Task A: Earth as an Exoplanet

The VPL team has developed a well-validated 3-D spectral Earth model to produce a definitive dataset of the photometric and spectral characteristics for the Earth as seen in direct imaging and transit.  

The VPL Earth model is now the most comprehensive and rigorous model of its type, simulating reflectance and emission from realistic planetary surfaces, with line-by-line treatment of atmospheric emission, absorption and scattering, and explicit phase dependent scattering from realistic clouds. The model outputs spatially- and spectrally-resolved synthetic observations of Earth, including disk-integrated photometry and spectra, for an arbitrary observational vantage point. It can be used to explore the appearance of Earth over all observed phases (from full to crescent), throughout any wavelength interval from the far UV to the far-IR, and on timescales from hours to years (Robinson et al., 2011; Misra et al., 2012, in prep).

The model has been extensively validated from the ultraviolet to the thermal infrared, and is in excellent agreement with data from NASA’s Atmospheric Infrared Sounder (Aqua/AIRS), and temporally- and spectrally-resolved observations of Earth from NASA’s Deep Impact flyby spacecraft (a VPL collaboration with the NASA EPOXI mission) (Robinson et al., 2011). The model has also been validated as a function of phase using Earthshine and EPOXI data (Robinson et al., 2010).

Products from the Earth model can be used to explore the detectability of signs of habitability and life for the Earth, to validate the retrieval techniques developed in Task E, and to expand our 3-D spectral visualization capability to planets other than the Earth. We are currently working on upgrades to the Earth model that will further increase its versatility.

Highlights of scientific applications of the VPL 3-D Spectral Earth model are described below. 

Detecting Alien Oceans Using Glint

In Robinson et al. (2010), we used the Earth model to simulate Earth's appearance in reflected light over a year, including the realistic evolution of cloud, snow, and sea ice cover. We used this VPL-generated dataset (which is publicly available on this website) to investigate the detectability of "glint", the mirror-like reflection of sunlight off a body of water, in the Earth’s disk-integrated brightness. Including the possibly confusing effects of realistic forward scattering clouds, our models of the Earth's phase-dependent brightness show that the crescent-phase Earth is as much as 100% brighter than an identical non-glinting Earth at some near-infrared wavelengths. Such an excess in brightness may be detectable by NASA's James Webb Space Telescope if it were to fly with an external occulter.

Detecting Exomoons

The VPL 3-D Spectral Earth model has also been used to explore the detectability of a moon around an Earth-like exoplanet as a function of wavelength and observed phase (i.e. whether the exoEarth and moon are observed at full, or near crescent phase) (Robinson, 2011). The models showed that the contribution of the exomoon to the exoEarth spectrum is very strongly phase dependent, and more likely to be detectable in the exoEarth’s carbon dioxide absorption bands.  

Figure: True color image of the Earth-Moon system, taken as part of NASA’s EPOXI mission compared to a simulated image using 10 _m brightness temperatures from our models. The spectra on the right shows the corresponding flux at 10 pc from the Moon (grey), Earth (blue), and the combined Earth-Moon flux (black), not including transit effects. The panel below the spectra shows the wavelength dependent lunar fraction of the total signal. Images and spectra are for a phase angle of 75.1◦. See Robinson, 2011.

Earth in Transmission 

Recently, we modified our line-by-line radiative transfer model to simulate transmission spectroscopy, that is, the backlighting of a planet’s atmosphere seen when it passes in front of its parent star. This work was validated against ATMOS-1 observations of the Earth’s transmission, and is currently being used to determine sensitivity to atmospheric pressure (Misra et al., 2011; Misra et al., 2012, in prep).