Surface and Temporal Biosignatures

Brief

Schwieterman et al. survey every major surface and temporal biosignature class accessible to remote observation, using Earth analogues as proof of concept. The vegetation red-edge (VRE), a reflectance jump near 700 nm shared by all oxygenic photosynthesizers, remains the benchmark: Earth's disk-averaged NIR albedo contribution is estimated at ~2–10%, and detecting a VRE analogue at R=20 requires SNR ≥ 100, roughly six times the SNR needed to detect the O2-A band. Alternative signatures, including halophile-dominated salt ponds (producing up to ~13% albedo at 0.68 μm) and bacteriochlorophyll edges shifted into the NIR, expand the search space but face mineral false positives and atmospheric water-vapor confusion. Temporal biosignatures, seasonal CO2 drawdown, surface reflectance modulation, bioluminescence, are identified as the least-studied class yet carry information unavailable from time-averaged atmospheric composition alone.

Metadata

Category

Search

Venue

Handbook of Exoplanets (Springer)

Type

Peer-reviewed

Year

2018

Authors

Edward W. Schwieterman, Christopher T. Reinhard, Stephanie L. Olson, Chester E. Harman, Timothy W. Lyons

DOI

10.1007/978-3-319-55333-7_69

arXiv

1803.05065

Access

Open access

Length

4.2 M

Programs

Virtual Planetary Laboratory, LUVOIR, HabEx, Exo-Earth Mapper

Instruments

MODIS (Terra satellite), Galileo probe, Earthshine photometry, LUVOIR (proposed), HabEx (proposed), Exo-Earth Mapper (proposed)

Data sources

ASTER spectral library, USGS spectral library, VPL Biological Pigments Database, biosignatures.astro.cornell.edu (Hegde et al. 2015 microbial spectral database), Robinson et al. 2011 VPL 3D Earth spectral model

Tags

biosignature, astrobiology, exoplanet, direct imaging, spectroscopy, photosynthesis, SETI

Key points

• VRE break wavelengths for all oxygenic photosynthesizers (moss, lichen, plants, seagrass, algae) cluster between 0.69–0.73 μm; the feature is produced by the contrast between chlorophyll absorption at red wavelengths and NIR scattering from cell/leaf structure.p.6
• Earth's disk-averaged NIR spectral albedo attributable to the red-edge is ~2–10% (Arnold 2008), with significant variability from cloud cover, vegetation fraction, and Earth-Moon viewing geometry.p.7
• Brandt and Spiegel (2014) find that detecting the VRE requires SNR ≥ 100 at R=20 (10% vegetation, 50% cloud cover), and that this SNR is approximately six times that required to detect the O2-A band.p.8
• Bacteriochlorophyll b has an in vivo long-wavelength absorption peak at 1000–1040 nm, approaching the ~1100 nm hypothetical thermodynamic limit for single-photon photosynthetic excitation.p.5
• A halophile-dominated surface (bacterioruberin + bacteriorhodopsin) could produce an albedo signature as high as ~13% at 0.68 μm assuming ocean-fraction coverage (~70%) and 50% cloud cover.p.11
• Mineral false positives, sulfur (edge at 0.45 μm) and cinnabar (edge at 0.6 μm), can mimic biological reflectance edges; Io's disk-averaged spectrum demonstrates this effect is planetary-scale, not merely theoretical.p.12
• Land plants have existed for only ~400 My; (anoxygenic) photosynthetic life extends to 3.5 Ga, meaning the dominant surface biosignature for most of Earth's history would have been bacterial mats, not vegetation.p.9
• Hegde et al. (2015) measured reflectance spectra of 137 pure microbial cultures from 0.35–2.5 μm, finding the strongest spectral edges in the visible-to-NIR (>1.0 μm) and consistent hydration bands at 0.95, 1.15, 1.45, and 1.92 μm across all samples.p.11

Verbatim

• “The VRE is the sharp increase in the reflectance of oxygenic photosynthesizers near the boundary between visible and near-infrared wavelengths (~700 nm), particularly apparent in green vascular plants (Gates et al. 1965).”

  p.6

• “The increase in Earth's disk-averaged NIR spectral albedo attributable to the red-edge has been estimated from Earthshine measurements and spectral models to be between ~2-10% (Arnold 2008) with significant variability caused by differences in cloud cover, vegetation covering fraction, and Earth-Moon viewing geometry.”

  p.7

• “Brandt and Spiegel (2014) further find that the SNR to detect the VRE is approximately six times that required to detect the O2-A band.”

  p.8

• “Land plants have existed for only ~400 My of Earth's history (Kenrick and Crane 1997), though evidence for (anoxygenic) photosynthetic life extends to 3.5 Ga or earlier (Buick 2008).”

  p.9

• “an exoplanet biosignature is unlikely to ever be completely unambiguous, so it is always best to think of an exoplanet biosignatures as a "potential biosignature."”

  p.2

Most interesting

• The SNR required to detect the vegetation red-edge is ~6× that needed to detect the O2-A band, meaning photometric detection of surface life is substantially harder than spectroscopic detection of its atmospheric byproduct.
• Bacteriochlorophyll b absorbs at 1000–1040 nm in vivo, near the ~1100 nm theoretical ceiling for photosynthetic electron excitation; no confirmed pigment yet pushes past this limit.
• Jupiter's moon Io already provides an empirical, planetary-scale example of a mineral reflectance 'edge' (from sulfur compounds) that would score as a biosignature false positive if Io were an exoplanet candidate.
• A planet orbiting an M-dwarf may harbor a NIR-shifted red-edge that is actually more detectable than Earth's VRE, because the shifted edge falls outside the confounding water-vapor absorption bands that partially mask the terrestrial signal near 0.72 μm.
• The NDVI of the Moon's surface more closely matched NDVI values for vegetated Earth than the disk-averaged NDVI of the whole Earth, illustrating how broadband vegetation indices break down at disk-averaged scales.
• For most of Earth's 4.5 Ga history, the dominant surface biosignature would have been bacterial mats, not green vegetation; land plants only appeared ~400 Ma, making them a geologically recent and potentially atypical biosignature target.