The Earthshine Spectrum in the Near Infrared M. Turnbull 1, W. Traub 2, K. Jucks 3, N. Woolf 4, M. Meyer 4, N. Gorlova 4, M. Skrutskie 5, J. Wilson 5 1 Carnegie Institute, Washington, DC, USA 2 JPL, Pasadena, CA, USA 3 Harvard-Smithsonian Center for Astrophysics, Cambridge, USA 4 University of Arizona, Tucson, AZ, USA 5 University of Virginia, Charlottesville, VA, USA What is the Earthshine spectrum? The spectrum of the light reflected from the Earth to dark side of the moon and back to Earth. The ratio of the spectrum of the dark side to the bright side gives a spectrum of light that has passed through the Earth’s atmosphere, reflected off a surface, and back through the atmosphere (see figure 1). It represents the illuminated disk average spectrum of the Earth. This is exactly the type of observation that would be observed from an extra-solar planet around it’s star. The resulting spectrum contains a combination of many conditions, like reflection off of the ocean, plants, soil, and clouds of various heights and types. All these paths must be considered in analyzing these data. Figure 1: Schematic of the observational geometry for the Earthshine observations. How were the observations made? Used the near infrared CorrMass Spectrograph telescope on Mt. Graham in Arizona. Spectrometer disperses the grating orders onto a NICMOS3 CCD detector (see figure 3). The orders are set for the edges to fall within the atmospheric water vapor bands (see figure 5). Spectra were taken for the Moon dark side, the Moon bright side, and background at a distance from the bright side that is similar to the distance of the dark side to account for scattering. Raw data were reduced using IRAF. Earthshine is calculated with: ES = (Moon D -sky s )/Moon B x R B /R D Figure 2: Ephemeris images of the Earth at the time Of the Earthshine observations. The right image shows the superimposed GOES-12 cloud image. The illuminated portion of the Earth contains a combination of ocean, clouds vegetated and non-vegetated land. Figure 3: Sample NICMOS image of the dispersed orders for the spectrum of the light scattered off the bright side of the moon. Figure 4: Sample overlapped raw extracted spectra from the different orders for the light scattered off the bright side of the moon. The long Wavelength end shows some emission From the telescope dome. Figure 5: Example of the background spectra obtained which must be subtracted off the dark moon spectra. The spectra show many significant Meinel band spectral features. Figure 6: The punchline! The final averaged near infrared Earthshine spectra from this study. The spectral fit included the 3 scenarios shown in the plot using a linear least square reduction. Figure 7: The “merged” Earthshine spectra combining this near infrared data with the visible data from Woolf et al The spectral features comprising the primary chromophores are denoted. What we learned We could reasonably model the observed data using a rather simple model that includes a combination of 3 atmospheric transmission/reflection schemes; Reflection off the surface Reflection off 10 km ice clouds Reflection off 4 km water clouds In the near infrared, one can observe spectral features from H 2 O, CO 2, O 2, O 3, and CH 4, as well as clouds. The reflectance signatures from the surface ( such as the red edge at.7 microns) are difficult to observe. The subtraction of the background light must be carefully handled to properly interpret these types of data. These types of studies are good test cases for what might be expected for the future observed spectra of extra-solar planets like what could come from future missions like TPF-C and Kepler.