Initial data from the Mars Global Surveyor thermal emission spectrometer experiment: Observations of the Earth

The thermal emission spectrometer (TES) on the Mars Global Surveyor spacecraft acquired observations of the Earth from a distance of 4.7 million km for instrument performance characterization on November 24, 1996. The data were calibrated using an internal reference surface and deep space in a manner identical to that which will be used in Mars orbit, and scaled to account for the fact the Earth filled only 9.3% of the field of view. These first, calibrated, in-flight spectra from the TES confirm the expected instrument performance and radiometric accuracy and precision. The data provide the firs known whole-disk thermal infrared spectral observations of the Earth. These spectra represent how an Earth-like planet would appear during a search for "extrasolar" planets. Spectral features in the Earth's atmosphere are readily apparent; CO₂ (centered at 668 cm^-1), ozone (1000-1075 cm^-1), and water vapor (200-550 and 1260-1650 cm^-1) absorptions are evident. Radiation at the center of the CO₂ band arises mainly from the lower stratosphere; near 650 and 700 cm^-1 from near the tropopause; and further into the band wings from the troposphere and surface. Thus, in the disk-averaged sense, the spectrum indicates a warm stratosphere above a tropopause somewhat colder than 215 K, in good agreement with results from similar instruments previously flown in low Earth orbit. The atmospheric window between approximately 800 and 1200 cm^-1 is relatively featureless, as expected, given the observing geometry centered over the Pacific Ocean and the partial obscuration by clouds. The derived window brightness temperature, assuming an emissivity of unity, is 270 K, a reasonable average temperature of the ocean surface, the polar regions, and the cloud tops, especially given the uncertainties in the exact scaling to account for the fact that the Earth did not fill the field of view.