New progress has been made in the research direction of astrobiology by NIAOT----Nanjing institute of Astronomical optics & Technology National Astronomical O bservatories .CAS

Astrobiology studies on the origin, evolution, and distribution of life in the universe, and the detection of extraterrestrial life is one of the most concerned research fields. The Vegetation Red Edge (VRE), as a unique reflection characteristic signal of vegetation, has been studied in the field of remote sensing. In recent years, as a direct surface biosignature, VRE has been deeply concerned with and widely studied by astrobiologists.

To improve the detectability of VRE signals of exoplanets and extract surface features of exoplanets based on VRE, the research team of He Jinping professor from National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences recently made some new progress on the detection of VRE time-series variation features of exoplanets. They retrieved the continent distribution of exoplanets based on the VRE diurnal variation signal. The related work was published in The Astrophysical Journal (https://doi.org/10.3847/1538-4357/abd6ff) on March 2nd.

The team established a model to study the VRE diurnal variation of the Earth and exoplanets under different observation geometries (as shown in Figure 1) and found that the detectability and the VRE index vary greatly with different observation conditions. Some observation conditions (such as φ =-60, Full & Gibbous phase) can't detect the positive VRE index even if observed all day, while under specific observation conditions (such as φ = 60, Full & Gibbous phase), the obvious vegetation reflectance signal (positive VRE value) can be observed all the time.

Figure 1. The VRE diurnal variation curve under different observation geometries

This shows that the optimized observation strategy is very important for the detection of extraterrestrial plants based on VRE. The team also established a simplified reverse model and realized the inversion of terrestrial planets' continental distribution and cloud cover by using VRE time-variation curve. Figure 2 shows the recovered continental distributions of modern and ancient Earth, which are in good agreement with the actual distribution curves. The continental distribution and the cloud fraction can be recovered simultaneously with high accuracy.
Figure 2. Land fraction recovered from the VRE variation data. Left: present Earth; right: Late Triassic Earth. The real land fraction derived from continental distribution maps is shown for reference (blue curve). The red and black dotted lines are the land fraction data retrieved at the phase of full and crescent, respectively.

To guide the follow-up observation, the team made mock observation validation for both the forward model and the reverse model. The mock observation is based on the low-resolution spectrometer (quantum efficiency= 25%) of LUVOIR-A (contrast ratio =10^ (-10), aperture = 15m), NASA's next-generation high-contrast direct imaging space telescope. Figure 3 shows the reflection spectrum curve obtained by observing the Earth at a distance of 10pc. The spectral resolution is 140, the signal-to-noise ratio is 35.6@760nm, and the time required to obtain the spectrum is 50 hours. As shown in Figure 4, based on the mock observation spectrum, the influence of spectral signal-to-noise ratio on the VRE time-varying curve is discussed, and on this basis, the distribution of continents is further recovered (SNR=30, Figure 5).
Figure 3. LUVOIR-A mock observation spectrum at the optical band (515–1030 nm) with a spectral resolution of 140.

Figure 4. Left: observational VRE data. The blue region is the prediction intervals of the time-series VRE signal smoothing. Right: averaged p-value curve with the varying S/N; the dashed region indicates the 1σ variability obtained from the 100 generated mock observations of each S/N.
Figure 5. Recovered land fraction computed with the mock observation data of S/N = 30 at quadrature (black dashed curve). Red dashed curve: land fraction recovered from the noise-free spectra.

This work will advance the study of extraterrestrial life based on VRE feature, and provide a new idea for the study of exoplanet surface characterization. In the future, the team will focus on the research of multi-messenger biosignature characterization of exoplanets and seek optimal observation strategy, which will provide a reference for the future exoplanets direct imaging program and the atmospheric and surface characterization of exoplanets in China.

The research was supported by the general project (No.11773045, No.11973009) and the key project (No.11933005) of the National Natural Science Foundation of China.