Identifying Surface Degeneracies in Single-Visit Reflected Light Observations of Modern Earth using the Habitable Worlds Observatory

Abstract

Characterizing the surface and atmosphere of Earth-like planets in reflected light is a key goal for upcoming direct imaging surveys. NASA's next flagship-class astrophysics mission concept, the Habitable Worlds Observatory (HWO), is a space-based Ultraviolet/Optical/Near-Infrared observatory with a mission design requirement to reach the $10^{-10}$ contrast necessary to characterize Earth-like planets around Sun-like stars. While reflected light from planetary surfaces provides a unique opportunity to constrain the coverage of surface materials and biopigments, detailed predictions of HWO's ability to retrieve surface fractions are necessary but have not been conducted. Here, we model photon-counting noise from astrophysical, instrumental, and post-processing sources for the HWO Exploratory Analytic Case 5 design equipped with a charge-6 vector-vortex coronagraph. By combining our photon-counting noise with five distinct modern Earth models at quadrature, we simulate single-visit HWO observations and perform spectral retrievals using the open-source code $\texttt{POSEIDON}$ to assess our ability to constrain both the surface and atmospheric composition. We find that degeneracies between planetary radius, surface pressure, surface material, and cloud coverage in reflected-light retrievals can significantly complicate the classification of surface features. These degeneracies can complicate the detection of surface biopigments, such as the chlorophyll-induced red edge on modern Earth. Our work shows that developing concrete strategies for detecting surface features and breaking degeneracies in reflected-light observations of Earth-like planets is a critical priority for mission design and data analysis.

Figures (18)

• Figure 1: (A) P-T and chemical mixing ratio profile from kaltenegger_high-resolution_2020 used in the MODIS-derived surface coverage model. Solid and dashed-dotted curves show the vertical mixing ratios of the molecular species contributing to the spectra. The dashed black line represents the P-T profile, and the gray horizontal bar indicates the water cloud height and thickness. (B) The top panel shows the surface spectra derived from MODIS observations, as outlined in Sec.\ref{['sec:MODIS']}, as well as the simpler four-component surface model for the three self-consistent POSEIDON spectra. The bottom panel shows the four spectra used in the three self-consistent forward models and in all retrievals.
• Figure 2: (A) Coronagraph core throughput $\tau_{\rm core}$ and aperature of the EAC 5 design. (B) Optical throughput $\mathcal{T}$ and quantum efficiency $q$ used to model the HWO EAC 5. $\mathcal{T}$ was computed using the full EAC 1 optical path, as specified by the HWO GOMAP Science-Engineering Interface. We acquired the QE values for Teledyne e2v EMCCD (VIS) and the SAPHIRA HgCdTe LmAPD (NIR) from the GOMAP Science-Engineering Interface repository. (C) On-axis PSF of the EAC 5 aperture shown in (A). (D) Off-axis PSF of the EAC 5 aperture shown in (A) at a separation of $\lambda/D=16$.
• Figure 3: Simulated $t_{\rm exp}=100$ hr. observation at $\alpha=90^\circ$ of the self-consistent POSEIDON spectra where the light gray curve denotes the high resolution ($\mathcal{R}=10^4$) forward model, the black points denote the data from the simulated HWO EAC 5 observations, and the red step curve shows the high-resolution model binned down to the simulated observation wavelength bins. Gas-phase atmospheric absorption features for H$_2$O, O$_2$, and O$_3$ are labeled along with the location of the red edge (red bar). The top, middle, and bottom panels are for the $f_{\rm cloud}=0$, $0.5$, and $1.0$ cases, respectively.
• Figure 4: Simulated $t_{\rm exp}=100$ hr. observation at $\alpha=90^\circ$ of the two complex spectral models. The light gray curve shows the high resolution ($\mathcal{R}=10^4$) MODIS-derived forward-modeled spectra and the spectra from roccetti_planet_2025 in the top and bottom panels, respectively. The black points denote the data from the simulated HWO EAC 5 observations and the red step curve shows the model spectrum binned down to the simulated observation wavelength bins. Gas-phase atmospheric absorption features for H$_2$O, O$_2$, and O$_3$ are labeled along with the location of the red edge (red bar).
• Figure 5: Individual photon count rates for the MODIS-derived spectra. The forms of the count rates are outlined in App. \ref{['sec:counts']}. The planet count rate is consistently within an order of magnitude of the total background except at the reddest wavelengths.