LUCAS experiment:
Spectroscopic observations of Earthshine in Antarctica
Danielle BRIOT
with the contribution of Luc ARNOLD
Main collaborators :
Eric Aristidi (LUAN, Nice)
Luc Arnold (Obs. Haute-Provence) Jérome Berthier (Obs.Paris-Meudon) Danielle Briot (Obs.Paris-Meudon) Eric Fossat (LUAN, Nice)
Stéphane Jacquemoud (IPGP, Paris) Pierre Riaud (Obs. Paris-Meudon) Jean Schneider (Obs. Paris-Meudon) and the winterover observers in
Antarctica : Karim Agabi, Eric
Aristidi, Erick Bondoux and Zalpha Challita.
Looking for life
in Extrasolar Planets
• Today, 16 April 2008,
287 extrasolar planets are known.
No telluric plane located in the Habitability Zone has been discovered up to now, as
well as no image of an extrasolar planet has been obtained.
However, we have to prepare the analysis of the results to come from future space
missions.
• Preparation of High contrast imaging space missions
– Darwin (infrared) Despite of ESA Cosmic Vision conclusions
– TPF-C coronagraph (visible) – More: See-Coast, Luciola, etc.
These instruments 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
Shall we be able to detect life on an
unresolved Earth-like extrasolar planet ?
• Future space missions will provide us with the first images and low-resolution spectra of these planets.
• Let us consider a unresolved extrasolar Earth-like planet, the spectrum of the light reflected by the planet, when normalized to the parent star
spectrum, gives the planet reflectance spectrum revealing its atmospheric and ground colour.
What would the spectrum of an
unresolved Earth-like planet look like ?
• Life on an extrasolar planet would be probably very different from life on Earth, which is the only known planet sheltering life.
• So a way to answer this question is to consider how the spectrum of our Earth would look like if
observed from a very large distance, typically several parsecs. This can be done from a space probe
traveling into the Solar System and looking back at the Earth as Voyager 1 did in 1990, or Mars Express in 2003.
Earth viewed from 6.4 109 km 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, following an idea of Jean
Schneider (1998), consists in taking a spectrum of the Moon Earthshine, i.e. the Earth light
backscattered by the nonsunlit 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.
Earthshine
Pathway of the Light
when observing Earthshine
Some historic points….
• It seems that Leonardo Da Vinci was the first who clearly understood the origin of the phenomenon of Earthshine, but the first publication was made by Galileo !
• The potential of the Moon’s Earthshine for
providing global data on the Earth was identified during the XIXe century (Flammarion, 1877), and may be earlier.
• In 1912, Arcichovsky suggested looking for
chlorophyll absorption in the Earthshine spectrum to calibrate chlorophyll in the spectrum of other planets. This approach was completely forgotten up to 1998…
What shall we look for in this spectrum ?
• Look for the signature of molecules in the atmosphere (biogenic products ?)
O2, O3, CH4, H2O, CO2
• Look for biologic activity:
the planet colour: vegetation, pigments
• Look for missing photons used in a photosynthesis
• Possible artifacts (i.e. false positive detection):
minerals, rocks
Detection of vegetable life
• Vegetation indeed has a very high
reflectivity in the near infrared, higher than in the visible spectrum by a factor of
approx. 5.
• This produces a sharp edge around
approx. 700 nm, the so-called Vegetation
Red Edge (VRE).
Vegetation and water at 1.1µm
Vegetation appears very bright and water very dark
Red edge
(Clark 1999) Reflectance spectra of green vegetation,
dry vegetation and soil
• Since 2002, several observations have been made of the VRE.
• Even with a VRE of only a few percents, vegetation signature is detectable in an integrated (or disk-averaged) Earth
spectrum.
When an ocean is facing the Moon, the VRE is smaller than when Africa or Asia is
facing the Moon.
A blue Earth, O3, O2, H2O, vegetation signature (Arnold et al 2002)
morning evening
O3
O2 O2
H2O Rayleigh
H2O (O2)2
H2O
Clark 1999
Hamdani et al. 2006 (A&A) VRE = 1 to 4%
Observation from Chile (NTT@ESO with EMMI)
Dark Earth in near-UV (<360nm) Ozone absorption ! Rayleigh visible down to 360nm
VRE values from spectroscopy
or models.
Other features of the Earth spectrum revealed by the Earthshine observations
• In addition to the Vegetation Red Edge, - the red side (600 - 1000 nm) of the Earth
reflectance spectrum shows the presence of O
2and H
2O absorption bands,
- while the blue side (320 - 620 nm) clearly shows the Huggins and Chappuis ozone
(O
3) absorption bands (Hamdani et al. 2006)
« Restrictions » of Earthshine observations
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. So observations 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).
Perspectives
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.
From six to eight 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.
Preliminary observations at Concordia
Checking the feasibility 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 conclusive observations.
In 2006, photographic tests made by Eric Aristidi clearly showed that Earthshine observations can be possible.
Eathshine observed by Eric Aristidi
at Concordia, 26 February 2006
So, it appears that Earthshine observations are possible from Concordia, and we plan to make observations to observe Vegetation Red Edge and biomarkers during the southern winter of 2008.
• The funds have been obtained from the P7 University, and the IPGP (Institut de Physique du Globe de Paris). 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.
LUCAS design
The telescope is a Celestron 8 Diametre : 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
Δλ = 461 → 900 nm, resolution >100
3D assembly diagram of LUCAS
LUCAS instrumentation during testing
at the Haute-Provence Observatory
LUCAS technology
Acquisition and storage of observational data will be provided by a computer located in a « igloo » at around twenty meters of the telescope.
Testings 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.
Current situation
From Haute-Provence Observatory (South of France) to Concordia the travel was:
• Plouzané (Institut Paul Emile Victor, in the western part of France),
• Roissy (Paris airport),
• Hong-Kong (China),
• Hobart (Australia),
• Dumont-d’Urville (Antarctica)
• and finally attained the Concordia station at Dome C on the 9th of January 2008.
Actually, LUCAS takes place among the first « purely » astrophysical experiments at the Dome C site.
LUCAS at Concordia
Possible observing times
• Observations of Earthshine are possible during 6 to 8 periods in a year, around the Equinoxes.
• Observations around the equinox of March correspond to the last days of the Lunar
Cycle. Observations around the equinox of
September correspond to the first days of
the Lunar cycle.
Possible observations dates of Earthshine at Concordia in 2008
Ephemerids have been calculated by Patrick Rocher and Jérôme Berthier.
1) 28 February - 04 March 2) 29 March - 04 April
3) 27 April - 01 May 4) 27 May - 29 May 5) 07 July - 10 July
6) 04 August - 08 August
7) 02 September - 07 September 8) 01 October - 10 October
Conclusion
• Actually, LUCAS is the first program with spectroscopic observations at Dome C.
As such, it is also a test for the design and improvement of small instrumentation, data collecting and management of observations in Concordia extremely cold
environnement.
The dream of every exoplanetologist
A planet with a very easily detectable
Vegetation, without clouds and without oceans
The baobabs
from « Le Petit Prince », Antoine de Saint-Exupéry
