Systematic Uncertainties in Rocky Exoplanet Reflected Light from Incomplete Laboratory BRDF Measurements

Abstract: The first observations of short-period rocky exoplanets have initiated the era of terrestrial exoplanet characterization. The interpretation of these observations relies on forward models used in atmospheric and surface retrieval frameworks. While atmospheric modeling is supported by extensive laboratory datasets of molecular opacities, the experimental basis for rocky surface characterization remains limited. Most reflectance libraries of planetary analog materials provide spectra measured at a single illumination and viewing geometry, constraining the spectral behavior but not its full angular scattering properties. We show that incomplete angular coverage in laboratory bidirectional reflectance measurements introduces an intrinsic systematic uncertainty in the interpretation of rocky exoplanet observations. We demonstrate that multiple combinations of photometric parameters can reproduce the same laboratory reflectance spectrum while predicting different angular scattering behaviors and disk-integrated observables. We propagate this non-uniqueness to reflected-light phase curves, geometric albedos, and planet-star contrast ratios between $0.5$--$1.7\,μ$m, representative of the spectral range targeted by future direct-imaging missions such as the HWO. We find that single-geometry laboratory spectra produce large uncertainties in predicted signals, particularly at small phase angles. Simplified assumptions adopted in exoplanet studies, such as isotropic scattering can additionally introduce systematic biases. Using multi-angular laboratory measurements from the Bloomsburg University Goniometer dataset, we show that the uncertainty in predicted observables collapses once angular scattering behavior is directly constrained. Our results demonstrate that multi-angular measurements are required to construct physically predictive reflected-light models of rocky exoplanets.