NASA's Exoplanet Nexus — Part II: Looking to the Stars
+ This is part 2 of a 2-part feature. You can read Part 1 here.
NASA's Goddard Institute for Space Studies (GISS) has a long history in climate studies, which led to the development of one of the most accurate and complete models of Earth's global climate, the General Circulation Model (GCM). Now, scientists at GISS are lending this expertise to a new NASA initiative that will to coordinate research on exoplanets.
The GCM may be one of the most useful tools for studying Earth's own climate, but a great deal of research on other planetary bodies, including Venus, helped shaped the way we now think about planetary climates (see Part I). Experts at GISS are turning their sights away from our home planet once again as part of the Nexus for Exoplanet System Science (NExSS), and will provide key insights into the atmospheres and climates of planets that orbit distant stars.
Developments in Comparative Planetology at GISS
While GISS was opening the eyes of the world to new areas of Earth systems science in the 1990s (see Part I), the research roots in planetary science continued. What began with climate studies of Venus continued for planets like Mars as new space missions sent back huge amounts of data about Earth's other planetary cousin. Scientists who were planning space missions also benefited from the clearer picture of Earth that GISS was helping build.
"Planets in our Solar System with atmospheres can be divided into two types: What we call rocky planets, i.e., those with solid surfaces and relatively thin atmospheres, and giant planets, which are primarily gaseous with very thick atmospheres," explained Anthony Del Genio, a planetary scientist at GISS. "Only planets in the first category (Earth, Mars, Venus) might be habitable now or might have been at some time in their past, so those are the ones we are interested in for the NExSS project."
By 1993, the NASA/GISS GCM was being applied to Mars. In 1999, GISS's Michael Allison and colleagues used an updated Mars-adapted version of the GCM to simulate martian meteorology through the seasons as the planet circled around the Sun4,5. Allison and Megan McEwan of the Department of Astronomy at Columbia University also used data from the NASA Pathfinder mission to determine the exact length of a martian day (sol)6. Studies like these have helped inform many missions to Mars. This work has also been a benchmark for understanding Mars' past and present climate, and is essential in determining whether or not life was possible on ancient Mars.
When the Cassini mission shot through the rings of Saturn, Del Genio and colleagues used the knowledge gained from Earth studies to better understand bodies in the Saturn system7.
"The type of climate model one uses to simulate Earth is fundamentally different from the type of model one uses to simulate the giant planets (Jupiter, Saturn, Uranus, Neptune), so those would not be appropriate targets for us to apply the GCM," explained Del Genio. "That having been said, we have learned quite a bit about Saturn from Cassini. As different as it is from Earth, it has some things in common with Earth."
Saturn has a dynamic atmosphere, and Cassini sent back photos of spectacular cloud formations. Scientists had assumed that these beautiful, swirling patterns were created by a jet stream that pushed the clouds around. However, studies from GISS proved otherwise. Using an algorithm to track clouds in images from Cassini, Del Genio and his colleagues concluded that the opposite process was happening. The jet stream in Saturn's atmosphere was actually powered by huge, rotating storms known as eddies.
"Processes that maintain Saturn's jets are basically the same processes that are responsible for the low and high and pressure centers in mid-latitudes on Earth," said Del Genio. "This is responsible for much of our weather and helps maintain our jet stream.
Saturn's moon Titan, another target of the Cassini mission, provided an opportunity for more direct comparisons with Earth. Titan has an incredibly dense atmosphere that once shrouded its surface in mystery. Cassini released the Huygens probe toward Titan when the spacecraft entered the Saturn system. As it descended through the atmosphere, Huygens revealed Titan as a world with a strange and active climate.
"By studying Saturn's moon Titan, which although being an icy moon can be considered something like the Solar System's rocky planets, we have discovered rainstorms produced by methane rather than water, and we have found that Titan's meteorology has things in common with Earth's tropics," said Del Genio.
Studying the climates of other worlds in the Solar System helps scientists test their theories about how the Earth works as a whole. We know more about Earth's climate than any other planet, but using this knowledge to predict weather and climate on other planets is a true test of how well we actually understand the fundamental physics. The new developments in Earth System Science associated with aerosols and chemistry that were developed in recent decades turn out to be crucial for generalising that understanding for Solar System bodies like Titan.
"Generalizing an Earth climate model to simulate other planets is a kind of 'stress test' for the model. By taking the model to a different planetary environment, extreme climate conditions are magnified and inaccuracies not previously known are detected," said Del Genio. "Our early efforts to generalize our climate model for other planets have identified several such problems and allowed us to go back and make improvements that produce a better Earth model."
Beyond the Solar System
In 1995, astronomers confirmed that a massive gas giant planet was orbiting the star 51 Pegasi. Since then, many hundreds of planets big and small have been detected around distant stars.
"Now that we are faced with trying to characterize the habitability of the multitude of planets that are being discovered in other stellar systems, the need is greater than ever to place Earth's climate in a larger context," said Del Genio. "We study planets to learn about them, not to learn about Earth, but what that does is give us a perspective on Earth that we would not otherwise have.”
Astrobiologist Nancy Kiang was the motivating force behind gathering GISS scientists to explore theories about exoplanet atmospheres, climates and habitability. She had been conducting studies of the diversity of life on Earth to figure out how processes such as photosynthesis might provide alternative signs of life if adapted to stars that are different from our own Sun8,9.
"I saw the need to be able to constrain potential alien photosynthesis with climates consistent with the parent star and atmosphere," Kiang said, "and that gathering the climate experts here at GISS was the next important step." Studying the climates of well-observed Solar System planets provides the necessary foundation for understanding habitability of exoplanets for which there will be less detailed information.
As technology improves and our observations of exoplanets become more and more detailed, GISS is positioned to play a key role in comparative planetology beyond the Solar System. Earth systems science at GISS is now well established, and the time has come to apply this knowledge to a new area of exoplanet systems science. Researchers at GISS hope to adapt the originally Earth-based climate model to newly discovered exoplanets in the same way it has been used with Venus and Mars.
The Nexus for Exoplanet System Science (NExSS)
There are now almost 2,000 exoplanets that have been confirmed by long-range telescopes, such as Hubble and Kepler. With each passing month, the catalogue of planet discoveries continues to grow. Astronomers have found gas giants with orbits that hug close to their host star, and super-sized Earths that might be capable of hosting life as we know it. There are planets of varying densities, some light and fluffy and others that could be as hard as diamond. In the past decade our eyes have opened to the incredible variety of planet-types that exist in our universe.
Scientists now hope go beyond simply spotting extrasolar planets, and to begin studying the potential climates of these fantastic worlds. We do not yet have the technology to observe extrasolar planets directly, or with resolutions similar to the observations we make of planets in our own solar system. However, with data on their size, density and orbits, we can begin to understand their potential environments by adapting our three-dimensional Earth system models.
NASA has decided to form a coordinated network of exoplanet research in order to bring together expertise from different programs at NASA and other institutions in the United States. The Nexus for Exoplanet Systems Science (NExSS) is a new inter-divisional initiative managed by the NASA Science Mission Directorate (SMD).
"Exoplanetary science has grown explosively, which is great, but that can also fragment the science," said Caleb Scharf, director of astrobiology at Columbia University, a NExSS partner. "Chasing habitable planets is going to be extremely challenging, but if we can bring coherence to the effort we stand a much better chance of not just advancing our understanding but also making a good case for the kind of resources that are going to be necessary."
The virtual network will support geographically widespread communication, research, training and educational activities between groups of exoplanet investigators working in different disciplines, divisions and organizations. The institutions involved include GISS, Goddard Space Flight Center, Columbia University, NASA Ames Research Center, University of Washington, Weber State University, Planetary Science Institute, Southwest Research Institute, and the American Museum of Natural History. The goal of the initiative is to foster new collaborations that will address interdisciplinary topics in the study of extrasolar planets.
"NExSS is a visionary new approach to doing science at NASA," said Anthony Del Genio. "Science at NASA has traditionally operated within disciplinary categories: planetary science, Earth science, astrophysics, heliophysics. The emergence of exoplanet research, spurred by the breath-taking discovery of so many new planets in the past decade, is at once tremendously exciting and at the same time unbelievably challenging, given the difficulty in detecting and characterizing these planets from so great a distance."
NExSS will help lay the groundwork for answering important questions about how to search for signatures of life on distant worlds using existing technology. The initiative will also help determine what future technology we need in order to identify truly Earth-like planets among the stars.
"The role of GISS in NExSS is to use our climate model to characterize other planetary climates and to help assess their habitability," said Del Genio. "We will use past eons in the history of Earth, Mars, Venus and Titan as snapshots of climates that might be relevant to conditions on exoplanets."
With input from partnering institutions, researchers at GISS will use their simulations as a starting point for studying both known and hypothetical exoplanets. Their hope is to produce a library of potential planetary climates in the Universe, which can then be used for research on habitability and planet detection. Ultimately, this work will inform the development of future exoplanet missions.
Del Genio sums it up: "What we at GISS bring to this effort is not only a useful tool — our global climate model and many of the people who develop it — but also a scientific perspective grounded in many years of thinking about Earth's climate and Solar System climates that we hope will complement the different perspectives of the other NExSS teams."
Gavin Schmidt, the current director of GISS and collaborator on the NeXSS effort highlighted the connections between the new effort and GISS’s roots: “The study of planetary climates, wherever they are, involves the basic sciences of radiative transfer, atmospheric composition and circulation. It is tremendously impressive that the tools and basic research that GISS developed in the 1960s to understand Venus came to inform our understanding of Earth. GISS is now applying advances in climate simulations from the 1980s to known exoplanets in order to accelerate new discoveries.”
GISS Science Brief: Simulating Martian Weather with the GISS GCM
GISS Software: Mars24 Sunclock — Time on Mars
4 Allison, M., 1997: Accurate analytic representations of solar time and seasons on Mars with applications to the Pathfinder/Surveyor missions. Geophys. Res. Lett., 24, 1967-1970, doi:10.1029/97GL01950.
5 Allison, M., J.D. Ross, and N. Solomon,1999: Mapping the Martian meteorology. In Fifth International Conference on Mars. LPI Contribution No. 972. Lunar and Planetary Institute. Houston.
6 Allison, M., and M. McEwen, 2000: A post-Pathfinder evaluation of aerocentric solar coordinates with improved timing recipes for Mars seasonal/diurnal climate studies. Planet. Space Sci., 48, 215-235, doi:10.1016/S0032-0633(99)00092-6.
7 Del Genio, A.D., J.M. Barbara, J. Ferrier, A.P. Ingersoll, R.A. West, A.R. Vasavada, J. Spitale, and C.C. Porco, 2007: Saturn eddy momentum fluxes and convection: First estimates from Cassini images, Icarus, 189, 479-492, doi:10.1016/j.icarus.2007.02.013.
8 Kiang, N.Y., J. Siefert, Govindjee, R.E. Blankenship, and V.S. Meadows, 2007: Spectral signatures of photosynthesis I: Review of Earth organisms. Astrobiology, 7, 222-251, doi:10.1089/ast.2006.0105.
9 Kiang, N.Y., A. Segura, G. Tinetti, Govindjee, R.E. Blankenship, M. Cohen, J. Siefert, D. Crisp, and V.S. Meadows, 2007: Spectral signatures of photosynthesis II: Coevolution with other stars and the atmosphere on extrasolar worlds. Astrobiology, 7, 252-274, doi:10.1089/ast.2006.0108.