during planetary accretion can play an important role in forming chondrules. This arises because protoplanet-planetesimal collisions can achieve an impact velocity of about 2.5 km s −1 or higher, as protoplanets approach the isolation mass (M p,iso ). Assuming that the ejected mass is a fraction (F ch ) of the colliding planetesimals' mass, we show that the resultant abundance of chondrules is expressed well by F ch M p,iso , as long as the formation of protoplanets is completed within a given disk lifetime. We perform a parameter study and examine how the abundance of chondrules and the timingof their formation change. We find that the impact jetting scenario generally works reasonably well for a certain range of parameters, while more dedicated work would be needed to include other physical processes that are neglected in this work and to examine their effects on chondrule formation.

Thermal History of CBb Chondrules and Cooling Rate Distributions of Ejecta Plumes

“…If both formed in plumes, there were collisions of very different energy for Type II and CB chondrules, lower for the older Type II chondrules. This is consistent with the results of Hasegawa et al (2016), collisions with earlier smaller bodies producing more incompletely melted (porphyritic) chondrules. However, other formation mechanisms cannot be excluded (Connolly & Jones 2016).…”

“…Initial modeling considered impacts of asteroid-sized bodies (Asphaug et al 2011;Dullemond et al 2014;Morris et al 2015), but later work compares calculations where the target is asteroidal to those where it is a planetary embryo (Johnson et al 2015;Hasegawa et al 2016). The latter situation leads to higher impact velocities and therefore greater efficiency of chondrule formation.…”

“…The latter situation leads to higher impact velocities and therefore greater efficiency of chondrule formation. The comparison of models with different target body size shows that most of the collision-generated chondrules would have formed with the largest bodies, i.e., the final protoplanets (Hasegawa et al 2016).…”

“…This indicates a late event in a gas-poor disk. A consensus is developing for an origin of the "non-normal" chondrules by melting, evaporation, and condensation in an impact plume (Krot et al 2005), and the physics of collisions, jetting, and ejecta transport is being studied (Asphaug et al 2011;Dullemond et al 2014;Johnson et al 2015;Morris et al 2015;Hasegawa et al 2016). Here we have developed two tests of this concept: the determination of the thermal history of the chondrules (1) using dynamic crystallization experiments on CB b chondrule compositions, and (2) using models of impact plumes with the Eulerian adaptive mesh refinement (AMR) code FLASH4.3 (Fryxell et al 2000).…”

Impact splash chondrule formation during planetesimal recycling

“…Recently proposed chondrule formation scenarios considered melt spray from subsonic collisions ('splashes') between similar-sized planetesimals, which were fully melted by decay heat from 26 Al (Asphaug et al, 2011;Sanders and Scott, 2012) or impact 'jetting' via collisions of planetesimals with undifferentiated protoplanets (Johnson et al, 2015;Hasegawa et al, 2016;Wakita et al, 2017). Collisional mechanisms were suggested previously and offer attractive solutions to many chondrule features (Krot et al, 2005;Sanders and Scott, 2012;Stammler and Dullemond, 2014;Dullemond et al, 2014Dullemond et al, , 2016Marrocchi et al, 2016).…”