Variability of Water and Oxygen Absorption Bands in the Disk-Integrated Spectra of the Earth

TL;DR

This study investigates the diurnal variability of Earth's water and oxygen absorption bands in disk-integrated spectra, linking observed variations to cloud cover and atmospheric composition, with implications for analyzing Earth-like exoplanets.

Contribution

It introduces a simple cloud model to explain spectral variability and demonstrates its effectiveness using Earth observation data, highlighting potential for exoplanet atmospheric analysis.

Findings

Water vapor and oxygen bands vary by 5-20% due to cloud cover.

The cloud model successfully reproduces observed spectral variations.

Differences in variability patterns reveal atmospheric composition inhomogeneity.

Abstract

We study the variability of major atmospheric absorption features in the disk-integrated spectra of the Earth with future application to Earth-analogs in mind, concentrating on the diurnal timescale. We first analyze observations of the Earth provided by the EPOXI mission, and find 5-20% fractional variation of the absorption depths of H2O and O2 bands, two molecules that have major signatures in the observed range. From a correlation analysis with the cloud map data from the Earth Observing Satellite (EOS), we find that their variation pattern is primarily due to the uneven cloud cover distribution. In order to account for the observed variation quantitatively, we consider a simple opaque cloud model, which assumes that the clouds totally block the spectral influence of the atmosphere below the cloud layer, equivalent to assuming that the incident light is completely scattered at the cloud top level. The model is reasonably successful, and reproduces the EPOXI data from the pixel-level EOS cloud/water vapor data. A difference in the diurnal variability patterns of H2O and O2 bands is ascribed to the differing vertical and horizontal distribution of those molecular species in the atmosphere. On the Earth, the inhomogeneous distribution of atmospheric water vapor is due to the existence of its exchange with liquid and solid phases of H2O on the planet's surface on a timescale short compared to atmospheric mixing times. If such differences in variability patterns were detected in spectra of Earth-analogs, it would provide the information on the inhomogeneous composition of their atmospheres.