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Project Progress

The first generation of NASA and ESA space-based observatories to characterize extrasolar terrestrial planets will only be able to obtain disk-averaged spectra. This year, we completed a 3-D model of the Earth’s environment, complete with clouds, and used it to simulate its disk-averaged spectrum and lightcurves (Tinetti et al., 2005). This model includes realistic surface and atmospheric properties from NASA Atmospheric Infrared Sounder (AIRS), and moderate-resolution Imaging Spectrophotometer (MODIS) products and clouds from the International-Satellite-Cloud-Climatology-Project (ISCCP) products. The model was validated against disk-averaged visible to infrared observations of the Earth taken by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES), the ESA Mars Express Omega instrument, and ground-based observations of earthshine reflected from the unilluminated portion of the Moon. This comprehensive model produced excellent fits in all three cases and is versatile enough to be used to model the geometry, illuminated and cloud cover on the day an Earthshine dataset is observed. Comparison between the data and the model indicate that several atmospheric species can be identified in disk-averaged earth spectra. At optical wavelengths (0.4-0.9 microns) O₃, H₂O, O₂ and the oxygen dimer (O₄) are clearly apparent. In the mid-IR (5-20 microns) CO₂, O₃ and H₂O are present. CH₄, N₂O, CO₂, O₃ and H₂O are visible in the near-IR (1-5 microns).

We can discriminate different surface types from the Earth’s spectra in the visible, but there are almost no differences in the IR. We showed that disk-averaged optical spectra are sensitive to observing phases when clouds are present. Clouds produce dramatic changes in the shapes and intensities of he spectra (variation of up to 400% in the visible and up to 50% in the mid-IR). While the photosynthetic red-edge near 0.6 microns can produce 50% changes in the spectrum on a cloud-free Earth, this signature can be obscured by clouds in disk-averaged spectra. The signature of phytoplankton was searched for, but is sufficiently weak that it is unlikely to be detected.

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