OVIRS generates spectra, measurements of how much of each wavelength of sunlight a spot reflects. OVIRS measures reflected sunlight in wavelengths ranging from 0.4 to 4.3 micrometers (visible light spans 0.4 to 0.7 micrometers). In this region, inorganic materials and organic molecules preferentially absorb some of the light wavelengths. These absorptions create dips in the spectra. Spectroscopists read those dips to determine what minerals or chemicals are present. Because of the Pacific point of view during the flyby, our EGA+1 spectra mostly show spectral features signaling the presence of water and carbon dioxide. Unlike OVIRS, OTES does not need sunlight to work; it takes spectral readings in thermal infrared wavelengths, where materials radiate heat, from 5.6 to 100 micrometers. Like OVIRS, OTES measured absorptions due to water vapor and carbon dioxide gas, and we also saw absorptions due to ozone and methane in the spectra. The depths and widths of the absorption features could tell us the abundances of each atmospheric gas, but extracting that information requires computer modeling that we do not plan to do because it’s not applicable to Bennu science. OTES is also sensitive to temperature, and the radiance (intensity) of different locations on Earth increases or decreases with temperature. At wavelengths where we could see the ocean’s surface (8 to 9.5 micrometers), OTES measured temperatures of about 13 degrees Celsius (55 degrees Fahrenheit). At other wavelengths where there are atmospheric absorptions, we measured the lower temperatures in the atmosphere. Bennu won’t have an atmosphere to complicate our interpretations of the thermal infrared spectra, thankfully.
INSET: The OTES instrument gathered its spectral data at nearly the same time that MapCam took this photo. The yellow circles, each 800 kilometers (500 miles) in diameter, show the approximate locations and sizes of the spots on Earth scanned by OTES to produce these curves.