Planetary Climate Dynamics

Abstract:

Growing observational evidence has suggested active meteorology in the atmospheres of brown dwarfs (BDs) and directly imaged extrasolar giant planets (EGPs). In particular, a number of surveys have shown that near-infrared brightness variability is common among L and T dwarfs. Despite the likelihood from previous studies that atmospheric dynamics is the major cause of the variability, the detailed mechanism of the variability remains elusive, and we need to seek a natural, self-consistent mechanism. Clouds are important in shaping the thermal structure and spectral properties of these atmospheres via their opacity, and we expect the same for inducing atmospheric variability. In this work, using a time-dependent one-dimensional model that incorporates a self-consistent coupling between the thermal structure, convective mixing, cloud radiative heating/cooling, and condensation/evaporation of clouds, we show that radiative cloud feedback can drive spontaneous atmospheric variability in both temperature and cloud structure under conditions appropriate for BDs and directly imaged EGPs. The typical periods of variability are 1 to tens of hr, with a typical amplitude of the variability up to hundreds of K in effective temperature. The existence of variability is robust over a wide range of parameter space, but the detailed evolution of the variability is sensitive to model parameters. Our novel, self-consistent mechanism has important implications for the observed flux variability of BDs and directly imaged EGPs, especially for objects whose variability evolves on short timescales. It is also a promising mechanism for cloud breaking, which has been proposed to explain the L/T transition of BDs.

Abstract:

Hydrogen cyanide (HCN) is a key feedstock molecule for the production of life's building blocks. The formation of HCN in an N2-rich atmospheres requires first that the triple bond between N≡N be severed, and then that the atomic nitrogen find a carbon atom. These two tasks can be accomplished via photochemistry, lightning, impacts, or volcanism. The key requirements for producing appreciable amounts of HCN are the free availability of N2 and a local carbon to oxygen ratio of C/O ≥ 1. We discuss the chemical mechanisms by which HCN can be formed and destroyed on rocky exoplanets with Earth-like N2 content and surface water inventories, varying the oxidation state of the dominant carbon-containing atmospheric species. HCN is most readily produced in an atmosphere rich in methane (CH4) or acetylene (C2H2), but can also be produced in significant amounts (>1 ppm) within CO-dominated atmospheres. Methane is not necessary for the production of HCN. We show how destruction of HCN in a CO2-rich atmosphere depends critically on the poorly-constrained energetic barrier for the reaction of HCN with atomic oxygen. We discuss the implications of our results for detecting photochemically produced HCN, for concentrating HCN on the planet's surface, and its importance for prebiotic chemistry.