  An increasing number of potentially habitable terrestrial planets and planet
candidates are found by ongoing planet search programs. The search for
atmospheric signatures to establish planetary habitability and the presence of
life might be possible in the future. We want to quantify the accuracy of
retrieved atmospheric parameters which might be obtained from infrared emission
spectroscopy. We use synthetic observations of hypothetical habitable planets,
constructed with a parametrized atmosphere model, a high-resolution radiative
transfer model and a simplified noise model. Classic statistical tools such as
chi2 statistics and least-square fits were used to analyze the simulated
observations. When adopting the design of currently planned or proposed
exoplanet characterization missions, we find that emission spectroscopy could
provide weak limits on surface conditions of terrestrial planets, hence their
potential habitability. However, these mission designs are unlikely to allow to
characterize the composition of the atmosphere of a habitable planet, even
though CO2 is detected. Upon increasing the signal-to-noise ratios by about a
factor of 2-5 (depending on spectral resolution) compared to current mission
designs, the CO2 content could be characterized to within two orders of
magnitude. The detection of the O3 biosignature remains marginal. The
atmospheric temperature structure could not be constrained. Therefore, a full
atmospheric characterization seems to be beyond the capabilities of such
missions when using only emission spectroscopy during secondary eclipse or
target visits. Other methods such as transmission spectroscopy or orbital
photometry are probably needed in order to give additional constraints and
break degeneracies. (abridged)
