12 research outputs found
A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum
We report spectroscopic observations (400 to 800nm, R = approx 100) of
Earthshine in June, July and October 2001 from which normalised Earth albedo
spectra have been derived. The resulting spectra clearly show the blue colour
of the Earth due to Rayleigh diffusion in its atmosphere. They also show the
signatures of oxygen, ozone and water vapour. We tried to extract from these
spectra the signature of Earth vegetation. A variable signal (4 to 10 +/-3%)
around 700nm has been measured in the Earth albedo. It is interpreted as being
due to the vegetation red edge, expected to be between 2 to 10% of the Earth
albedo at 700nm, depending on models. We discuss the primary goal of the
present observations: their application to the detection of vegetation-like
biosignatures on extrasolar planets.Comment: 7 pages, 7 figures. A&A, accepted 6 May 200
Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants
Earth's deciduous plants have a sharp order-of-magnitude increase in leaf
reflectance between approximately 700 and 750 nm wavelength. This strong
reflectance of Earth's vegetation suggests that surface biosignatures with
sharp spectral features might be detectable in the spectrum of scattered light
from a spatially unresolved extrasolar terrestrial planet. We assess the
potential of Earth's step-function-like spectroscopic feature, referred to as
the "red edge", as a tool for astrobiology. We review the basic characteristics
and physical origin of the red edge and summarize its use in astronomy: early
spectroscopic efforts to search for vegetation on Mars and recent reports of
detection of the red edge in the spectrum of Earthshine (i.e., the spatially
integrated scattered light spectrum of Earth). We present Earthshine
observations from Apache Point Observatory to emphasize that time variability
is key to detecting weak surface biosignatures such as the vegetation red edge.
We briefly discuss the evolutionary advantages of vegetation's red edge
reflectance, and speculate that while extraterrestrial "light harvesting
organisms" have no compelling reason to display the exact same red edge feature
as terrestrial vegetation, they might have similar spectroscopic features at
different wavelengths than terrestrial vegetation. This implies that future
terrestrial-planet-characterizing space missions should obtain data that allow
time-varying, sharp spectral features at unknown wavelengths to be identified.
We caution that some mineral reflectance edges are similar in slope and
strength to vegetation's red edge (albeit at different wavelengths); if an
extrasolar planet reflectance edge is detected care must be taken with its
interpretation.Comment: 19 pages, 6 figures, to appear in Astrobiolog
Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works
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 (,
, 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.Comment: 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 tabl