Surface biosignatures of exo-Earths: Remote detection of extraterrestrial life

Abstract

Exoplanet discovery has made remarkable progress, with the first rocky planets having been detected in the central star's liquid water habitable zone. The remote sensing techniques used to characterize such planets for potential habitability and life rely solely on our understanding of life on Earth. The vegetation red edge from terrestrial land plants is often used as a direct signature of life, but it occupies only a small niche in the environmental parameter space that binds life on present-day Earth and has been widespread for only about 460 My. To more fully exploit the diversity of the one example of life known, we measured the spectral characteristics of 137 microorganisms containing a range of pigments, including ones isolated from Earth's most extreme environments. Our database covers the visible and near-infrared to the short-wavelength infrared (0.35-2.5 μm) portions of the electromagnetic spectrum and is made freely available from biosignatures. astro.cornell.edu. Our results show how the reflectance properties are dominated by the absorption of light by pigments in the visible portion and by strong absorptions by the cellular water of hydration in the infrared (up to 2.5 μm) portion of the spectrum. Our spectral library provides a broader and more realistic guide based on Earth life for the search for surface features of extraterrestrial life. The library, when used as inputs for modeling disk-integrated spectra of exoplanets, in preparation for the next generation of space-and ground-based instruments, will increase the chances of detecting life.biosignatures | spectral library | reflectivity | extremophiles | pigments I n the last decade, the field of exoplanet research has transitioned rapidly from detection to detection and characterization, with the first rocky exoplanets detected in the central star's liquid water habitable zone. Much of the excitement of this research in both the astrobiology community and the general public is motivated by the quest to discover a second genesis of life. The great distances that separate us from even the most nearby stars dictate that all measurements of the exoplanet must be made through remote sensing techniques for the foreseeable future. Thus, it is critical for us to determine the types of biosignatures that we should be looking for when designing the next generation of ground-and space-based instruments that will observe these planets at high spectral and possibly spatial resolutions.Since the mid-1960s a primary life-searching strategy has been to look for a specific combination of an oxidizing and a reducing gas in the exoplanetary atmosphere, such as the O 2 and CH 4 in our atmosphere, because this is a thermodynamically unstable situation suggesting that an active agent such as life is responsible for the chemical disequilibrium (1, 2). Of particular interest, both from an observational and modeling perspective, is to complement those indirect life detection studies with surface features that are direct properties of the organisms themselves (3).Alt...

The First Space-Related Study of a Kombucha Multimicrobial Cellulose-Forming Community: Preparatory Laboratory Experiments

“…A promising approach in the search for traces of Earth-like life is also focused on the examination of surface and subsurface rock/dust material of meteorites, comets or the nearest Earth neighbors (McKay et al 1996 ; 4/30 Kebukawa et al 2009 ;De Gregorio et al 2013 ). For the remote detection of biosignatures, photosynthetic and non-photosynthetic pigments (Varnali and Edwards 2013 ;Hegdea et al 2015 ;Schwieterman et al 2015 ), biogenic gases from anoxygenic photosynthesis in microbial mats or other biosignature gases (Seager and Bains 2015 ) are new biosignatures of life which have been proposed by researches. One of the strategies for substantiation of the extraterrestrial life existence probability in the Universe is to investigate the stability of selected putative biosignatures in the extraterrestrial analog environments, e.g., within martian simulation chambers or under low Earth orbit in specially designed devices with Marslike atmosphere .…”

How to Characterize Habitable Worlds and Signs of Life

“…Vegetation is only one among many surface features life produces on Earth (see, e.g., Kiang et al 2007a, Cockell et al 2009, Huet al 2012, Sanroma et al 2013, Hegde et al 2015. Land plants have been widespread on Earth for only approximately 460 Myr (e.g., Zahnle et al 2007), whereas much of the history of life has been dominated by single-celled microbial life.…”

“…Biota on Earth generates a wide range of characteristic reflectivity and colors. To mimic detectable surface reflection features of known life, Hegde et al (2015) measured the spectral characteristics of 137 phylogenetically diverse microorganisms containing a range of pigments, including ones isolated from Earth's most extreme environments. The team used an integrating sphere, which mimics the observing geometry of an exoplanet that is modeled as a Lambertian sphere.…”

Surface and Temporal Biosignatures

“…Note the diversity in edge wavelengths caused by absorption at different wavelengths and higher reflectance where pigments are not absorbing. Hegde et al (2015) measured the reflectance spectra from 0.35 to 2.5 µm of pure cultures of 137 microorganisms with a variety of pigments suited to different functions, generating a publically available spectral database (biosignatures.astro.cornell.edu). In general, these authors found that the strongest features (e.g., 'edges') in the ensemble of microorganisms were present in the visible to NIR (> 1.0 µm) range due to pigment absorption.…”