A radiative heating model for chondrule and chondrite formation

Abstract

We propose that chondrules and chondrites formed together during a brief radiative heating event caused by the close encounter of a small (m to km-scale), primitive planetesimal (SPP) with incandescent lava on the surface of a large (100 km-scale) differentiated planetesimal (LDP). In our scenario, chondrite lithification occurs by hot isostatic pressing (HIP) simultaneously with chondrule formation, in accordance with the constraints of complementarity and cluster chondrites. Thermal models of LDPs formed near t=0 predict that there will be a very narrow window of time, coincident with the chondrule formation epoch, during which crusts are thin enough to frequently rupture by impact, volcanism and/or crustal foundering, releasing hot magma to their surfaces. The heating curves we calculate are more gradual and symmetric than the "flash heating" characteristic of nebular models, but in agreement with the constraints of experimental petrology. The SPP itself is a plausible source of the excess O, Na and Si vapor pressure (compared to a solar nebula environment) that is required by chondrule observations. Laboratory experiments demonstrate that FeO-poor porphyritic olivine chondrules, the most voluminous type of chondrule, can be made using heating and cooling curves predicted by the "flyby" model. If chondrules are a by-product of chondrite lithification, then their high volume abundance within well-lithified chondritic material is not evidence that they were once widespread within the Solar System. Relatively rare events, such as the flybys modeled here, could account for their abundance in the meteorite record.

The Macroporosity of Rubble Pile Asteroid Ryugu and Implications for the Origin of Chondrules

“…The chondrule and chondrite formation epoch is, therefore, defined by that narrow window of time, t = 1.8 -4 Myr, when large asteroids were nearly fully molten and lava could easily extrude onto their surfaces, if ruptured. The suggested lithification mechanism for the chondrites is hot isostatic pressing (HIP) (Herbst and Greenwood 2019). This model is consistent with many known properties of chondrites, including their formation epoch, chondrule textures, complementarity, cluster chondrites, remnant magnetism, and others.…”

“…They calculated a series of heating/cooling curves that would be expected as a small primitive planetesimal (SPP) orbited a LDP under the influence of its gravity and, importantly, verified experimentally that objects with chondrule-like, porphyritic textures could be made in laboratory simulations based on those curves. This model potentially satisfies the constraint that the gaseous atmosphere be rich in O and Na and accounts for the epoch of chondrule formation in the same way as the Sanders and Scott (2012) model, The efficiency of the flyby mechanism depends on whether SPPs typically orbit LDPs many times before accreting, but if chondrules are rare, then it is probably efficient enough (Herbst and Greenwood 2019).…”

“…The results reported here support the view that, in fact, most primitive material never was lithified to the extent of chondrites. This opens the door for consideration of a mechanism that could lithify a small fraction of primitive material in space before it accretes to a larger body, namely hot isostatic pressing (HIP) (Herbst and Greenwood 2019). This idea is bolstered by the increasingly strong evidence that chondrite lithification and chondrule formation occurred simultaneously or nearly simultaneously.…”

“…Is this conceivable? Yes, if we allow that chondrite lithification could occur immediately in space via hot isostatic pressing (HIP), as the chondrules form, rather than later on parent bodies (Herbst and Greenwood 2019). In their scenario, porous SPP's accreting to LDP's that are exposed to a radiation blast from extruding lava will lithify into chondrites, while forming chondrules within them and trapping enough of the atoms released in the process to account for the complementarity signatures of the whole meteorites.…”

Chondrule formation during low‐speed collisions of planetesimals: A hybrid splash–flyby framework

“…But the required radiation can only be liberated to space briefly and episodically by rupturing the surface of an LDP and the pre-chondrule material (primitive dust or porous blocks) must be within kilometers of it to receive a sufficient dose of energy to melt. Herbst and Greenwood (2019) envisioned a volcanic magma extrusion on the surface of an LDP, modeled as a circular hot spot, irradiating orbiting material within a circumplanetesimal accretion disk. They calculated the heating/cooling curves expected in such a case and demonstrated via laboratory experiments that objects with realistic chondrule textures could be synthesized from mineral precursors, building on the classic work of Hewins and Radomsky (1990) and complementing the more recent studies reviewed by Jones et al (2018).…”