NASA's Exoplanet Nexus — Part 1: A History in Climate Studies
Every time a new exoplanet is spotted orbiting a distant star, there's one question in the back of our minds: "Could it have life?"
With each new exoplanet discovery, astronomers are getting closer and closer to finding Earth-like planets among the stars, but we still lack the technology to find a planet with life. A new inter-divisional initiative at NASA has been announced to coordinate research on exoplanets and to push forward efforts to pinpoint planets that could be habitable.
The Nexus for Exoplanet System Science (NExSS) includes researchers at institutions around the country, each studying different aspects of the habitable planet puzzle. For its part, NASA's Goddard Institute for Space Studies (GISS) is providing expertise in the area of exoplanet atmospheres and climate. GISS has been a key player in the study of planetary climates and atmospheres for decades, and this work will be a major component of NExSS.
A History of Planetary Atmospheres
GISS's history with planetary atmospheres dates back to the founding of the institute in the 1960s, including the early work of the previous director, James Hansen. Hansen’s reputation is mostly based on his work in climate science on Earth, but his early work focused on understanding the climate of Venus.
Hansen arrived at GISS in 1967 after his doctorate at the University of Iowa, where he studied astrophysics as part of a NASA trainee program under James Van Allen (the namesake of Earth's Van Allen radiation belts).
Despite being a similar size to Earth, Venus's environment is vastly different. Conditions on the surface are a blistering 863°F (462°C), and the atmosphere is filled with clouds of sulphuric acid, rendering the planet uninhabitable. Venus also has the densest carbon dioxide atmosphere in the Solar System. Hansen modelled the way sulphate particles ("aerosols") and an extremely high concentration of carbon dioxide combine to produce Venus' extreme climate.
Combinations of these theories and data from the Pioneer Venus mission revealed important insights into the composition of Venus' clouds and how they behave, adding further knowledge of Venus' global climate.
Around the same time, NASA's Voyager missions were dipping and diving through the Outer Solar System, where they paid a visit to the planet Jupiter. Data from Voyager and ground-based telescopes on Earth revealed a complex atmosphere surrounding the giant planet, with separate cloud systems spiralling around at different altitudes (1).
With views of Venus and Jupiter, our understanding of planetary atmospheres expanded beyond the Earth. Scientists drew comparisons between the planets, providing a new perspective on the dynamics of Earth's climate and atmospheric processes. Anthony Del Genio, a planetary scientist who arrived at GISS in 1978 as a postdoctoral researcher working on the Pioneer Venus Orbiter mission, said the broader viewpoint led to new thinking about Earth's place in the cosmos.
"Throughout the history of planetary science, going all the way back to the Mariners, Pioneers, and Voyagers, we have been continually surprised and humbled by what we found the first time we visited a planet," said Del Genio. "That humility is good because it opens our minds to think about things more broadly and fundamentally. Then, as we better understand other planets, it allows us to place the Earth in a larger context."
As early as the 1970s, Hansen's work on Venus was being adapted to Earth. In a 1971 publication in the journal Science, Ichtiaque Rasool and Stephen Schneider used Hansen's Venus Mie scattering code to study how light is scattered by spherical particles in Earth's atmosphere (2). It was the early days of a new scientific field known as comparative planetology. Data from vastly different planets could be compared and contrasted, providing an entirely new perspective on how planetary climates work.
Mission to Planet Earth
In the 1990s, NASA introduced a new initiative called Mission to Planet Earth (MTPE). In addition to designing missions that would launch to distant worlds, MTPE also directed NASA resources inward to study our own planet and its habitability. The goal of MTPE was to see the Earth as a whole system, and to identify the mechanisms behind both natural and human-induced changes in the global environment. In designing MTPE, NASA pioneered a new discipline known as Earth system science.
Space-based satellites like the Terra (EOS AM-1) mission provided a unique vantage point above our planet to observe the atmosphere, oceans and land. GISS research played a key role and climate models developed at the center were put to use in studying our planet's global environment. These models helped us understand how all the pieces of Earth's environment — from oceans to atmosphere, geosphere to biosphere — fit together.
NASA GISS began a dedicated effort of global climate modeling aimed at coupling together atmospheric and oceanic models of the Earth, two of the most prominent regions that affect our planet's climate. The General Circulation Model (GCM) at GISS in the 1980s was one of the earliest models to provide predictions of climate change using simulations of the atmospheric circulation. The important drivers of current change on Earth — aerosols and greenhouse gases — turned out to be the same as those that had proved crucial to understanding the Venusian atmosphere.
Combined with evidence of on-going global warming, these studies showed that increasing greenhouse gases would have a lasting impact decades into the future and could have dramatic effects on life. These eye-opening results pushed the issue of Earth's changing climate to the forefront of public awareness.
In the 1990s, NASA GISS continued to improve the realism of the climate model. With the work of scientists like Dorothy Koch and Drew Shindell, more and more interactions were added to the calculations, including detailed information about the composition of aerosols and the chemical reactions occurring in Earth's atmosphere.
"I arrived at GISS as a postdoc in the mid-1990s and at that point the aerosols in the climate model came from a simple radiative treatment based on offline simulations, so none of the variability that occurs in the real world was captured," said Dorothy Koch, now with the Climate and Environmental Sciences Division at the U.S. Department of Energy in Washington.
Koch noted that one exception to the lack of real-world variability was the addition of dust by Ina Tegen, now of the Leibniz Institute for Tropospheric Research in Germany. Tegen showed that wind and moisture affected the sources of dusty regions, and that dust played a role in affecting radiation.
Rainout of dust was treated simply because dust is not soluble (i.e., it doesn't dissolve in rain). The same is not true for molecules amongst the clouds. The aerosol sulfate, for instance, can fall out of the sky when it dissolves in rain. Koch adapted Tegen's code for atmospheric dust so that the behavior of aerosols could also be studied.
"Adding aerosols required embedding aerosol chemistry and removal carefully into the cloud processes, such as condensation, updraft/downdraft motion, and evaporation," said Koch. "It also required addition of new deposition processes, which Drew Shindell and I worked on together."
An ever-growing team of scientists at GISS began contributing to the GCM, improving the model with each additional component that was added. The additions included sulphate, sea-salt, black and organic carbonaceous aerosols, and others.
Susanne Bauer of GISS and Columbia University added nitrate aerosol chemistry as well as aerosol microphysics, which provided insight into the mixing and size distribution of aerosols. Kostas Tsigaridis, also of GISS and Columbia, brought in secondary organic aerosol chemistry. Surabi Menon (GISS/Columbia) led an effort show how aerosols affect the brightness and lifetime of clouds. Koch showed how black carbon deposits affect snow albedo, a measure of the amount of sunlight reflected off a snowy surface. Another important contribution made under the direction of Jim Hansen was the use of emissions data to include historical sources of aerosols that have been added to the Earth's atmosphere over time.
"I worked with Jim and other scientists who were experts on combustion and soot, like David Streets and Tica Novakov, to assemble a carbonaceous aerosol emission history for the past 150 years," said Koch. "As a result of this work, GISS was the first group to include carbonaceous aerosol histories in the Intergovernmental Panel on Climate Change's Coupled Model Intercomparison Project experiments."
This historical view helped paint a picture of humankind's role in shaping Earth's climate over the past 150 years.
Another important first at GISS was the “tagging” of aerosols by source regions and types. This allowed scientists to determine which regions of the globe were having the biggest impact on Earth's climate.
"Jim and I wrote a paper showing that significant black carbon from Asia travels to the Arctic where it could have major climate-warming impacts," said Koch (3). "The community continues to react to that result."
All this work culminated in the GCM we have today, which remains one of the most accurate and complete models of Earth's global climate, and is an essential tool for studying Earth system science.
+ This is part 1 of a 2-part feature. You can read Part 2 here.
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1 Flaser, F.M. and P.J. Gierasch, 1986: Mesoscale waves as a probe of Jupiter's deep atmosphere. J. Atmos. Sci., 43, no. 22, 2683-2707, doi:10.1175/1520-0469(1986)043&lht;2683:MWAAPO>2.0.CO;2.
2 Rasool, S.I., and S.H. Schneider, 1971: Atmospheric carbon dioxide and aerosols: Effects of large increases on global climate. Science, 173, 138-141, doi:10.1126/science.173.3992.138.
3 Koch, D., and J. Hansen, 2005: Distant origins of Arctic black carbon: A Goddard Institute for Space Studies ModelE experiment. J. Geophys. Res., 110, D04204, doi:10.1029/2004JD005296