Chondrule Formation During Low-Speed Collisions of Planetesimals: A Hybrid Splash-Flyby Framework

By Keith Cowing
Status Report
February 21, 2024
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Chondrule Formation During Low-Speed Collisions of Planetesimals: A Hybrid Splash-Flyby Framework
A schematic chronology of the early asteroid belt based on the evidence discussed in the text. CAI formation defines t = 0 and primary planetesimals are assumed to form quickly. Within a few hundred thousand years most of the mass is in the form of large planetesimals, which become dominated by magma interiors, as shown by the red line labeled magma fraction. The energy source is 26Al, whose level declines with a half-life of 0.717 Ma from an assumed initial value of 4 at t = 0, where a level of 1 corresponds to the amount of energy (1.6 kJ/g) needed to just reach the liquidus of a perfectly insulated cold primitive starting material. Chondrules formed between ∼1-4 Ma when exposed to hot magma released during a collision, as illustrated by the drawing in the right hand panel by David A. Harvey ( originally depicting the formation of a ring around the Earth. Chondrule formation ends when the LDPs have cooled sufficiently that magma does not reach their surfaces even during collisions. Jupiter may form at around 3-4 Ma and begin clearing the asteroid belt of many of its larger objects. — astro-ph.EP

Chondrules probably formed during a small window of time ∼1-4 Ma after CAIs, when most solid matter in the asteroid belt was already in the form of km-sized planetesimals.

They are unlikely, therefore, to be “building blocks” of planets or abundant on asteroids, but more likely to be a product of energetic events common in the asteroid belt at that epoch.

Laboratory experiments indicate that they could have formed when solids of primitive composition were heated to temperatures of ∼1600 K and then cooled for minutes to hours. A plausible heat source for this is magma, which is likely to have been abundant in the asteroid belt at that time, and only that time, due to the trapping of 26Al decay energy in planetesimal interiors.

Here we propose that chondrules formed during low-speed (≲1 km s−1) collisions between large planetesimals when heat from their interiors was released into a stream of primitive debris from their surfaces. Heating would have been essentially instantaneous and cooling would have been on the dynamical time scale, 1/(√Gρ)∼30 minutes, where ρ is the mean density of a planetesimal.

Many of the heated fragments would have remained gravitationally bound to the merged object and could have suffered additional heating events as they orbited and ultimately accreted to its surface. This is a hybrid of the splash and flyby models: we propose that it was the energy released from a body’s molten interior, not its mass, that was responsible for chondrule formation by heating primitive debris that emerged from the collision.

William Herbst, James P. Greenwood

Comments: 17 pages, 3 figures. Accepted for publication in Meteoritics and Planetary Science
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Cite as: arXiv:2402.12534 [astro-ph.EP] (or arXiv:2402.12534v1 [astro-ph.EP] for this version)
Submission history
From: William Herbst
[v1] Mon, 19 Feb 2024 20:47:16 UTC (5,915 KB)

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