Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works

Abstract

The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet's atmosphere ($O_2$, $O_3$, $C{O}_2$ etc.) have been identified as possible global biomarkers, we review here the detectability of a signature of life from the planet's surface, i.e. the green vegetation. The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the "Vegetation Red Edge" (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant. Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum. The egetation spectral signature is detected as a small 'positive shift' of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earth's phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better. Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of 'red edge' universality in the Universe.

Invited talk in "Strategies for Life Detection" (ISSI Bern, 24-28 April 2006) to appear in a hardcopy volume of the ISSI Space Science Series, Eds, J. Bada et al., and also in an issue of Space Science Reviews. 13 pages, 8 figures, 1 table

References in corpus (6)

• Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants
• The Spectrum of Earthshine: A Pale Blue Dot Observed from the Ground
• A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum
• Vegetation signature in the observed globally-integrated spectrum of Earth: Modeling the red edge strength using simultaneous cloud data and application for extrasolar planets
• Globally integrated measurements of the Earth's visible spectral albedo
• Biomarkers in disk-averaged near-UV to near-IR Earth spectra using Earthshine observations

Cited by in corpus (16)

• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life
• Exoplanet Atmospheres
• How to Characterize Habitable Worlds and Signs of Life
• Exoplanet Biosignatures: A Framework for Their Assessment
• The Earth as an extrasolar transiting planet: Earth's atmospheric composition and thickness revealed by Lunar eclipse observations
• Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars
• Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy
• Phase Variation of Earthshine Polarization Spectra
• The Earth as an extrasolar planet: The vegetation spectral signature today and during the last Quaternary climatic extrema
• Polarimetric signature of the oceans as detected by near-infrared Earthshine observations
• Tohoku-Hiroshima-Nagoya planetary spectra library: A method for characterizing planets in the visible to near infrared
• Climates of Warm Earth-Like Planets III: Fractional Habitability from a Water Cycle Perspective
• Surface and Temporal Biosignatures
• Purple is the new green: biopigments and spectra of Earth-like purple worlds
• Photometric Variability in Earthshine Observations
• The influence of Jupiter, Mars and Venus on Earth's orbital evolution