NASA Spacecraft Maps Earth's Global Emissivity

Our planet emits energy as thermal infrared radiation at wavelengths we cannot normally see but can feel -- for example, if we stand close to a hot surface. The strength of the energy emitted depends on both the temperature of the surface and how efficiently it can emit radiation, known as its emissivity. The emissivity of most natural Earth surfaces is a unitless quantity and ranges between approximately 0.6 and 1.0, but surfaces with emissivities less than 0.85 are typically restricted to deserts and semi-arid areas. Vegetation, water and ice have high emissivities above 0.95 in the thermal infrared wavelength range.

Certain NASA sensors such as the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA's Terra spacecraft, the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Terra and Aqua spacecraft, and the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft can sense changes in emissivity. This emissivity information is used to help measure the water vapor in the lower atmosphere and how much energy is emitted to balance Earth's energy budget. A small error in emissivity can result in a large error in the models used to predict climate change.

ASTER has been making very detailed measurements of the emissivity of Earth's surface since 2000. Recently NASA's Jet Propulsion Laboratory, Pasadena, California, used all the cloud-free images from ASTER to generate the most detailed global map ever of the land surface of Earth, known as the ASTER Global Emissivity Database (ASTER GED). ASTER GED provides an average emissivity of the surface every 100 meters at five different wavelengths in the thermal infrared. This map is approximately 100 times more detailed than any previous emissivity map. ASTER GED is being used to improve estimates of surface temperature and atmospheric water vapor and improve the accuracy of climate models.

The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate, Washington, D.C.

More information about ASTER is available at http://asterweb.jpl.nasa.gov/.