Earthshine: Earth's Spectra, Albedo, Clouds, Climate, and the Search for Earth-Like Exoplanets
Research by Traub, W A (wtraub@jpl.nasa.gov) , Jet Propulsion Laboratory, 4800 Oak Grove Drive, M/S 301-451, Pasadena, CA 91109 United States; Kaltenegger, L (lkaltenegger@cfa.harvard.edu) , Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 United StatesTurnbull, M C (turnbull@dtm.ciw.edu) , Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20008 United States Jucks, K W (kjucks@cfa.harvard.edu) , Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 United States
Earth-like planets around distant stars may be too far away to be reached by spacecraft, but scientists could still investigate whether they harbor life.
The search for life on extrasolar planets requires us to discover terrestrial-mass planets around nearby stars, and to characterize these planets for habitability and signs of life.
The Terrestrial Planet Finder missions -- a coronagraph (TPF-C) and an Interferometer (TPF-I in the US, also Darwin in Europe) -- are designed to carry out these tasks.
In the visible and near-infrared, TPF-C could measure O2, H2O, O3, Rayleigh scattering, and the red-edge reflection of land planet leaves; on an early-Earth twin it also could measure CO2 and CH4. In the mid-infrared, TPF-I/Darwin could measure CO2, O3, H2O, and temperature. To validate some of these expectations, Traub et. al. observed Earthshine spectra in the visible and near-infrared, and modeled these spectra with their line-by-line radiative transfer code.
They found that the major gas and reflection components are present in the data, and that a simple model of the Earth is adequate to represent the data, within the observational uncertainties. They also determined that the Earth appears to be habitable, and shows signs of life. However to validate the time variable features, including the continent-ocean differences, the presence of weather patterns, the large-scale variability of cloud types and altitude, and the rotation period of the planet, they need to obtain a continuous time-series of observations covering multiple rotations; these observations could be carried out in the coming years, using, for example, a site on the surface of the Moon.
Implications
Essential to these observatories' success will be a new generation of instrumentation capable of seeing past the blinding glare of the parent star to pick out only the faint light reflected off the distant world's surface. It is a hard task -- the parent star is likely to be a billion to 10 billion times brighter than its tiny companion -- but recent experiments suggest the technologies are getting close to the sensitivities required.
Significance to Solar System Exploration
Traub et al's modeling has tried to work out what the Earth's planet shine would have looked like at various stages in geologic history -- to get a set of "profiles" planet hunters could use to gauge what stage in the evolution of life a newly discovered world might have reached. Through these experiments, one can calculate the chemical composition of what existed before life arose on Earth. There would not be any oxygen and no ozone line, because the ozone comes from oxygen. As an Earth-twin evolves, CO2 disappears, and oxygen increases, leading to a possible formation of animal life.
Written by Samantha Harvey
For more information about NASA Science Highlights and information on publication, please contact Samantha Harvey, Samantha.K.Harvey@jpl.nasa.gov.
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