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

Abstract

The EPOXI Discovery Mission of Opportunity reused the Deep Impact flyby spacecraft to obtain spatially and temporally resolved visible photometric and moderate resolution near-infrared (NIR) spectroscopic observations of Earth. These remote observations provide a rigorous validation of whole-disk Earth model simulations used to better understand remotely detectable extrasolar planet characteristics. We have used these data to upgrade, correct, and validate the NASA Astrobiology Institute's Virtual Planetary Laboratory three-dimensional line-byline, multiple-scattering spectral Earth model. This comprehensive model now includes specular reflectance from the ocean and explicitly includes atmospheric effects such as Rayleigh scattering, gas absorption, and temperature structure. We have used this model to generate spatially and temporally resolved synthetic spectra and images of Earth for the dates of EPOXI observation. Model parameters were varied to yield an optimum fit to the data. We found that a minimum spatial resolution of *100 pixels on the visible disk, and four categories of water clouds, which were defined by using observed cloud positions and optical thicknesses, were needed to yield acceptable fits. The validated model provides a simultaneous fit to Earth's lightcurve, absolute brightness, and spectral data, with a root-mean-square (RMS) error of typically less than 3% for the multiwavelength lightcurves and residuals of *10% for the absolute brightness throughout the visible and NIR spectral range. We have extended our validation into the mid-infrared by comparing the model to high spectral resolution observations of Earth from the Atmospheric Infrared Sounder, obtaining a fit with residuals of *7% and brightness temperature errors of less than 1 K in the atmospheric window. For the purpose of understanding the observable characteristics of the distant Earth at arbitrary viewing geometry and observing cadence, our validated forward model can be used to simulate Earth's time-dependent brightness and spectral properties for wavelengths from the far ultraviolet to the far infrared.

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

“…But only a subset of hundreds of these are volatile enough to enter Earth's atmosphere at more than trace concentrations. Only a few of the gases produced by life on Earth-O 2 (and O 3 ), CH 4 , and N 2 O-have been detected in Earthshine and spacecraft observations of the spatially unresolved ''Earth as an exoplanet'' (e.g., Christensen and Pearl, 1997;Turnbull et al, 2006;Palle et al, 2009;Robinson et al, 2011). These observations show us what might be possible from space telescopes capable of finding and characterizing Earth-like planets orbiting Sun-like stars in reflected light observations (e.g., Stapelfeldt et al, 2015).…”

Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth

“…Results from the photochemical code that exhibited potentially detectable biosignature gases were used as inputs to the Spectral Mapping Atmospheric Radiative Transfer (SMART) model (developed by D. Crisp), which produced simulated spectra for these model planets. SMART has been validated against observations of Earth, Mars, and Venus (Meadows & Crisp 1996, Crisp 1997Halthore et al 2005;Robinson et al 2011;Robinson et al 2014), and has been used to simulate spectra of extrasolar planets (Kiang et al 2007;Domagal-Goldman et al 2011;Robinson 2011). …”

A Statistical Comparative Planetology Approach to the Hunt for Habitable Exoplanets and Life Beyond the Solar System

“…The systems science framework aims to reveal the nature of individual planets based on a combination of empirical data and theoretical modeling. The empirical data should be as complete as possible to minimize model dependencies, while the theoretical models are benchmarked on the Solar System planets to maximize their accuracy (e.g., Robinson et al 2011Robinson et al , 2014. The ultimate goal of the systems science approach is to identify particular exoplanets that are habitable and to make statements about the possibility that they harbor life.…”