Simulating and Characterizing the Pale Blue Dot
Robinson, Tyler David
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The goal of this work is to develop and validate a comprehensive model of Earth's disk-integrated spectrum. Earth is our only example of a habitable planet, or a planet capable of maintaining liquid water on its surface. As a result, Earth typically serves as the archetypal habitable world in conceptual studies of future exoplanet characterization missions, or in studies of techniques for the remote characterization of potentially habitable exoplanets. Using spacecraft to obtain disk-integrated observations of the distant Earth provides an opportunity to study Earth as might an extrasolar observer. However, such observations are rare, and are often limited in wavelength range, spectral resolution, temporal coverage, and viewing geometry. As a result, modeled observations of the distant Earth provide the best means for understanding the appearance of the Pale Blue Dot across a wide range of wavelengths, times, and viewing geometries. In this work, I discuss a generalized approach to modeling disk-integrated spectra of planets. This approach is then used to develop a model capable of simulating the appearance of Earth to a distant observer---the Virtual Planetary Laboratory (VPL) three-dimensional, line-by-line, multiple-scattering spectral Earth model. This comprehensive model incorporates absorption and scattering by the atmosphere, clouds, and surface, including specular reflectance from the ocean, and direction-dependent scattering by clouds. Data from Earth-observing satellites are used to specify the time- and location-dependent state of the surface and atmosphere. Following the description of this tool, I validate the model against several datasets: visible photometric and near-infrared (NIR) spectroscopic observations of Earth from NASA's EPOXI mission, mid-infrared spectroscopic observations of Earth from the Atmospheric Infrared Sounder (AIRS) aboard NASA's <italic>Aqua</italic> satellite, and broadband visible observations of Earth's brightness from Earthshine observations. The validated model provides a simultaneous fit to the time-dependent visible and NIR EPOXI observations, reproducing the normalized shape of the multi-wavelength lightcurves with a root-mean-square error of typically less than 3%, and residuals of ~10% for the absolute brightness throughout the visible and NIR spectral range. Comparisons of the model to the mid-infrared <italic>Aqua</italic>/AIRS observations yield a fit with residuals of ~7%, and brightness temperature errors of less than 1 K in the atmospheric window. There is also good agreement between the Earthshine observations and the model over a wide range of phase angles, with the model always being within one standard deviation of the observations. The validated Earth model is used to study techniques for detecting oceans on Earth-like exoplanets, and for detecting moons around such planets. The former study shows that glint, or specular reflection of sunlight, off Earth's oceans can reveal the presence of oceans on exoplanets. I find that the simulated glinting Earth can be as much as 100% brighter at crescent phases than simulations that do not include glint, and that the effect is dependent on both orbital inclination and wavelength, where the latter dependence is caused by Rayleigh scattering limiting sensitivity to the surface. The latter study shows that, for an extrasolar twin Earth-Moon system observed at full phase at IR wavelengths, the Moon consistently comprises about 20% of the total signal, approaches 30% of the signal in the 9.6 μm ozone band and the 15 μm carbon dioxide band, makes up as much as 80% of the total signal in the 6.3 μm water band, and more than 90% of the signal in the 4.3 μm carbon dioxide band. These excesses translate to inferred brightness temperatures for Earth that are too large by about 20-40 K, and demonstrate that the presence of an undetected satellite can have a significant impact on the spectroscopic characterization of terrestrial exoplanets. However, since the thermal flux contribution from an airless companion depends strongly on phase, it may be possible to detect such a companion by differencing IR observations of an Earth twin with a companion taken at both gibbous phase and at crescent phase. In general, the VPL three-dimensional spectral Earth model is a well-validated tool which the exoplanetary science community can use to better understand techniques for the remote characterization of habitable worlds.
- Astronomy