Characterizing the Purple Earth: Modeling the Globally Integrated Spectral Variability of the Archean Earth

Abstract

Ongoing searches for exoplanetary systems have revealed a wealth of planets with diverse physical properties. Planets even smaller than the Earth have already been detected and the efforts of future missions are aimed at the discovery, and perhaps characterization, of small rocky exoplanets within the habitable zone of their stars. Clearly, what we know about our planet will be our guideline for the characterization of such planets. However, the Earth has been inhabited for at least 3.8 Gyr and its appearance has changed with time. Here, we have studied the Earth during the Archean eon, 3.0 Gyr ago. At that time, one of the more widespread life forms on the planet was purple bacteria. These bacteria are photosynthetic microorganisms and can inhabit both aquatic and terrestrial environments. Here, we use a radiative transfer model to simulate the visible and near-infrared radiation reflected by our planet, taking into account several scenarios regarding the possible distribution of purple bacteria over continents and oceans. We find that purple bacteria have a reflectance spectrum that has a strong reflectivity increase, similar to the red edge of leafy plants, although shifted redward. This feature produces a detectable signal in the disk-averaged spectra of our planet, depending on cloud amount and purple bacteria concentration/distribution. We conclude that by using multi-color photometric observations, it is possible to distinguish between an Archean Earth in which purple bacteria inhabit vast extensions of the planet and a present-day Earth with continents covered by deserts, vegetation, or microbial mats.

Theoretical Reflectance Spectra of Earth-like Planets through Their Evolutions: Impact of Clouds on the Detectability of Oxygen, Water, and Methane with Future Direct Imaging Missions

“…Previous studies have modeled the reflectance spectra of modern Earth-like planets considering the effect of clouds in the atmospheres (e.g., Des Marais et al 2002;Tinetti et al 2006a,b;Robinson et al 2011;Kitzmann et al 2011;Rugheimer et al 2013;Sanromá et al 2013;Kitzmann et al 2013;Sanromá et al 2014;Rugheimer et al 2015a;Feng et al 2018;Wang et al 2018). In addition to modern Earth-like planets, Kaltenegger et al (2007) and Rugheimer & Kaltenegger (2018) modeled the reflectance spectra of planets similar to the Earth at earlier geological epochs orbiting around Sun-like stars, and those around F, G, K, and M stars, respectively.…”

Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures

“…Evolution of anoxygenic photosynthesizers was followed by oxygenic photosynthesizing cyanobacteria and ultimately eukaryotic algae and plants. Interestingly, a distinct hypothesis that purple sulphur bacteria may have dominated euxinic oceans during the mid-Proterozoic eon has also been proposed, resulting in a second Purple Earth (Brocks et al, 2005;Sanromá et al, 2014), which would have been long after the retreat of retinal-based life dominating the first Purple Earth to ecological niches resembling those of today. The development of eukaryotic algae and complex plants and their spread throughout the terrestrial environment allowed the evolution of land animals and ultimately intelligent life (Catling et al, 2005;Reinhard et al, 2016).…”

“…The capacity to detect surface biosignatures is an ongoing consideration in the design mandate of large, space-based telescopes (Fujii et al, 2018;Schwieterman et al, 2018) such as the conceived HabEx and LUVOIR/HDST missions (Dalcanton et al, 2015;Mennesson et al, 2016;Rauscher et al, 2016;Bolcar et al, 2017). Direct imaging spectra and spectrophotometry will allow characterization of the surfaces of terrestrial planets in the habitable zone, producing constraints on surface types, including surface biosignatures, providing the cloud covering fraction is sufficiently low (Sanromá et al, 2014(Sanromá et al, , 2013. The detectability potential of retinal photopigments and halophile-analogs suggests wavelengths shortward of the traditional VRE (λ< 700 nm) will be important to observe and analyse for 'edge' features suggestive of diverse phototrophic pigments and should be considered in the search for life outside the solar system.…”

Surface and Temporal Biosignatures

“…Sanromá et al (2013) found that if bacterial mats containing oxygenic cyanobacteria constituted 50% land cover or more on the early Earth, they could have been identified through spectrophotometry and distinguished from vegetation because they are dimmer at wavelengths greater than ~1.3 µm (e.g., see Figure 2). Similarly, Sanromá et al (2014) found that bacterial mats composed of anoxygenic photosynthesizers such as "purple bacteria" will have NIR 'edges' longward of bacteriochlorophyll absorption (a ~1…”