Lixiang Gu1*, Zhao-Cheng Zeng2,3, Siteng Fan4, Vijay Natraj5, Jonathan H. Jiang5, David Crisp5, Yuk L. Yung2,5, Yongyun Hu1
[1]Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, 100871, China;
2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA;
3Institute of Remote Sensing and Geographic Information System, School of Earth and Space Sciences, Peking University, Beijing, 100871, China;
4LMD/IPSL, Sorbonne Université, PSL Research Université, École Normale Supérieure, École Polytechnique, CNRS, Paris 75005, France;
5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA;
*Corresponding author: lixianggu@pku.edu.cn
ABSTRACT
Analyzing time-resolved disk-integrated spectral images of the Earth can provide a baseline for future exoplanet characterization. The Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) provides ~5000 full-disk sun-lit Earth images each year in ten wavelengths from the ultraviolet to the near-infrared. A whole-disk radiative transfer model can improve our understanding of the temporal variation of Earth’s disk-integrated reflected radiance (“light curves”) at different wavelengths and create a pool of possible observations of Earth-like exoplanets. We use the 2-stream-exact-single-scattering line-by-line radiative transfer model to build the Earth Spectrum Simulator (ESS) and reconstruct DSCOVR/EPIC spectral observations. Atmospheric effects, such as scattering by air molecules, clouds, and aerosols, and gaseous absorption, are included. Surface contributions are treated using appropriate bidirectional reflectance distribution functions. We simulate ~300 images in each channel for observations collected in 2016, with a spatial resolution of ~2000 pixels over the visible disk. ESS provides a simultaneous fit to the observed light curves, with time-averaged reflectance differences typically less than 7% and root-mean-square errors less than 1%. The only exceptions are in the oxygen absorption channels, where reflectance biases can be as large as 19.55%; this is a consequence of simplified assumptions about clouds, especially their vertical placement. We also recover principal components of the spectrophotometric light curves and correlate them with atmospheric and surface features.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China under grant 41888101. All computations were done at the High-performance Computing Platform of Peking University. A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Y.L.Y. was supported in part by a Virtual Planetary Laboratory grant from the University of Washington. S.F. acknowledges funding support from CNES. We acknowledge funding support from the NASA Exoplanet Research Program NNH18ZDA001N.
CITATION
Gu L, Zeng Z C, Fan S, et al. Earth as a Proxy Exoplanet: Simulating DSCOVR/EPIC Observations Using the Earth Spectrum Simulator[J]. The Astronomical Journal, 2022, 163(6): 285.
