Abstract
Thermochemical calculations of metastable equilibria are used to evaluate
the stability of condensed polycyclic aromatic hydrocarbons (PAHs)
in cooling thermal gases and hydrothermal fluids on ancient Mars,
which are roughly similar to their terrestrial counterparts. The
effects of temperature, pressure, the extent of PAH alkylation, and
the relative stability of PAHs and alkanes are considered. Inhibition
of methane and graphite formation favors synthesis of metastable
mixtures of hydrocarbons from aqueous or gaseous CO, CO2, and H-2
below 200 degrees-300 degrees C. High-temperature quenching of H-2
and CO in volcanic and impact gases and dynamic hydrothermal fluids
also favor the synthesis of hydrocarbons. In addition, an excess
of CO in cooling systems relative to equilibrium makes the synthesis
from CO and H-2 more favorable energetically than from CO2 and H-2.
Both the CO-H-2 reactions through Fischer-Tropsch (FT) type processes
and the CO2-H-2 reactions could be catalyzed by magnetite. Volcanic
gases and hydrothermal fluids related to mafic and ultramafic magmas
and rocks are more favorable for FT type synthesis than those associated
with oxidized Fe2O3-bearing rocks and regolith. We conclude that
PAHs and aliphatic hydrocarbons on Mars and Earth could be formed
without the contribution of biogenic carbon. Some PAHs could be formed
because of pyrolysis of other hydrocarbons formed earlier by the
FT type synthesis or other processes. If the PAHs found in the ALH
84001 martian meteorite formed together with other hydrocarbons through
FT type synthesis, it may be possible to bracket the temperature
of the synthesis. The approach presented here call be generalized
to study the synthesis of hydrocarbons in terrestrial volcanic and
hydrothermal processes.
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