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arXiv · 2605.00700

The Rocky Planet Picture Show: Implementation of Surface Reflection and Emission in $\texttt{POSEIDON}$ with Application to and Interpretation of JWST Data

Elijah Mullens·Ryan J. MacDonald·Marina E. Gemma·Ishan Mishra·Esteban Gazel·Nikole K. Lewis
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Abstract

The surface characterization of rocky exoplanets via emission spectroscopy represents a frontier of current (JWST) and future (HWO) observational efforts. Here, we implement new features in the open-source retrieval code $\texttt{POSEIDON (v1.4)}$ to fully account for an emitting and reflecting planetary surface and an overlying absorbing and scattering atmosphere. We show that realistic rocky surfaces (with wavelength-dependent albedos derived from laboratory measurements) affect emission spectra by imparting mid-infrared diagnostic absorption features, imprinting pseudo-features due to atmospheric transparency windows, and flipping absorption features to emission via surface-atmosphere interface pseudo-temperature inversions. We demonstrate that current JWST spectral data can distinguish between tenuous (low surface pressure, $\leq$ 1 bar) and thick (high surface pressures, $\geq$ 0.1 bar) atmospheres by performing atmosphere + surface retrievals on published JWST emission data of the rocky worlds TOI-1685b and 55 Cancri e. We then explore JWST MIRI LRS's capability to constrain surface geology of rocky worlds, finding that with sufficient SNR retrievals can distinguish between granite-like and basaltic surfaces for synthetic datasets. Finally, we provide an open-source database of lab-derived surface albedos (in the form of directional-hemispherical reflectances), organized by geologic classification and include supplemental tables developed to foster future collaboration between geology and exoplanet science. Our atmosphere + surface retrieval technique provides a pathway to probe geologic processes on rocky exoplanets, showing that upcoming JWST data for terrestrial worlds will enable a deeper exploration of rocky surfaces beyond our Solar System.

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Elijah Mullens, Ryan J. MacDonald, Marina E. Gemma, Ishan Mishra, Esteban Gazel, Nikole K. Lewis. 2026-05-01. The Rocky Planet Picture Show: Implementation of Surface Reflection and Emission in $\texttt{POSEIDON}$ with Application to and Interpretation of JWST Data. https://arxiv.org/abs/2605.00700

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