Earth as a Proxy Exoplanet: Decomposing and Recomposing Spectral Images

Abstract:

Albeit highly convoluted, information about spatial features on Earth-like worlds is encoded in single-point spectrophotometric light curves. Spatially resolving these light curves is critical for assessing time-varying surface features and the gross characteristics of its climate, 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 sun-lit 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 decomposing and recomposing these spectral images, different spatial features on Earth are successfully separated and tested using known ground truth data. Our analysis shows that the first and fourth principal components of single-point light curves, contributing ~83.23% of the light curve variation, contain information about low and high clouds, respectively. Surface information relevant to land and ocean contrast is contained in the second principal component, while individual land sub-types are not distinguishable (<0.04% variance). The Earth model with real clouds and decomposed spectra can serve as a baseline for analyzing light curves of Earth-like exoplanets and guide wavelength selection and sampling strategy for future observations.