A New Mechanism for the Formation of Meteoritic Kerogen-Like Material

Abstract

The carbon in ancient carbonaceous chondritic meteorites is mainly in a hydrocarbon composite similar to terrestrial kerogen, a cross-linked structure of aliphatic and aromatic hydrocarbons. Until recently, the composite has been commonly thought to have been produced in the early solar nebula by a Fischer-Tropsch-type process, involving the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen on grain surfaces. Instead, the aromatic hydrocarbons may form in gas-phase pyrolysis of simple aliphatics like acetylene and methane by a mechanism developed recently to explain formation of soot in combustion and of aromatic molecules in circumstellar envelopes. Nonequilibrium chemical kinetic calculations indicate that this mechanism can produce meteoritic aromatics if the initial concentration of simple hydrocarbons in the solar nebula was sufficiently but not unreasonably high.

Polycyclic Aromatic Hydrocarbons Orbiting HD 233517, an Evolved Oxygen-rich Red Giant

“…The disk's approach to this thermodynamically predicted state in a sufficiently short time can be facilitated by Fischer-Tropsch (FT) catalysis on the surface of iron grains at phases when the material was warmer than 400 K, as has been suggested to explain the presence of carbonaceous material in meteorites (see, e.g., Kress &Tielens 2001 andZolotov &Shock 2001). Alternatively, gas-phase chemistry may be important (Morgan et al 1991). Although detailed models are required, we imagine a scenario somewhat similar to what Willacy (2004) has computed for the role of FT reactions in the outflow from the carbon-rich star IRC ϩ10 216 in which CO and H 2 are converted to H 2 O and hydrocarbons.…”

“…However, a contrary view is that even in the oxygen-rich solar nebula, Fischer-Tropsch catalysis on the surface of metal grains at T 1 400 K was an important route for the synthesis of carbon-rich molecules (Kress & Tielens 2001;Zolotov & Shock 2001). Alternatively, gas-phase synthesis of such molecules may be important (Morgan et al 1991). Since the disk around HD 233517 is free of interstellar contamination, it can serve as an indirect test of these models.…”

Nanodiamonds around HD 97048 and Elias 1

“…Thus, the temperature, pressure, and composition (ie., molecular precursors) in these planetary disks and in the solar nebula would favor the condensation of carbonaceous compounds (Morgan et al 1992;Frenklach & Wang 1991). Theoretical calculations show that the long residence time in such disks more readily allow carbon dust formation than in stellar outflows (Morgan et al 1992).…”

Establishing a molecular relationship between chondritic and cometary organic solids

“…Acetylene condensation reactions (i.e., soot chemistry) at temperatures in the range of 900 to 1,100 K have also been proposed as a plausible inner solar system origin of IOM (14). Alternatively, it has been proposed that organic solids formed in plasmas close (approximately 1 AU) to the young sun (15) and may have been incorporated into asteroids.…”