  Some super Earths and mini Neptunes will likely have thick atmospheres that
are not H2-dominated. We have developed a photochemistry-thermochemistry
kinetic-transport model for exploring the compositions of thick atmospheres on
super Earths and mini Neptunes, applicable for both H2-dominated atmospheres
and non-H2-dominated atmospheres. Using this model to study thick atmospheres
for wide ranges of temperatures and elemental abundances, we classify them into
hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and
hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO2
rather than CH4 or CO in a H2-depleted water-dominated thick atmosphere, and
that the preferred loss of light elements from an oxygen-poor carbon-rich
atmosphere leads to formation of unsaturated hydrocarbons (C2H2 and C2H4). We
apply our self-consistent atmosphere models to compute spectra and diagnostic
features for known transiting low-mass exoplanets GJ 1214 b, HD 97658 b, and 55
Cnc e. For GJ 1214 b like planets we find that (1) C2H2 features at 1.0 and 1.5
micron in transmission and C2H2 and C2H4 features at 9-14 micron in thermal
emission are diagnostic for hydrocarbon-rich atmospheres; (2) a detection of
water-vapor features and a confirmation of nonexistence of methane features
would provide sufficient evidence for a water-dominated atmosphere. In general,
our simulations show that chemical stability has to be taken into account when
interpreting the spectrum of a super Earth/mini Neptune. Water-dominated
atmospheres only exist for carbon to oxygen ratios much lower than the solar
ratio, suggesting that this kind of atmospheres could be rare.
