The Importance of Experiments: Constraints on Chondrule Formation Models

Introduction: John Wood in his Masursky Lecture of 2001 made the case that many problems in meteoritics would remain unsolved unless rigorous astrophysical modeling were brought to bear on them. He emphasized the differences in cultures between the fields of meteoritics and astrophysics, one data-driven and fixated on minutiae such as per mil isotopic shifts, the other able to model physics as accurately as needed but often unconcerned with details like factors of 2. Problems like chondrule formation have indeed required rigorous astrophysical modeling; but we prefer to emphasize that it has been the marriage of meteoritics and astrophysics that has allowed progress on the issue. Astrophysicists are proficient at spinning theories to explain the few facts they often have to work with--perhaps too proficient. But only the experimental constraints, culled over decades by researchers like Roger Hewins and others, have allowed us to distinguish between dozens of theories of chondrule formation. Among recent theories for chondrule formation are: asteroid impacts [1]; asteroidal magmatism [2]; nebular lightning [3-4]; bipolar outflows near the protoSun [5]; sudden exposure to sunlight or magnetic flares within the purported X wind environment [6-7]; and nebular shocks [8-13], driven either by gravitational instabilities in the disk [14], planetesimal bow shocks [15], or X-ray flares impinging on the disk [16]. The fit to constraints on chondrule formation have been reviewed before [17-20]. Here we will focus on the Xwind and nebular shock models. We will show how experimental constraints allow us to test either model, and we conclude that the X-wind model can be ruled out for most chondrules. The most important constraints on chondrule formation concern their thermal histories. Chondrules are observed to contain primary sulfur [21], so that their temperatures before melting must have been < 650 K. The time spent at elevated temperature before the melting is believed to be < 1 hr, on the basis of lack of isotopic fractionation of S [22]. Melting changed the textures of the chondrules, with the result that roughly 85% of chondrules have porphyritic textures; the majority of the remainder are barred olivine (BO) or radial chondrules [23]. Furnace experiments reveal that to create porphyritic textures requires preservation of hundreds of seed nuclei, while BO and radial textures require essentially all nuclei to be destroyed. These needs constrain the peak temperatures to be 1770 – 2120 K for most chondrules [24-27] and > 2200 K for BO chondrules [28]. Retention of sulfur following peak heating restricts the time above the liquidus to minutes, implying cooling rates > 10 K/hr [29]. Following this initial, rapid cooling, the cooling rate slows; furnace experiments reproduce chondrule textures only if chondrules take hours to cool. Porphyritic chondrules generally require cooling rates 5 – 1000 K/hr [30-31; see also 10]. BO and radial textures require cooling at rates 250 – 3000 K/hr [32; see also 10]. These constraints on chondrule thermal histories are represented as the red curve in Figure 1 (for porphyritic chondrules). The nebular shock model [10, 13] reproduces these thermal histories very well. Chondrules form in the disk at low ambient temperatures; they are suddenly heated by the passage of the shock; their initial temperatures are enhanced by frictional drag, which persists for an aerodynamic stopping time, typically 1 minute for chondrules in the disk; thereafter chondrules are heated by thermal exchange with the gas and by radiation from other chondrules, and the temperature drops on timescales needed to travel several optical depths from the shock front. Dust is likely to evaporate in the post-shock region for chondruleforming shocks [13], and so the opacity is set by the chondrules themselves. For canonical densities of chondrules, the cooling rates are ~ 10 K/hr. The only difference between the experimental constraints and the shock model is in the time for which chondrules are heated (by radiation) before encountering the shock. Significantly higher opacities in the pre-shock region could reduce this time; or, the lack of isotopic fractionation may be limited by non-thermal processes (e.g., high vapor pressures). Higher cooling rates in the crystallization temperature range can be accommodated by higher post-shock chondrule densities, e.g., if the shock overtakes a dense clump of chondrules. Overall the agreement between the thermal model and the experimental constraints is excellent.