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Executive Summary

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

Because of the vast distances to even the nearest stars, NASA and ESA’s search for life outside our solar system will be undertaken using astronomical “remote-sensing” techniques, of which spectroscopy is the most powerful. The Virtual Planetary Laboratory is a suite of computer models that allow us to explore the spectra of simulated extrasolar environments, to provide a theoretical foundation to ultimately improve our ability to identify signs of habitability and life in distant planetary spectra.

The Virtual Planetary Laboratory Lead Team

The NAI Virtual Planetary Laboratory Lead Team is an interdisciplinary group of researchers whose broader scientific focus is the search for habitable planets and life beyond the Solar System. The team’s expertise encompasses astronomy, geology, planetary science, Earth science, ecology and biology. Under the auspices of the NAI, the VPL team was an experiment in scientific research techniques: to pioneer truly interdisciplinary research across geographical distances, using information technology to enable remote collaboration.

The VPL’s Interdisciplinary Concept

The VPL was founded on the concept that interdisciplinary research was most likely to occur when a core team agreed to tackle pre-defined scientific questions of such breadth that more than one discipline was required to make significant progress. To that end, the VPL’s computer models link life and planetary processes, planet-star interactions, and astronomical instrument performance to identify plausible, self-consistent extrasolar terrestrial planet environments, and to simulate spectra similar to those that might be seen by future NASA planet detection and characterization missions. By analyzing spectra of planets in our Solar System as well as simulated spectra of a larger range of possible extrasolar planets, and the environmental interactions that influenced them, we worked to improve out understanding of the use of spectra to identify the most likely characteristics for planetary habitability and life. 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.

In essence, the need for interdisciplinary interaction was built into the research plan from the very beginning. The development of the modeling suite served as a universal focus for the entire team, while also allowing us to use published results and expertise from NAI colleagues in other teams. Interest, scientific interaction and identified additional expertise grew our team from the initial 17 on the 2000 CAN-2 Proposal, to the 40 members it now encompasses in 2006. The models still serve as our central focus, and also provide the foundation for the scientific research that gathers necessary scientific inputs to the models, and the applications of those models to theoretical exploration of extrasolar terrestrial planet environments.

The VPL Research Goals, Astrobiology and NASA

The VPL’s suite of computer models and supporting observational and field work are most directly relevant to Astrobiology Roadmap Goals 1 and 7, on the nature of planetary habitability and remote-sensing signs of life. Over the course of this project, core research undertaken by the VPL Team touched on all 7 Astrobiology Roadmap goals, and was relevant to 17 of the 18 Astrobiology Roadmap Objectives.

During the 5 year course of this project, the VPL Team trained or had significant interaction with a total of 23 early career astrobiologists, from undergraduate students to postdoctoral scholars. Several of our postdoctoral scholars subsequently obtained faculty positions, and now teach astrobiology and interdisciplinary science as part of their university curricula.

Over the past 5 years, VPL research also supported 16 international missions 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, Kepler, ESA COROT, and GAIA. This year, we also added VPL research relevant to human exploration of the Moon and Mars. 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 provided two-way communication between the astrobiology community and NASA missions as they participated in mission science definition and science activities for TPF-C, TPF-I, the Orbiting Carbon Observatory, Venus Express and the Cassini-Huygens mission.

The Virtual Planetary Laboratory: Models and Research

The VPL is designed around 5 major modeling tasks and several supporting activities that successively increase our ability to model a diversity of environments. This year, we also worked to make as many of our modeling tools and products available to the larger community as possible, and also added many new scientific projects that use our existing modeling capability, or otherwise address the question of planetary habitability and remote-sensing life detection.

Figure 1. Click for larger image.

These tasks and highlighted accomplishments are briefly summarized below. More details on task scope and accomplishments can be found in the individual project reports.

Modeling Tasks

VPL 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 Earth model (Tinetti et al., 2006a), publication of a paper that used the Task 1 models to model vegetation with an alternative photosynthetic scheme around another star (Tinetti et al., 2006b), and acceptance of a second paper on results with the Earth model which included cloud effects 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 (Tinetti et al. 2006c). We also continued remote-sensing observations of Solar System planets as input to the models.

VPL 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. Our simple/fast climate model (Kasting) was modified to accept time-dependent solar insolation. Our more complex model (Crisp) continued development to implement an even faster radiative transfer scheme.

VPL 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 model was completed and is now a generalized model for terrestrial planet atmospheres ranging from CO₂ to N₂ dominated, that includes reaction and species lists for present-day and Early Earth, Mars, Venus, Titan and Jupiter, for a total of 616 species and 2247 reactions. It fully interfaces with the VPL database and can be run from the VPL GUI interface. A paper using this model for Snowball Earth was submitted this year (Liang et al., 2006).

VPL 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). Progress continues to be made on reactive transport models for rock weathering to understand volatile fluxes. Fluxes for O2 and CO2 through the surface will be incorporated later this year. Research was performed on the effects of photosynthesis on sulfur cycling.

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. This year we worked to set up an ecology model to couple with the VPL 1-D climate-chemical models. This new model coupling will allow simulation of vegetation cover, surface exchanges and spectral reflectance on an Earth-like planet’s surface. We also performed two other supporting modeling tasks for microbial mat gas fluxes, and vegetation canopy modeling. This theoretical work was complemented by field work at sites that provide good proxies for early Earth bacterially dominated systems. Research highlights include the submission of a paper exploring global productivity and biosignatures for photosynthetic pigments adapted to different parent star spectra and extrasolar planetary atmospheric composition (Kiang et al., 2006).

Supporting Tasks

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 a new isotope spectra page, and to provide a new interactive plotting tool to view HITRAN and PNNL line list data. We added full wavelength range M star spectra to our publicly available stellar spectral database, and worked on obtaining time-dependent spectra characterizations of active M stars for use the tasks below. Both databases, molecular and spectral can be found at: http://vpl.ipac.caltech.edu/sci/ “VPL Spectra.”

The Virtual Planetary Laboratory: Synthesis, Architecture and Release

This task focuses on interfacing and integration of the core model components to produce the VPL planetary models described in Tasks 3-5, and in releasing these models to the community. This year, the component models were all housed in a version controlled online database. We also made publicly available our 1-D coupled-climate model for either download (http://vpl.ipac.caltech.edu/sci/ “VPL Models” ) or to be accessed via the VPL online interface, which includes environment specification, computation of results, and analysis tools.

We will continue to work this year to release via the interface the radiative transfer tools, and the VPL Task 1 3-D spectral Mars and Earth models. We are also working to finalize the full generic terrestrial planet climate-chemistry model, including interaction wthh the ecology model. In future work, we also plan to provide a data interface for the Task 1 models that will upload temperature profiles from 3-D datasets for different environments.

Research Tasks

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 planet formation, human exploration, habitability and the detectability of biosignatures. The scope of these projects is summarized below, and more details on the research can be found in the relevant project report on this website.

Modeling Terrestrial Planet Formation and Composition

This task, provides the most realistic simulation to date of the final stages of terrestrial planet formation to explore the likely distribution and starting composition for habitable planets. This year, we explored the potential formation of habitable planets around low mass stars, planetary systems with hot Jupiters, binary star systems, and for the known sample of planetary systems around other stars. In addition new results from 3-D hydrodynamical simulations indicate that protoplanetary disks may be seeded with ~100 m sized bodies, thereby averting a problem for the growth of larger bodies. We also explored the possible formation of “sootlines” for carbon in protoplanetary disks.

Characterizing the Earth’s Early Environment

This task focused on using the isotopic records of oxygen and sulfur to better understand the evolution of Earth’s atmosphere, which serves as a potential analog for young extrasolar terrestrial planets. A mechanism was proposed to evolve the O isotope composition of seawater by changing ridgecrest depth. This might imply that the early Earths’s ocean was not as hot as has been believed. Alternative mechanisms including glaciation and using models to study the formation of an organic CH4-photolysis induced haze were also proposed to explain sulfur fractionation anomalies between 2.8 and 3.2Ga which would otherwise imply very low O₂ concentrations at that time.

The Astrophysical Environment and Planetary and Lunar 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 understanding the variation of cosmic ray flux in the habitable zone as a function of stellar age and passage through the interstellar medium, with an emphasis on recreating the Earth’s radiation flux history due to its passage through the Galaxy. We also used radiative transfer models and the observed statistics of solar flares to estimate the effects of X-ray flares on human Lunar and Martian EVA risk. While we find that lunar risk is appreciable, and estimates are given for the degree of shielding required, the Martian EVA risk is marginal.

Searching for Life on Mars

This task focuses on understanding the recent discovery of methane on Mars, in the context of the planetary environment to help determine whether the methane has an abiotic or biological origin. As part of this effort this year, VPL participated in the organization of the NAI Distributed workshop on Martian methane, in collaboration with several other NAI teams. This interdisciplinary forum suggested possible sources for the methane, and discussed ways to distinguish biologically produced methane from abiotic sources, including isotopic discrimination, and the co-generation of other species that might be characteristic of different sources. More details can be found in the project report.

Exploring Conditions for Habitability in our Solar System: Mars and Enceladus

In this task, VPL team members published work on long-term climate, surface and sub-surface characteristics of Mars, including the possibility of surface liquid water for present and Early Mars, and the effects of obliquity changes on the surface and sub-surface environment. 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. This year, in response to the Cassini discovery of water vapor on Enceladus we published a paper on the astrobiological implications of this phenomenon, and also successfully acquired Spitzer spectra of Enceladus, to search for signatures of the plume composition.

Biosignatures for Earth-like Planets Around Other Stars

This research task is arguably the most interdisciplinary work performed by the VPL and concentrates on understanding the planet-star interaction, and the effects on a planet’s atmosphere and gaseous biosignatures, for parent stars of different spectral type. This year results from our coupled climate-chemical modeling of planets around M stars were published. We found that planets around quiescent and active M dwarfs with similar biogenic fluxes to the Earth’s may have larger amounts of atmospheric methane, nitrous oxide and methyl chloride than modern Earth, as a result of longer atmospheric lifetimes. This year, we completed a new coupled-climate chemistry model that models the atmospheric chemistry of a planet affected by time-dependent radiation from a stellar flare. We also performed a set of simulations for habitable, but uninhabited planets with different atmospheric CO₂ abundances for stars of high UV flux to determine the possibility of false positive generation of large amounts of O₂. We found that even for the best possible scenario for the production of abiotic O₂, the O₂ and O₃ concentrations produced were extremely small, and unlikely to be detectable to the first generation of planet detection and characterization missions. We also found that in the MIR, CO₂ dominated atmospheres may produce sufficiently strong features to allow the determination of the isotopic ratios of O in the atmosphere.

The Detectability and Characteristics of Extrasolar Planets: Techniques and Observations

This task focuses on techniques and observations for detecting and characterizing extrasolar Jovian and terrestrial planets. This year, team members continued work on algorithms for planet signal extraction for TPF-I, techniques for detecting planets via analysis of circumstellar disk structure, found several new extrasolar planets, and explored the use of polarimetry for planet detection and characterization. This year VPL team members resolved an ambiguity in the location and orbital direction of the unseen planets around Vega, and performed high precision polarimetry of the Tau Boo hot Jupiter system, placing significant limits on the nature of the scatterers in that planet’s atmosphere.

VPL Education and Public Outreach

This year, VPL 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 in the areas of Astronomy, Geology, Biology and Atmospheric Sciences. Last October, Design A Planet was listed as Newsweek Magazine’s top website for the week, and in November, VPL scientists participated in an interactive webcast focused on this new site. VPL also subsidized the cost of offering 120 in-service teachers the opportunity to earn three graduate semester credits in an Internet-delivered Astrobiology for Teachers course where participants connect to their classes and interact with each other and the instructor.

In addition to supporting our more formal EPO elements, this year, VPL scientists continued to contribute organizational and scientific expertise at science forums, winter schools, workshops and university seminars that reached undergraduate and graduate audiences, and also developed and/or participated in 6 university courses that included astrobiological elements. VPL scientists also interacted with the media in magazine, newspaper and radio interviews and web events and articles.

...and Beyond

After 5 years of interaction, the NAI Virtual Planetary Laboratory Lead Team has forged interdisciplinary collaborations that will likely endure past our 5-year term, via regular meetings and continuation of our ongoing research efforts. For the remainder of this year, the team is focused on continued research, finalizing and documenting models and model products, and making them available to other members of the scientific community. As always, we welcome new collaborations, including those across disciplines and distances.