Abstract EANA2024-3

  In the search for extraterrestrial life on exoplanets, the detection of biosignatures is crucial. Previous studies have examined the detection of oxygen and ozone in the atmosphere through atmospheric spectroscopy, as well as the reflection spectra of bio-derived pigments on the surface.

  Many of these biosignatures are related to photosynthetic organisms. Particularly, the reflection spectral pattern of terrestrial plants, known as the Vegetation Red Edge (VRE), is expected to be detectable on Earth-like habitable planets. The VRE refers to the spectrum where the reflectance significantly increases around 700 nm, transitioning from red light to near-infrared, due to the absorption of red light by chlorophyll and light scattering. On Earth, this VRE characteristic is translated into the NDVI (= (ρNIR - ρRed) / (ρNIR + ρRed)) standard and is used to evaluate forest areas and crop growth.

  However, habitable planets with liquid water might have their surfaces entirely covered by water due to excess water, as reported in some studies of planetary formation. In a water-rich environment, infrared reflection by  algae and aquatic plants is attenuated by the surrounding water. Only plants with floating leaves could show VER equivalent to terrestrial plants.

  This study examined the detectability of VRE in various aquatic plants, from controlled laboratory conditions to natural vegetation observed by Earth observation satellites. The reflectance spectra of floating plants, when measured on individual leaves, exhibited VRE similar to those of terrestrial plants. Specifically, when the floating leaves completely covered the water source of the container and overlapped each other, the VRE was comparable to that of moderate terrestrial vegetation. In the analysis of Earth observation satellite data, the magnitude of VRE evaluated by NDVI was a few times smaller than the forest and grassland, reflecting the difference in vegetation structures.

  We surveyed hundreds of lakes in Japan over the years and analyzed seasonal variations of NDVI with location and vegetation information. The lakes in mid-latitude regions show significant seasonal changes in NDVI. Aquatic plants mature during hot summers, dormancy, and then death, completely removing them from the water surface during cold winters. Thus, reflections from the water surface are detected instead of the plant body, leading to significant changes in VRE.

  This indicates that the variability of VRE, in addition to signal strength, can be utilized, and this variability holds promise as a biosignature, particularly on water-covered planets.