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Executive Summary

The Virtual Planetary Laboratory @ The University of Washington — Executive Summary

The Search for Life Beyond the Solar System

While humanity’s robotic explorers search for signs of past and present life on Mars and the satellites of the Jovian planets, current technology does not allow us to extend this robotic exploration beyond our own Solar System. Because of the vast distances to even the nearest stars, NASA’s search for life outside our solar system will be undertaken instead using large telescopes to gather and analyze light from distant planets to search for remotely-detectable signs of habitability and life.

Life’s fundamental requirements include molecular building blocks, a liquid medium and a reliable energy source. In our universe, these requirements are most likely to be met on terrestrial planets, rocky worlds like our own Earth circling a main sequence star. So the search for life beyond the Solar System starts with the search for extrasolar terrestrial planets. Until recently, the planets found around other stars were the more easily detectable gas giant planets, like Jupiter and Saturn (Marcy et al., 2005) that are several hundred times more massive than the Earth. However, improved radial velocity detection techniques and sensitivity have started to reveal the existence of “super-Earths”, terrestrial planets up to ten times more massive than the Earth. Statistical inferences from gravitational microlensing observations indicate that these smaller planets may be extremely common in other planetary systems (Bennett et al., 2008), even though they are currently extremely difficult to observe. In the next few years, via the NASA Kepler mission and ongoing ground-based efforts, we may have identified several candidate planets of only a few Earth masses, orbiting at a distance from their parent star that makes the maintenance of liquid water oceans on the surface likely. However, although we will know which stars they orbit, our ability to directly image terrestrial planets and obtain spectra of their environments will remain highly observationally challenging, and will likely require the development over the next decade of visionary missions such as NASA’s Terrestrial Planet Finder, and the European Space Agency’s Darwin mission.

In the interim, while we await the observational capabilities to study extrasolar terrestrial planets in detail, theoretical modeling studies allow us to explore the potential diversity and likely locations for habitable planets; the modification of terrestrial planetary characteristics as a function of planetary mass, composition and parent star; and the nature and detectability of planetary-level signs of habitability and life. In doing so, modeling studies can inform and guide the science, instrumentation and observational strategies for the large observational missions under development.

The Virtual Planetary Laboratory

The Virtual Planetary Laboratory is a suite of computer models that can be used to address questions relating to the formation, nature, location and evolution of habitable terrestrial planets, and the detectability of global photometric, spectral and temporal signatures of habitability and life. Research in these areas strongly supports Goals 1 and 7 of the NASA Astrobiology Roadmap, and elements of our research additionally touch on aspects and objectives of the remaining 5 Roadmap Goals. To undertake this work, The Virtual Planetary Laboratory at The University of Washington Lead Team of the NASA Astrobiology Institute is necessarily highly interdisciplinary, with over 40 researchers in fields ranging from stellar astrophysics to biometeorology, at over 18 institutions worldwide. Our research interests focus primarily on habitable planets, and span topics such as terrestrial planet formation, dynamical and radiative interactions between a terrestrial planet and its parent star, the stellar habitable zone, the early Earth, super-earths, evolution of the planetary surface and life, the generation of and detectability of planetary biosignatures, and the nature of photosynthesis on planets around other stars.

Summary of Research

This year, the first in our second term within the NAI, was necessarily smaller in scope than originally envisioned, due to budgetary pressures. Nonetheless, the team made significant progress in developing model code to augment our existing suite of software, and continued to make scientific progress via modeling studies, astronomical observations and field work. Below, we summarize this year’s progress. More detailed information on these efforts, and full references for citations given here can be found in the Individual Project reports that accompany this Executive Summary.

VPL Environmental Modules: Progress was made on code development for the VPL climate model, radiative transfer model, surface weathering model, atmospheric escape module, and vegetation model.

Our spectrum-resolving, line-by-line radiative transfer model, SMART, was modified to improve its speed for use within our 1-D radiative convective climate model and upgraded to include ocean glint, non-LTE effects and airglow, and line-mixing with speed dependence (Martin-Torres et al., 2007). In addition, the code was parallelized to run on a super-cluster. These upgrades make both the radiative transfer model and the climate model more versatile for modeling a wide range of planetary environments. Work also continued on the development of the VPL Community tools which now include interactive molecular absorption coefficient visualizers, as well as a user-specified interface to radiative transfer and coupled-climate chemistry modules.

Our surface weathering model, a reactive-transport model, was upgraded with a larger mineral set (Bolton et al., 2007) and work is underway to link codes for gaseous diffusion to aqueous phase flow and reaction codes to create a state of the art model for regions of the subsurface where atmospheric exchange is important. Similarly, mineralogic compositions for typical igneous rock types were compiled in preparation for modeling a range of terrestrial environments, and the atmospheric and surface implications of hypothesized high temperature Archean conditions on granite and basalt weathering are being explored (Rye et al., 2007).

We also developed a 1-D hydrodynamic model of Earth’s upper atmosphere to understand atmospheric escape (Tian et al., 2008), We have used this versatile model to study thermal escape of C and O from a hypothetical CO₂-rich atmosphere on early Mars (Tian et al., 2008, submitted), Because the Sun’s EUV luminosity was high during early Solar System history, and because Mars’ gravity is low, these heavier gases may also have been able to escape quite readily, perhaps contributing to the thinness of Mars’ present atmosphere.

In Life Modules development, the Ent Dynamic Global Terrestrial Ecosystem models photosynthesis and vegetation conductance were coupled to two atmospheric general circulation models and the modules for soil biogeochemistry and seasonal growth were successfully tested on field data for several vegetation types (Karecha et al., 2007). The VPL Life Modules group is currently collaborating with the Columbia Astrobiology Center (CAC) at Columbia University to couple an extrasolar version of the Ent DGTCM to the Terrestrial Planet Global Climate Model to allow 3-D modeling of extrasolar planet environments.

Planet Formation and Dynamical Evolution

In a continued exploration of the likely locations for habitable planets, we have extended models of terrestrial planet formation to new domains such as around low-mass stars and binary stars (Raymond et al., 2007, 2008, Haghighipour and Raymond, 2007). To better understand the likely composition of extrasolar terrestrial planets we are developing a new interdisciplinary model for the origin of carbon on the Earth and other habitable planets, by combining studies of meteoritics, and models of planetary accretion with models of protoplanetary disk chemical and physical structure (Kress et al., 2008). Team members also studied the long-term tidal-orbital evolution of habitable zone planets around low-mass stars, and found that in some cases planets may form in the habitable zone but evolve in to hotter orbits because of tidal effects, interactions between the planet and star (Barnes et al., 2008). We have studied the formation of close-in terrestrial planets (e.g., “hot Earths”) and have shown that with high-precision transit and radial velocity information it may be possible to uniquely determine the formation mechanism (Raymond et al, 2008) We have also studied the dynamics of extra-solar multiple-planet systems — in 2007 we were the first to successfully predict the mass and orbit of a subsequently discovered exoplanet in a multiple planet system (Barnes et al.,2007, 2008).

The Early Earth

When exploring plausible environments for extrasolar terrestrial planets, we necessarily rely on information on non-Earth-like terrestrial planet environments in our Solar System, for which we can obtain measurements and constraints. These environments include Venus and Mars, the two other terrestrial planets with significant atmospheres, and the early Earth, which serves as an example of an inhabited planet with an environment and biosphere quite unlike the present day Earth’s.

The team’s early Earth work this year concentrated on understanding the climate of the early Earth and whether it was warm or hot. Shields and Kasting (2007) argued that existing isotope data need not imply hot early oceans. Team members also ran climate calculations to determine the effectiveness of a combined CH₄/C₂H₆ greenhouse for warming the early Earth, and found that its overall effectiveness would likely have been limited by the formation of organic haze (Haqq-Misra et al., 2008). Photochemical modeling showed that organic haze could have explained the presence of both a glaciation and the smaller range in Δ³³S values seen in the mid-Archean (by shielding SO₂ from UV photolysis and creating an anti-greenhouse effect Domagal-Goldman et al., 2008). We also examined feedbacks between the Archean sulfur cycle and climate by modeling how a biological feedback between oxygen production and oceanic sulfate would throttle methane production, a positive feedback on the rise of atmospheric oxygen. (Claire, Catling and Zahnle, 2007).

Photochemical modeling of Archean “Snowball Earth” type events revealed sustained production and sequestration of hydrogen peroxide during the Snowball phase, which would have been released upon melting. These processes could explain global oxidation events in the aftermath of the Snowball seen in the rock record (Yung and Liang, 2007) and could have driven the evolution of oxygen-mediating enzymes and oxygenic photosynthesis.

Molecular modeling techniques were also used to examine, and support, using Fe isotopes as a tracer of the evolution of the oxidation state of Earth’s ocean (Domagal-Goldman and Kubicki, 2008 accepted; Domagal-Goldman et al., 2008, accepted).

Planetary Habitability and Biosignatures.

This research area focuses on improving our modeling of planetary atmospheres to better understand factors governing planetary habitability, and to better understand the likely habitability of recently discovered super-Earths around other stars. This work also allows us to explore and determine observational signatures, and false positives for habitable environments and life

We continued our investigations of planets around M stars as possible abodes for life (Scalo et al., 2007), and are currently developing 3-D dynamical models for the habitability of tidally locked planets (Edson et al., 2008). We also addressed the question of whether O₂ might be produced abiotically in CO₂-rich atmospheres on planets orbiting young G stars, and would therefore act as a “false-positive” for photosynthetic life. We found that for planets in the habitable zone, this false positive is highly unlikely, even with high stellar UV (Segura et al., 2007). We worked with Franck Selsis and colleagues (Selsis et al., 2007a) to study the possibility that hot ocean planets might be observed with the ESA’s CoRoT and with NASA’s Kepler mission. While the study showed that the planets would likely be identifiable, if they exist, their dense steam atmospheres would likely preclude habitability. We have also explored habitability on on Enceladus, a planetary moon in our Solar System that our outside the classical habitable zone (Parkinson et al., 2007, 2008). Team members also used climate models to study the habitability of the observed planets around Gliese 581 concluding that 581c, initially believed to be in the habitable zone was probably too close to its star, but that 581d, near the outer limits of the habitable zone, had a higher probability of being habitable (Selsis et al., 2007b.

Observations and Field Work.

Astronomical observations focus on deepening our understanding of the atmospheres and detectable surface characteristics of Venus and Mars, and understanding the history of water on these planets. This year, new spectroscopic observations of Venus were obtained in an international coordinated program that supported the ESA Venus Express mission at Venus. These observations provided an improved estimate of the temperatures structure of the upper atmosphere via measurements of oxygen airglow on the nightside of the planet (Bailey et al., 2007, 2008a,b). In undertaking analysis of this data we also discovered substantial discrepancies in spectral line lists for water vapor in the near-IR spectral range. We are currently assessing the impact of these discrepancies on past estimates of Venus water vapor. A revision of these values may also affect our understanding of the Venus greenhouse mechanism.

To test the limits for life on Earth in extreme, cold environments, similar to those that might be found at the outer edge of the habitable zone, or in Mars-like environments, we have undertaken field work in the Arctic to measure several environmental parameters associated with the distribution of microbial life in Mars analog regolith in Svalbard. Some of the factors that influence the distribution and functional diversity of microbial life are wavelengths and intensity of available light, bioavailability of water, mechanical stability, mineral abundances, optical properties of the minerals for rock-dwelling microbes, temperature, wind speed and direction, radiation environment, etc.

Field work also continued at Cuatro Cienegas, Mexico in phosphorus-limited pools that house freshwater stromatolites. This work focused on co-evolution of microbial communities and biogeochemical adaptation to extreme environments. This environment also may be representative of some found on the early Earth. Work to date includes a functional estimation of the bacterial (Breitbart et , 2008, in press,) and viral metagenomes (Desnues et al, 2007) of two different stromatolite morphologies. A model of possible genome architecture agents of change and their requirement for phosphorus (Souza et al, 2008), revealed that phosphorus limitation likely enhances speciation. The whole genome sequence of a Bacillus isolate from Cuatro Ciénegas (Alcaraz et al, PNAS, 2008) showed direct evidence for geographic adaptation via horizontal gene transfer with non-related bacterial constituencies.