2009 Annual Science Report
VPL at University of Washington Reporting | JUL 2008 – AUG 2009
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 this planet?
This question is relevant to Astrobiology Roadmap Goals 1 and 7, and specifically 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). This research provides a scientific foundation for interpretation of Kepler results, and for NASA’s Terrestrial Planet Finder mission concept.
While this science focus was developed prior to the detection of extrasolar terrestrial planets, subsequent observations have brought us much closer to this goal. Radial velocity surveys, initially only sensitive to planets larger than Jupiter, are now able to detect “superEarths”, small, presumably rocky, planets up to 10 times more massive than the Earth. At least one of these superEarths. Gl581d, appears to reside in the circumstellar habitable zone, that region around the star where the star’s radiation allows a terrestrial planet with an atmosphere to support liquid water on its surface. The recent successful launch of NASA’s Kepler Mission promises to improve our understanding of how common terrestrial planets are in the Galaxy. Following on from Kepler, NASA plans to develop mission concepts for spaceborne telescopes capable of directly imaging extrasolar terrestrial planets and studying them using multiwavelength photometry and spectroscopy.
The terrestrial planets that we discover in the coming decades will likely display a much higher degree of diversity than those found in our Solar System. They will also appear “unresolved” with no direct spatial resolution, providing only disk-integrated information from which we must disentangle planetary environmental characteristics and signs of life.
To provide a scientific foundation for these extrasolar terrestrial planet detection and characterization missions the Virtual Planetary Laboratory uses information from our own planet through its stages of evolution, and planets in our Solar System, to validate more comprehensive models of terrestrial planets that can simulate environments unlike those currently observable. These model environments and supporting field work are then used to understand stellar effects on planetary habitability, and to understand which metabolically produced gases can survive in a planetary atmosphere and produce a “biosignature”, a global sign of life that can be detected even with unresolved, globally-averaged observations.
The team required to develop and run these models is necessarily highly interdisciplinary. Our research encompasses single disciplinary 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 this year can be divided into three major efforts: understanding the environments and observational characteristics of 1. the Earth and Solar System planets; 2. the early Earth and early Mars, and 3. extrasolar planets. Subheadings in this section refer to the corresponding Project Report, where more information is available.
The Earth and Solar System Planets
The Earth as an Extrasolar Planet : The Earth serves as the only known example of a habitable planet. By studying the global appearance of the Earth through the course of a day and over seasons, we can better understand how to recognize the global imprint of the Earth’s habitability and life.
The VPL’s Earth model is a 3-dimensional spectroscopic model of the Earth that uses input data from NASA Earth Observing satellites and a line-by-line, multiple-scattering radiative transfer code to generate synthetic lightcurves and spectra for the Earth at a user specified wavelength range, viewing geometry and spatial resolution. This year, we validated this model against observations of the Earth taken by NASA’s EPOXI mission, greatly improving the model (Robinson et al., 2009). The resulting model reproduces the Earth’s lightcurve (temporal variability linked to spatial inhomogeneity) to within 3%, the absolute brightness of the Earth to within 5%, and can simultaneously provide a good fit to the Earth’s spectrum from 0.3-5.0um. The validated model is currently being used to explore the detectability of ocean glint and surface temperature in disk-averaged visible and mid-infrared (MIR) data.
We also used EPOXI data to test analysis techniques for retrieval of spatial information from time-resolved photometry (Cowan et al., 2009)
Figure 1. (Figure 2 from Earth as Extrasolar Planet Report – Blue and white spatial map of Earth)
Astronomical Observations of Planetary Atmospheres and Exoplanets: In this project we ran a systematic comparison of available water vapor line lists, identifying the BT2 line list as the best one to use for Venus spectral modeling (Bailey, 2009). We also took and analyzed ground-based observations of Venus’s lower atmosphere, and measured the temperature of O2 1-delta emission in the Venus mesosphere (Bailey et al., 2009) for comparison with results obtained by the ESA Venus Express spacecraft. We also obtained and analyzed the first detailed MIR spectrum of extrasolar Jovian HD189733b, showing the first definitive detection of water vapor via its band shape at 6.3um (Grillmair et al., 2009).
Limits of Habitability: In this project we visited field sites in the Mojave Desert and Svalbard to explore the range of environmental parameters compatible with habitability (Conrad et al., 2009). We also collected reflectance spectra on 1m scales for comparison with orbital or sub-orbital reflectance data to determine the detectability of habitable environments on different spatial scales.
Team member Yung collaborated in efforts to determine the potential habitability of the icy moon Enceladus (Parkinson et al., 2009), and to understand the role of atmospheric pressure in regulating terrestrial planet climates (Li et al., 2009).
Other team members started work on using climate models to re-examine the limits of the habitable zone. Proposed mechanisms for extending the habitable zone include the formation of organic hazes at the inner edge, and the presence of strong visible atmospheric absorption to warm the lower atmosphere and surface of planets near the outer edge.
The Early Earth and Early Mars
Understanding Past Earth Environments: In this project, team members used nitrogen isotopes (Garvin et al., 2009), Archean sediments (Shen et al., 2009) and organic geochemistry (Buick et al., 2009) to show that metabolisms using aerobic nitrification, sulfate reduction and oxygenic photosynthesis existed well before 2.3 billion years ago, when the atmosphere became permanently oxygenated. Work also continues on efforts to measure the atmospheric pressure in the Archean.
Team members also made progress on developing models to simulate the Archean atmosphere and ocean. Our 1-D photochemical code was improved to simulate the transition from anoxic to oxic conditions, and we added new greenhouse gases and particle species to our climate. These models were used to simulate the Archean environment to explore the climatic effects of methane, ethane and hazes (Haqq-Misra et al., 2009)
Detectability of Biosignatures: We used an improved photochemical model of the Archean Earth to simulate the effects of sulfur metabolisms on the composition and spectral features of an anoxic atmosphere. We simulated the fluxes, lifetimes and ultimate atmospheric concentrations of methyl mercaption, dimethylsulfide, dimethyldisulfide, carbon disulfide and oxygen carbonyl sulfide, for planets around both Sun-like and M dwarf stars. We found that methyl mercaptan is the only likely detectable gas for this metabolic suite, and is most detectable at values 30 times the modern day flux and for planets in orbit around M stars, where photolytic destruction is reduced.
Figure 2 (Figure 2 from Detectability of Biosignatures Report: CH3SH as a Biosignature).
Stromatolites In the Desert: Analogs to Other Worlds: This field work component, led by team member Siefert and in collaboration with the NAI ASU team concentrates on studying freshwater stromatolites, an ancient form of life, in phosphorous poor environments found at Cuatro Cienegas, Mexico. No field trips to collect samples were possible this year, but scouting for future field trips were carried out. Plans are in place to acquire stromatolite communities and grow them under phosphorous and CO2-rich environments, with metatranscriptome sequencing acquired prior to and after exposure to the enriched environments.
Understanding the Early Mars Environment: This project was primarily supported as a DDF activity but includes work by other team members in related areas, and was led by team member Mark Claire, so is also discussed in his Postdoctoral Report. We investigated if SO2 gas could warm early Mars as suggested by Halevy et al. (2007) but found that high concentrations of SO2 inevitably lead to sulfate aerosol formation, which cools the climate (Claire, 2009, Tian et al, 2009). We proposed a novel mechanism for warming early Mars via visible absorption by NO2 gas, (Kasting et al, 2009).
To aid in explanations for high abundances of perchlorates found by the Phoenix Lander, we have also developed the first photochemical model for the atmospheric production of perchlorate in the Atacama (Catling et al. 2009.), and enhanced the FREZCHEM cold temperature chemistry model to include perchlorate salts (Marion et al., 2009). Zahnle et al. (2008) modeled a cold, dry ancient Mars and showed that CO2 was not necessarily stable against conversion to CO, providing a testable hypothesis for future Mars missions.
DesMarais et al., (2008) discuss the potential for life on Mars in light of environmental constraints for Earth life and the characteristics of two regions explored by the Mars Exploration Rovers.
Hydrodynamic escape: In this project we further developed a 1-D, multi-component, hydrodynamic thermosphere-ionosphere model and used it to investigate the stability of CO2 atmospheres of Noachian Mars and superEarths. We found that Noachian Mars was unlikely to have been warm and wet due to rapid loss of the atmosphere from strong solar XUV radiation (Tian et al., 2009) . For superEarths our model indicates that they should be
able to maintain their CO2 inventory despite strong XUV from their parent stars (Tian et al., 2009).
Delivery of volatiles: This project encompasses 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 dynamical models of the effect of Oort cloud showers on the Earth (Kaib et al., 2008), 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 the realization that PAHs entering a planetary atmosphere are likely to release small aromatic compounds that may form a UV protective haze around young planets (Carter et al., 2009; Pevyhouse et al., 2008).
Planet Formation and Dynamical Modeling: In this task we concentrate on understanding terrestrial planet formation and dynamical effects on planetary habitability. Highlights include the first self-consistent model of planetary growth from 1km planetesimals, which proceeds quickly, and points to terrestrial planets being common around other stars (Barnes et al., 2009). Simulations of planet-planet scattering indicate that this mechanism may be responsible for the tight packing seen in known planetary systems, and that it can add and remove terrestrial planets from the habitable zone (Raymond et al., 2008a; 2009b,c). We also explored how tidal interactions between star and planet could affect the surface properties of planets in known planetary systems (Barnes et al., 2009b) and have simulated stability in specific planetary systems, predicting which can support terrestrial planets in their habitable zones ( Kopparapu et al., 2009).
Planetary Surface and Interior Models: We continued to develop and use a reactive transport model to simulate weathering at planetary surfaces, expanding the mineral set and adding surface flux computation for several volatiles. We are also pursuing the role of tidal interactions between a planet and its parent star in affecting planetary habitability. Several team members are working to understand these effects using a 1-D planetary thermal evolution model (without tidal heating) and a tidal dissipation (heating) model. Other team members looked at the linkage between global geochemical cycles, metabolisms and the environment of the early Earth finding that volcanic activity and serpentinization were reliable sources of energy (via H2) for early pre-photosynthetic organisms, and potentially for those on extrasolar planets. Biological enhancement of weathering likely led to the development of sandstones, shales, carbonates, and (indirectly) granites. Models are also being developed to understand the chemistry of mafic rocks exposed to superheated fluids during the early Hadean (Sleep et al., 2009).
Stellar Effects on Planetary Habitability: This project looked at the radiative and gravitational effects of the star on planetary habitability. We used a coupled photochemical-climate model to study the effects of a large stellar flare on the photochemistry of an Earth-like planet in the habitable zone of the M dwarf AD Leonis.
We found that the flare’s UV radiation does not significantly change the ozone column depth or the UV flux reaching the surface of the planet, and such flares may not be hazardous to surface life (Segura et al., 2009). We also considered the range of tidal heating that is possible in the habitable zones of low mass stars, and suggest that a “tidal habitable zone” concept is required to identify planets with a range of tidal heating bounded by Io-like volcanism and insufficient tidal energy to drive plate tectonics (Barnes et al., 2009). We also explored the statistical probability of detrimental effects on habitability due to astrospheric collapse for stars of different mass (Smith & Scalo, 2009)
Thermodynamic efficiency of electron-transfer reactions in the Chlorophyll d-containing cyanobacterium, Acharyochloris marina: This laboratory-based project explores the efficiency of photosynthesis at the extreme red end of the spectrum, as a means of understanding whether photosynthesis might be possible on planets around M dwarfs, or on haze-covered planets. The bacterium A. Marina is the only organism to use Chlorphyll d, rather than Chlorophyll a, to perform photosynthesis with red wavelengths of light. Our laboratory technique uses a laser to measure energy storage in A. Marina cells as a means of determining the relative efficiency of Chl a, vs Chl d, which will help constrain the longest useable wavelength for photosynthesis. This work forms a significant part of the research of NAI postdoc Steve Mielke, and is also described in his postdoctoral report.
VPL Life Modules: We are developing two models to simulate the interaction of the biosphere with the surface and atmosphere, the Ent Dynamic Global Terrestrial Ecosystem Model (Ent DGTEM) effort, involving Nancy Kiang, and the NASA Ocean Biogeochemistry Model (NOBM) lead by Watson Gregg. This year the Ent DGTEM development included simulations of Earth’s seasonal vegetation, forced with observed meteorology. The NOBM is being integrated into two modern Earth system models, and is being modified to simulate the Archean biosphere.
VPL databases: As part of the on-going VPL Community Tools, we have are developing 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. The result, called the Virtual Planetary Spectral Library, will provide a common source of input data for modelers and a single source of comparison data for observers. We also continue development on tools to access and analyze information stored in the databases.
EPO and Education Efforts
Due to a funding shortfall in the previous year, his was our first year to implement or EPO efforts and detailed descriptions of these efforts can be found in the corresponding EPO reports. We have started work on a web interactive “Extreme Planet Makeover” and on development of an Astrobiology Outreach Toolkit for the Night Sky Network. Additionally VPL scientists taught 5 astrobiology courses to nonscience majors and engaged over 1500 members of the public via public talks, museum exhibits and media interviews. Astrobiology graduate students at the University of Washington also organized and hosted the Astrobiology Graduate Student Conference 2009.