Venus as an Exoplanet: I. An Initial Exploration of the 3-D Energy
Balance for a CO2 Exoplanetary Atmosphere Around an M-Dwarf Star
Abstract
The chemical evolution of an exoplanetary Venus-like atmosphere is
dependent upon the ultraviolet to near ultraviolet (FUV-NUV) radiation
ratio from the parent star, the balance between CO2
photolysis and recombination via reactions that depend on the water
abundance, and various catalytic chemical cycles. In this study, we use
a three-dimensional (3-D) model to simulate conditions for a Venus-like
exoplanet orbiting the M-dwarf type star GJ436 by varying the
star/planet distance and considering the resultant effects on
heating/cooling and dynamics. The simulation includes the middle and
upper atmosphere (<40 mbar). Overall, these model comparisons
reveal that the impact of extreme ultraviolet to ultraviolet (EUV-UV)
heating on the energy balance shows both radiative and dynamical
processes are responsible for driving significant variations in zonal
winds and global temperature profiles at <
10-5 mbar. More specifically, CO2
15-μm cooling balances EUV/UV and Near InfraRed (NIR) heating at
altitudes below 10-7 mbar pressure with a strong
maximum balance for pressures at ~10-5
mbar, thus explaining the invariance of the temperature distribution at
altitudes below 10-5 mbar pressure for all cases. Our
model comparisons also show that moderate changes in NIR heating result
in relatively small changes in neutral temperature in the upper
atmosphere, and virtually no change in the middle atmosphere. However,
with larger changes in the NIR heating profile, much greater changes in
neutral temperature occur in the entire upper and middle atmosphere
studied.