The vegetation red edge (VRE) is a unique spectral fingerprint of light-harvesting vegetation on Earth and provides a robust remote detectable surface biosignature of exoplanets. To improve the detectability and sensitivity, we have studied the diurnal variability of VRE in the disk-integrated spectra of Earth and also Earth analogs in the case of different observing geometry conditions. Simulation results show that the VRE index varies from <−0.4 to >0.6 at a diurnal timescale for both present and also Late Triassic Earth, and the maximum variation of VRE in 1 day changes by >3 times with different observing geometry conditions. This means that the extraterrestrial light-harvesting vegetation (even if it really exists) will not be efficiently detectable without proper observing geometry conditions and time, especially in the case of the exoplanets covered with thick clouds. The VRE temporal variation curve can also be used to retrieve the cloud cover fraction and continent distribution of exoplanets with relatively high precision. Several observational strategies are proposed to detect the light-harvesting vegetation and retrieve the planetary information from the planet’s VRE variation signals, and a mock observation is also demonstrated.
Direct-imaging spectra hold rich information about a planet’s atmosphere and surface, and several space-based missions aiming at such observations will become a reality in the near future. Previous spectral retrieval works have resulted in key atmospheric constraints under the assumption of a gray surface, but the effect of wavelength-dependent surface albedo on retrieval has not been shown. We explore the influence of the coupling effect of cloud and wavelength-dependent surface albedo on retrieval performance via modeling suites of Earth-like atmospheres with varying cloud and surface albedo parameterizations. Under the assumption of known cloud scattering properties, the surface spectral albedos can be reasonably recovered when the surface cover represents that of Earth-like vegetation or ocean, which may aid in characterizing the planet’s habitability. When the cloud scattering properties cannot be assumed, we show that the degeneracy between the cloud properties and wavelength-dependent surface albedo leads to biased results of atmospheric and cloud properties. The multiepoch visible-band observations offer limited improvement in disentangling this degeneracy. However, the constraints on atmospheric properties from the combination of the UV band (R ∼ 6) + visible band (R ∼ 140) are consistent with input values to within 1σ. If short-bandpass data are not available, an alternative solution to reduce the retrieval uncertainties would be to have the prior constraints on the planetary cloud fraction with less than 20% uncertainty.
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