Biomarkers in disk-averaged near-UV to near-IR Earth spectra using Earthshine observations

Context. The detection of exolife is one of the goals of very ambitious future space missions or extremely large ground-based telescopes that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet's atmosphere (O-2, O-3, CO2 etc.) have been identified as possible global biomarkers, we analyse here the detectability of vegetation on a global scale on Earth's surface. Aims. Considering its specific reflectance spectrum showing a sharp edge around 700 nm, vegetation can be considered as a potential global biomarker. This work, based on observational data, aims to characterise and to quantify this signature in the disk-averaged Earth's spectrum. Methods. Earthshine spectra have been used to test the detectability of the Vegetation Red Edge (VRE) in the Earth spectrum. We obtained reflectance spectra from near UV (320 nm) to near IR (1020 nm) for different Earth phases (continents or oceans seen from the Moon) with EMMI on the NTT at ESO/La Silla, Chile. We accurately correct the sky background and take into account the phase-dependent colour of the Moon. VRE measurements require a correction of the ozone Chappuis absorption band and Rayleigh plus aerosol scattering. Results. The near-UV spectrum shows a dark Earth below 350 nm due to the ozone absorption. The Vegetation Red Edge is observed when forests are present (4.0% for Africa and Europe), and is lower when clouds and oceans are mainly visible (1.3% for the Pacific Ocean). Errors are typically +/- 0.5, and +/- 1.5 in the worst case. We discuss the different sources of errors and bias and suggest possible improvements. Conclusions. We showed that measuring the VRE or an analog on an Earth-like planet remains very difficult (photometric relative accuracy of 1% or better). It remains a small feature compared to atmospheric absorption lines. A direct monitoring from space of the global (disk-averaged) Earth's spectrum would provide the best VRE follow-up.