Task A: Solar System Analogs for Extrasolar Planet Observations

Observations of Solar System planets and moons are used to explore processes and remote-sensing discriminants for habitable environments, and as exoplanet model validation. We have we addressed these goals using observations of Europa, models and observations of Venus, and the properties of Earth, Mercury, Mars, and the asteroid belt. 

Krissansen-Totton et al. (2016) collated spectra of planets in our Solar System with a large number of VPL planetary spectral models to determine optimum filter bands for discrimination of Earth-like spectra from other planetary types. These authors found that, for exoplanet observations in which noise is dominated by dark current, planet color can provide an efficient means of preliminary characterization.

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).

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. 

First Detection of Earth's Primary Atmospheric Gas in Disk Spectrum

In Schwieterman et al. (2015b), VPL team members made the first-ever detection of an absorption feature in Earth’s whole-disk spectrum due to molecular nitrogen (N2). This feature—the 4.2 μm N2-N2 collision-induced absorption band—was found using comparisons between observations of the distant Earth from NASA’s EPOXI mission and simulations from the VPL 3-D spectral Earth model. Molecular nitrogen can be a major constituent in planetary atmospheres, but its abundance is notoriously difficult to constrain due to a lack of spectral features. Thus, this work provides a new means for determining N2 concentrations in exoplanet atmospheres, which could be used to constrain surface pressure (thus helping to indicate liquid water stability) and could also rule out certain mechanisms for producing abiotic atmospheric oxygen.

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.

In collaboration with LCROSS mission scientists, Robinson et al. (2014) compared predictions from the VPL 3-D spectral Earth model with UV to infrared spectra of the Earth obtained by the LCROSS mission. This comparison was used to validate our predictions of the detectability and spectral dependence of glint from the Earth’s ocean, and it also revealed an error in the spectral calibration of data from the LCROSS mission, which we were able to help correct.

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). 

Venus as an Exoplanet

We study Venus to understand what can happen to an evolved, runaway terrestrial planet. Arney et al. (2014) published work on the first simultaneous near-infrared spectral mapping observations of the Venus atmosphere, an analog for a hot, haze-covered planet. These observations were used to produce simultaneous spatially-resolved maps of H2O, HCl, CO, OCS, and SO2 abundances in the Venusian lower atmosphere, revealing unexpected dichotomies, and showing spatial correlations that were indicative of chemical interactions between several atmospheric species. Gao et al. (2014) validated a generalized 1-D microphysical and vertical transport cloud model for use in the VPL 1-D Climate Model against Venus data, and revealed an oscillatory “rain out” of the Venus clouds in the process.

Titan as an Exoplanet

Saturn's giant moon, Titan, represents a very alien world, where methane exists in multiple phases, rather than water. 

Robinson et al. (2014) used Cassini occultation observations of Titan to simulate a transit transmission observation of a haze-enshrouded world. The observations showed a strong slope in the spectrum due to the hydrocarbon haze, and these data were combined with our existing knowledge of the Titan atmosphere to quantify the atmospheric depths probed in transit transmission.

Charnay et al. (2015) explained the observed eastward propagation of dunes on this planet using a coupling between tropical methane storms and superrotation. This work provides insights into volatile cycles on dry planets, as might occur for habitable worlds that form dry or lose most of their water.