Earth as a Proxy Exoplanet: Deconstructing and Reconstructing Spectrophotometric Light Curves

Point source, spectrophotometric light curves of Earth-like planets contain a surprising amount of information about the spatial features of those worlds. Spatially resolving these light curves is critical for assessing time-varying surface features and the existence of a climate system, which in turn is critical to life on Earth and significant for determining habitability on exoplanets. Given that Earth is the only celestial body confirmed to harbor life, treating it as a proxy exoplanet by analyzing its time-resolved spectral images provides a benchmark in the search for habitable exoplanets. The Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) provides such an opportunity, with observations of ~5000 full-disk sunlit Earth images each year at ten wavelengths with high temporal frequency. By integrating over the Earth’s disk to create a single point source and then deconstructing and reconstructing these spectral images, different spatial features on Earth are successfully separated and compared with known ground truth data. Our analysis reveals that the first and fourth principal components of single-point light curves derived from DSCOVR measurements, contributing ~83.23% of the light curve variability, contain information about low and high clouds, respectively. Surface information relevant to the contrast between land and ocean reflectance is contained in the second principal component, while individual land sub-types are not easily distinguishable (<0.1% total light curve variation). The Earth model with real clouds and reconstructed spectra can serve as a baseline for analyzing light curves of Earth-like exoplanets and guide wavelength selection and sampling strategies for future observations.