The discovery of planets outside our Solar System continues at a rapid pace, and some of these planets may potentially be habitable1. However, these planets have been observed through indirect means — through detection of the gravitational wobble due the tug between planet and parent star, or through the dimming of the star's light as the planet passes in front of it. Until the next-generation space observatories are able to directly measure spectral details, we must rely on theoretical understanding and on planets close to home to ascertain the possibility of water and life for diverse planets elsewhere.
The Solar System is home to the only known inhabited planet: Earth. Our Solar System's history, including Earth and the other terrestrial planets, has driven early thinking about concepts of planetary habitability, such as the "habitable zone", concepts that scientists apply to evaluate the habitability of planets discovered orbiting other stars. Three-dimensional (3D) dynamic models are now needed to address outstanding questions about how Earth became and remained habitable despite wide swings in Solar radiation, atmospheric chemistry, and other climate forcings; whether these different eras of habitability manifest themselves in signals that might be detected from a great distance; whether and how planets such as Mars and Venus were habitable in the past; how common habitable exoplanets might be; and how we might best answer this question with future observations.
The NASA Goddard Institute for Space Studies (GISS) is part of the ROCKE3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics) team, a multi-institution collaborative project to address these questions as a member of a new NASA program, the Nexus for Exoplanet System Science (NExSS). GISS's role in this project is to perform 3D general circulation model (GCM) simulations of past climates of Earth and other rocky Solar System planets to broaden our understanding of planetary habitability, to use similar simulations to assess the habitability of rocky exoplanets, and to produce synthetic disk-integrated spectra of these planets.
Our project will use Solar radiation patterns and planetary rotation rates from simulations of spin-orbit dynamical evolution of planets over Solar System history provided by our colleagues at the Columbia Astrobiology Center and at other institutions in the NExSS team. In turn, the synthetic disk-integrated spectra we produce from the GCM will be used as input to a whole planetary system spectral model that emulates observations that candidate future direct imaging exoplanet missions might obtain (See the HAYSTACKS project). Further support for GCM simulations of other planets and periods in history is provided by the NASA Planetary Atmospheres and Exobiology Programs.
Photosynthesis and Astrobiology
Photosynthesis produces signs of life we can see from space at the planetary scale: atmospheric oxygen and the vegetation "red-edge," a spectral reflectance signature of vegetation on land. Oxygen is produced through the splitting of water molecules during photosynthesis. The vegetation "red-edge" is so-called because healthy foliage absorbs light in the red with chlorophyll a (Chl a), which contrasts with strong scattering in the near-infrared. However, the Earth harbors a greater diversity of photosynthetic organisms than vascular plants, and includes algae, cyanobacteria, and anoxygenic photosynthetic bacteria, all of which occur in a wide array of colors, due to adaptation and acclimation to different light and chemical environments. This diversity raises questions about how photosynthesis developed on Earth, and provides clues as to what might dominate over the course of a planet's history and yield alternative "biosignatures" on a planet orbiting a different kind of star from our Sun.
At GISS, through membership in the Virtual Planetary Laboratory team of the NASA Astrobiology Institute, research is probing the long wavelength limit of oxygenic photosynthesis. The cyanobacterium Acaryochloris marina is being studied as a model organism for alternative pigment signatures of oxygenic photosynthesis adapted, for example, to the light of red dwarf stars. As recently as 1996, Japanese scientists discovered A. marina living on the light left over by Chl a organisms and able to stretch into the far-red and near-infrared through the novel pigment Chl d instead of Chl a. Research at GISS has quantified the photon energy use efficiency of A. marina in comparison to Chl a organisms, and is now turned toward investigating its light regime in nature to ascertain its kinetics of light use and competitive ecological niche. The above work informs NExSS GCM modeling investigations with alternative stellar spectra, the Young Sun as well as other star types. Simulation experiments aim to understand how different irradiance spectra to a planet affect its habitability and climate dynamics and its radiance signature, due to both the spectral absorbance properties of different atmospheres and different surface spectral albedos resulting from the gradual spread of life over land and adaptations of photosynthetic pigments to other light regimes.
1.) Mars: Development continues on a "desert world" that mimics numerous aspects of the current Martian atmosphere. It is currently lacking Dust, CO2 and H2O clouds, but has CO2 condensation and dynamical atmospheric mass, as well as the correct Martian calendar, solar insolation, planetary mass, rotation rate, and most atmospheric constituents.
2.) Probing the Inner Edge of the Habitable Zone. As seen in the figure above we have successfully replicated many of the results from Yang et al. 2014 showing the effects of slow rotation and higher solar insolations. For day lengths equivalent to 64 times present day earth length it is possible to reach Paleo-Venus type solar insolations, and for 128 and 256 day lengths it is possible to reach present day Venus solar insolation.
3.) Snowball Earth: A preliminary set of experiments has been started to simulate the Sturtian glaciation. At the time of this glaciation, 715 million years ago, the geography and climate drivers were radically different than today, with a greatly altered continental configuration, solar insolation, and potentially large variations in atmospheric CO2. In our first experiment using S0X=0.94 and 40 ppm CO2 the sea ice front has advanced to about 30° latitude in both hemispheres after 700 years of simulation. This leaves a broad swath of open ocean in the low latitudes with sea surface temperatures along the equator remaining as warm as 12°C. Additional experiments are planned that will explore the contributions of rotation rate, CO2 surface condensation, and both water and CO2 clouds.
GISS/Columbia University core Team Members: Anthony del Genio [GISS] (Clouds), Nancy Kiang [GISS] (Photosynthesis), Linda Sohl [CU] (Paleo Earth/Geology), Mark Chandler [CU] (Paleo Earth), Caleb Scharf [CU] (Orbital Dynamics), Michael Way [GISS] (Astrophysics/Development), Maxwell Kelley [GISS] (Development), Igor Aleinov [CU] (Development), Kostas Tsigaridis [CU] (Chemistry), Andrew Ackerman [GISS] (Clouds), Gavin Schmidt [GISS].
GSFC core Team Members: Tom Clune (Development/Orbital Mechanics), Shawn Domagal-Golden (Mars), Scott Guzewich (Mars), Michael MacElwain (Exoplanet Missions), Avi Mandell (Exoplanets), Alex Pavlov (Mars), Melissa Trainer (Planetary Aerosols).
The Astrobiology group at GISS was formed in January 2013 as part of a GSFC "Science Innovation Funding" effort to bring together Planetary and Earth Scientists, Astronomers and Computer Scientists with interests in astrobiology, paleoclimate, and comparative planetary climates. The rapidly growing discoveries of exoplanets since the 1990s have brought to light the need for 3D GCMs to understand the climate phenomena of diverse planets, and the prime motivation of the GISS Astrobiology group is to fill that need with the first such community rocky planet climate model. Almost unique to efforts elsewhere around the world, the group leverages the in-house team of developers that support the current GISS Earth GCM (Model E2). Currently the group is funded via a GSFC "Science Task Group", the Planetary Atmospheres Program, the Exobiology Program. The group was recently informed that it will be a member of a new NASA program called the Nexus for Exoplanet System Science.