Abstract
Thermodynamic calculations of metastable equilibria were used to evaluate
the potential for abiotic synthesis of aliphatic and polycyclic aromatic
hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALII) 84001.
The calculations show that PAHs and normal alkanes could form metastably
from CO, CO2, and H-2 below approximately 250-300 degrees C during
rapid cooling of trapped magmatic or impact-generated gases. Depending
on temperature, bulk composition, and oxidation-reduction conditions,
PAHs and normal alkanes can form simultaneously or separately. Moreover,
PAHs can form at lower H/C ratios, higher CO/CO2 ratios, and higher
temperatures than normal alkanes. Dry conditions with H/C ratios
less than approximately 0.01-0.001 together with high CO/CO2 ratios
also favor the formation of unalkylated PAHs. The observed abundance
of PAHs, their low alkylation, and a variable but high aromatic to
aliphatic ratio in ALH 84001 all col respond to low H/C and high
CO/CO2 ratios in magmatic and impact gases and can be used to deduce
spatial variations of these ratios. Some hydrocarbons could have
been formed from trapped magmatic gases, especially if the cooling
was fast enough to prevent reequilibration. We propose that subsequent
impact heating(s) in ALH 84001 could have led to dissociation of
ferrous carbonates to yield fine-grain magnetite, formation of a
CO-rich local gas phase, reduction of water vapor to H-2, reequilibration
of the trapped magmatic gases, aromatization of hydrocarbons formed
previously, and overprinting of the synthesis from magmatic gases,
if any. Rapid cooling and high-temperature quenching of CO-, H-2-rich
impact gases could have led to magnetite-catalyzed hydrocarbon synthesis.
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