Detectability of Planetary Characteristics in Disk-Averaged Spectra. I: The Earth Model

Abstract

Over the next 2 decades, NASA and ESA are planning a series of space-based observatories to detect and characterize extrasolar planets. This first generation of observatories will not be able to spatially resolve the terrestrial planets detected. Instead, these planets will be characterized by disk-averaged spectroscopy. To assess the detectability of planetary characteristics in disk-averaged spectra, we have developed a spatially and spectrally resolved model of the Earth. This model uses atmospheric and surface properties from existing observations and modeling studies as input, and generates spatially resolved high-resolution synthetic spectra using the Spectral Mapping Atmospheric Radiative Transfer model. Synthetic spectra were generated for a variety of conditions, including cloud coverage, illumination fraction, and viewing angle geometry, over a wavelength range extending from the ultraviolet to the farinfrared. Here we describe the model and validate it against disk-averaged visible to infrared observations of the Earth taken by the Mars Global Surveyor Thermal Emission Spectrometer, the ESA Mars Express Omega instrument, and ground-based observations of earthshine reflected from the unilluminated portion of the Moon. The comparison between the data and model indicates that several atmospheric species can be identified in disk-averaged Earth spectra, and potentially detected depending on the wavelength range and resolving power of the instrument. At visible wavelengths (0.4-0.9 microm) O3, H2O, O2, and oxygen dimer [(O2)2] are clearly apparent. In the mid-infrared (5-20 microm) CO2, O3, and H2O are present. CH4, N2O, CO2, O3, and H2O are visible in the near-infrared (1-5 microm). A comprehensive three-dimensional model of the Earth is needed to produce a good fit with the observations.

Clouds in the atmospheres of extrasolar planets

“…Thus, the mean Earth cloud cover yields an increase of nearly 300% in the planetary albedo. With 100% low-level clouds, this increase would be even larger as demonstrated using observations and synthetic spectra by Tinetti et al (2006a). In contrast to reflection spectra, the absorption bands of O 2 and H 2 O can be more easily identified in the calculated albedos.…”

“…satellite viewing angle, partial illuminated Earth etc.) that would be necessary for a direct comparison with the Earth observations of Tinetti et al (2006a). A realistic quantitative comparison with observed Earth spectra would require a more advanced radiative transfer model.…”

“…the strong absorption bands of H 2 O in the VIS, the Fraunhofer A-band of O 2 , and the Chappuis band of O 3 . Owing to the high resolution of their spectra, Tinetti et al (2006a) were also able to identify the weak Fraunhofer B-band of O 2 , which, however, is too narrow and too weak to be observable at low resolution. Our results also agree with the principle radiative effects of clouds reported by Tinetti et al (2006a,b) and Kaltenegger et al (2007), namely that they cause an increase in scattered radiation and depths of absorption bands.…”

“…Depending on the type and amount of clouds, reflection spectra allow for the detection of several important atmospheric chemical species (Tinetti et al 2006a;Turnbull et al 2006). Corresponding calculations of synthetic high-resolution reflectance spectra of Earth including the effects of different cloud types have been performed by e.g.…”

The atmospheric chemistry of the warm Neptune GJ 3470b: Influence of metallicity and temperature on the CH4/CO ratio

“…7). Emission spectra are calculated using the line-by-line radiative transfer codes described in Tinetti et al (2005Tinetti et al ( , 2006. Transmission spectra are calculated using the line-by-line radiative transfer codes described in Hollis et al (2013).…”