  An important goal within the quest for detecting an Earth-like extrasolar
planet, will be to identify atmospheric gaseous bio-signatures. Observations of
the light transmitted through the Earth's atmosphere, as for an extrasolar
planet, will be the first step for future comparisons. We have completed
observations of the Earth during a Lunar eclipse, a unique situation similar to
that of a transiting planet. We aim at showing what species could be detected
in its atmosphere at optical wavelengths, where a lot of photons are available
in the masked stellar light. We present observations of the 2008 August 16 Moon
eclipse performed with the SOPHIE spectrograph at the Observatoire de
Haute-Provence. Locating the spectrograph fibers in the penumbra of the
eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and
solar light that has passed through the Earth's limb. This mixture essentially
reproduces what is recorded during the transit of an extrasolar planet. We
report here the clear detection of several Earth atmospheric compounds in the
transmission spectra, such as ozone, molecular oxygen, and neutral sodium as
well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover,
we present a method that allows us to derive the thickness of the atmosphere
versus the wavelength for penumbra eclipse observations. We quantitatively
evaluate the altitude at which the atmosphere becomes transparent for important
species like molecular oxygen and ozone, two species thought to be tightly
linked to the presence of life. The molecular detections presented here are an
encouraging first attempt, necessary to better prepare for the future of
extremely-large telescopes and transiting Earth-like planets. Instruments like
SOPHIE will be mandatory when characterizing the atmospheres of transiting
Earth-like planets from the ground and searching for bio-marker signatures.
