Earth as an Exoplanet: Thermal Emission and Time Variability using Spatially Resolved MODIS Data

The search for habitable and inhabited worlds beyond Earth is a key topic in exoplanetary science. Future space missions will bring exoplanetary characterization to the next level due to their capability to detect Earth-like planets around nearby stars, which are potentially habitable. To inform future mission concepts and their (atmospheric) characterization potential, it is desirable to explore the full range of spectral signatures and variability of the only known inhabited planet, i.e. Earth.

We use Earth observation data sets that are constructed from 14 years of observations by MODIS aboard the Aqua satellite, containing 16 discrete bands located in the Mid Infrared (3.66 to 14.40 microns) wavelength regime. We analyze the data and infer the spectral radiance, spectral energy distributions (SEDs), and power spectral densities (PSDs) on four different locations on the Earth: (1) Sahara Desert, (2) Arctic, (3) Antarctica, and (4) Indian Ocean. The probed satellite frames had a maximum spatial coverage of 2330 km by 2030 km. We investigate the behavior of their thermal emissions and search for evidence of planetary obliquity therein.

Our findings suggest that (1) viewing geometry does play an important role in case the thermal emission of an Earth-twin exoplanet were to be analyzed in the future, (2) thermal fluxes do vary by up to plus/minus 10-15% around 10 micron irrespective of viewing geometry, and (3) the thermal emission spectrum from an Earth-twin does encode information about seasons/planetary obliquity if the object is ob- served sufficiently frequently along its orbit over several years.