Our planetary system formed out of a rotating accretion disk of gas and dust, often called the solar nebula. In the outer disk where temperatures were cold enough to harbor ices and interstellar materials, comet nuclei accreted. Yet, comets also accreted Mg-rich crystalline silicate mineral grains that formed in the hot inner disk close to the young Sun. Hence, cometary Mg-rich crystalline silicates are the touchstone for radial transport of grains that formed in the inner protoplanetary disk out to the comet-forming zone.
Mg-rich crystalline silicate minerals are the expected condensates from the "arid" canonical solar nebula, i.e., condensates from a gas of solar composition. In comparison to Mg-rich crystalline silicates, Fe-rich crystalline silicates are recently discovered in Stardust Mission return samples from comet 81P/Wild 2 and in the Deep Impact coma of comet 9P/Tempel 1. We discuss how Fe-rich crystalline silicates, as well as phyllosilicates (aqueous minerals) can form in more "humid" weather conditions (higher oxygen fugacity) than Mg-rich crystalline silicates. The inward migration of cometesimals (small icy bodies) can raise the humidity of the solar nebula and allow these more oxidized minerals to condense. By comparing the relative abundances of Mg- and Fe-rich crystalline silicates in comets with those in chondrites (asteroidal samples), and by looking at recent dating of chondrules, we derive a picture of the solar nebula as initially arid that then suffers fluctuating epochs of humid weather by 0.6 Myr to 3Myr after the formation of the first condensates. Cometary mineralogy probes the meterology of the solar nebula.
This idea that fluctuating solar nebula "weather" caused the formation of a variety of recently discovered cometary materials challenges recent assertions that cometary Fe-rich crystalline silicates, carbonates, and phyllosilicates are aqueously altered asteroidal materials (selective tiny bits of asteroids that were accreted by comets), or products of selective aqueous alteration on submicron scales in comet nuclei. Instead, if comets do not contain aqueous alteration products from asteroids, then cometary dust grains that formed in the disk are pre-accretionary with respect to asteroids. If comets contain pre-accretionary nebular grains then comets indeed probe the earliest planet-forming processes. Comets contain both the interstellar ingredients for and the products of transmutation in the inner nebula.