  Motivated by recent spectroscopic evidence for carbon-rich atmospheres on
some transiting exoplanets, we investigate the influence of the C/O ratio on
the chemistry, composition, and spectra of extrasolar giant planets both from a
thermochemical-equilibrium perspective and from consideration of disequilibrium
processes like photochemistry and transport-induced quenching. We find that
although CO is predicted to be a major atmospheric constituent on hot Jupiters
for all C/O ratios, other oxygen-bearing molecules like H2O and CO2 are much
more abundant when C/O < 1, whereas CH4, HCN, and C2H2 gain significantly in
abundance when C/O > 1. Disequilibrium processes tend to enhance the abundance
of CH4, NH3, HCN, and C2H2 over a wide range of C/O ratios. We compare the
results of our models with secondary-eclipse photometric data from the Spitzer
Space Telescope and conclude that (1) disequilibrium models with C/O ~ 1 are
consistent with spectra of WASP-12b, XO-1b, and CoRoT-2b, confirming the
possible carbon-rich nature of these planets, (2) spectra from HD 189733b are
consistent with C/O ~< 1, but as the assumed metallicity is increased above
solar, the required C/O ratio must increase toward 1 to prevent too much H2O
absorption, (3) species like HCN can have a significant influence on spectral
behavior in the 3.6 and 8.0 um Spitzer channels, potentially providing even
more opacity than CH4 when C/O > 1, and (4) the very high CO2 abundance
inferred for HD 189733b from near-infrared observations cannot be explained
through equilibrium or disequilibrium chemistry in a H2-dominated atmosphere.
We discuss possible formation mechanisms for carbon-rich hot Jupiters. The C/O
ratio and bulk atmospheric metallicity provide important clues regarding the
formation and evolution of the giant planets.
