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2013 Annual Science Report

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

Executive Summary

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 – which may reside 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, and the planned James Webb Space Telescope (JWST) will probe the atmospheric composition of super-Earths. In the longer term, we anticipate spaceborne telescopes, such as NASA’s Terrestrial Planet Finder concept, 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, JWST and the TPF. 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 data that has no direct spatial resolution and may be quite limited in other ways.

The team required to develop and run these models is necessarily 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: Earth as an Extrasolar Planet, Early Earth and Mars, The Habitable Planet, and The Living Planet and The Observer.

Earth as an Extrasolar Planet.

In this task we use observations of our home planet 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 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, 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. This year we also added N2-N2 collisionally-induced absorption (CIA) to the VPL Earth model. The updated model has been used to demonstrate that N2-N2 CIA is required to fit the spectral region near 4.1um 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. 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.

Early Earth and Mars:

In this task we work to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. In our early Mars studies on surface environments, David Catling and collaborators completed work on understanding the origin and abundance of carbonates on Mars (Niles et al., 2013), and the environmental implications of clay minerals (Ehlmann et al., 2013). Photochemical modeling of the formation of salts from the oxidation of gases in the atmosphere of early Mars was also performed (Smith et al., 2013). Ongoing efforts include work on the possibility of oceans on Mars and the formation of salts in the soil. Kasting, Ramirez, Kopparapu, Robinson, Freedman and Zugger collaborated on studies of the warming of Early Mars by using CO2 and H2 (Ramirez et al., 2013). They modeled the origin, abundance and lifetime of CO2 in the early Martian environment, and investigated the possible abundance of H2 on early Mars, since H2 can act as a secondary greenhouse gas via collision-induced absorption.

For studies of the early Earth’s environment, we made progress in the areas of Earth’s geochemical history and its implications for life, climate evolution, and the evolution of atmospheric oxygen levels. Studying the Earth’s geochemistry, Buick and colleagues discovered a soluble and reactive phosphorus species (phosphite) in early Archean carbonates (Pasek et al., 2013) that was likely of meteoritic origin. Phosphite’s delivery during the Late Heavy Bombardment may have driven chemistry to form cell membranes and make nucleotides, a precursor to the RNA World hypothesis for the origin of life. Team members also explore the effectiveness of H2 as a greenhouse gas on the early Earth (Wordsworth and Pierrehumbert, 2013) with commentary on the paper from team member Kasting (Kasting 2013c). In understanding the history of O2 in the Earth’s atmosphere we outlined possible causes for the first rise of oxygen at ~2.5 Ga. (Kasting, 2013a, 2013b), and a framework for understanding both the first and second rise of oxygen was provided (Catling, 2013a, 2013b). Zahnle reviewed the importance of hydrogen escape for the oxygenation of the Earth’s atmosphere (Zahnle et al., 2013). Catling and Krissansen-Totton are currently re-examining 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. Domagal-Goldman and Robinson are working to generalize our 1-D atmospheric chemistry and climate models and automate changes in boundary conditions. This will allow us to rapidly run the models for a variety of conditions, to quantify the effects of specific metabolism on the atmospheric evolution of the Earth.

Habitable Planets

VPL’s core research is in the area of planetary habitability, and this task explores the effect of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy. This year, exploration of these factors that affect habitability included work on planet formation, internal properties, atmospheric evolution, orbital dynamics, and the limits of the habitable zone. Raymond provided in-depth reviews of planet formation for the periodic international meeting Protostars and Planets 6. (Raymond et al., 2013a; Davies et al., 2013). He and his students also considered numerous accretional phenomena such as a final accumulation of volatiles (Raymond et al., 2013b), the distribution of dust left over from formation (Bosnor et al., 2013), and planetary migration (Pierens, et al., 2013). Additionally, Timpe, Barnes, Kopparapu, Raymond and colleagues demonstrated that the observed orbital dynamics of multi-planet systems naturally arises from the ejection of fully formed planets at the very end of planet formation (Timpe et al., 2013).

VPL also modeled the environments at the planetary system-galaxy interface. These regions are far from the host star and hence features can be more easily detected as the starlight can be separated. We showed that systems that eject planets can have cometary reservoirs that are closer in than that in the Solar System (Raymond & Armitage, 2013). We also showed that distant stellar companions can disrupt planetary systems on long timescales after close stellar passages perturb the binary star’s orbit (Kaib et al., 2013). Ongoing work by Deitrick and Barnes explored the stability of planets on mutually-inclined orbits (Dietrick et al., in prep).

We also explored the direct effect of the star’s gravity on planetary habitability for a number of different scenarios. In Barnes et al (2013), Barnes, Goldblatt, Meadows, Kasting and colleagues showed that tidal heating can trigger a runaway greenhouse on planets in the habitable zones of M dwarf stars, and especially for those that have planetary companions that can maintain the planet’s orbital eccentricity. A “tidal greenhouse” can be maintained for upwards of 1 Gyr prior to circularization and hence planets can evolve into the habitable zone after losing all their primordial water content. Barnes & Heller (2013) showed that a tidal-heating generated greenhouse is also possible on planets in the habitable zones of white dwarfs and brown dwarfs. For these planets, eccentricities as small as 10-6 can be sufficient to trigger the runaway greenhouse, severely limiting the chances that they may support life. Heller & Barnes (2013) showed that exomoons are also in danger of a tidal greenhouse. Luger and Barnes explored the evolution of atmospheric loss for small gas-dominated planets under the combined effects of stellar radiation and orbital evolution (Luger et al., in prep). Armstrong, Barnes, Domagal-Goldman and Meadows worked together on understanding the coupled effects of obliquity and orbital evolution on planetary climate and surface ice coverage (Armstrong et al., submitted).

VPL also looks at internal planetary processes and how these may impact habitability, and in turn be impacted by life. This year Driscoll wrote a numerical program to model the internal evolution of the mantle and core of a rocky terrestrial planet. This model includes radioactive heat generation, heat loss due to mantle melting, inner core growth, and magnetic field generation in the outer core. Work is underway to add tidal heating to explore the effect of this process on outgassing and habitability. Sleep et al., (2013, submitted) discuss 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 and colleagues also looked at the partitioning of radionuclides in the Earth’s mantle, crust and hydrosphere as the result of biologically mediated geochemical evolution (Sleep et al., 2013). Bolton is working on code efficiency improvements for quantifying CO2 draw-down from the atmosphere by weathering of soils derived from idealized granite and basalt rock types. This modeling is being done to find the influence of atmospheric composition, temperature, and infiltration rates on CO2 consumption.

We also continued to refine the climatic limits of planetary habitability and the habitable zone. Kopparapu, Ramirez, Robinson, Kasting and collaborators updated and validated a 1-D radiative-convective climate model with new water and carbon-dioxide absorption coefficients to recalculate habitable zone limits around stars of different spectral types (Kopparapu et al., 2013a). Using these results Kopparapu and colleagues estimated the occurrence rate of Earth-size planets around Kepler M-stars to be between 40%-50% (Kopparapu et al., 2013b). Shields, Meadows, Bitz, Pierrehumbert and collaborators studied the near-IR ice-albedo effect on planets in the middle of the habitable zone of M-stars, and found that planets orbiting M stars are more stable against sustained low-latitude glaciation than planets orbiting F or G stars (Shields et al., 2013). Goldblatt, Robinson, Zahnle and Crisp also used updated spectral databases to calculate a new limit for a planet undergoing a runaway greenhouse, and showed that the Earth is closer to this limit than previously thought (Goldblatt et al., 2013).

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the biosphere with its environment, and the limits of life. Kiang, Mielke and colleagues continued testing the photon use efficiency of the bacterium Acarychloris marina, which photosynthesizes at longer than usual wavelengths, serving as an example of an organism adapted to the longer wavelength light environments that might be found around M dwarf stars. They used models to identify the likely trap wavelengths (Mielke et al., 2013) and further work being prepared for publication has confirmed dips in efficiency between the trap wavelengths. Blankenship and Kiang conducted field expeditions to gather further long-wavelength-using cyanobacteria, which are currently being cultured. Hoehler and Parenteau are initiating studies of the impact of anoxygenic photosynthesis on its environment, Parenteau conducted field reconnaissance to Yellowstone to locate anoxygenic mats that will subsequently be analyzed for biogenic gases and volatile organics. A new anaerobic chamber has been purchased and installed at NASA Ames and small vessels are being developed to allow testing of the mats under different environmental parameters such as starting atmosphere and radiation environment. The Biological Pigments Database of the VPL Spectral Library was also officially launched this year and is intended for community use: ( Seifert continues to study the biology of Cuatro Cienegas, in collaboration with the NAI ASU team and astrobiologists in Mexico. They conducted two field trips last year with several papers in progress. Black and Keller performed laboratory studies that suggest that the joining of RNA and fatty acids to form the first cells may have been assisted by a natural chemical affinity between these components (Black et al., 2013).

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 retrieve environmental properties of Solar System planets and exoplanets, and generate improved retrieval algorithms for exoplanet data.

This year Misra, with Crisp and Meadows, completed modification of the VPL’s line-by-line radiative transfer model (SMART) to generate transit transmission spectra (Misra et al., in prep). The model includes 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, which provide a proxy for Earth seen in transmission. We find that the 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., in prep). 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 atmospheres and biosignatures (Misra et al., 2013, in press).

In other biosignature research, Domagal-Goldman, Segura, Claire and Meadows completed a study into the generation of abiotic false positives for ozone for early Earth-type planets in orbit around M dwarf stars (Domagal-Goldman, in prep). Segura and colleagues calculated a possible maximum methane production for the abiotic process of serpentinization and for a planet of Earth-like composition (Guzmán-Marmolejo et al., 2013). Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to explore the detectability of non-photosynthetic pigments, especially for halophiles, in an Earth-like planet’s disk-averaged spectra (Schwieterman et al., in prep). Catling, with Robinson and Krissanson-Totton initiated a study of thermodynamic disequilibrium in planetary atmospheres. A first step in understanding physical entropy in planetary atmospheres resulted in a paper that improved our generalized understanding of the origin and maintenance of atmospheric vertical temperature and pressure structures (Robinson and Catling, 2013). This understanding can also be used as an initial framework to improve retrieval of exoplanetary atmospheric properties and structure.

In planetary observation research, observations of Venus, an analog for a hot, haze covered exoplanet, were obtained by Arney and Meadows at the Apache Point Observatory. These observations were used to produce the first simultaneous spatially-resolved maps of retrieved H2O, HCl, CO, OCS, and SO2 abundances in the Venusian lower atmosphere, revealing surprising hemispherical dichotomies that are still unexplained, but that may be due to cloud processes (Arney et al., in prep). 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., 2013, in press). Bailey, Agol, Barnes, Raymond and collaborators continued their work searching for extrasolar terrestrial planets using radial velocity surveys and Kepler data and discovered some of the most habitable planets to date, as well as a unique system with definitely 3, and possibly 5 planets in the habitable zone of its parent M dwarf star (Agol et al. 2013; Anglada-Escude et al., 2013). Deming, Agol, Dobbs-Dixon and Wilkins used HST to observe transiting giant exoplanets with a new spatial scanning mode, greatly improving the observational sensitivity (Deming et al., 2013). They detected water vapor in several of these planets and are now extending the new technique to smaller planets. Deming and Sheets pioneered a new statistical technique to study the atmospheres of super-Earth planets discovered by Kepler. 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).

Education and Public Outreach

We completed our interactives “Extreme Planet Makeover” where users get to change parameters to control the appearance and habitability of their world, and a second interactive Eyes on Exoplanets 3D to allow users to visualize the position of known planetary systems on the sky, and learn about their potential habitability. Looking forward to outreach products under the new award, we have organized two Science Café experiences where VPL scientists (Rory Barnes and Aomawa Shields) present to and mingle with the public in bars and cafes. The theme for our two Science on a Sphere shows at the Pacific Science Center in Seattle have been determined, they are The Earth Through Time and Signatures of Habitability and Life, and work is underway to develop these shows in collaboration with VPL scientists. 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, Lucianne Walkowicz pioneered Science Train, an interaction between scientists and the public in the New York subway, and VPL research was featured in numerous popular science magazines, newspapers and television documentaries.

The limits of the habitable zone shown as a function of incident stellar radiation. The limits were recalculated using improved molecular line lists, and extended down to include smaller and cooler stars. Planets within our own Solar System are shown for reference, along with recently discovered, potentially habitable planets (Kopparapu et al., 2013)
Oxygen and Oxygen Dimer Features as Biosignatures in Earth-Like Atmospheres
Transmission spectra of an Earth-like atmosphere with different total atmospheric pressures. (a) Difference in flux from the stellar flux. (b) Percent of stellar flux absorbed by the atmosphere. Features from both the oxygen monomer (O2) and the oxygen dimer (O4) can be seen. The 1.06 μm dimer feature is strong only in the spectra corresponding to atmospheric pressures greater than ∼0.5 bar. The spectra for atmospheres with pressure ≥1 bar are nearly identical except for an offset because there is a fundamental limit on which heights in an atmosphere can be probed using transmission spectroscopy. The strong dimer bands may be the most detectable biosignature in Earth-like planetary atmospheres (Misra et al., 2013).

The location of the tropopause depends on two key parameters---the strength at which sunlight is attenuated in a planet's stratosphere (shown as contours labeled k_strato), and the ratio of the energy flux budgets of the upper atmosphere to the lower atmosphere (shown as the horizontal axis).  Both the upper and lower atmosphere absorb solar energy, and the lower atmosphere may have an internal energy source.  Values for solar system worlds are indicated.  Note that the tropopause pressure is near 0.1 bar over a range of parameter space that exceeds what is seen in the solar system. This relationship can be used to estimate the atmospheric structure for extrasolar planets.
Schematic of the orbits and habitable zone for the Gl 667C planetary system based on radial velocity observations. The green annulus is the nominal habitable zone, and the red and blue annuli are plausible extensions for planets with certain properties. Orbital stability requires all the potentially habitable planets to have masses less than 10 Earth-masses, suggesting that Gl 667C hosts at least 3, and maybe 5, potentially habitable planets. Mercury's orbit is shown for scale.