Using synthetic disk-integrated reflectance spectra to constrain direct imaging sensitivity requirements for a Mars-like exoplanet

The 2020 Decadal Survey on Astronomy and Astrophysics ¹ recommended the prioritization of a space-based telescope capable of directly characterizing Earth-like exoplanets in reflected light. The planned suite of instruments onboard such a mission are expected to provide disk-integrated spectra with moderate spectral resolution and signal-to-noise (SNR). Although the detection and characterization of Earth-like exoplanets remains the primary focus of such a mission, land planets with limited available water, such as Mars, may be much more common. Mars-like exoplanets, therefore, are an equally significant set of targets when investigating the diverse climatologies and potential habitability of other worlds, especially if our own Solar System is any indication of planetary diversity. In this study, we constrain the direct imaging sensitivity requirements for observing and characterizing Mars-like exoplanets with the goal of informing future telescope design and mission planning. Employing an instrument noise model simulating a coronagraph-equipped, space-based telescope, spatially- and spectrally-resolved synthetic observations of Mars are produced. We evaluate the direct imaging sensitivity requirements across a range of wavelengths, from the ultraviolet (UV) to near-infrared (near-IR), to enable the spectral characterization of key atmospheric and surface features from disk-integrated reflectance spectra. Detectability at a given SNR is assessed through optical wavelength integration times for a range of phase angles, host star spectral types, and levels of atmospheric dustiness. Our results indicate that a Decadal-recommended space telescope featuring an aperture of 6-m is likely only proficient in detecting Mars-like exoplanets around K-type stars located within a 10 parsec (pc) radius from Earth. Furthermore, we demonstrate that when integrating over visible and near-IR wavelengths, required exposure times to detect such a planet are reasonable, especially near full phase angles. In the context of upcoming and proposed observatories, such as the Habitable Exoplanet Observatory (HabEx) and Large UV/Optical/IR Surveyor (LUVOIR), our findings provide valuable insights into the direct imaging capabilities and optimal observational strategies needed for detecting and studying Mars-like exoplanets.