The Impact Of Planetary Rotation Rate On The Reflectance Spectrum Of Terrestrial Exoplanets Around Sun-like Stars

Recent studies indicate that habitable zone planets around Sun-like stars can be tidally-locked (Barnes et al. 2017). It is plausible that Earth-like planets around G-dwarfs, which are the primary exoplanet targets for future NASA concept missions like the Habitable Exoplanet Imaging Mission (HabEx) & the Large Ultra-Violet Optical Infra-Red (LUVOIR) Surveyor, have slower rotational periods than the Earth. We present results from 3-D climate model (ROCKE-3D) simulations of an Earth-like planet around a Sun-like star, varying the rotational period between 1-day to 365-days. From these simulations, we generate reflectance spectra as a function of (monthly) orbital phase over one year for each case, and compare it with the standard Earth spectrum. The reduced rotation period broadens the Hadley circulation, eliminates the characteristic westerly jet streams, and enhances clouds and precipitation around the planet (Yang et al. 2014). In some cases, the characteristic upwards Rayleigh scattering slope seen in the standard 1-day rotation Earth spectra at short visible wavelengths reduces significantly with orbital phase. This could be due to high-altitude clouds reaching optical depth unity for downwelling stellar radiation before Rayleigh scattering reaches an optical depth of unity. This gives a less pronounced Rayleigh peak that appears to have the same grey opacity as the continuum out to longer wavelengths. Our results show that the rotation period has a significant effect on the reflectance spectra of Earth-like planets around Solar-type stars, and direct imaging spectral characterization missions should consider the likely possibility that present-day Earth-like planets may not be pale-blue dots.