The NExSS ROCKE3D Project
The discovery of exoplanets (planets outside our Solar System) has occurred at a rapid pace. Some of these planets may be habitable at their surfaces, in the sense that they are warm enough to sustain liquid water, a necessary ingredient for life as we know it. Knowing which ones are most likely to be habitable would facilitate the search for life. However, to date exoplanets have only been observed indirectly — through detection of the gravitational wobble due to 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 next-generation space observatories are able to directly measure spectral details, we must rely on theoretical understanding and on planets closer 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, especially Earth, Mars, and Venus, has driven early thinking about concepts such as the "habitable zone" that have traditionally been applied to evaluate the habitability of planets discovered orbiting other stars. Three-dimensional (3D) planetary general circulation models (GCMs) derived from the models that we use to project 21St Century changes in Earth's climate can now be used 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 new NASA Nexus for Exoplanet System Science (NExSS), a cross-discipline effort that brings 17 teams of astrophysicists, heliophysicists, planetary scientists, and Earth scientists together to understand all the factors that determine exoplanet habitability, from stars, to their protoplanetary disks, to the rocky planets that form from them, to the atmospheres that determine their climates, to the planet-star interactions that determine which atmospheres remain stable over geologic time. The ROCKE3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics) team, a multi-institution collaborative project, is addressing these questions as part of NExSS. GISS's role in this project is to perform 3D 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 that are part of our 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 NASA Goddard Space Flight Center Haystacks project).
Further support for GCM simulations of other planets and periods in history is provided by the NASA Planetary Atmospheres, Exobiology, and Habitable Worlds Programs, and by internal GSFC Science Task Group funding.
NASA/GISS: Anthony Del Genio, Michael Way, Nancy Kiang, Andrew Ackerman, and Gavin Schmidt.
Columbia University: Caleb Scharf, Linda Sohl, Mark Chandler, Igor Aleinov, Kostas Tsigaridis, David Amundsen, and Sean Solomon.
Trinnovim LLC: Maxwell Kelley.
NASA/GSFC: Shawn Domagal-Goldman, Thomas Clune, Aki Roberge, Alexander Pavlov, Avi Mandell, Melissa Trainer, and Michael MacElwain.
NASA Postdoctoral Program: Yuka Fujii Ebihara (GISS) and Scott Guzewich (GSFC).
Other institutions: Rory Barnes (U. Washington), John Armstrong (Weber State U.), Mark Marley (NASA/ARC), Roxana Lupu (BAER), Ty Robinson (UCSC), Mark Bullock (SWRI), David Grinspoon (PSI), Christopher Stark (STSci), and Rebecca Oppenheimer (AMNH).
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 by Nancy Kiang 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. Development is ongoing to include dust as well as CO2 and H2O clouds, but the model already has CO2 condensation and dynamic atmospheric mass, as well as the correct Martian calendar, insolation, planet mass, rotation rate, and most atmospheric constituents. The final model will allow us to address questions related to the history of water on Mars raised by current spacecraft observations.
2.) Inner Edge of the Habitable Zone: As seen in Fig. 2 above we have successfully replicated many of the results from Yang et al. (2014; Astrophys. J., doi:10.1088/2041-8205/787/1/L2) showing the effects of slow rotation and higher insolation. For day lengths equivalent to 64 times present day earth length it is possible to reach Paleo-Venus type insolation, and for 128 and 256 day lengths it is possible to reach present day Venus insolation, while maintaining surface temperatures that are conducive to liquid water.
3.) Snowball Earth: We have simulated the Sturtian glaciation that occurred 715 million years ago. The geography and climate drivers were radically different than today, with a greatly altered continental configuration, insolation (94% today's value), and potentially much less CO2 (40 ppm). The sea ice front advances to about 30° latitude, leaving a broad swath of open ocean in the tropics with sea surface temperatures at the equator as warm as 12°C. Simulations of the earlier Huronian glaciation (2.1 billion years ago), when the Sun was even weaker, result in a fully ice-covered Earth.
The Astrobiology group at GISS was formed in 2013 from a GSFC "Science Innovation Fund" effort to bring together planetary and Earth scientists, astronomers and computer scientists with interests in astrobiology, paleoclimate, and comparative planetary climates to develop 3D planetary models. Unique compared to efforts elsewhere around the world, our group leverages the in-house team of developers that support the current GISS Earth GCM (Model E2).
Way, M.J., A.D. Del Genio, N.Y. Kiang, L.E. Sohl, D.H. Grinspoon, I. Aleinov, M. Kelley, and T. Clune, 2016: Was Venus the first habitable world of our solar system? Geophys. Res. Lett., in press, doi:10.1002/2016GL069790.