Abstract
Abstract NASA’s EPOXI mission used the Deep Impact spacecraft to observe the disk-integrated Earth as an analog to terrestial exoplanets’ appearance. The mission took five 24 hr observations in 2008–2009 at various phase angles (57.°7–86.°4) and ranges (0.11–0.34 au), of which three equatorial (E1, E4, E5) and two polar (P1, North and P2, South). The visible data taken by the HRIV instrument ranges from 0.3 to 1.0 μm, taken trough seven spectral filters that have spectral widths of about 100 nm, and which are centered about 100 nm apart, from 350 to 950 nm. The disk-integrated, 24 hr averaged signal is used in a phase angle analysis. A Lambertian-reflecting, spherical planet model is used to estimate geometric albedo for every observation and wavelength. The geometric albedos range from 0.143 (E1, 950 nm) to 0.353 (P2, 350 nm) and show wavelength dependence. The equatorial observations have similar values, while the polar observations have higher values due to the ice in view. Therefore, equatorial observations can be predicted for other phase angles, but (Earth-like) polar views (with ice) would be underestimated.
Highlights
After almost three decades of exoplanet discoveries, we know that possibly up to 50% of solar-type stars have a small, terrestrial-type planet in its habitable zone (Bryson et al 2020)
The visible data taken by the High-Resolution Instrument Visible camera (HRIV) instrument ranges from 0.3 to 1.0 μm, taken trough seven spectral filters that have spectral widths of about 100 nm, and which are centered about 100 nm apart, from 350 to 950 nm
We used the wavelength dependent geometric albedos that we derived from the Equatorial observation 1 (E1) data and computed the phase curves for a Lambertianreflecting planet (see Equation (1)), to allow a comparison with the data taken at the different phase angles
Summary
After almost three decades of exoplanet discoveries, we know that possibly up to 50% of solar-type stars have a small, terrestrial-type planet in its habitable zone (Bryson et al 2020). The Large UV/Optical/IR Surveyor (LUVOIR) is another proposed mission that will have the capabilities to observe and characterize a more diverse range of exoplanets. Most Earth observation data comes from nearEarth/Earth orbiting spacecraft (NASA EODIS, ESA GMES; Pfeifer et al 2012; Aschbacher & Milagro-Pérez 2012; Kansakar & Hossain 2016) Those data are often highly detailed, large volume and recreating Earth exoplanet-like data can be very tedious as it entails data reduction/selection, numerical modeling to reconstruct a simulated view of the whole disk from a distance and/or require special treatments (data gaps, spacecraft maneuvers; Hearty et al 2009; Robinson et al 2011; Mettler et al 2020).
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