Astrobiology Science Conference 2017 (LPI Contrib. No. 1965)
3515.pdf
A PHASE-DEPENDENT SPECTRAL EARTH DATABASE WITH APPLICATIONS FOR DIRECTLY
IMAGED EARTH-LIKE EXOPLANETS. E. W. Schwieterman1,2,3,4,5, J. Lustig-Yaeger2,5, V.S. Meadows2,5, T.D.
Robinson2,6, and W. B. Sparks2,7. 1Department of Earth Sciences, University of California, Riverside
([email protected]), 2NASA Astrobiology Institute Virtual Planetary Laboratory, 3NASA Postdoctoral Program
Fellow, 4Blue Marble Space Institute of Science, Seattle, WA, 5University of Washington Astronomy Department
and Astrobiology Program, 6University of California, Santa Cruz, 7Space Telescope Science Institute
Introduction: Earth will always be our best example of a living planet. To simulate the spatially unresolved "pale blue dot" of prospective future exoplanet
observations, it is important that the Earth views used
as analogs include the whole planetary disk. While
Earth data model comparisons have been effectively
used in recent years to validate spectral models, observations by interplanetary spacecraft are limited to
“snapshots” in terms of viewing geometry and Earth’s
dynamic surface and atmosphere state. Here we use the
well-validated Virtual Planetary Laboratory 3D spectral Earth model [1] to generate both simulated diskaveraged spectra and high resolution, spatially resolved
spectral data cubes of Earth at viewing geometries
consistent with Lunar viewing angles at wavelengths
from the far UV (0.1 µm) the to the far IR (200 µm).
Earth Model Properties: The radiative transfer
model calculations include line-by-line gaseous absorption including natural, Doppler and pressure
broadening; multiple scattering including Rayleigh
scattering by gaseous molecules, Mie scattering by
spherical liquid cloud droplets, and geometric scattering from distributions of hexagonal ice cloud particles;
a heterogeneous surface composition including nonLambertian reflectance from ocean surface; geographically distributed surface temperatures and altitudedependent temperatures informed by low-Earth orbiting satellite observations; and cloud coverage, optical
thicknesses, and altitudes informed by satellite data
products.
Database Properties: The database includes diskaveraged spectra from dates 03/19/2008 to 04/23/2008
at one-hour cadence and fully spectral data cubes for a
subset of those times. These data include the phase and
time-dependent changes in spectral biosignatures (O2,
O3, CH4, N2O, VRE) and habitability markers (N2,
H2O, CO2, ocean glint) at their empirical abundances.
The advantages of the VPL Earth model data products
over 1D spectra traditionally used for testing instrument architectures include accurate modeling of
Earth’s surface inhomogeneity (continental distribution
and ice caps), cloud cover and variability, pole to equator temperature gradients, obliquity, phase-dependent
scattering effects, and rotation. Figure 1 shows UVMIR spectra at four viewing geometries, illustrating
the changes in the spectra as a function of phase.
Applications: These spectral products have a wide
range of applications including data-model comparisons of Earth’s spectrum [2], calibration of spacecraft
instrumentation [3], and testing the detectability of
atmospheric and surface features of an Earth-like planet orbiting a distant star with a large space-based telescope mission concepts such as LUVOIR [4]. We present a subset of this spectral data including anticipated
signal-to-noise calculations of an exoEarth twin at different phases using a coronagraph instrument model
[5]. We also calculate time-dependent UBVRIJHK
absolute magnitudes of Earth and binned intensities in
wavelength ranges relevant for planet detection with
proposed space telescope missions.
References:
[1] Robinson et al. (2011) Astrobiology,
11 , 5, 393-408. [2] Schwieterman et al. (2015) ApJ,
810:57. [3] Robinson et al. (2014) ApJ, 787:171,
[4] Dalcanton et al. (2015),
http://www.hdstvision.org/report/ [5] Robinson et al.
(2016), PNAS, 128:025003.
Figure 1 - UV-MIR spectra of Earth at four phase angles: A) full phase (α=0°), B) gibbous phase (α=60°),
C) quadrature phase (α=90°), D) crescent phase
(α=130°). Major gas absorption features are labeled in
(A).