Solar nebula shock waves

Four principal models have been proposed to explain the presence of alteration phases in chondritic meteorites. These models can be summarized as follows (i) reaction of anhydrous, high-temperature condensate phases with water vapor as the solar nebula cooled to temperatures below —375 K (e.g., Grossman and Larimer, 1974]) (ii) hydration of anhydrous dust in icy regions of the nebula during the passage of shock waves (Ciesla et al., 2003) (iii) alteration within small (tens of meters), ephemeral parent bodies that were subsequently disrupted and their altered components accreted with unaltered materials into the final asteroidal parent bodies (pre-accretionary alteration) (Metzler et al., 1992 Bischoff, 1998) and (iv) alteration within asteroidal parent bodies (Kerridge and Bunch, 1979 Zolensky and McSween, 1988). 

A second interpretation of carbonaceous chondrites is as primary condensates of the solar nebula. By this view, their hydrolytic alteration is due to melting in cometary nuclei during close passes with the Sun, or due to transient heating events by shock waves or collisions (McSween, 1999). Other carbonaceous chondrites show metamorphic alteration with minerals similar to those in Earth formed during deep burial under elevated temperatures and pressures (Brearley, 1999). Like soils and paleosols on Earth and Mars, carbonaceous chondrites demonstrate the great antiquity of hydrolytic weathering in dilute acidic solutions, presumably of carbonic acid derived from water vapor and CO2. These remain the principal gases released from volcanoes, and soils remain important buffers for this environmental acid. 

Interstellar dusts could have a layered structure, as proposed by Greenberg (1998), of an icy mantle covering an organic-silicate core. On the way from an interstellar molecular cloud to protosolar nebula, interstellar dust could be modified by any thermal events, e.g., nebula gas heated by shock waves on an accreting disk or the absorption of solar radiation. 

Table 1 lists the extensive isotopic evidence for these extinct radionuclides, which demonstrates that the time between the cessation of nucleosynthesis and the formation of meteorites was so short that it supports the hypothesis that a last minute injection of nucleosynthetic products may have accompanied a supernova shock wave which triggered the collapse of the solar nebula. In this case it is likely that some of the nucleosynthetic products may not have been well mixed in the molecular cloud from which the Solar System evolved. 

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