Vegetative Red-Egde and it's applications in Astrobiology

Introduction

Plant life on Earth is photosynthetic, it uses light to absorb energy from the sun. A process we know as photosynthesis. Throughout Earth’s history plants and organisms have utilized the strong light that reaches earth to harvest energy, store it, and convert it into growth and reproduction. While there are various mechanisms for plants to do this, the dominant process in plants that converts light into energy is broken down into two cellular processes. Known as Photosystem I (PS I) and Photosystem II (PS II). These processes use multiple molecules, specifically proteins and pigments, to capture light photons and convert them into usable stored energy. The main pigments utilized by plants in PS I and PS II are chlorophyll a (Chl a), and chlorophyll b (Chl b). Chlorophyll a and b are energized by specific wavelengths of light, and in turn help feed plants the energy necessary to sustain life. By capturing light, these pigments absorb light photons, meaning that specific wavelength of light is no longer available to be reflected or seen by other organisms. In the case of Chl a, the wavelengths absorbed are at 680 nm and 700 nm. In Chl b, the specific wavelength of light absorbed is at 453 nm and 642 nm. When plant leaves absorb light at these wavelengths, they pick out light which appears red and blue in the electromagnetic spectrum, leaving out green light. This green light, is then reflected by the plant, and produces the wonderful greenery we see when looking at trees and plant life.

The Vegetative red-Edge (VRE)

From a satellite, the green forests and plants we see aren’t what tell us that there is plant life on Earth. Instead, what informs us there is plant life on Earth is what we don’t see. When Chl a absorbs red light at the wavelengths of 680 and 700 nm, we can no longer see this light reflecting out at us. What we observe is a drastic drop in the amount of light shining off the planet at these wavelengths. This drastic change is what scientists have named the Vegetative Red-Edge (VRE) and we owe this feature to the small chlorophyll absorbing light all across the surface of Earth. Seen below is a general graph depicting the VRE.

Remotely Estimating Aerial N Uptake in Winter Wheat Using Red-Edge Area Index From Multi-Angular Hyperspectral Data - Scientific Figure on ResearchGate. Available from:https://www.researchgate.net/figure/Schematic-representation-of-red-edge-reflectance-curve_fig2_325371767 [accessed 13 Sep, 2022]

Astrobiology Applications

The VRE is known to scientists as a biosignature, a distinct sign that biological processes are taking place on Earth. Biosignatures are not limited to observations of light, and may include atmospheric inputs of biotic molecules such as the O2 produced by plants, or the CO2 breathed out by humans. Other biosignatures include chirality, which is the phenomenon of biotic organisms to produce only one variation of a molecule, denoted as Left-handed molecules (as opposed to Right-handed molecules). When life is present, the concentration of Left-handed molecules build-up in comparison to Right-handed ones, and provide strong evidence for life. In other words, there are a range of biosignatures, each of which includes specific chemical and biological constraints that provide justification for why it is a sign of biotic life. However, telescope and satellite observations are the best technology we have to see distant planets at this time, and looking for biosignatures that can be seen this way provides the highest likelihood of detection.

Variations of VRE

Research in astrobiology has found that there is a relationship between the strongest available wavelength and the wavelength at which pigments absorb energy. Meaning that the wavelength absorbed by the plant is typically the most available, or strongest wavelength that reaches the surface of the planet. Pigments will essentially adapt to absorb light at a range where the light is strong. Knowing this relationship, scientists predict that a similar absorption/reflectance feature may occur on other planets, but it will be tuned to the light given off by the planet’s host star. In other words, different stars can cause similar ‘edge-like’ features on their planets, but they will occur at a different peak wavelength. A team of scientists in partnership with NASA Ames created a model based on this idea, and have developed a hypothesis about the wavelengths used by vegetation around other stars. Using the Earth’s current atmosphere as a model, they analyzed different star types for what wavelengths of light they would provide to life on their planets. For constraint, these hypothetical planets were placed within the Habitable Zone of each star, which is the distance at which liquid water could be present on the planet. The results below show that variations in host stars have an effect at what wavelength a planet’s ‘edge-like’ biosignature will occur. An F2V star is more massive than our sun, and would show an edge in the blue spectrum. While K2V and both M type stars are less massive than our sun and would show an edge further in the red spectrum. This research allows scientists to further understand what type of biosignatures they might look for on other planets.

Guo, Bin-Bin, et al. "Remotely estimating aerial N uptake in winter wheat using red-edge area index from multi-angular hyperspectral data." Frontiers in plant science 9 (2018): 675.

Lehmer, Owen R., et al. "The peak absorbance wavelength of photosynthetic pigments around other stars from spectral optimization." arXiv preprint arXiv:2107.04120 (2021).