Abstract
We have obtained the first time-resolved, disc-integrated observations of Earth's poles with the Deep Impact spacecraft as part of the EPOXI Mission of Opportunity. These data mimic what we will see when we point next-generation space telescopes at nearby exoplanets. We use principal component analysis (PCA) and rotational lightcurve inversion to characterize color inhomogeneities and map their spatial distribution from these unusual vantage points, as a complement to the equatorial views presented in Cowan et al. (2009). We also perform the same PCA on a suite of simulated rotational multi-band lightcurves from NASA's Virtual Planetary Laboratory 3D spectral Earth model. This numerical experiment allows us to understand what sorts of surface features PCA can robustly identify. We find that the EPOXI polar observations have similar broadband colors as the equatorial Earth, but with 20-30% greater apparent albedo. This is because the polar observations are most sensitive to mid-latitudes, which tend to be more cloudy than the equatorial latitudes emphasized by the original EPOXI Earth observations. The cloudiness of the mid-latitudes also manifests itself in the form of increased variability at short wavelengths in the polar observations, and as a dominant gray eigencolor in the south polar observation. We construct a simple reflectance model for a snowball Earth. By construction, our model has a higher Bond albedo than the modern Earth; its surface albedo is so high that Rayleigh scattering does not noticeably affect its spectrum. The rotational color variations occur at short wavelengths due to the large contrast between glacier ice and bare land in those wavebands. Thus we find that both the broadband colors and diurnal color variations of such a planet would be easily distinguishable from the modern-day Earth, regardless of viewing angle.
Highlights
Observations of exoplanets will be limited to disk-integrated measurements for the foreseeable future
We presented time-resolved, disk-integrated observations of Earth’s polar regions from the Deep Impact spacecraft as part of the EPOXI mission of opportunity
We found that both of the polar observations have broadband colors similar to the equatorial Earth, but with uniformly higher albedos
Summary
Observations of exoplanets will be limited to disk-integrated measurements for the foreseeable future This is true whether a planet can be spatially resolved from its host star (direct imaging) or not (as in current studies of short-period transiting planets). Thermal phases inform us about the diurnal heating patterns of the planet: the dayside temperature, the nightside temperature, and the hottest local time on the planet (Cowan & Agol 2008). Depending on whether it has an atmosphere, such observations can constrain a body’s rotation rate as well as its average Bond albedo, thermal inertia, emissivity, surface roughness, and wind velocities (Spencer 1990; Cowan & Agol 2011a)
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