3:00 PM - 3:15 PM
[PCG19-16] On the origin of reflective aerosols in close-in giant exoplanets: Haze composition evolution
Keywords:Exoplanet, Atmosphere, Cloud and haze
JWST has started to explore the atmosphere of warm exoplanets for which abundant photochemical hazes were predicted. One of the surprising discoveries from the early JWST results is the potential prevalence of reflective hazes. Reflective hazes with very high single scattering albed was first suggested by a too cold dayside of canonical hazy sub-Neptune GJ1214b and also suggested by the lack of temperature inversion in the emission spectrum of warm giants, such as WASP-80b. The indication of reflective hazes has challenged the pre-JWST theories of photochemical haze formation, which predicted the formation of dark, absorbing particles as similar to Titan's haze and soot particles produced by flame chemistry.
In this study, we propose a completely new idea for explaining the existence of reflective aerosols: diamond formation via chemical vapor deposition (CVD) is operating in exoplanetary atmospheres. The CVD process is a well-established method to synthesize diamond under low pressure (~10 mbar) environments in the industory community. The industorial diamond formation via CVD process requires (1) the mixture of hydrogen and carbon-bearing gases as an ingredient (2) hot environments of ~1000 K, (3) energy source (e.g., hot filament) to produce atomic hydrogen that acts to stabilize diamond at low pressure. Intringuingly, close-in exoplanets with hydrogen-rich atmospheres satisfy all of these requirements; for example, intense stellar UV can act as an energy source for producing atomic hydrogen instead of hot filament used in experiments.
We first constrain the atmospheric metallicity and C/O ratio that may yield CVD diamond by utilizing the compilation of CVD diamond experiments, known as the Batchmann's C-H-O diagram for diamond domain. We found that super-solar metallicity with C/O~1 is favorable condition for CVD diamond formation. In addition, we found that all existing experiments of exoplanetary haze synthesis had not satisfied the required gas compositions. Thus, the previous non-detection of diamonds in exoplanetary haze experiments is a natural consequence. To further test the hypothesis, we performed a series of photochemical simulations using the public code VULCAN and utilized those results to estimate the diamond growth timescale based on the experimentally validated CVD diamond model. Lastly, we investigate how planetary equilibrium temperature, atmospheric metallicity and C/O ratio affects the vertical distribution of diamond fraction using a novel microphysical diamond-haze model. We will also discuss the origin of reflective aerosols reported by ongoing JWST surveys.
In this study, we propose a completely new idea for explaining the existence of reflective aerosols: diamond formation via chemical vapor deposition (CVD) is operating in exoplanetary atmospheres. The CVD process is a well-established method to synthesize diamond under low pressure (~10 mbar) environments in the industory community. The industorial diamond formation via CVD process requires (1) the mixture of hydrogen and carbon-bearing gases as an ingredient (2) hot environments of ~1000 K, (3) energy source (e.g., hot filament) to produce atomic hydrogen that acts to stabilize diamond at low pressure. Intringuingly, close-in exoplanets with hydrogen-rich atmospheres satisfy all of these requirements; for example, intense stellar UV can act as an energy source for producing atomic hydrogen instead of hot filament used in experiments.
We first constrain the atmospheric metallicity and C/O ratio that may yield CVD diamond by utilizing the compilation of CVD diamond experiments, known as the Batchmann's C-H-O diagram for diamond domain. We found that super-solar metallicity with C/O~1 is favorable condition for CVD diamond formation. In addition, we found that all existing experiments of exoplanetary haze synthesis had not satisfied the required gas compositions. Thus, the previous non-detection of diamonds in exoplanetary haze experiments is a natural consequence. To further test the hypothesis, we performed a series of photochemical simulations using the public code VULCAN and utilized those results to estimate the diamond growth timescale based on the experimentally validated CVD diamond model. Lastly, we investigate how planetary equilibrium temperature, atmospheric metallicity and C/O ratio affects the vertical distribution of diamond fraction using a novel microphysical diamond-haze model. We will also discuss the origin of reflective aerosols reported by ongoing JWST surveys.