Forming chondrules in impact splashes. I. Radiative cooling model

TL;DR

This paper models the radiative cooling of molten droplets ejected during planetesimal collisions, supporting the impact splash hypothesis for chondrule formation with detailed numerical and analytical results.

Contribution

It introduces a radiative cooling model for impact-ejected chondrule droplets, providing new analytical and numerical insights into their thermal evolution.

Findings

Temperature remains constant initially before cooling begins.

Cooling follows a specific power-law decay after t_cool.

Volatile element vapor saturation prevents large volatile losses during early expansion.

Abstract

The formation of chondrules is one of the oldest unsolved mysteries in meteoritics and planet formation. Recently an old idea has been revived: the idea that chondrules form as a result of collisions between planetesimals in which the ejected molten material forms small droplets which solidify to become chondrules. Pre-melting of the planetesimals by radioactive decay of 26Al would help producing sprays of melt even at relatively low impact velocity. In this paper we study the radiative cooling of a ballistically expanding spherical cloud of chondrule droplets ejected from the impact site. We present results from a numerical radiative transfer models as well as analytic approximate solutions. We find that the temperature after the start of the expansion of the cloud remains constant for a time t_cool and then drops with time t approximately as T ~ T_0[(3/5)t/t_cool+ 2/5]^(-5/3) for t>t_cool. The time at which this temperature drop starts t_cool depends via an analytical formula on the mass of the cloud, the expansion velocity and the size of the chondrule. During the early isothermal expansion phase the density is still so high that we expect the vapor of volatile elements to saturate so that no large volatile losses are expected.