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Executive Summary

Overview

The Virtual Planetary Laboratory’s interdisciplinary research effort focuses on answering a single key question in astrobiology: If we were to find a terrestrial planet orbiting a distant star, how would we go about recognizing signs of habitability and life on that planet? This question is relevant to the search for life beyond our Solar System, as outlined in NASA’s Astrobiology Roadmap Goals 1 and 7. VPL research spans many of the Roadmap objectives, but is most relevant to Objectives 1.1 (Formation and Evolution of Habitable Planets), 1.2 (Indirect and Direct Observations of Extrasolar Habitable Planets) and 7.2 (Biosignatures to be Sought in Nearby Planetary Systems).

Recent observations have brought us much closer to identifying extrasolar environments that could support life. The successful Kepler Mission has found over three thousand planetary candidates – many of them smaller than twice the diameter of the Earth – and several residing in the habitable zones of their parent stars. Kepler’s large survey has improved our understanding of how common terrestrial planets are in the Galaxy. The K2 and TESS missions will help find potentially habitable targets for the planned James Webb Space Telescope (JWST), which will probe the atmospheric composition of super-Earths. In the longer term, we anticipate large spaceborne telescopes, such as NASA’s LUVOIR and AT-LAST concepts, that can directly image and obtain spectroscopy of terrestrial extrasolar planets.

The VPL provides a scientific foundation for interpretation of data from extrasolar terrestrial planet detection and characterization missions such as Kepler, K2, TESS, JWST and the AT-LAST. To do this, the VPL uses information from Earth’s stages of evolution, and data provided by NASA’s Earth observing and planetary exploration programs, to validate and develop more comprehensive models of terrestrial planets. These models allow us to simulate and explore the likely diversity of extrasolar planet environments in advance of the more challenging observations. These models are primarily used to understand the radiative and gravitational effects of stars on the planets that orbit them. Combinations of model and fieldwork are also used to understand which biologically produced gases can produce a detectable biosignature in globally-averaged planetary observations. Finally, models and instrument simulators are used to understand how we can best extract information on a planet’s environment from astronomical observations that have no direct spatial resolution and may be quite limited in other ways.

The team required to develop and run these models is necessarily large, and highly interdisciplinary. Our research encompasses single discipline efforts that produce results pertinent to our overarching habitability and biosignatures focus, all the way through to highly interdisciplinary efforts where stellar astrophysicists, planetary climate modelers, orbital dynamicists, atmospheric chemists and biologists work together to determine the effects of stellar radiation and gravitation on the habitability of terrestrial planets.

Our Research This Year

Our research can be divided into five main tasks: Solar System Analogs, Early Earth and Mars, The Habitable Planet, and The Living Planet and The Observer. The first four tasks explore known and simulated environments to understand the factors that affect habitability, the plausible range of terrestrial planet environments, both inhabited and uninhabited, and the global impact of life on its environment. This knowledge can be used to help prioritize newly discovered potentially habitable planets for more-detailed observational follow-up, and to generate new biosignatures to be sought in planetary spectra. The fifth task uses the models and data generated and gathered in the first four tasks to assess the remote detectability of newly postulated global signs of habitability and life.

Solar System Analogs for Extrasolar Planet Observations.

In this task we use observations of our home planet and other planets in our Solar System to explore the detectability of signs of habitability and life on terrestrial planets. In collaboration with LCROSS mission scientists, this year Robinson, Meadows and Sparks published a paper comparing 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., 2014). 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. We also discuss using the UV Hartley band of ozone as a biosignature. We completed models of the Earth as seen from the Moon through a lunar month. Schwieterman, Meadows, Misra and Crisp demonstrated the first detection of the nitrogen dimer (N₂-N₂) in the Earth’s disk-averaged near-infrared spectrum, and showed that it produced a 40% modification of the spectrum. This band may be the only way to complete the inventory of atmospheric bulk gases for extrasolar terrestrial planets. In another exoplanet observation analog, Robinson led a paper that 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 (Robinson et al., 2014). Arney, Meadows, Crisp and Bailey published work on the first simultaneous near-infrared spectral mapping observations of the Venus atmosphere, an analog for a hot, haze-covered planet (Arney et al., 2014). 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, Yung, Crisp and colleagues 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 (Gao et al., 2014).

Early Earth and Mars:

In this task we worked to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. Atmospheric chemistry models were used to explore the formation of surface salts through the oxidation of volcanic gases, and the redox state of the early atmosphere due to volcanic outgassing. Assuming past volcanic outgassing in the expected range, we found that the early Martian atmosphere could have been anoxic, weakly reducing, and CO-rich. These conditions are favorable for an origin of life, and may leave geochemical traces in the soil (Sholes et al 2015). Work also tested the hypothesis that the perchlorate, sulfates and nitrates seen by the Phoenix Lander were produced from atmospheric deposition over the last 3 billion years (Smith et al 2014). Although atmospheric deposition works well for sulfates, we found that gas phase reactions for making perchlorates are insufficient to explain levels of ~0.5wt% in the soil sampled by Phoenix. Thus, unknown gas-solid reactions are likely required. Work was also published that places such results in the broader context of the evolution of Mars and the general behavior of planetary atmospheres (Catling 2014, Catling 2015, Haberle et al 2015). Conrad and Domagal-Goldman developed models that account for the evolution of the Martian interior and how the process of differentiation works with interior cooling to off-gas volatiles, and subsequent processes that may lead to atmospheric escape. This work was used to interpret results from MSL and in reviews of MSL science (Conrad 2014, Mahaffey et al., 2014, Webster et al., 2014). In laboratory work, Toner, Catling and colleague studied the properties of super-cooled brines, which may have existed on early Mars, to determine how much salt solutions will supercool and remain amorphous (Toner et al., 2014a,b). They found that perchlorate-rich (ClO₄) salt solutions tending to supercool down to -120°C – much more than analogous chlorides and sulfates – and then transition into an amorphous glass. Such glasses could potentially preserve organic molecules and structures, such that organisms can remain viable upon rewarming. Improvements to models and additional laboratory data were also used to identify a suite of potential habitable brines (Toner et al., 2015a).

For studies of the early Earth’s environment, we made progress in the areas of Earth’s atmospheric and geochemical history and its implications for life, climate evolution, and the evolution of atmospheric oxygen levels. Stüeken and Buick conducted N, S and Se isotope analyses to investigate the oxygenation state of early Earth environments. They discovered extreme nitrogen isotope enrichments, that indicated that lakes in the Archean may have been significantly alkaline, and by comparison, the ocean was not as alkaline as previously thought (Stüeken et al., 2015). This high pH will affect the free energy for chemosynthetic energy pathways, and this new information on the Archean environment will be included in energy-based ecosystem models currently under development by our team. Claire, Buick, Domagal-Goldman, Kasting and Meadows increased the spectral resolution of our photochemical models to explore UV effects on S isotope fractionation to reassess hypotheses for the preservation of these signals in the rock record (Claire et al., 2014). Updated k-coefficients were incorporated into our climate models to show that at low atmospheric pressures, the addition of H₂ to the early Earth atmosphere has a negligible effect on warming, and to recalculate the limits of the radiative habitable zone. Byrne and Goldblatt (2014) developed a library of forcings for different greenhouse molecules applicable to studies of early Earth and Mars.

We reconsidered haze formation in the Archean, and computed the first self-consistent solutions for Archean Earth’s atmospheric chemistry and climate with a fractal hydrocarbon haze (Arney et al 2014a,b,c; Arney et al 2015). These models indicated that the fractal haze that covered Earth could remain habitable, even under a fainter early Sun, and that the haze provided a significant surface UV shield in the pre-oxygenated atmosphere. Transit transmission models of these hazy Archean environments show a strongly sloped spectrum similar to that seen for Titan. We also explored haze formation for planets orbiting stars of different spectral type (Domagal-Goldman, 2013) and found that hazes did not form at very high or low UV fluxes. Charnay analyzed hot climates of the late Hadean/early Archean with high amounts of CO₂ and explored the climate and carbon cycle for these hot early Earths.

Catling and Krissansen-Totton re-examined the multi-billion year record of marine carbon and sulfur isotopes to derive optimal statistical estimates of their implications for oxygen fluxes into the atmosphere and ocean (Krissansen-Totton and Catling, 2014). They show that fractional organic carbon burial has increased in the last 3.6 Gy by a factor from 1.2-2, with the larger value consistent with the rise of oxygen in the Proterozoic. Higher fractionation in the later Archean may be consistent with the advent of photosynthesis at 2.8 Gya or earlier. Zahnle and Catling (2014) considered the 0.5 Gy gap between the advent of oxygenic photosynthesis and the rise of O₂, exploring possible explanations for the delay, including the rate of hydrogen escape to space, and the size of the reduced reservoir that needed to be oxidized before O₂ became favored. They also discussed how hydrogen escape may have been linked to the history of continental growth. Domagal-Goldman wrote a pair of review papers exploring the geochemical constraints that make higher- and lower-than-accepted amounts of O₂ in the Archean unlikely, and that argue that parts of the Earth system, including the ocean, may track redox states unrelated to atmospheric proxies (Domagal-Goldman, 2014).

The Habitable Planet

VPL’s core research is in the area of planetary habitability. This VPL task explores habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy.

In planet formation, Quinn and Backus ran supercomputer simulations of planet formation and migration around M dwarf stars, where planet-forming material can be relatively scarce near the star. As large planets are unlikely to form under these conditions, this work explores mechanisms for super-Earth migration and “parking” in the habitable zone. Raymond and colleagues explored the late stages of terrestrial planet formation, including migration of terrestrial planets (Raymond et al. 2013, 2014a, 2014b; Izidoro et al. 2014) water delivery (O’Brien et al. 2014), the composition of the Earth (Jacobson et al. 2014), and the formation of known systems such as Kepler-186 (Bolmont et al. 2014), which houses the first Earth-sized planet found in the HZ.

To explore the star’s gravitational influence on planetary habitability, and in particular how planetary orbital, rotational and climate evolution are coupled, Armstrong, Barnes, Domagal-Goldman, Meadows and colleagues studied hypothetical systems with large relative inclinations between planetary orbital planes (Armstrong et al. 2014). They found that planets that experience strong orbital and rotational evolution could support habitable conditions at larger stellar distances than planets with more stable orbital and spin configurations. Dietrick, Barnes, Quinn, Luger and colleagues used orbital stability models to reveal the full 3-dimensional orbital architecture of the Upsilon Andromedae system, and showed that a potentially habitable planet in this system would have its orbit strongly perturbed by the mutual inclination of its companions (Deitrick et al. 2015). Kress and colleagues used stability arguments to investigate the plausibility of an Earth-sized planet existing in the habitable zone of the known multi-planet system Gl 581 (Joiner et al., 2014). Barnes and colleagues discovered that terrestrial exoplanets in mean motion resonances with non-planar orbits can evolve chaotically for at least 10 Gyr. In extreme cases, the simulated orbital eccentricities grow so large that planets would graze the host star, compromising habitability (Barnes et al. 2015). Barnes also showed that tidal theory applied to a large ensemble of transiting planets (e.g. Kepler data) could reveal the boundary between rocky and gaseous exoplanets based on the different tidal circularization timescales experienced by these different types of bodies (Barnes 2015).

Looking to the effects of companion planets and moons, Barnes and colleagues showed that a wide range of companion properties can sustain modest tidal heating in potentially habitable exoplanets for more than 50 billion years, offering a stable energy source that could sustain geochemical cycles conducive to life (van Laerhoven et al., 2014). Barnes and colleagues also explored the respective roles of tidal heating and planetary radiation on the habitability of exomoons (Heller & Barnes, 2014) and contributed to an invited review on exomoon habitability (Heller et al. 2014). Barnes, Dietrick, Luger and Quinn worked on the development of the VPLanet model architecture, which can calculate the coupled effects of orbital and spin/obliquity evolution of a planetary system much more quickly than standard N-body algorithms. This new model will allow for efficient calculation of constraints on planetary orbits for newly-discovered potentially-habitable planets.

VPL team members also studied the effects of stellar radiation on planetary habitability and evolution. Kopparapu, Domagal-Goldman, Kasting and colleagues used climate models to calculate the habitable zone boundaries for planets of different masses. They found that the habitable zone moves outward for smaller mass planets due to the enhanced greenhouse effect from an increased water column depth (Kopparapu et al., 2014). Kopparapu, Domagal-Goldman and collaborator determined the occurrence rate of potential Venus analogs from Kepler data, and showed that this type of planet is more likely to be common around more Sun-like – rather than smaller – stars (Kane et al., 2014). Shields, Bitz, Meadows and colleagues used a hierarchy of climate models to explore of the effect of a star’s spectrum on the rate at which a planet can exit a snowball state. They found that planets orbiting M-dwarf stars require a smaller stellar flux to initiate deglaciation and melt out of a snowball state than planets orbiting hotter, brighter stars (Shields et al. 2014). Wordsworth and Pierrehumbert undertook a range of calculations to explore water photolysis and hydrogen loss for terrestrial exoplanets, and found that CO₂ can only cause significant water loss by increasing surface temperatures over a narrow range of conditions. They concluded that many “Earth-like” exoplanets in the habitable zone may have ocean-covered surfaces, stable CO₂/H₂O-rich atmospheres, and high mean surface temperatures (Wordsworth and Pierrehumbert, 2013). Pierrehumbert and Ding continued to work on development of a generalized 3-D GCM model for exoplanet atmospheres. Charnay and Wordsworth explored possible solutions to the faint young Sun problem, and the possible climates of the Archean Earth with a 3-D GCM (Charnay et al., 2013). Work by Hawley and colleagues (Hawley et al., 2014; Davenport et al., 2014) characterized the frequency and characteristics of stellar flares on M dwarf stars from the Kepler data. These data will feed into future work on characterizing the effect of stellar flares on habitability.

In pioneering work on star-planet interactions and planetary evolution, Luger and Barnes (2015) showed that the extended pre-main sequence super-luminous phase of an M-dwarf can lead to strong atmospheric escape of water, and dessication of terrestrial planets that form within the star’s main sequence habitable zone. This result has important implications for the search for habitable worlds using transit transmission. Luger, Barnes, Meadows and colleagues also used coupled atmospheric escape and tidal orbital evolution models to illustrate how the pre-main sequence evolution of M-dwarfs could strip the gaseous envelopes from migrating mini-Neptunes, transforming them into potentially-habitable, Earth-sized rocky bodies (Luger et al., 2015). Together, the papers imply that habitability is more likely for planets that have migrated into the habitable zones of M dwarf stars, rather than on those that formed in the HZ.

VPL researchers also explored how internal planetary processes may impact habitability. Sleep and colleagues discussed the state of the mantle and crust soon after the moon-forming impact, the influence of lunar induced earth-tides on these zones, and the fate of CO₂ in the mantle and atmosphere in the Hadean (Sleep et al., 2014). Sleep and Lowe (2014) examined the physics of crustal fracturing and dike formation triggered by a large meteorite impact in the Barberton greenstone belt, South Africa and speculated on the formation of habitable fracture in the Earth’s crust. Barnes and Kopparapu participated in the ASU NAI Stellar Stoichiometry Workshop Without Walls and contributed to the conference’s review paper (Young et al., 2014). Driscoll and Barnes explored thermal-orbital planetary evolution with the coupling of orbital tidal evolution and interior models, showing different rates of circularization for inner and outer edge HZ M dwarf planets that result in either an early burst of tidal heating or an extended influx tidal energy for much of the lifetime of the planet. Bolton continued work on weathering models to understand the effects of weathering in low oxygenation states that may have been present in the Proterozoic.

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the environment and biosphere, and aspects of life’s global impact that could potentially be detected remotely as biosignatures. Recent work has focused on understanding the long-wavelength limit for photosynthesis, which is relevant to haze enshrouded planets such as the early Earth, and for planets orbiting red M dwarf stars. Kiang, Parenteau, Blankenship and Seifert continued field and laboratory experiments to isolate new strains of far-red utilizing oxygenic photosynthetic organisms, and to understand their natural light regime. Kiang undertook field work with the far-red light utilizing Acaryochloris marina in the Salton Sea. Kiang and Parenteau collected red algae samples at three sites on the California coast and isolated the longer-wavelength Chlorophyl d. Seifert analyzed samples taken from Cuatro Cienegas, where freshwater stromatolites are found, and found evidence for Acaryochloris. Additional samples for culture in Blankenship’s lab are being sought. Field measurements of the light environments for these organisms will also be used to drive kinetic models of photon energy use to ascertain light thresholds of survival. Kiang published a white paper exploring theoretical challenges in quantifying the long wavelength limit of oxygenic photoysynthesis in a special issue on Astrobiology in The Biochemist magazine (Kiang, 2014).

Parenteau, Hoehler, Kiang and Blankenship measured the reflectance spectra of pure cultures and field in situ samples of a variety of anoxygenic photosynthesizers, searching for spectral signatures. They found a rise in reflectivity just past absorption maxima for bacteriochlorophyll pigments. This is a near-infrared (NIR) bacterial analog to the vegetation red edge, that is comparable in magnitude to that found in vegetation (Parenteau et al., 2014). Some of these measurements were used as input to models of the Archean Earth, to test for the detectability of anoxygenic photosynthesizers on a global scale (Palle et al., 2014). Future work on these samples includes measurement of biogenic gases and volatile organics for mats housed under different environmental parameters such as starting atmosphere and radiation environment. Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to conduct an interdisciplinary study of the diversity and detectability of non-photosynthetic pigments as biosignatures, especially for halophiles (Schwieterman et al., 2015). This study included reflectance spectra measurements of a diverse collection of pigmented non-photosynthetic organisms and an analysis of the remote detectability of analogs of these organisms in disk-averaged spectra, with a planetary environment that includes other surfaces, an atmosphere and clouds. These spectra will be made available through the existing VPL Pigment Database.

This year the VPL Team also explored the generation of false positives for the oxygen atmospheric biosignature, and ways in which such a false positive could be identified and discriminated from a true biosignature. Domagal-Goldman, Segura, Claire and Meadows published a study into the generation of abiotic false positives for ozone for early Earth-type planets in orbit around M dwarf stars (Domagal-Goldman et al., 2014). This work took a conservative approach to the conditions for false positive production, yet still produced some model atmospheres with detectable amounts of abiotically-generated O₃. Wordsworth and Pierrehumbert (2014) explored the potential buildup of photolytically produced O₂ in the upper atmosphere of planets with a low abundance of non-condensible atmospheric species, such as N₂. These less-dense atmospheres are less likely to trap water vapor near the surface, and if water escapes to the stratosphere then it can be photolyzed to produce O₂. Another possible mechanism to photolytically generate abundant oxygen could occur on planets orbiting M dwarfs due to the high FUV/NUV flux ratio of some M dwarfs in comparison to that of Sun-like stars. VPL team members Gao and Yung explored the stability of CO₂ under this incident UV spectrum and showed that a CO₂-dominated atmosphere can be converted into a CO₂ /CO/O₂ -dominated atmosphere in 10³-10⁴ years by CO₂ photolysis. Our results indicate that it is unlikely that CO₂ atmospheres can remain stable on terrestrial planets around M dwarfs with high FUV/NUV flux ratios. Luger and Barnes (2015) explored the pre-main-sequence evolution of terrestrial HZ planets under the influence of super-luminous young M-dwarf stars, and also discovered a potential abiotic oxygen source. Pre-main sequence M dwarfs are powered by nuclear reactions in the core as well as energy of contraction as they slowly collapse to their main-sequence radius. For these low-mass stars, the process can be extremely slow, lasting over a billion years, and subjecting planets that form in the main-sequence HZ to extended periods of high radiation. Luger and Barnes showed that this can result in the potential loss of several oceans of water, and the photolysis of this escaping water abiotically generates a dense, O₂ rich atmosphere. Schwieterman, Meadows, Domagal-Goldman, Robinson, Misra, Crisp, Luger, Barnes and Arney are now working on possible spectral discriminators for these different scenarios, including looking for CO, CH₄ and O₄ in the planetary spectra.

We also worked on two projects this year to understand how to quantify disequilibrium biosignatures, and to develop a coupled environment-ecosystem model that can be used to predict the magnitude and type of biosignatures for a range of different planetary environments. Krissansen-Totton, with Catling and Robinson, developed a method to quantify the degree of thermodynamic disequilibrium in planetary atmospheres, as a means of detecting a potential biosignature. They found that the Earth’s atmosphere-ocean disequilibrium, was maintained by biology, and was an order of magnitude larger than for any other Solar System atmosphere (Krissansen-Totton et al., 2015). Hoehler, Domagal-Goldman, Som, Kasting and Meadows have begun modeling chemosynthetic-based biospheres by completing the coupling and benchmarking of two models: one model codifies a thermodynamic energy balance concept for habitability (Hoehler, 2009); the other model predicts a planet’s atmospheric composition and climate while balancing the redox state of the surface environment (Domagal-Goldman et al., 2014). These models will allow an ecosystem to be coupled into an interactive planetary environment including atmospheric, oceanic and subsurface components. This will allow us to predict standing biomass size, net biological productivity, and resulting gas fluxes to and from a biosphere (and therefore the nature and magnitude of any potential biosignature) with a given set of volcanic and hydrothermal inputs to the planetary surface.

The Observer

In this task we explore the detectability of signs of habitability and life for modeled observations from the previous tasks. We also observe and develop new observational, analysis and retrieval techniques to improve our understanding of the environmental properties of exoplanets for current and future observations. In exoplanet observations this year, Raymond was part of the team that discovered Kepler 186f, the first Earth-sized planet found in the habitable zone of an M dwarf star (Quintana et al., 2014). Agol was involved in detecting and characterizing new planets in the Kepler data, including the first 7-transiting planet system, Kepler-90 (Lissauer et al., 2014; Rowe et al., 2014), and the planetary system found by 'Citizen Scientists’ (Deck et al., 2014, Schmitt et al., 2014). Agol and Deming helped analyze and interpret Hubble and Spitzer Space Telescope observations of short-period giant planets (Deming et al., 2013), including detection of water absorption features and potential planet variability, which are being interpreted with the help of 3D simulations (Dobbs-Dixon and Agol, 2013). They are now extending these new technique to smaller planets.

Misra, Meadows and Crisp completed modification of the VPL’s line-by-line radiative transfer model (SMART) to generate a state-of-the-art transit transmission model that includes the effects of gas absorption, cloud and aerosol extinction, refraction, and the effects of stellar limb darkening. The model has been validated against ATMOS limb spectra of the Earth and lunar eclipse spectra. We have used the model to show that simultaneous measurements of the absorption features from the O₂-O₂ dimer molecule and molecular oxygen (O₂) can be used as a new technique to probe planetary atmospheric pressure for oxygenated terrestrial atmospheres and biosignatures (Misra et al., 2014a). We have shown that inclusion of refraction decreases the detectability of spectral absorption features in transit transmission, and that this effect is dependent on atmospheric composition, the size of the star, and the planet-star distance (Misra et al., 2014b), and we have postulated that refraction effects in transit transmission observations could be used to discriminate between planets with and without clouds (Misra et al., 2014c). Work is currently underway on a publication that describes the effects of scattering on terrestrial exoplanet spectra.

Deming and Sheets used the predictions from these models to begin searching for refracted light near transit, to estimate the degree of cloud coverage and scale height of super-Earth atmospheres as an important precursor to JWST spectroscopy during transit. Deming and Sheets also measured the albedos of small transiting exoplanets by coadding Kepler data for each planet class, to search for secondary eclipses. Observations for planets of similar radius were grouped and transformed to a common orbital phases scale so that data from multiple planets could be coadded (Sheets and Deming, 2014). They were able to detect reflected light from close-in super-Earth planets between one and two Earth radii and conclude that these objects have a relatively low albedo, indicating significant absorption in their atmospheres or surfaces. Kopparla and Yung have undertaken a modeling study using the vector radiative transfer model VLIDORT to study the phase space of expected atmospheric composition and the observable polarization signal for a range of potential exoplanets (Kopparla et al., 2014). Line, Crisp and Yung developed, tested, and published the relative performance of three commonly used remote sensing retrieval algorithms (optimal estimation, Markov-Chain Monte Carlo, and Bootstrap Monte Carlo) for interpreting realistic, synthetic spectra of exoplanets. (Line et al., 2013) and used the retrieval algorithms to search for chemical disequilibria in observations of exoplanetary atmospheres (Line &Yung, 2013) and to undertake a systematic retrieval of secondary eclipse spectra of nine planets to determine their C/O ratios. Lustig-Yaeger analyzed existing exoplanet spectra to determine that broadband observations were often inadequate for detection of molecular species, but that their accuracy could be enhanced with even a small section of spectroscopic data (Lustig-Yaeger et al., 2015). Lustig-Yaeger, Meadows and Crisp are currently developing VPL’s retrieval model, which is at the core of the proposed fifth task. This model will combine VPL’s existing forward models of planetary environments developed by Crisp, Robinson, Misra and Meadows, with instrument models for direct imaging and transit transmission missions, obtained or developed by Meadows and Deming, and the sophisticated retrieval models developed by Line. The resulting model will also use all known information about the planet and planetary system to more robustly determine the family of environmental conditions that best fit the observed planetary spectrum.

Education and Public Outreach

This year we completed the prototype for our first Science on a Sphere show in collaboration between the VPL science team and the Pacific Science Center in Seattle. This show uses the modern Earth as a setting for understanding Signatures of Habitability and Life, and Harnett, Robinson and Meadows served as the principal liaisons with the PSC for this EPO product. Barnes and Shields delivered Science Café experiences, presenting and mingling with the public in bars and cafes.

Work was initiated on our database for educational materials for teaching undergraduates astrobiology, led by Harnett. The UW VPL contingent hosted the Lakewood High Astrobiology class again this year for a field trip to UW to learn about astrobiology research. Several of our scientists again gave public lectures this year, and we also hosted public lectures at UW on the latest Kepler results and the origin of life. VPL research was featured in numerous popular science magazines, newspapers and television documentaries, including a feature article in the Atlanitic, which highlighted work by Meadows, Barnes and Kiang.

Publications

Armstrong, J. C.; Barnes, R.; Domagal-Goldman, S., Breiner, J.; Quinn, T. R.; Meadows, V. S., (2014) Effects of Extreme Obliquity Variations on the Habitability of Exoplanets, Astrobiology, vol. 14, issue 4, pp. 277-291. doi: 10.1089/ast.2013.1129

Arney, G., Meadows, V., Crisp, D., Schmidt, S. J., Bailey, J. and Robinson, T. (2014), Spatially resolved measurements of H2O, HCl, CO, OCS, SO2, cloud opacity, and acid concentration in the Venus near-infrared spectral windows, JGR: Planets 119(8) 1860–1891. doi: 10.1002/2014JE004662

Bolmont, Emeline, Raymond, Sean N., von Paris, Philip, Selsis, Franck, Hersant, Franck, Quintana, Elisa V., and Barclay, Thomas, 2014, Formation, Tidal Evolution, and Habitability of the Kepler-186 System, The Astrophysical Journal, 793. doi: 10.1088/0004-637X/793/1/3

Byrne, B. and Goldblatt, C. (2014) Radiative forcings for 28 potential Archean greenhouse gases, Clim. Past, 10, 1779-1801. doi: 10.5194/cp-10-1779-2014

Cady Lawrence P., Brack André, Bueno Prieto Jorge E., Cockell Charles, Horneck Gerda, Kasting James F., Lineweaver Charles H., Raulin François, Schopf J. William, Sleep Norman, von Bloh Werner, Westall Frances, Deamer David, Lehman Niles, and Pérez-Mercader Juan. Astrobiology. August 2014, 14(8): 629-644. doi:10.1089/ast.2014.1405.

Catling DC. 2014. Mars Atmosphere: History and Surface Interactions. In Encyclopedia of the Solar System (2nd Ed.), ed. T Spohn, TV Johnson, D Breuer, pp. 343-57. New York: Elsevier

Charnay, B.; Forget, F.; Wordsworth, R.; Leconte, J.;Millour, E.; Codron, F.; Spiga, A., (2013) Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM, JGR – Atmospheres, 118 (18), 10,414-10,431. doi: 10.1002/jgrd.50808

Claire, M.W., Kasting, J.F., Domagal-Goldman, S.D.,Stüeken, E. E., BUICK, R. & Meadows, V.S. (2014) Modeling the signature of sulfur mass-independent fractionation produced in the Archean atmosphere. Geochimica et Cosmochimica Acta,141, 365-380. doi: 10.1016/j.gca.2014.06.032

Conrad, P. G. (2014). Scratching the surface of martian habitability. Science, 346(6215), 1288-1289. doi: 10.1126/science.1259943

Davenport, J.R.A., and 14 colleagues (2014) Kepler Flares. II. The Temporal Morphology of White-light Flares on GJ 1243. The Astrophysical Journal 797, 122. doi: 10.1088/0004-637X/797/2/122

Deck, K. M., E. Agol, et al. (2014). “TTVFast: An Efficient and Accurate Code for Transit Timing Inversion Problems.” The Astrophysical Journal 787(2): 132. doi: 10.1088/0004-637X/787/2/132

Deitrick, R. et al. (2015) The Three-dimensional Architecture of the υ Andromedae Planetary System. Astrophys. J., 798, 46. doi: 10.1088/0004-637X/798/1/46

Izidoro, Andre;, Morbidelli, Alessandro, and Raymond, Sean. N., 2014, Terrestrial Planet Formation in the Presence of Migrating Super-Earths, The Astrophysical Journal, 794. doi: 10.1088/0004-637X/794/1/11

Deming, D., A. Wilkins, et al. (2013). “Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope.” The Astrophysical Journal 774(2): 95. doi: 10.1088/0004-637X/774/2/95

Dobbs-Dixon, I. and E. Agol (2013). “Three-dimensional radiative-hydrodynamical simulations of the highly irradiated short-period exoplanet HD 189733b.” Monthly Notices of the Royal Astronomical Society 435(4): 3159-3168. doi: 10.1093/mnras/stt1509

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