A Re-evaluation of Chondrule Formation in Large-scale Shocks.m

Chondrules are significant indicators of processes occuring in the early solar nebula. However, the method of their formation is a continuing source of debate. There are a number of constraints on chondrule formation provided by careful chemical and isotopic analysis, as well as experiments undertaken in order to reproduce chondrules in the lab. Any proposed formation method must meet this wide range of constraints. Shocks in the solar nebula remain the most promising chondrule-forming mechanism, due to their ability to meet the majority of constraints, especially those on the thermal histories experienced by chondrules [1]. The type of nebular shock has also been hotly debated, but has largely been narrowed down to two: large-scale shocks, such as those due to disk instabilities and bow shocks around large protoplanets [2,3]. Recent work [4] has shown that bow shocks around large protoplanets, while still a likely candidate for some types of chondrules, are not ideal for producing the majority (those with porphyritic textures). Hence, we feel it necessary to re-evaluate chondrule formation in large-scale shocks. The large-scale shock model of [2] met most of the constraints on chondrule formation, especially their thermal histories, with the exception of excess “pre-heating”. Solids will be pre-heated ahead of the shock (in the pre-shock region; Figure 1), due to the propagation of a radiation front (known as a Marshak wave) originating in the hot, postshock region [2]. Pre-heating of chondrules ≥ 1300 K is constrained to under 30 minutes, in order to prevent isotopic fractionation of sulfur [5]. In the model of [2], the solids were pre-heated to temperatures exceeding 1300 K for over 4 hours. It has been demonstrated that the inclusion of a population of smaller particles will eliminate this problem [6], due to an increase in opacity in the pre-shock region that does not affect the cooling rates of chondrules in the post-shock region. Here we examine this problem in more detail and discuss the ramifications.