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The atmospheric composition of transiting exoplanets can be characterizedduring transit by spectroscopy. For the transit of an Earth twin, modelspredict that biogenic $O_2$ and $O_3$ should be detectable, as well as watervapour, a molecule linked to habitability as we know it on Earth. The aim is tomeasure the Earth radius versus wavelength $\lambda$ - or the atmospherethickness $h(\lambda)$ - at the highest spectral resolution available to fullycharacterize the signature of Earth seen as a transiting exoplanet. We presentobservations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Eartheclipses the Sun and opens access to the Earth atmosphere transmissionspectrum. We used HARPS and UVES spectrographs to take penumbra and umbrahigh-resolution spectra from 3100 to 10400 Ang. A change of the quantity ofwater vapour above the telescope compromised the quality of the UVES data. Wecorrected for this effect in the data processing. We analyzed the data by 3different methods. The 1st method is based on the analysis of pairs of penumbraspectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of allpenumbra and umbra spectra. Profiles $h(\lambda)$ are obtained with the threemethods for both instruments. The 1st method gives the best result, inagreement with a model. The second method seems to be more sensitive to theDoppler shift of solar spectral lines with respect to the telluric lines. The3rd method makes use of umbra spectra which bias the result, but it can becorrected for this a posteriori from results with the first method. The 3methods clearly show the spectral signature of the Rayleigh scattering in theEarth atmosphere and the bands of H$_2$O, O$_2$, and O$_3$. Sodium is detected.Assuming no atmospheric perturbations, we show that the E-ELT is theoreticallyable to detect the $O_2$ A-band in 8~h of integration for an Earth twin at10pc.

The Earth as an extrasolar transiting planet - II: HARPS and UVES detection of water vapor, biogenic O$_2$, and O$_3$