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2010 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2009 – AUG 2010

Executive Summary

The Virtual Planetary Laboratory

The Virtual Planetary Laboratory is an interdisciplinary research effort focused 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, and the steps towards that are 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 discovery earlier this year of the planet Gl 581g may be our very first example of a likely rocky planet (~ 3 Earth masses) residing squarely in the circumstellar habitable zone, and potentially able to support life. In the next few years, the successful Kepler Mission will improve our understanding of how common terrestrial planets are in the Galaxy, and the soon-to-be-launched James Webb Space Telescope (JWST) will probe the atmospheric composition of super-Earths. In the longer term, we anticipate the development and launch of spaceborne telescopes that can directly image extrasolar planets as small as the Earth, and obtain multiwavelength photometry and spectroscopy.

The VPL provides a scientific foundation for interpretation of data from extrasolar terrestrial planet detection and characterization missions such as Kepler, JWST and the Terrestrial Planet Finder. 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 globally-detectable “biosignature”. 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, 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 Planets, Early Earth and Mars, The Habitable Planet, and The Living Planet. Under each main task, Italicized text refers to a Project Report, with more information.

Solar System Planets

In this task, we use remote-sensing observations of planets in our own Solar System and astronomical observations of extrasolar planets to explore the detectability of planetary characteristics. The Earth still serves as the only known example of a habitable planet, and Venus and Mars show us the end states of alternative terrestrial evolutionary paths.

In the Earth as an Extrasolar Planet project we study the global appearance of the Earth through the course of a day and over seasons, to better understand how to recognize the global imprint of the Earth’s habitability and life. The principal model in this effort is the VPL’s 3D Spectral Earth model, which was further validated this year against phase-dependent EPOXI and ground-based Earthshine measurements of the Earth’s brightness. The model was used it to simulate the Earth’s appearance through an entire orbit, as seen by a distant extrasolar observer (Robinson et al., 2010), and used to disentangle the phase-dependent effects of realistic clouds and ocean reflectivity. We showed that ocean glint is distinguishable from an isotropically scattering surface even in the presence of Earth-like cloud cover. Simulations were also run to look at the detectability of this phenomenon for planet detection and characterization missions (Robinson et al., 2010).

This plot shows the increase in reflectivity near crescent phases for an Earth-like planet with an ocean as it orbits around a distant star. The bell-shaped curve shows the Earth’s brightness as a function of phase, modeled by the VPL 3-D spectral Earth model over the course of a year. Full phase is at 180 degrees, crescent phases are closer to 0 and 360 degrees phase. The U-shaped curve shows the model predictions for the Earth’s reflectivity, normalized to that expected from a surface that scatters isotropically (a Lambert sphere). If Earth behaved like a Lambert sphere, then the ratio curve would be a straight line at about 0.3 albedo, at all phases. Instead, our model (and the Earthshine and EPOXI data plotted over it) show that the apparent reflectivity of the Earth deviates strongly from a Lambertian sphere at phases shortward of 90 degrees and longward of 270 degrees. This is due to the addition of both forward scattering from clouds, which dominates this behavior, and specular reflectance from the Earth’s ocean. The difference between the black and grey U-curves shows the effect from ocean, as distinct from clouds, and this difference is plotted in the window below the curves. Our modeling suggests that the presence of a reflecting ocean can be disentangled from the forward-scattering effect of clouds, and may produce an enhancement in the Earth’s reflectivity of as much as 50% at some crescent phases.

For the Astronomical Observations of Planetary Atmospheres and Exoplanets project we performed and/or analyzed ground-based and Spitzer observations of Venus and Titan, and improved models for hot Jupiters and terrestrial exoplanets. VPL team members participated in the Anglo-Australian Telescope Rocky Planet Search, a high-precision radial velocity search, which put constraints on the fraction of stars with super-Earth planets (O’Toole et al., 2009). We also completed a review chapter on terrestrial planet atmospheres (Meadows and Seager, 2010) for the upcoming Exoplanets book.

The Early Mars and Early Earth

In the area of Understanding the Early Mars Environment we completed our interdisciplinary work on climate and photochemical modeling of the effects of SO2 on the early Mars environment. We were able to show that large CO2 abundances, combined with SO2 abundances in excess of 10ppm, could theoretically warm Mars above the freezing point. However, photochemical modeling of a representative 3 bar CO2-SO2-H2O atmosphere revealed that SO2 abundances of just a few ppm would result in dense sulfuric acid aerosol formation. The corresponding increase in planetary albedo outweighs the SO2 greenhouse effect (Figure 2) We conclude that if early Mars was warm it must be due to some other characteristic, other than SO2 (Tian et al., 2010). Additional work on early Mars this year included using energy balance models to simulate the effects of surface temperature on putative Martian rainout. This can serve as a stabilizing feedback to maintain “cool” temperatures at the boundary between evaporation-driven and sublimation-driven hydrological cycles (Breiner et al., 2009) We also provided the first explanation of gas-phase atmospheric production of perchlorate over Earth’s Atacama Desert (Catling et al. 2010) as a first step in understanding perchlorate formation on Mars.

Panel 'A’-aerosol extinction optical depth at 5500 Å. The solid curve is for sulfate aerosols under normal (terrestrial) rainout conditions. The diamond symbols are for S8 aerosols under normal rainout condition. The crosses are for sulfate aerosols for rainout reduced by a factor of 100. The star is for sulfate aerosols for rainout reduced by a factor of 1000. Panel 'B’-planetary albedo. Panel 'C’-surface temperature. Panel 'D’-combined (SO2+H2S) fluxes, with the same symbols as those in panel A. These calculations are for a 3-bar atmosphere with S/S0 = 0.75. Dotted curves in panels B and C show similar calculations in which sulfate aerosols are neglected

For Understanding the Earth Earth we made further progress in developing a techniques to put geological constraints on the Archean atmospheric pressure. These involve measuring and modeling gas bubble sizes in sea level basalt flows, and from preserved raindrop impact craters (Som and Buick, in progress). Goldblatt et al., (2009) discussed the possibility that there was more nitrogen in the Archean atmosphere than today, which would have enhanced greenhouse warming via pressure broadening. Goldblatt & Zahnle (2010) have explored how clouds could affect the Faint Young Sun paradox. VPL team member Sleep (Rosing et al. 2010) has suggested a new constraint on pCO2 in the Archean, which would make this paradox harder to solve. Tian and Kasting (manuscript in review) are suggesting that NH3 may have been relevant as an Archean greenhouse gas, contrary to earlier work. Catling reviewed the importance of the oxygen cycle in determining planetary redox state (Catling, 2010). Work in progress by Haqq-Misra uses a passive tracer in a general circulation model (GCM) to constrain the transport of photolytically generated oxygen down to the planetary surface. Additionally we enhanced our atmospheric chemistry model to include isotopic sulfur species and sub-Angstrom wavelength resolution The model predicts magnitudes of sulfur fractionation that are at odds with other recent papers, and show that the situation is more complex that originally described (Claire & Kasting, 2010; Postdoctoral Fellow Report: Mark Claire).

The Stromatolites In the Desert: Analogs to Other Worlds project is a field component (led by J. Siefert) that complements our Archean modeling research by studying freshwater stromatolites, an ancient form of life, in phosphorous poor environments found at Cuatro Cienegas, Mexico. A successful field trip was executed this year with in situ studies of organism calcification and the collection of samples. The samples are being naturalized and prepared for experiments under higher atmospheric CO2 concentrations.

The Habitable Planet

In this task we explore the many planetary and planetary system processes and characteristics required to initiate and maintain planetary habitability.

The Formation of Terrestrial Planets project uses detailed numerical simulations to model formation of rocky worlds. This year, we published models of the rate of planet growth from 1km planetesimals, showing that it proceeds quickly, and that terrestrial exoplanets, which could provide sites for life, may be common (Barnes et al., 2009). We performed simulations to explore how the shape of outer belts of minor planets in a planetary system are sculpted by the architecture of interior planets (Raymond et al 2010).

The Delivery of Volatiles project encompases modeling of the dynamical delivery of volatiles during the planet formation process, and the fate of carbon and volatiles on atmospheric entry. Results this year include publication of a new model showing polycyclic aromatic hydrocarbons (PAHs) to be the dominant form of carbon included in terrestrial planet formation (Kress et al., 2009), and a laboratory investigation of meteor/atmosphere reactions that shows that complex organics containing aromatic bonds can undergo further reaction in the upper atmosphere.
Significant progress was made in Dynamical Effects on Planetary Habitability this year. We published work demonstrating that comets are unlikely to have produced more than one mass extinction event on Earth (Kaib & Quinn 2010). We also published a new empirical description of where in a planetary system orbits will be stable, which allowed us to calculate the fraction of known habitable zones that can support a terrestrial planet (Kopparapu & Barnes 2010). Team members were instrumental in the discovery that two known exoplanets have orbital planes inclined by 30 degrees to each other (McArthur, Benedict, Barnes et al 2010). Work is ongoing to explain this phenomenon. Additionally, progress was made in estimating the tidal heating of known exoplanets, coupling tidal evolution to atmospheric mass loss, and contributions to radial velocity detection of planets. Other ongoing work includes modeling of systems with planets on orbits with high inclination; coupling orbital oscillations to obliquity and climate; modeling of planetary obliquity under the influence of tides; and the development of a coupled interior-tidal model.

The Planetary Surface and Interior Models and Super-Earths project is concerned with planetary geological processes that may affect habitability. We continued to develop and use a reactive transport model to simulate weathering at planetary surfaces, expanding the CO2 reaction set and adding soil-atmosphere volatile exchange for 8 gases (Bolton, work in progress). We have made significant progress towards an integrated model to simulate the thermal evolution of planets, including super-Earths, that are subjected to tidal heating (Rye and Barnes, work in progress). In other work we have found that Earth’s cratons are near the transition from chemical lid to stagnant lid, suggesting that chemical lids may easily develop on super-earths. We have also found that life may significantly affect the geology of a planet, as well as the atmosphere, with work that suggests that life greatly increases sulfur in arc volcanics (Sleep et al., 2010).

In the Super-Earth Atmospheres project we are developing the capabilities to model the atmospheres and spectra of super-Earths using atmospheric escape, climate, chemistry and radiative transfer models. Work this year included atmospheric escape modeling for super-Earth planets in M star habitable zones, indicating that these planets could lose a significant fraction of their volatile inventory if not dominated by CO2 (Tian et al., in progress). Work also continued on a generalized climate model for terrestrial planets and modifications to our radiative transfer model to allow us to simulate transit transmission spectra.

In the Stellar Effects on Planetary Habitability project we look at the radiative effects of the parent star on planetary habitability. We worked this year to enhance a model of the evolution of Solar flux over the age of the Solar System as input to models of the evolution of planetary atmospheres (Claire, 2010). We also completed and published our work on the effect of stellar flare UV and particles on planetary habitability for Earth-like M dwarf planets (Segura et al., 2010). This photochemical-climate modeling shows that the incoming UV has very little effect on ozone (Figure 3), but particle driven chemistry can severely deplete a planet’s protective ozone layer. However, the corresponding enhanced UV fluxes at the planet’s surface only exceed those received on Earth for about 100 seconds, minimizing flare damage. We are moving into the next phase of this research by modeling the effect of flares on CO2-rich planetary atmospheres.

Time evolution of the ozone column depth compared to the initial steady state before, during, and after a big UV flare event. The lines show simulations made with different time steps after the flare ended. Times used for each run are listed
in the figure (Segura et al. 2010).

Time evolution of the ozone column depth compared to the initial steady state. These results show the combined influence of the flare’s incident UV radiation and a proton event at the peak of the flare. Line with diamonds: O3 fraction
change for a simultaneous UV and proton flux peak. Line with crosses: O3 fraction change for a proton event with a maximum delayed by 889 s with respect to the UV flare peak. Vertical dotted lines indicate the time for the peak of the UV
flare and the end of the UV flare (Segura et al. 2010).

Flux received at the top of the atmosphere of a planet on the AD Leo habitable zone (0.16 AU). The dotted line is the solar flux received by Earth. Times listed on the right lower corner of each panel correspond to the flare fluxes plotted on that
panel. AD Leo spectrum during quiescence is always shown in a black continuous line to be used as a reference (Segura et al. 2010).

The Living Planet

The VPL Life Modules task encompasses development of 3-D ecosystem models to look at biosphere/planet interactions. This year we collaboratively incorporated the NASA Ocean Biogeochemistry Model (NOBM) of VPL Co-I Gregg into the ocean model of the GISS GCM, providing the capability to model a full carbon cycle system. We are continuing evaluation and testing of the coupled carbon dynamics. The NOBM, coupled to an ocean general circulation model was used to explore conditions for early life on Earth. Simulations were run with cyanobacteria as the only photosynthesizers (modern diatoms, coccolithophores and chlorophytes were removed) showing that the early Earth would have had 19% less productivity and 35% more nitrate due to slower growth of the cyanobacteria. In addition, our Ent ecosystem model’s canopy radiative transfer scheme for mixed canopies and clumped foliage was published (Kiang et al. 2010)

The Thermodynamic efficiency of electron-transfer reactions in the Chlorophyll d-containing cyanobacterium, Acharyochloris marina and Postdoctoral Report: Steve Mielke project reports describe a laboratory-based project to explore the efficiency of photosynthesis at the extreme red end of the spectrum, using chlorophyll d in the bacterium A. marina, as a means of understanding whether photosynthesis might be possible on planets around M dwarfs, or on haze-covered planets. Recent results indicate that chlorophyll d is just as efficient as chlorophyll a, showing that photosynthesis can procede efficiently at far red wavelengths, and implying that we have not yet found the wavelength limit for oxygenic photosynthesis.

This year in the Detectability of Biosignatures task we continued our research into remote-sensing biosignatures for metabolisms other than oxygenic photosynthesis. This year we used photochemical and radiative transfer modeling to explore the potential of various sulfur-bearing biogenic gases to act as biosignatures for anoxic planets similar to the early Earth, and in orbit around stars of different spectral type (Domagal-Goldman, et al., submitted). For this type of biosphere, we find that the methyl mercaptan and ethane signatures are most likely to build up to detectable levels on planets orbiting M dwarf stars. (Figure 4). We also started research to explore the potential for abiotic false positives from ozone, for early Earth like planets orbiting cooler stars. In parallel, we have obtained and are now running simulators for Terrestrial Planet Finder Coronograph (S. Heap and D. Lindler) and Interferometer (T. Velusamy) mission concepts. We are using these simulators to explore the detectability of signs of habitability and biosignatures from oxygenic photosynthetic, as well as the other metabolisms described here (Evans et al., 2010).

<span class="caps">MIR</span> Spectra of Sulfur-Based Biosignatures

Mid-infrared spectra showing the position and relative strengths of absorption bands of biogenic gases produced by sulfur-based metabolisms. The three panels show the results for a planet orbiting the Sun, an active M dwarf and an M dwarf with no stellar activity. The difference colored spectra in each panel correspond to different levels of planetary surface flux of biologically generated sulfur gases or methane. The most detectable feature of the sulfur metabolism is ethane, which can be produced both by methane photolysis, and the release of methyl groups from biologically generated sulfur gases. Quantifying the amount of methane in the planet’s atmosphere may help to discriminate between these two sources.

As part of the on-going VPL Community Tools, we have developed a comprehensive database of molecular, stellar, pigment, and mineral spectra useful in developing extrasolar planet climate models and interpreting the results of NASAs current and future planet-finding missions. This year work focused on improving our molecular line lists for gases such as methane and ethane (lead by Brown), and significantly expanding our photosynthetic pigment database (lead by Kiang).

EPO and Education Efforts

This year in EPO we made significant progress on the Extreme Planet Makeover Interactive, which allows users to change the appearance and habitability of a planet they create by modifying factors such as star-planet distance, and planetary age and size. Initial development of our Night Sky Network Astrobiology Outreach Toolkit has been completed, and the kit is currently in test phase, with an anticipated rollout date to all qualified member astronomy clubs in the summer of 2011. We have also started a partnership with Lakewood High School in Washington state to pioneer a high school level Astrobiology course. Additionally VPL scientists taught astrobiology courses to nonscience majors, and engaged members of the public via public talks, museum exhibits and media interviews.