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2013 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2012 – AUG 2013

Earth as an Extrasolar Planet

Project Summary

Earth is our only example of a habitable planet, or a planet capable of maintaining liquid water on its surface. As a result, Earth 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. We seek to accurately simulate the time-, phase-, and wavelength-dependent appearance of the Pale Blue Dot, and to use these models to understand how to best recognize and characterize potentially Earth-like exoplanets.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

In this task we use observations of our home planet to explore the detectability of signs of habitability and life on terrestrial planets. At the center of this development is the VPL 3-D spectral Earth model, which is a community tool that simulates the time-dependent disk-integrated spectrum of our planet from any arbitrary viewing geometry. In collaboration with LCROSS mission scientists, this year Robinson, Meadows and Sparks performed a comparison of predictions from the VPL 3-D spectral Earth model with UV to infrared spectra of the Earth obtained by the LCROSS mission (Robinson et al., submitted). This comparison was used to validate our predictions of the detectability and spectral dependence of glint from the Earth’s ocean, by comparing the model predictions of the Earth’s brightness at different phases with the data taken by LCROSS. This data-model comparison also revealed an error in the spectral calibration of data from the LCROSS mission, which we were able to help correct. We also discussed using the 255nm UV Hartley band of ozone, one of the strongest spectral features in the Earth’s spectrum, as a biosignature. This project was made possible by collaboration between the VPL, NASA/LCROSS mission scientists, and the Space Telescope Science Institute (STScI).

The updated model has been used to demonstrate that N2-N2 CIA is required to fit the spectral region near 4.1μm in Earth spectra taken by the NASA/EPOXI mission (Schwieterman et al., in prep). Detection of N2-N2 CIA in a planet’s spectrum can help constrain surface pressure and thus surface habitability. This year we also completed spectral libraries of the Earth’s appearance through a Lunar month as a simulated dataset for studies of observations of the Earth from a lunar platform.

N2-N2 Dimer Absorption in Earth's Observed Disk-Averaged Spectrum
Spectral radiance from 3.4-4.6 μm of EPOXI Earth observations on March 18, 2008 with a sub-spacecraft longitude of 145° East (black), the VPL model from Robinson et al. 2011 (red), the model with N[~2~]-N[~2~] CIA included (blue), the model with both N[~2~]-N[~2~] and N[~2~]-O[~2~] CIA included (green). Being able to detect the N[~2~]-N[~2~] absorption may provide an additional probe of the bulk composition of planetary atmospheres, which is important for assessing planetary habitability.

Images of Earth from the NASA/LCROSS spacecraft (top) and the VPL Earth model (bottom). Left column is reflected light, from 0.9-1.7 microns, and middle column is infrared light, from 6-10 microns. Right-top demonstrates the viewing geometry (crescent phase) and right-bottom is a true-color image from the model. Note that the entire disk of Earth is visible in the infrared image, as Earth emits thermal radiation at these wavelengths. These observations have been used to validate Sun glint from Earth's oceans in the VPL Earth model, which is a process that could be used to detect surface oceans on exoplanets.