Spectral Evolution of an Earth‐like Planet

Abstract

We have developed a characterization of the geological evolution of the Earth's atmosphere and surface in order to model the observable spectra of an Earth-like planet through its geological history. These calculations are designed to guide the interpretation of an observed spectrum of such a planet by future instruments that will characterize exoplanets. Our models focus on planetary environmental characteristics whose resultant spectral features can be used to imply habitability or the presence of life. These features are generated by H 2 O, CO 2 , CH 4 , O 2 , O 3 , N 2 O, and vegetation-like surface albedos. We chose six geological epochs to characterize. These epochs exhibit a wide range in abundance for these molecules, ranging from a CO 2 -rich early atmosphere, to a CO 2 /CH 4 -rich atmosphere around 2 billion years ago, to a present-day atmosphere. We analyzed the spectra to quantify the strength of each important spectral feature in both the visible and thermal infrared spectral regions, and the resolutions required to optimally detect the features for each epoch. We find a wide range of spectral resolutions required for observing the different features. For example, H 2 O and O 3 can be observed with relatively low resolution, while O 2 and N 2 O require higher resolution. We also find that the inclusion of clouds in our models significantly affects both the strengths of all spectral features and the resolutions required to observe all these. Subject headingg s: astrobiology -Earth -planetary systems

Flux and polarisation spectra of water clouds on exoplanets

“…It is well known that clouds have strong effects on flux spectra of planets in the Solar System and beyond. Examples of simulated flux spectra of exoplanets with water clouds can be found in Marley et al (1999); Tinetti et al (2006); Kaltenegger et al (2007). Far less work has been done regarding polarisation spectra of cloudy planets.…”

Background

“…Observations of Earth-like planets at different geological ages will certainly help us understand the diversity and some common features of terrestrial planets, including ours. For our own planet, models show varying biomarkers over geological times that could be detected in the spectrum (see, e.g., Schindler and Kasting 2000;Pavlov et al 2000;Kaltenegger et al 2007) to characterize it in terms of habitability and the possible presence of life. …”

“…Spectra of Earth in Reflection, Emission, and Transmission Figure 1 shows observations and model fits to the spectra of the Earth in three wavelength ranges, the visible and near infrared represent the reflected starlight, while the infrared (IR) spectrum is generated by the emitted heat flux of the planet (Kaltenegger et al 2007). For the data shown in Fig.…”

“…The spectrum of the Earth has exhibited a strong infrared signature of ozone for more than 2 billion years and a strong visible signature of O 2 for an undetermined period of time between 2 and 0.8 billion years (depending on the required depth of the band for detection and also the actual evolution of the O 2 level; Kaltenegger et al 2007). This difference is due to the fact that a saturated ozone band appears already at very low levels of O 2 (10 À4 ppm), while the oxygen line remains unsaturated at values below the present atmospheric level (Segura et al 2003).…”

“…The 9.6-mm O 3 band is highly saturated and is thus a poor quantitative indicator but an excellent qualitative indicator for the existence of even traces of O 2 . CH 4 is not readily identified using low-resolution spectroscopy for present-day Earth, but the methane feature at 7.66 mm in the IR is easily detectable at higher abundances (e.g., 100Â on early Earth; Kaltenegger et al 2007), provided that the spectrum contains the whole band and a high enough signal-to-noise ratio. Taken together with molecular oxygen, abundant CH 4 can indicate biological processes (see also Sagan et al 1993;Segura et al 2003).…”