Habitable Worlds Observatory Living Worlds Working Group: Surface Biosignatures on Potentially Habitable Exoplanets

TL;DR

This paper assesses the detectability of surface biosignatures on potentially habitable exoplanets using the Habitable Worlds Observatory, focusing on pigment-based reflectance features as a complementary line of evidence to atmospheric biosignatures. It develops an Earth-through-Time framework, compiling a spectral library of pigments (including chlorophyll, bacteriochlorophylls, carotenoids, and bacteriorhodopsin) and uses radiative transfer and instrument-noise modeling to evaluate detectability across Archean, Proterozoic, and Modern atmospheres. The study derives practical observational requirements, finding that achieving $SNR$ in the range $20$–$40$ across $500-1100$ nm with a resolving power $R\lesssim 130$ and broad, gapless coronagraph coverage is crucial to identify surface pigment edges at 15% surface coverage under 50% cloud. These results guide mission design and demonstrate how surface biosignatures could enable not only corroboration of atmospheric signals but also potential mapping of photosynthetic life and detection of primitive, non-oxygenic biospheres, thereby informing target selection and survey strategies.

Abstract

The Habitable Worlds Observatory (HWO) is the first NASA Astrophysics flagship mission with a key science goal of searching for signs of life on rocky habitable exoplanets beyond our solar system. The Living Worlds Community Working Group was charged with investigating how HWO could characterize planets orbiting stars in the solar neighborhood, search for signs of life, and interpret potential biosignatures within a false positive and false negative framework. The Surface Biosignatures Task assessed the measurement requirements and instrument needs to detect these biosignatures under an 'Earth through time' scenario. Surface biosignatures are planetary-scale spectral features resulting from absorption and/or scattering of radiation by organisms containing photosynthetic and non-photosynthetic pigments. This secondary class of biosignature can be used to corroborate atmospheric biosignatures by providing multiple lines of evidence to aid in assessing their biogenicity. Furthermore, surface biopigments are the only way to detect more primitive forms of anoxygenic photosynthesis if oxygenic photosynthesis never evolved. Key Findings: To detect biopigments on the surface of planets under Archean, Proterozoic, and Modern atmospheric compositions (15 percent coverage, 50 percent cloud cover), an SNR of 20-40 would be needed over 500-1100 nm. However, there may be some cases in which lower SNR is required; studies are ongoing. Coronagraph requirements: (1) The detection of surface biosignatures would be greatly enhanced by having as many parallel coronagraph channels as possible across the entire wavelength range with no or minimal gaps between channels. (2) Retrieval studies revealed that restricted wavelength ranges (e.g., 0.4 - 0.7 microns), such as may be used during initial survey strategies, are not sufficient to deconvolve the biopigment features from the abiotic background.

Figures (12)

• Figure 2: Requirements for life. An environment must provide these resources and conditions simultaneously in order to sustain habitable conditions (Hab) for life. Adapted from a graphic by Tori Hoehler.
• Figure 3: Left: Reflectance spectra of photosynthetic organisms on rock, mineral, and snow/ice abiotic surfaces with Archean, Proterozoic, and Modern atmospheric compositions (planetary spectra provided by Anna Grace Ulses, Univ. of Washington). The thick angled lines show the pigment "edge" features in the spectrum. Right: Top: purple anoxygenic phototrophs with bacteriochlorophyll pigments absorbance and reflectance features at 805-880 nm at 15% planet coverage under an Archean atmosphere with 50% cloud cover. Middle: cyanobacterial microbial mat (oxygenic phototroph) with chlorophyll a absorption at 660-680 nm at 15% coverage under a Proterozoic atmosphere with 50% cloud cover. Note that the 3-D structure of the mat is effective at scattering light to create a "red edge" feature similar to the vegetation red edge (VRE) shown in the bottom panel. Top two spectra provided by N. Parenteau, unpublished data.
• Figure 4: Reflectance spectra of bacteriochlorophylls a and b (red), as well as carotenoids (yellow) from purple non-sulfur bacteria. Note absorption near-infrared features from Bacteriochlorophylls span between 800-1010 nm. Data from Coelho et al., 2024.
• Figure 5: Glacier algae carotenoids covering the surface of Glacier Austre Brøggerbreen (Svalbard), image by Ligia F. Coelho (Cornell University).
• Figure 6: Aerial photograph of Iwo island (left), transmitted light spectrum measured at the depth of 5 m (center), and comparison of albedos between a typical blue ocean and a green ocean. Adapted from Matsuo et al., 2025.).