Thermometric Soots on Warm Jupiters?

Abstract

We use a 1D thermochemical and photochemical kinetics model to predict the disequilibrium stratospheric chemistries of warm and hot Jupiters (800 < T < 1200 K). Thermal chemistry and vertical mixing are generally more important than photochemistry. At 1200 K, methane is oxidized to CO and CO2 by OH radicals from thermal decomposition of water. At T < 1000 K, methane is reactive but stable enough to reach the stratosphere, while water is stable enough that OH levels are suppressed by reaction with H2. These trends raise the effective C/O ratio in the reacting gases above unity. Reduced products such as ethylene, acetylene, and hydrogen cyanide become abundant; further polymerization should lead to formation of PAHs (Poly-Aromatic Hydrocarbons) and soots. Parallel shifts are seen in the sulfur chemistry, with CS and CS2 displacing S2 and HS as the interesting disequilibrium products. Although lower temperature is a leading factor favoring hydrocarbons, higher rates of vertical mixing, lower metallicities, and lower incident UV radiation also favor organic synthesis. Acetylene (the first step toward PAHs) formation is especially favored by high eddy diffusion coefficients Kzz > 10^10 cm2/s. In most cases planetary compositions inferred from transit observations will differ markedly from those inferred from reflected or emitted light from the same planet. The peculiar properties of HD 189733b compared to other hot Jupiters - a broadband blue haze, little sign of Na or K, and hints of low metallicity - can be explained by an organic haze if the planet is cool enough. Whether this interpretation applies to HD 189733b itself, many organic-rich warm Jupiters are sure to be discovered in the near future.