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  • Cited by 3
  • Print publication year: 2010
  • Online publication date: February 2010

8 - Thermal processing in protoplanetary nebulae


Abstract Crystalline silicates around other stars demonstrate that protoplanetary material is often heated or processed. Similarly, primitive Solar System materials (chondrule components, IDPs, Stardust samples, comet grains) provide multiple lines of evidence for repeated dramatic heating events that affected most or all the protoplanetary materials in the first few million years. The existence of such powerful heating events is not predicted or understood from planet-formation models, yet may have had important implications on the status and composition of planetary raw materials. Here we synthesize the astronomical and meteoritic evidence for such events and discuss proposed models. By matching astronomical analogs to events in the young Solar System we attempt to reconstruct a possible scenario for the thermal processing of protoplanetary materials consistent with all evidence. We also highlight details where the astronomical and cosmochemical views are difficult to reconcile and identify key directions for future research.

Crystalline silicates are tracers of high temperatures (<1000 K), yet they are often observed in cool outer regions of protoplanetary disks (>350 K). Their origin is one of the puzzles that offer insights into the thermal evolution of protoplanetary disks. These crystals may have formed in situ in the cold disk in past heating events or may have been mixed outward from the hot inner disk; perhaps both mechanisms played a role. Similarly, once-molten silicate spherules – chondrules – and refractory inclusions delivered from the cold Asteroid Belt (∼180 K) by primitive chondritic meteorites are products of high temperatures. In many chondritic meteorites chondrules account for over 80% of the total mass, revealing that most of the primordial material in the Asteroid Belt, and perhaps elsewhere, has been heated and compacted.