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Executive Summary

THE VIRTUAL PLANETARY LABORATORY

Laying the Scientific Foundation for the Search for Life Beyond the Solar System

Because of the vast distances to even the nearest stars, the search for life outside our solar system will be undertaken using astronomical “remote-sensing” techniques. The first-generation missions to use these techniques, the NASA Terrestrial Planet Finder and ESA Darwin missions, and the next generation NASA LifeFinder mission will not be able to spatially resolve features on the surface of the planet. While basic information can be gleaned about an extrasolar planet based on the characteristics of the host star and the planet’s position in its planetary system, spectroscopy is still our most powerful technique for extrasolar planet characterization. The NASA Astrobiology Institute’s Virtual Planetary Laboratory (VPL) uses computer models of terrestrial planets to understand the nature and potential range of spectroscopic signs of planetary habitability and life. The VPL simulates the environments of terrestrial planets, starting with planets in our own Solar System, and moving outward to a larger range of planetary environments and host stars. The disk-averaged spectra of these bodies are then analyzed to identify and assess the detectability the spectral biosignatures that might be encountered by future planet detection and characterization missions. More specifically, the VPL explores the limits of our capability to characterize a distant environment from disk-averaged spectra, by researching the likely atmospheric gases (e.g. O₂, H₂O, O₃, CH₄ and CO₂), surface signatures (e.g. the vegetation red edge) and temporal variability (e.g. due to spatial variations in surface type, or the seasonal “breathing” of a biosphere) that may be seen in this type of data.

The VPL Research Goals, Astrobiology, and NASA

The work undertaken by this group is heavily interdisciplinary, drawing on and synthesizing the expertise of an extremely wide range of disciplines spanning the NASA Science Mission Directorate, including astronomy, geology, planetary science, Earth science, ecology and biology to address a common, focused scientific goal. The VPL’s suite of computer models and supporting observational and field work are being used to provide the fundamental research needed to support the remote-sensing detection of life, by improving our understanding of the use of spectra to discriminate between extrasolar planets with and without life. This work is most directly relevant to the Astrobiology Roadmap Goals 1 and 7, on the nature of planetary habitability and the remote-sensing signs of life. However, this year, work undertaken by the VPL Team to understand the remote-sensing signs of habitability and life touched on all 7 Astrobiology Roadmap goals, and was relevant to 15 of the 18 Astrobiology Roadmap Objectives.

The VPL’s interdisciplinary research is targeted to provide results relevant to the design and search strategies for future NASA planet detection and characterization missions. However, to achieve this goal, VPL research this year supported 16 international mission or mission concepts, including TPF-C, TPF-I, LifeFinder, the Orbiting Carbon Observatory, ESA Venus Express, Cassini-Huygens, MARVEL , Phoenix, Mars Odyssey, Mars Science Laboratory, the Space Interferometry Mission, Spitzer, Kepler, ESA COROT and GAIA. In addition, the VPL used data from 8 active or previously flown spaceflight missions (Spitzer, Cassini-Huygens, Mars Express, Mars Odyssey, Aqua, Terra, The Hubble Space Telescope and the International Ultraviolet Explorer) as input to its science activities. VPL Team members also provided two-way communication between the astrobiology community and NASA missions as they participated in mission science activities for TPF-C, TPF-I, the Orbiting Carbon Observatory, Venus Express and the Cassini-Huygens mission.

The Virtual Planetary Laboratory, NAI Year 7.

Building the VPL: (8 Research Tasks)

This year, the VPL team continued its efforts to develop and combine computer models of planetary processes into a series of progressively more comprehensive terrestrial planet models. Significant milestones, particularly in model integration, were achieved. The completed VPL model suite will provide the capability to model terrestrial planet environments in a self-consistent fashion, and output the spectral appearance of these environments to remote-sensing observations. The resultant models are being applied to a number of scientific questions on the environments of early Earth and extrasolar terrestrial planets.

Figure 1.

This effort is characterized by 5 successive component model tasks, the supporting tasks of compilation of a molecular spectral database and a stellar spectra database, and the overarching task of model integration. These tasks and highlighted accomplishments briefly summarized below. More details on task scope and accomplishments can be found in the individual project reports.

Task 1: Spatially-resolved models of terrestrial planets. These models use observed or interpolated data to provide 3-D spectral representations of planets in our own Solar Systems. Research highlights for this year include publication of the Mars model results, and completion and submission of the full Earth model with realistic clouds. Modeling highlights included an improved understanding of the effect of clouds on 1) the disk-averaged spectrum as a function of phase, 2) the visible and MIR light-curves and 3) the detectability of biosignatures in the Earth’s spectrum.

Task 2: 1-D Climate Models for Extrasolar Terrestrial Planets. These models yield a globally-averaged description of the surface temperature and vertical temperature distribution for a terrestrial planet. Progress this year included development of a new, faster radiative transfer scheme and the incorporation of a cloud model. The resultant model was used to produce annual cycles of soil and atmospheric temperature for Earth in an elliptical orbit. The model with clouds is being validated against the environments of planets in our own solar system.

Task 3: A Chemistry Model for Extrasolar Terrestrial Planets. This task focuses on the development of a generalized, yet comprehensive, photochemical model for terrestrial planet atmospheres which will interact with the Task 2 climate model to create a self-consistent climate-chemical model. This year the master reaction file set and the coupling with the climate model via the VPL database was completed. The chemistry model was used to model the chemistry of Titan, and a Snowball Earth. Work was also completed on atmospheric fractionation processes on Mars as an aid to identifying the source of Mars methane.

Task 4: The Abiotic Planet Model: This task develops models of processes at the upper and lower boundaries of the coupled climate-chemical model, including exogenic mass fluxes (dust/meteors/asteroids, atmospheric loss) and surface-atmosphere interactions (chemical weathering, mantle outgassing). This past year saw the interior model, supporting stagnant lid and plate tectonics, integrated into the VPL. Progress was also made on reactive transport models for rock weathering to understand volatile fluxes with a specific study of high-temperature weathering of granite during the Archean. Work was also done on the ablation of organics from meteorites and the chemical kinetics of polycyclic aromatic hydrocarbons in impacts and circumstellar disks.

Task 5: The Inhabited Planet Model: This task provides computer models of life’s interaction with the planetary environment for integration with the Task 4 model. Progress this year included the publication of an Archean ecosystem model, and work to understand the photosynthetically active radiative and light harvesting pigments that might be present on Earth-like planets around F, G, and K stars. Progress was also made on coupling a land biosphere model to the VPL. This task also included field trips to sites that provide analogues to possible Early Earth environments. The goals of the field work are to analyze life in highly alkaline aquifers and springs associated with terrestrial serpentinizing bodies, and to work on horizontal gene transfer in environments that contain freshwater stromatolites.

Figure 2. Task 5: VPL calculated surface photon flux densities and candidate wavelengths for peak photosynthetic pigment radiation absorbance on Earth-like planets in the habitable zone of F, K, and M stars. Included for comparison are the Earth’s surface photon flux density and the absorbance spectrum of various purple bacteria.

Task 6 and 7: Spectroscopic Databases to Support Extrasolar Planet Modeling: Molecular and Stellar.

This task focuses on collecting molecular absorption information, and preparing full-wavelength, continuous stellar spectra for use by planetary climate and chemistry models. This year the molecular database was updated to include the new HITRAN 2004 database and molecular information over UV and visible wavelength ranges relevant TPF-C. We also worked on a larger sample of spectra of stars (46), and worked on obtaining time-dependent spectra characterizations of active M stars. An initial version of this database, containing continuous UV-FIR spectra of several stars was made public this year. Both databases, molecular and spectral can be found at: http://vpl.ipac.caltech.edu/spectra/ .

Task 8: The Virtual Planetary Laboratory: Synthesis and Architecture

This task focuses on interfacing and integration of the core model components to produce the VPL planetary models described in Tasks 3-5. This year, significant progress was made on interface design, with the VPL climate-chemistry interface developed, and the full coupling of Jim Kasting’s climate-chemistry code through the VPL architecture, including a validation run to achieve a convergent run of the Earth’s atmosphere. The PlanetDrafter tool to parameterize runs with the VPL was improved in the last year. A new feature, which parses input files to the spectrum generation code from the output of the climate-chemistry model was developed this year.

In addition to core work on the development of the VPL code, this year team members expanded into several relevant and complementary fields pertaining to habitability and the detectability of biosignatures. These projects are summarized below.

Earth-like Planets Around Other Stars

Continuing work completed in previous years on the environments and spectral appearance of Earth-like planets around F, G and K stars, this year we submitted a publication on models of environments and spectra for Earth-like planets around M. Our models indicated that ozone layers did form, and abundances of reduced biomarker gases were found to be higher on the M-star planets than on Earth, given the same assumed surface fluxes, due to the low near-UV flux from the parent stars. We also showed that the biomarker CH₃Cl, undetectable in the modern day Earth spectrum became greatly enhanced on Earth-like M star planets, being most detectable in the mid-infrared. Work was significantly advanced on modeling Earth-like planets around M stars with time-variable spectra, and we also started simulations of High-CO₂ planetary atmospheres to address whether O₂ and O₃ abiotic abundances resulted in false positives for life.

The Astrophysical Environment and Planetary Habitability.

This task focuses on the effects of transient astronomical radiation events, such as stellar flares and Galactic cosmic rays, on planetary atmospheres and biospheres over time. Work this year focused on identification and quantification of sources of cosmic-ray (CR) variations over a large range of timescales, including parent star CR variations, passage of planetary systems through interstellar density structure, and variations in the star formation rate during the parent star’s Galactic orbit.

Exploring Conditions for Habitability on Mars.

In this task, we explored long-term climate, surface and sub-surface characteristics of Mars, including the effects of obliquity changes, and looked for evidence of snow/ice interactions with the atmosphere on Mars today. Results indicated that the Martian atmospheric pressure is remarkably static over time and that a sub-surface water ice boundary would also be largely unaffected by changes in orbital cycle, except during periods of extreme obliquity.

Detectability of Planets and Biosignatures

This task focuses on the question of detectability of planets and biosignatures and uses instrument and data simulation models, retrieval techniques, and the amassed spectra of the Virtual Planetary Laboratory to date. This year, team members worked on algorithms for planet signal extraction for TPF-I, limits for astrometric detection, techniques for detecting planets via analysis of circumstellar disk structure, and explored the use of polarimetry for planet detection and characterization. Work was also done to assess the detectability of planetary characteristics using instrument simulator models for both TPF-C and TPF-I.

VPL Education and Public Outreach

VPL has provided budget and scientists support to develop the “Design a Planet” module of the AstroVenture interactive website for middle school students. AstroVenture highlights NASA careers and astrobiology research. VPL has also funded in-service teachers to earn graduate credits taking the “Astrobiology On-line Course for teachers “. The VPL E/PO team, with the support of the VPL scientists, has also created a new website with more information and new sections related to Education and Public Outreach. Additionally, there are numerous E/PO efforts and public talks by Dr. Meadows and VPL scientists and an on—line astrobiology course provided by Dr. Siefert at Rice University .

NAI-wide Collaborative Efforts

VPL team members worked in collaboration with CIW and the NAI Astronomy Focus Group to host and participate in the NAI Global Biosignatures Workshop, a pioneering effort to use videoconferencing to support a bi-coastal workshop. VPL team members also organized a second workshop on TPF and Biosignatures in collaboration with the TPF Project, and organized, held and participated in a workshop on Mars Methane. Team members also participated in the SETI workshop on M stars and Habitability.