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LUCAS experiment: Spectroscopy of Earthshine in Antarctica for Detection of Life. Danielle BRIOT (Observatoire de Paris). Main collaborators : Eric Aristidi (LUAN, Nice) Luc Arnold (Obs. Haute-Provence) Jérome Berthier (Obs.Paris-Meudon) Danielle Briot (Obs.Paris-Meudon)
Eric Aristidi (LUAN, Nice)
Luc Arnold (Obs. Haute-Provence)
Jérome Berthier (Obs.Paris-Meudon)
Danielle Briot (Obs.Paris-Meudon)
Stéphane Jacquemoud (IPGP, Paris)
Patrick Rocher (Obs.Paris-Meudon)
Jean Schneider (Obs. Paris-Meudon)
and the winterover observers in Antarctica :
Karim Agabi, Eric Aristidi, Erick Bondoux,
Zalpha Challita, Denis Petermann and Cyprien Pouzenc
347 extrasolar planetsare known.
Possibly one super-Earth detected in the habitable zone (Gliese 581 d)
Therefore, we have to prepare the analysis of the results to come from future space missions.
Theseinstruments will (hopefully) provide us with
1/ unresolved images of extra-solar planets in the HZ
2/ spectra to give us first insights into planet chemistry
Voyager-1, 14th feb. 1990
Shall we be able to detect life on an
unresolved Earth-like extrasolar planet ?
An alternative method to obtain the Earth-averaged spectrum, according to an idea of Jean Schneider (1998), consists in measuring the reflectance spectrum of the Moon Earthshine, i.e. the light back-scattered by the non-sunlit Moon.
A spectrum of the Moon Earthshine directly gives the disk-averaged spectrum of the Earth.
Because of the lunar surface roughness, any place
of the Earthshine reflects the totality of the enlighted
part of the Earth facing the Moon.
O2, O3, CH4, H2O, CO2
the planet colour: vegetation, pigments
Reflectance spectra of green vegetation, dry vegetation and soil
A blue Earth, O3, O2, H2O, vegetation signature (Arnold et al 2002)
As it is well known, at mean or low latitudes, Earthshine observations are possible during twilight: just after the sunset or just before the sunrise. Soobservations can only be made during a short period of time.
And roughly speaking, for one telescope, only two enlighted parts of Earth can be facing the Moon :
• either the part located at the West of the observing telescope for evening observations (beginning of the lunar cycle),
• or the part of Earth located at the East of the observing telescope for morning observations (last days of the lunar cycle).
However, there are other possibilities.
From an idea of Jean Schneider (2002), if observations are made from a site located at a high latitude, conditions of Earthshine observations are different. About 8 times in a year, around equinoxes, Earthshine can be observed during several hours, and even, in very high latitude places, during a 24 hour duration (total nycthemere).
During these long observing windows, and due to the terrestrial rotation, different « landscapes » alternately face the Moon.
Actually, Antarctica offers a very good opportunity for this kind of observations.
• Checking thefeasibility of Earthshine observations
considering the darkness of the sky was the first point. Is
the sky enough dark to allow observations of Earthshine ?
• The first tests have been planned during the first
Winterover campaign in 2005, by Karim Agabi.
Unfortunately, bad weather conditions did not allow some
• In 2006, photographic tests made by Eric Aristidi clearly
showed that Earthshine observations could be possible.
• Funding has been obtained from the University of Paris 7, the PID-OPV of the CNRS and the PNTS. A collaboration has been established with specialists of the Vegetation remote-sensing.
• We designed and built a dedicated instrumentation for Earthshine spectroscopic observations in the extreme conditions corresponding to Concordia station.
The telescope is a Celestron 8
Diameter : 203 mm, F/D = 10
The telescope has been « antarctized » by « Optique et Vision »
The spectrograph was designed by Luc Arnold and Pierre Riaud and built at the Haute-Provence Observatory.
Grating : 300t/mm
CCD : Camera Audine - KAF 402ME CCD
= 461 900 nm, resolution >100
Acquisition and storage of observational data will be provided by a computer located in a « igloo » at about 20 meters from the telescope.
Tests carried out at the Haute-Provence observatory validated the instrumentation.
Before their departure, winterover observers made a visit at the Haute-Provence observatory to see LUCAS.
A very detailed handbook intented for winterover observers has been written.
Position of the slit on the lunar surface : The observing slit is to be positionned • first on both the lighted crescent and the sky, • and then on both the Earthshine and the sky.
The places to be observed on the Moon are precisely defined to avoid different physical characters corresponding to different places :
- On East side,
the slit is centered on Mare Spumans,
- and on West, on crater Hevelius,
both near the Equator.
The feedback we got from the first observing campaign in 2008 was very important to detect, analyze and correct instrumental problems due to extreme temperature and extreme physical conditions.
Some important instrumental improvements were carried out for the 2009 winterover campaign.
3)The flip-mirror was very unconvenient for the observer and we replaced it by a static beam-splitter. Here again, moving elements are to be restricted as possible.
4) An eyepiece initially installed to verify the pointing reveals also highly unconvenient and almost unsuable. It as been replaced by a videocamera and the observer controls the accuracy of the pointing from the "igloo".
Today, we have obtained several Moon spectra, but adjustment of the pointing has to be improved to avoid a bright ghost from direct Moon light that currently still compromises the record of faint Earthshine spectra.
Tests are done right now to adjust the pointing.
The next observational run is from :
the 17th of May to the 20th of May.
Thanks are due to Luc Arnold, Stéphane Jacquemoud and Jean Schneider for their help during the preparation of this talk.
A planet with a very
clouds and without
from « Le Petit Prince »,
Antoine de Saint-Exupéry