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2011 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2010 – AUG 2011

Executive Summary

The Virtual Planetary Laboratory: Overview

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 endeavor 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 successful Kepler Mission has found over a thousand planetary candidates, many of them smaller than twice the diameter of the Earth, which may reside in the habitable zones of their parent stars. Kepler’s large survey will improve our understanding of how common terrestrial planets are in the Galaxy, and the planned James Webb Space Telescope (JWST) will one day probe the atmospheric composition of super-Earths. In the longer term, we anticipate the development and launch of spaceborne telescopes, such as NASA’s Terrestrial Planet Finder concept, 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, orbital dynamicists, 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 four main areas: 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, which will contain more information on that project.

I. 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.

Fig. 1.— True color image of the Earth-Moon system, taken as part of NASA’s EPOXI mission compared to a simulated image using 10 micron brightness temperatures from our models. The spectra on the right shows the corresponding flux at 10 pc from the Moon (grey), Earth (blue), and the combined Earth-Moon flux (black), not including transit effects. The panel below the spectra shows the wavelength dependent lunar fraction of the total signal. Images and spectra are for a phase angle of 75.1◦

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 (Robinson et al., 2011), which was used in conjunction with a model of the Moon to determine the detectability of an exomoon around an exoEarth as a function of wavelength and observed phase, i.e. whether the exoEarth and moon are observed at full, or near crescent phase (Robinson 2011). This work showed that the contribution of the exomoon’s contribution to the exoEarth spectrum is very strongly phase dependent more likely to be detectable in the exoEarth’s carbon dioxide absorption bands. In a separate project, we modified our line-by-line radiative transfer model to simulate transmission spectroscopy. This work was validated against ATMOS-1 observations of the Earth’s transmission, and is currently being used to determine sensitivity to atmospheric pressure (Misra et al., 2011).

II. The Early Mars and Early Earth: VPL team members have contributed to a number of projects this year which constrain the habitability and environment of both present and ancient Mars. We have critically investigated putative evidence for methane plumes on modern Mars as well as claims of ancient ocean. In a re-examination of the primary data for the discovery of methane on Mars, Zahnle et al. (2011) postulate a blue-shifted isotopic line of methane in Earth’s atmosphere may have been confused for primary methane in the Martian atmosphere.

In trying to determine if Mars displays a continental/ocean floor dichotomy suggestive of an early ocean, or if the same topography could be impact generated, Catling et al. (2011) reasoned that a global ocean would have led to formation of sedimentary rocks and searched for evidence within smaller impact craters in the northern plains. This work suggests that basaltic, rather than sedimentary rocks are present, and is supportive of the giant impact, rather than global ocean, theory. Climate modelers also have severe difficulties in determining geochemically self-consistent scenarios resulting in clement conditions that would have supported surface water on early Mars. To improve our understanding of this issue, Tian et al. (2010) are updating prior VPL work on Martian greenhouse effects to incorporate updated CO2 collision-induced parameterization into calculations for early Mars (Ramirez, 2011). Following up on the surprising results from NASA’s Phoenix Lander, we continue to investigate the formation of perchlorate in the Martian atmosphere, and in Earth analog environments such as the Atacama desert (Smith et al., 2011). Perchlorate formation is affected by concentrations of N2, total atmospheric pressure and volcanic Cl and Br emissions, and so may provide clues to atmospheric composition and pressure for early Mars.

The Understanding the Early Earth task focuses on the Earth’s history of feedbacks between the atmosphere, biosphere, and climate for planets with lower surface oxidation states than the modern Earth’s surface. This year we used models to better understand the radiative forcing due to clouds which could have warmed or cooled the Early Earth’s climate. A large parametric study on the radiative forcing of clouds (Goldblatt et al., 2011b) was able to show that hypotheses that less clouds could solve the so-called Faint Young Sun Paradox were physically implausible (Goldblatt et al., 2011a) and that the approximation in climate models of clouds as enhanced surface albedo would lead to a systematic overestimate of the strength of greenhouse warming. We also used a passive tracer in a general circulation model (GCM) to constrain the transport of photolytically generated oxygen down to the planetary surface (Haqq-Misra et al., 2011). We addressed the potential for greenhouse warming by biogenically produced nitrous oxide and methane in the Proterozoic Eon, a time when the Earth had a lower oxygen abundance in its atmosphere. Results from a coupled climate-chemical model indicated that the combined greenhouse effects of N2O and CH4 could have provided up to 10 degrees of surface warming, without requiring large amounts of CO2 (Roberson et al., 2011). In a cross-team interdisciplinary study between VPL and PSU, measurements in a microbiology lab were used to test modeling theories for mechanisms for the rise of oxygen in the Earth’s atmosphere. These studies cultivated microbes that can oxidize CH4 using sulfate at different levels of environmental sulfate, and showed that the oxidation of CH4 can continue down to much lower levels of sulfate than thought previously. This behavior would have resulted in a positive feedback for oxidation of the Earth’s surface (Beale et al., 2011).

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.

III. The Habitable Planet: In this task we explore the many planetary and planetary system processes and characteristics required to initiate and maintain planetary habitability.

Significant progress was made in linking dynamical effects to habitability in the Orbital Evolution and Planetary Habitability this year. We found a connection between the apparent brightness of debris disks around stars and the likelihood of terrestrial planets in or near the habitable zone (Raymond et al., 2011). We also explored gravitational tidal effects on planets orbiting main sequence stars and brown dwarfs. For main sequence stars we find that obliquities can be modified by tides very quickly around low-mass stars (Heller et al., 2011). The spin properties of habitable planets can also be modified by tides even for planets orbiting solar-mass stars if they have large eccentricities. Brown dwarf planets, however, may evolve quickly through habitablezones and hence have a decreased likelihood for habitability (Bolmont et al., 2011). We have explored the role of orbital architecture on the obliquity evolution of exoplanets in systems with large mutual inclination (Barnes et al., 2011). These planets may undergo wild obliquity swings (Armstrong et al., 2010) which actually increase the width of the habitable zone by suppressing the ice-albedo feedback at the outer edge (Domagal-Goldman et al., 2011). Finally we have begun to explore the possibility that tidal heating may be strong enough to produce a runaway greenhouse on planets in the traditional habitable zone (Barnes et al., in prep). This possibility can only be realized for low luminosity objects, but could lead to sterilization of habitable planet candidates. We also proposed that Jupiter performed an inward-thenoutward migration which can explain the masses and orbits of the terrestrial planets, crucially explaining the small mass of Mars (Pierens and Raymond, 2011; Walsh et al., 2011). We also examined how orbital migration of our Sun through the galaxy may have impacted the structure of the Kuiper Belt. In particular we find that the orbit of Sedna has a 25% chance of resulting from this migration (Kaib et al., 2011).

The Delivery of Volatiles project encompasses modeling of the dynamical delivery of volatiles during the planet formation process, and the fate of carbon and volatiles on atmospheric entry. Building on previous work on the release of meteoritic organics at high altitude in the Earth’s atmosphere, results this year focused on the fate of organic meteoritic compounds upon atmospheric entry, for a range of terrestrial planet sizes and atmospheric pressures (Pevyhouse et al., 2011), and for Titan (Templeton and Kress, 2011). This work found that the most important factor for survival of exogenic organics is to be released beneath a sufficient amount of atmosphere to block destruction by stellar UV, and that planetary properties were therefore not as important as stellar properties. We also showed that for Titan-like planets survival of organics was enhanced.

The work described above on Jupiter’s inward and then outward migration through the Solar System also feeds into issues of volatile delivery, as this mechanism can explain the S-type vs C-type dichotomy of the asteroid belt. In the context of this model, Earth’s water was delivered by C-type material that itself was implanted into the asteroid belt from beyond Jupiter’s orbit. (Pierens and Raymond, 2011; Walsh et al., 2011).

The Planetary Surface and Interior Models and Super-Earth 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 mineral and gas reaction set and tackling Fe redox kinetics and speciation (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, by combining 1-D non-tidal heating calculations with 3-D tidal dissipation models (Rye and Barnes, work in progress). In other work VPL team members contributed to a paper on the synchronously rotating hot super-Earth CoRoT 7-b that showed that the planet likely had a magma ocean that extended to 50 degrees from the sub-stellar point, and that were likely 45km deep. The possibility of a habitable terminator was considered but rejected as water would evaporate from this region too rapidly (< 1yr).

CoRoT 7b

Figure 2: Artist’s impression of CoRot-7b

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. This year we continued our work on atmospheric escape modeling for super-Earth planets in M star habitable zones, indicating that these planets could lose a significant fraction of their N2 inventory if not dominated by CO2 (Tian et al., 2011). A coupled model for high CO2 atmospheres was implemented to study the habitability of the super-Earth Gliese 581d. This work showed that Gl581d would need a minimum atmosphere of 7 bars of CO2 to elevate the surface temperature above freezing (Kaltenegger et al., 2011). Work also continued on a generalized climate model for terrestrial planets, including super-Earths, which uses a line-by-line radiative transfer model and Jacobians to produce a physically rigorous simulation of a terrestrial planet atmosphere. The model now converges in the clear-sky case, and clouds are currently being incorporated into the model.

Surface UV Flux for a CO2 Dominated Planetary Atmosphere Under a Stellar Flare

Figure 3. Incoming ultraviolet flux at the surface of a planet around AD Leonis during a big flare. The atmosphere has 1 bar surface pressure and a bulk composition of 0.2 CO2 and 0.8 N2. The dotted line shows the UV flux at the surface of a planet with the same bulk atmospheric composition around the Sun. In all cases, the CO2 planet is better protected.

In the Stellar Effects on Planetary Habitability project we look at the radiative effects of the parent star on planetary habitability. This year we published work on the effects of a large flare from a cool M dwarf star on an Earth-like planetary atmosphere (Segura et al., 2010) and worked on the next step of this research, which is to understand the effect of flares on CO2 dominated atmospheres like those of the early Earth. Our initial results suggest that these planets may be better protected from UV activity than planets around Sun-like stars with protective ozone layers (Sanchez-Flores & Segura, 2011). To determine characteristic flaring rates and intensities for planetary habitability, we used the first quarter of Kepler results to identify flare events in cool K and M dwarfs and found that stars with longer duration flares tend to flare less frequently (Walkowicz et al., 2011). The effects on planetary habitability of long-duration, less frequent flares vs more frequent but short duration flares will be explored in future work. VPL team members also contributed to climate modeling work that showed that the habitable zone may be significantly enlarged for planets with low water inventories, so-called Dune planets, because they can avoid the runaway greenhouse and ice-albedo feedbacks over a wider span of planet-star distance (Abe et al., 2011). We also improved error calculations for our 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; Claire 2011, submitted).

IV. The Living Planet: The VPL Life Modules task encompasses development of 3-D ecosystem models to look at biosphere/planet interactions. Last year we collaboratively incorporated the NASA Ocean Biogeochemistry Model (NOBM) which models the interaction of life and nutrients, into the ocean model of the GISS GCM, providing the capability to model a full carbon cycle system. Efforts continue to integrate the NOBM into the GMAO GEOS-5 Earth system models. To compare equilibrium behavior of carbon stocks and to identify climate biases we have worked to improve our realistic land vegetation cover using products from the NASA Earth Observing MODIS and ICESat/GLAS We are continuing evaluation and testing of the coupled carbon dynamics. Global tests with this dataset will help constrain the carbon budget of the Earth and processes in the Ent ecosystem model (Kiang et al., 2010), setting the stage for experiments for exoplanet biosphere modeling.

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, and possibly even more efficient than chlorophyll a, showing that photosynthesis can proceed efficiently at far red wavelengths, and implying that we have not yet found the wavelength limit for oxygenic photosynthesis. Current work includes distinguishing the contributions of Photosystems I and II to the overall efficiency of the pigments.

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 published our work using 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., 2011). 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. We are also writing up for publication new work on 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) Interferometer (T. Velusamy) and Occulter (W. Cash) 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).

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 developing more user-friendly search and display access to the molecular database (lead by Gupta), improving our molecular line lists for modeling high opacity atmospheres, gases such as methane, and for foreign broadening of gases (lead by Brown), and improving access to our significantly expanded photosynthetic pigment database (lead by Kiang).

EPO and Education Efforts

This year in EPO the Extreme Planet Makeover Interactive was launched ( This interactive 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. We also completed development and testing on the Night Sky Network Astrobiology Outreach Toolkit, which was rolled out to qualified member astronomy clubs in June 2011. This toolkit includes activities to enhance participant understanding of extremophiles, exoplanet detection, planetary characterization for life, the history of life on Earth and the probability for intelligent life in the universe. Our partnership with Lakewood High School in Washington state to pioneer a high school level Astrobiology course was successful, with the school offering the course again this year. Additionally VPL scientists taught astrobiology courses to nonscience majors, and engaged members of the public via public talks, museum exhibits and media interviews.

Amateur Astronomers Testing the Night Sky Network Extremophiles Activity.

Fig 4. Amateur astronomers field testing the Extremophiles module of the Night Sky Network Astrobiology Outreach Kit.