arXiv subjects

Xiao'ou Luo

Publications and source records attributed to Xiao'ou Luo.

4 recordsLinked to original sources

arXiv2026

Optical Constants of Photochemical Haze Analogs in N2-CH4-CO Atmospheres from 0.4 to 28.6 μm

Photochemical hazes play an important role in shaping the spectra and radiative balance of N2-dominated planetary atmospheres. We present newly acquired FTIR and retrieved optical constants (N=n+ik) of laboratory-generated haze analogs from N2/CH4 and N2/CH4/CO gas mixtures under plasma discharge conditions. The retrievals use particle densities newly measured for the CH4-series and previously published particle densities for the CO-series. The experiments systematically explored CH4 concentrations from 0.5% to 10% and CO concentrations from 0% to 5% with fixed 5% CH4. Using measured particle densities together with the Beer-Lambert law and subtractive Kramers-Kronig (SKK) relation, we derived optical constants over the 350-25000 cm-1 (0.4-28.6 μm) spectral range, with the 0.4-25 μm results presented in the main text. The infrared spectra reveal prominent absorption features associated with hydrocarbon-, nitrogen-, and oxygen-bearing functional groups. Increasing CH4 abundance enhances aliphatic hydrocarbon features and corresponds to decreasing particle density, whereas increasing CO abundance promotes oxygen incorporation, broader mid-infrared absorptions, and higher particle density. The derived k spectra exhibit strong absorptions near ~3 μm, ~4.6 μm, and ~6-10 μm, while the real refractive index n generally ranges from ~1.2 to 1.7. The controlled CH4- and CO-series establish composition-dependent variations in haze optical properties. A benchmark comparison among Titan-, Pluto-, and Triton-like haze analogs then uses these experimentally identified trends to interpret the optical differences among N2-dominated planetary haze compositions. The density and optical constants provide laboratory constraints for atmospheric radiative transfer models and for interpreting planetary and exoplanetary spectra from spacecraft and telescopes.

astro-ph.EP↗

arXiv2026

Seasonal Variability of Pluto's Haze Formation Revealed by Laboratory Simulations

Pluto possesses a thin atmosphere primarily composed of N2, with minor constituents including CO and CH4. Photochemical processes generate distinct haze layers as observed by the New Horizons spacecraft. However, the mechanisms governing haze formation, as well as the composition and physical properties of the hazes, remain poorly constrained. Due to Pluto's highly eccentric orbit and obliquity, its surface temperature and atmospheric composition undergo substantial seasonal variations, but it is unclear how such seasonal variations impact the chemical pathways and efficiency of haze formation in Pluto's atmosphere. To address this, we conducted a laboratory simulation of Pluto's atmospheric photochemistry, in which N2/CH4/CO gas mixtures with CH4 concentrations varying from 0.1% to 5% were exposed to a glow discharge to initiate photochemical reactions. Gas-phase composition was monitored in situ using a residual gas analyzer (RGA), while the solid-phase products were characterized by atomic force microscopy (AFM), a gas pycnometer, infrared spectroscopy (IR), and very high-resolution mass spectrometry (VHRMS) to determine particle sizes, density, and composition, respectively. Our results show that increasing the CH4 mixing ratio significantly enhances the yield of gas and solid products. Under low CH4 conditions, nitrogen is primarily incorporated into solids as cyanide groups; whereas CH4-rich conditions favor the formation of amino groups, greatly promoting nitrogen incorporation into organic solids. These findings not only shed light on how seasonal variations into Pluto's atmosphere composition influence haze formation pathways, but also provide critical parameters to interpret observational data and to improve photochemical and microphysical models of planetary hazes.

astro-ph.EP↗

arXiv2025

The impact of organic hazes and graphite on the observation of CO2-rich sub-Neptune atmospheres

Many sub-Neptune and super-Earth exoplanets are expected to develop metal-enriched atmospheres due to atmospheric loss processes such as photoevaporation or core-powered mass loss. Thermochemical equilibrium calculations predict that at high metallicity and a temperature range of 300-700 K, CO2 becomes the dominant carbon species, and graphite may be the thermodynamically favored condensate under low-pressure conditions. Building on prior laboratory findings that such environments yield organic haze rather than graphite, we measured the transmittance spectra of organic haze analogues and graphite samples, and computed their optical constants across the measured wavelength range from 0.4 to 25 μm. The organic haze exhibits strong vibrational absorption bands, notably at 3.0, 4.5, and 6.0 μm, while graphite shows featureless broadband absorption. The derived optical constants of haze and graphite provide the first dataset for organic haze analogues formed in CO2-rich atmospheres and offer improved applicability over prior graphite data derived from bulk reflectance or ellipsometry. We implemented these optical constants into the Virga and PICASO cloud and radiative transfer models to simulate transit spectra for GJ 1214b. The synthetic spectra with organic hazes reproduce the muted spectral features in the NIR observed by Hubble and general trends observed by JWST for GJ 1214b, while graphite models yield flat spectra across the observed wavelengths. This suggests haze features may serve as observational markers of carbon-rich atmospheres, whereas graphite's opacity could lead to radius overestimation, offering a possible explanation for super-puff exoplanets. Our work supplies essential optical to infrared data for interpreting observations of CO2-rich exoplanet atmospheres.

astro-ph.EP↗

arXiv2025

Formation of organic hazes in CO$_2$-rich sub-Neptune atmospheres within the graphite-stability regime

Super-Earths and sub-Neptunes are the most common exoplanets, with a "radius valley" suggesting that super-Earths may form by shedding sub-Neptunes' gaseous envelopes. Exoplanets that lie closer to the super-Earth side of the valley are more likely to have lost a significant fraction of their original H/He envelopes and become enriched in heavier elements with CO$_2$ gaining in abundance. It remains unclear which types of haze would form in such atmospheres, potentially significantly affecting spectroscopic observations. To investigate this, we performed laboratory simulations of two CO$_2$-rich gas mixtures (with 2000 times solar metallicity at 300 K and 500 K). We found that under plasma irradiation, organic hazes were produced at both temperatures with higher haze production rate at 300 K probably because condensation occurs more readily at lower temperature. Gas-phase analysis demonstrates the formation of various hydrocarbons, oxygen- and nitrogen-containing species, including reactive gas precursors like C$_2$H$_4$, CH$_2$O, and HCN, for haze formation. The compositional analysis of the haze particles reveals various functional groups and molecular formulas in both samples. The 500 K haze sample has larger average molecular sizes, higher degree of unsaturation with more double or triple bonds presence, and higher nitrogen content incorporated as N-H, C=N bonds, indicating different haze formation pathways. These findings not only improve the haze formation theories in CO$_2$-rich exoplanet atmospheres but also offer important implications for the interpretation of future observational data.

astro-ph.EP↗