Colors of a Second Earth. Ii. Effects of Clouds on Photometric Characterization of Earth-Like Exoplanets

Abstract

As a test-bed for future investigations of directly imaged terrestrial exoplanets, we present the recovery of the surface components of the Earth from multi-band diurnal light curves obtained with the EPOXI spacecraft. We find that the presence and longitudinal distribution of ocean, soil and vegetation are reasonably well reproduced by fitting the observed color variations with a simplified model composed of a priori known albedo spectra of ocean, soil, vegetation, snow and clouds. The effect of atmosphere, including clouds, on light scattered from surface components is modeled using a radiative transfer code. The required noise levels for future observations of exoplanets are also determined. Our model-dependent approach allows us to infer the presence of major elements of the planet (in the case of the Earth, clouds and ocean) with observations having S/N 10 in most cases and with high confidence if S/N 20. In addition, S/N 100 enables us to detect the presence of components other than ocean and clouds in a fairly model-independent way. Degradation of our inversion procedure produced by cloud cover is also quantified. While cloud cover significantly dilutes the magnitude of color variations compared to the cloudless case, the pattern of color changes remains. Therefore, the possibility of investigating surface features through light curve fitting remains even for exoplanets with cloud cover similar to the Earth's.

Properties of an Earth-Like Planet Orbiting a Sun-Like Star: Earth Observed by the EPOXI Mission

“…These observations measured the disc-integrated and time-averaged spectroscopy of Earth at the beginning and the end of a Northern Hemisphere spring; the time-resolved spectroscopy of the disc-integrated spectrum; and time-resolved spectrophotometry as an indicator of the inhomogeneous surface. Detailed investigations of some components of this data set have begun, including azimuthal mapping of Earth's surface units from multicolor light curves (Cowan et al, 2009); constraining models for Earth's emergent visible spectrum and light curve based on properties of the surface and atmosphere (Fujii et al, 2011;Robinson et al, 2011); and empirically categorizing Earth among the planets of our Solar System by using visible colors (Crow et al, 2011).…”

Geology and Photometric Variation of Solar System Bodies with Minor Atmospheres: Implications for Solid Exoplanets

“…The spin rotation period may also undergo later evolution by, for example, the presence of satellites. Second, knowing the planetary rotation period is necessary for phase folding the light curves, which could eventually allow us to map the surface and to take a closer look at localized geological features (Cowan et al, 2009(Cowan et al, , 2011Oakley and Cash, 2009;Fujii et al, 2010Fujii et al, , 2011Fujii et al, , 2013Fujii, 2010, 2011;Fujii and Kawahara, 2012).…”

“…In the case of Earth, disk-averaged scattered light exhibits diurnal variation coherent with the change in the composition in the illuminated and visible region. Because the continental distribution is highly inhomogeneous, and the global cloud pattern does not change significantly in a day, the spin rotation period may be successfully identified through periodogram analysis (Pallé et al, 2008), which then may allow for recovery of surface inhomogeneity along the equator (Cowan and Agol, 2008;Cowan et al, 2009Cowan et al, , 2011Oakley and Cash, 2009;Fujii et al, 2010Fujii et al, , 2011Fujii et al, , 2013 as well as estimation of the surface composition (Cowan and Strait, 2013).…”

Earth as an Extrasolar Planet: Earth Model Validation Using EPOXI Earth Observations

“…For example, the combination of temporal resolution and multiwavelength photometry could disentangle phase-or rotation-dependent differences in surface properties from variable cloud cover. The resulting maps could discriminate between large-scale surface inhomogeneities such as continents and oceans (Pallé et al, 2008;Cowan et al, 2009;Kawahara and Fujii, 2010;Fujii et al, 2011). Disk-integrated spectroscopy can potentially determine globally averaged atmospheric and surface composition to verify habitability and to search for global evidence of life in the planetary environment (Seager et al, 2005;Meadows, 2006;Montañ és-Rodríguez et al, 2006).…”