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Executive Summary

Towards Characterization of Extrasolar Terrestrial Planets

Are we alone? The richness and enormous vastness of our universe makes our solitariness seem almost unthinkable, and so this ancient musing has remained one of the most fundamental of human questions. Yet while the medieval monk Giordano Bruno could only speculate on the prevalence of other planetary systems that could harbor life, in the last decade we have been fortunate enough to see the search for other worlds evolve beyond the realms of heresy and science fiction, into science theory, and finally, into science fact.

As this report is being written, we now know of 107 planets outside our own solar system. These planets have been discovered using existing ground-based telescopes. However, in the coming decades NASA and the European Space Agency (ESA) will design, build and fly space-borne observatories to expand our knowledge of the number and distribution of planets in our Galaxy, and to determine whether or not these planets might support life. These missions, such as NASA’s Kepler, Terrestrial Planet Finder (TPF), and Life Finder will specifically search for habitable terrestrial worlds, that is, Earth-sized rocky worlds that can maintain liquid water on their surfaces for extended periods of time. TPF, planned for launch in 2015, will be designed to directly detect Earth-sized worlds around nearby stars, and to determine whether or not these sister planets are habitable, like our own Earth, and will initiate the search for the global signs of non-technologically advanced life. The most ambitious of these planned missions, Life Finder, is envisioned to build upon the discoveries of TPF, to provide a more detailed spectroscopic search for non-technologically advanced life on planets in our Solar neighborhood.

To optimize the designs and search strategies for these NASA missions, and to ultimately interpret the data that they return, we must expand our ability to recognize worlds that might have habitable conditions, and to discriminate between planets with and without life, based only on remote-sensing observations.

Characterizing Extrasolar Terrestrial Planets

Methods for characterizing terrestrial planets in our solar system have been refined for decades by planetary and Earth-observing scientists. Some of these techniques can be adapted for use in the characterization of terrestrial planets around other stars. Examples include time-resolved whole-disk photometry, spectroscopic remote sensing for the detection and retrieval of atmospheric and surface composition and physical parameters, and time-resolved spectroscopy of spectral features to look for diurnal or seasonal variations in surface albedo or atmospheric composition. However, since existing observing techniques are not yet sensitive enough to directly detect and gather information on Earth-sized planets around other stars, our principal means of advancing our understanding of these planets is via theoretical modeling studies. So, to improve our understanding of the potential range of environmental and spectroscopic characteristics for terrestrial planets in our galaxy, our team is developing a suite of innovative modeling tools to simulate the environments and spectra of a range of plausible extrasolar planets and for the early Earth.

These modeling tools comprise a Virtual Planetary Laboratory (VPL). This tool will incorporate models to couple the radiative fluxes, climate, geology and biology of a terrestrial planet to produce a self-consistent state for a broad range of candidate planetary environments. Self-consistency between a planet’s chemical, physical, and biological properties state is particularly important when trying to understand the detectability of biosignatures in the spectrum of the planet’s atmosphere and surface. This effort is strongly interdisciplinary, calling upon the expertise of planetary scientists, astronomers, biologists, geologists, chemists, mathematicians, computer scientists and statisticians to all work together towards a focused common goal. This effort therefore exemplifies the science of astrobiology, an interdisciplinary approach to understanding life in our universe.

The Virtual Planetary Laboratory

The core of the VPL is a coupled radiative transfer/climate/chemistry model, which is being assembled from existing models that have already been validated individually and used to address many key scientific problems in planetary and Earth sciences. This coupled-climate-chemistry model is being augmented by interchangeable modules currently under development. These modules consist of geological, exogenic, atmospheric escape, and life process models that will be used to characterize fluxes of heat and trace species at the upper and lower boundaries of a planetary atmosphere.

The VPL will be validated using data derived from terrestrial planets in our own solar system. It will then be used to explore the plausible range of atmospheric compositions and thermal structures, and to generate disk-averaged spectra for extrasolar planets and for early Earth. These models will be run with and without biological processes to improve our understanding of the effects of life on a planet’s atmospheric composition and spectrum. They will also be used to create a spectral catalog that can be used as a statistical sample space to explore the optimum wavelength range, spectral resolution, and instrument sensitivity required to characterize extrasolar terrestrial planets.

Figure 1. The suite of radiative transfer, climate, chemical, geological, and biological component models are shown as boxes,and their interactions with each other are shown as arrows. The information transferred between these component models is labeled at each interface. The order in which these component models are coupled to each other during the course of building the VPL is specified by the Task number. The radiative transfer, climate and atmospheric chemistry models already exist, and the remaining models are under development by our team. At each task development stage the model can be used to generate synthetic spectra to derive required capabilities for astronomical instrumentation.

Progress This Year

Assembling the Virtual Planetary Laboratory

In the second year of this program, we continued our long-term efforts to develop the suite of VPL models. Focusing initially on the architecture of the integration, we defined the basic structure and communication modes for the integrated model. This included defining the requirements for the model-to-model translators and the nature and content of the common database that will hold initialization information, model input and output, and control status flags. Integration of the radiative transfer and climate models is completed, with the climate and chemistry models to follow. The geological, exogenic and biological processes will initially be specified as “state vectors” in the common database, to allow testing on the larger model, and later will be replaced with the full analytical or numerical models.

We have updated our one-dimensional (1-D) radiative-convective equilibrium (RCE) climate model to better support interaction with the geological and life components of the planet. We upgraded the RCE model to incorporate our line-by-line, multiple scattering radiative transfer model in place of simpler band modeling and 2-stream methods. A mixing length approach has been implemented to provide a self-consistent description of the vertical transport of heat and volatiles throughout the environment. Both these new features enhance the model’s ability to provide climate constraints needed for the geological and life processes. We also added a simple cloud model for formation and dissipation of clouds, which will later include precipitation.

Work has also proceeded on defining the “boundary layer” geology and atmospheric escape models. We have planned a series of 1-D reactive transport models to characterize biogeochemical cycling in different environments, which will couple to the atmosphere and lithosphere via flux terms fed through the database. This design draws upon team work performed this year on modeling to analyze Archean paleosol isotopic measurements. This work has improved understanding of elemental cycling on ancient Earth and will provide empirical constraints for VPL models of the Archean Earth.

This year, team members also worked on geology models that link the planetary boundary layer with the planet’s mantle and core. This included the development of physical models to reconcile geochemical observations, and to understand the link between plate tectonics, thermal evolution and degassing for the Earth throughout its history. Team members have also explored the link between planetary thermal evolution and the generation of the planetary magnetic fields that can strongly affect long-term planetary habitability.

Looking to the upper planetary boundary layer, we have developed a new technique to model hydrodynamic loss processes from planetary atmospheres that overcomes previous difficulties and instabilities in modeling supersonic outflow. We have validated a preliminary model, which will be incorporated as an upper boundary module for the VPL. This model can be applied to both terrestrial planets and extrasolar gas giant planets.

To support the VPL life modules, we have conducted both field work and modelling studies. We have continued our field assessments of life in high-pH aquifers and springs associated with ultramafic rocks, an environment analagous to early Earth’s, or an undifferentiated water-rich terrestrial planet. Our objectives are to understand how microbial communities may have fared in primordial environments without prior chemical weathering, to understand nutrient limitations, and to look for biosignatures associated with these communities. Several organisms have been isolated, and a baseline for annual and seasonal variations in geochemical conditions is emerging.

We have also been developing an anaerobic early Archean ecosystem model to understand biological productivity and methane production prior to the origin of oxygenic photosynthesis. The model explicitly accounts for cycling of hydrogen and carbon through primary production by hydrogen utilizing methanogens, fermentation, methanogenesis and photolysis. The ecosystem model has been run in conjunction with an atmospheric chemistry model. Initial results indicate that for a volcanic flux of hydrogen at the modern rate, the atmosphere could contain up to ~350 ppmv of methane if hydrogen utilizing methanogens were the primary producers. This level of methane would produce a strong signature in the spectrum of the planet.

We have also initiated a dynamical model of a microbial ecosystem, which includes interaction with the external environment, including the stellar flux. In this multicomponent model, individual microbial species interact with each other, and their growth is governed by available energy and chemical resources, and limited by space. We continue to explore the stability of these equations, for both the autonomous and environment-coupled system. We are also working on expansion of the model to include stellar-driven photosynthetic life.

In addition to development of the architecture and individual models, we are also gathering spectroscopic information required as input for extrasolar planet models. Both as ongoing support to the planetary modeling efforts, and as part of independent research to explore the concept of the “G star analog” we have combined astronomical observations and photospheric models to create high-resolution, full-wavelength range spectra for stars of different spectral type. Team members have also collated an ongoing, yet already comprehensive, database of spectral line lists and absorption cross-sections for 45 molecules required by planetary atmosphere models.

In addition to initiating the assembly of the VPL itself, our team has also worked on two related proof-of-concept or precursor projects, both of which use component models of the VPL to explore the detectability of biosignatures via remote-sensing techniques. These modeling efforts and results are described below.

Spatially Resolved Planetary Models

The first such project uses the VPL’s radiative transfer model to produce spatially resolved spectral models of planets in our own solar system. The 3-D “datacube” of synthetic spectra that is generated for each planet is being used to determine the effects of spatial and spectral averaging and temporal variability on the detectability of biosignatures, and other spectral features that provide important constraints on a planet’s physical and chemical state, and its potential for habitability. These models are also being created as a validation standard to test progressive development of the larger VPL model. This year, we completed the Mars model, producing disk-averaged synthetic spectra and lightcurves, and used this model and TPF instrument models to understand our ability to detect the CO₂ ice cap in the disk-averaged spectrum of a successively more frozen planetary surface. We are currently working on the comparable Earth model. As an experimental validation component for this project, our collaborators at the Australian Center for Astrobiology (Bailey and Chamberlain) continue to acquire and analyze high-resolution full-disk spectra of Mars and Venus using ground-based telescopes.

Earths Around Other Stars

In collaboration with the Penn State NAI team, we have explored the environments and detectability of biosignatures from extrasolar terrestrial planets similar to the Earth. Using a coupled 1-D radiative-convective climate model, we have determined atmospheric compositions for Earth-like planets with oxygen levels from the present atmospheric level (PAL), down to 1/100,000th PAL, around stars hotter and cooler than our own Sun. We have also run self-consistent models for atmospheric conditions during the Earth’s early Proterozoic. We have the used the VPL radiative transfer model, and the detailed spectrum of the parent star, to generate high-resolution synthetic spectra for these atmospheres, which we have analyzed for detectability using TPF instrument simulator models. These spectra are the first steps in building a spectral library of terrestrial planets for use in TPF mission planning and data analysis.

Significant results included an improved understanding of the behaviour and detectability of ozone and other atmospheric constituents for an Earth-like atmosphere at different oxygen levels. These simulations have also provided significant insight into the combined effects of temperature and trace gas distributions on the detectability of biosignatures, especially around stars of different spectral type, and the results are directly relevant to the Terrestrial Planet Finder mission. As part of this project, we also calculated surface ultraviolet (UV) fluxes and UV dose rates for erythema and deoxyribonucleic acid (DNA) damage. We find that planetary UV surface fluxes are low for high-oxygen planets orbiting the high-UV output F stars, because of the creation of a protective “super” ozone layer. These results will help us to further understand what effects stellar type and planetary atmospheric composition might have on planetary habitability and biological evolution.

Extrasolar Hot Jupiters

We have developed a 1-D chemical model for extrasolar “hot Jupiters” for which the formation of hydrocarbons, oxygen chemistry, and hydrodynamical loss are significantly enhanced because of their environment. Significant results include an improved understanding of the chemical mechanism for H and H₂ production in these unusual planets. The modifications to the chemical model to incorporate new chemistry, database modeling, and to couple it to a hydrodynamic escape code are directly applicable to modifications required for the VPL terrestrial planet models.

Education and Public Outreach

Our EPO team this year has participated in planning meetings for the Cosmic Origins travelling museum exhibit and is developing a framework for a family guide on Life Elsewhere. The EPO team has also run workshops on astrobiology and astronomy at several conferences. Scientific team members have participated in or contributed to many courses, presentations, talks and media interviews, including 7 university courses, 19 university seminars, a talk to middle school students, 7 talks at museums or observatories, and 6 interactions with the media, including interviews for public television. Team scientists have also participated in preparing and reviewing EPO material for the web. Further information about the VPL can be found on our public website, which was launched this year: http://vpl.ipac.caltech.edu.

Figure 2. Plots showing a) atmospheric temperature and b) ozone mixing ratio, as function of altitude for Earth-like planets with successively lower abundances of oxygen in their atmospheres. The blue line shows results for an atmosphere with the present atmospheric level of O₂, and the other lines are labeled with the factor of 10 reduction in oxygen abundance from the present level. These plots show the gradual cooling of the stratosphere, and the descent of the peak concentration of ozone in the atmosphere as a function of the decreasing atmospheric O₂ abundance.

Figure 3. This plot shows the ozone band as a function of atmospheric oxygen abundance for a planet around a G2V star. The ozone column decreases, but remains appreciable down to O₂ levels of 0.01 PAL, but the 9.6um ozone band retains almost the same strength down to this O₂ level because the lower ozone column is offset by enhanced absorption due to the greater temperature difference between the stratosphere and the warm, emitting planetary surface.