Numerous petrologic, cosmochemical and isotopic measurements of chondrules, the igneous spherules found in abundance in chondrites, potentially can constrain physical conditions and processes in the forming solar system, but only if chondrule formation is well understood. Previous work by the PI (Desch & Connolly 2002) and his team, including studies funded a previous Origins of Solar Systems grant (Morris et al 2009a,b), have helped establish the viability of passage through shock waves in the solar nebula gas as the model for chondrule formation. We now turn our attention to using models of chondrule formation to constrain the physical conditions in the solar nebula, especially chondrule densities and clumpiness and the sizes and sources of the shocks. Doing so will require a refinement of current shock codes, expanding them to 2 dimensions (cylindrical geometry), to account for variations in chondrule density parallel to the shock front, and to model parabolic bow shocks associated with eccentric planetesimals. We propose to take the Perseus code written by the PI and his research team (Ouellette et al. 2007) to study supernova-driven bow shocks around protoplanetary disks and adapt it to the problem of chondrule formation. The critical modification will be the addition of modules to calculate the radiation field in cylindrical geometry. We describe details of how this and other changes will be achieved. We will apply this modified code to test 3 ideas about chondrule formation by shocks. 1.) Desch & Connolly (2002) hypothesized a correlation between compound chondrules and barred olivine textures because chondrules in clumps experience higher peak temperatures and faster cooling rates. But shock models assume uniform densities on scales of 10^5 km larger than the likely sizes of clumps, < 10^3 km, and this idea is not fully tested. 2.) Recent measurements of primary Na in olivine phenocrysts by Alexander et al. (2008) led them to conclude chondrule melts were in equilibrium with high partial pressures of Na, due to partial evaporation of chondrules in exceptionally dense clumps. We instead hypothesize that high vapor pressures result from total evaporation of chondrules in dense clumps, with chondrules melted normally, in nearby regions of lower chondrule density being focused by shocks into the dense, vapor-rich regions. Our code will quantify and test this effect. 3.) Gravitational instabilities are one proposed source of shocks, but bow shocks driven by eccentric planetesimals are a very likely alternative. Proponents of the bow shock model hypothesize that chondrule cooling rates would match observational constraints, but this is untested. Our modified code, which will test the above hypotheses, is highly relevant to NASA's Strategic Goals and Research Objectives, especially subgoals 3C and 3D, and to the goals of the Origins of Solar Systems program. The key to understanding the origin of our solar system is to put chondrule formation in its proper context.
|Effective start/end date||3/29/10 → 6/30/14|
- NASA: Goddard Space Flight Center: $402,100.00