Reflected light phase curves -- which measure the reflectivity of a world as a function of the star-world-observer (phase) angle -- encode a wealth of information about scattering processes in…

This is a $359,284.00 grant awarded by the Shared Services Center, a civilian federal agency, to the University of Arizona (parent prime contractor) to conduct research on reflected light phase curves for the planets Titan and Earth. The research aims to generate phase curve datasets that will be valuable for comparative planetology studies of exoplanets, as well as support analysis of glint effects for future direct imaging missions. The research will leverage existing datasets from the Cassini and NASA Clementine and MESSENGER missions. This award is not associated with a set-aside designation. The research supports goals of the Exoplanet Research Program in exoplanet detection and characterization.

Reflected light phase curves -- which measure the reflectivity of a world as a function of the star-world-observer (phase) angle -- encode a wealth of information about scattering processes in atmospheres and from surfaces. For example, phase curves can reveal enhancements from both aerosol scattering (due to, e.g., forward scattering or rainbow features) and specular reflection from a liquid (e.g., ocean glint for earth). The narrow phase angle coverage accessible for outer solar system worlds has limited our ability to use ground-based observing to capture phase curves for most solar system worlds with thick atmospheres. Thus, interplanetary spacecraft provide the best platforms for characterizing planetary phase curves. Such datasets will soon have much-enhanced value for comparative planetology, as both the roman coronagraph instrument (RCI) and the habitable worlds observatory (HWO) will be capable of observing reflected light phase curves for exoplanets. To date, however, phase curve information is incomplete for two important solar system worlds: titan and earth. Critically, phase curves from titan would serve as important comparison points for more haze-covered, warm giant exoplanets (as would be observed by roman) as well as an early/archean earth-like planet. Alongside this, the phase-dependent reflectivity of earth is a key input to future direct imaging missions. Phase curve information at high phase angles (i.e., at crescent phases) for both titan and earth can help US to understand how specular reflection from oceans and seas could be detected in disk-integrated observations. To enable both future efforts in comparative planetology as well as studies of glint detectability, we propose a much-needed analysis of existing titan and earth observations. For titan, cassini/vims-derived whole-disk observations will be reduced to provide hazy world phase curves that extend our understanding of phase-dependent scattering processes at key near-infrared wavelengths and geometries. These phase curves would be applicable to roman and would enable predictions for HWO. For earth, wholly unanalyzed data from NASA's clementine and messenger missions will be used to create first-of-their-kind, wavelength-resolved phase curves for the pale blue DOT. We will analyze the titan and earth results to better understand glint effects and will publicly release our detailed phase curves to empower roman and HWO science. Our efforts to better enable comparative planetology studies of exoplanet atmospheres are relevant to exoplanet research program goals in exoplanet detection and characterization. Given that modern solar system planetary science emphasizes resolved observations, our disk-integrated products primarily advance exoplanet science, as required by the exoplanet research program call.