Surface and Temporal Biosignatures

Abstract

Recent discoveries of potentially habitable exoplanets have ignited the prospect of spectroscopic investigations of exoplanet surfaces and atmospheres for signs of life. This chapter provides an overview of potential surface and temporal exoplanet biosignatures, reviewing Earth analogues and proposed applications based on observations and models. The vegetation red-edge (VRE) remains the most well-studied surface biosignature. Extensions of the VRE, spectral 'edges' produced in part by photosynthetic or nonphotosynthetic pigments, may likewise present potential evidence of life. Polarization signatures have the capacity to discriminate between biotic and abiotic 'edge' features in the face of false positives from band-gap generating material. Temporal biosignaturesmodulations in measurable quantities such as gas abundances (e.g., CO 2 ), surface features, or emission of light (e.g., fluorescence, bioluminescence) that can be directly linked to the actions of a biosphere -are in general less well studied than surface or gaseous biosignatures. However, remote observations of Earth's biosphere nonetheless provide proofs of concept for these techniques and are reviewed here. Surface and temporal biosignatures provide complementary information to gaseous biosignatures, and while likely more challenging to observe, would contribute information inaccessible from study of the time-averaged atmospheric composition alone.

Identifikasi Pohon Tropis di Daerah Perkotaan Menggunakan Multispectral Drone Imagery

“…Setiap citra dikelompokan berdasarkan keragaman objek-objek yang termuat dalam citra. Pengelompokan citra menggunakan hierarchical clustering [6,7]. Kami memilih citra dalam cluster yang memiliki area dengan rasio pohon lebih tinggi dari pada bukan pohon.…”

“…Kemampuan kedua citra dibandingkan dengan kemampuan citra drone berformat RGB. Kami memandang suatu pohon mempunyai biosignature yang berbeda dengan pohon yang lain [6,7], sehingga setiap jenis pohon mempunyai komposisi organik yang berbeda [8,9]. Oleh karena itu, kami menguji kemampuan citra cahaya tampak (green layer), citra red-edge dan near-infrared untuk deteksi jenis pohon karena ketiga citra menangkap pantulan cahaya dari unsur organik.…”

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

“…Even on Earth, green vascular planets have only existed for just the last ∼10% of the planet's history, about 470 million years out of 4.6-billion years (Kenrick and Crane, 1997). In order to address the limited time of presence of the VRE, astrobiologists interested in remote biosignatures have begun to consider and catalog surface reflectance signatures from a diverse array of known pigmented organisms including oxygenic and anoxygenic photosynthesizers, rhodopsin-based phototrophs and non-photosynthetic microbes that use pigments as a UV screen or antioxidant, or for other purposes (DasSarma, 2006;Kiang et al, 2007bKiang et al, , 2007aCockell, 2014;Hegde et al, 2015;Poch et al, 2017;Schwieterman, 2018;Schwieterman et al, 2018;2015). The possible existence of a Purple Earth extends and expands the possible biological history of a planet when alternate biosignatures may be detectable, and it also enhances the number of possible evolutionary trajectories for which surface biosignatures may be found.…”

Photosynthetic Fluorescence from Earthlike Planets around Sunlike and Cool Stars

“…In a comprehensive study of different pigment reflectivity, Schwieterman et al (2015) showed that both nonphotosynthetic pigments and photosynthetic pigments affect the disk-averaged spectra. Furthermore, as for false positive detection, the reflectance features of some minerals on the Earth are similar to the VRE ones (Seager et al 2005;Schwieterman 2018). Thus, extracting the VRE signal from reflected light should require knowledge of the surface environment on an exoplanet and high-resolution spectroscopic observations.…”