  Temperate terrestrial exoplanets are likely common objects, but their
discovery and characterization is very challenging. Concepts for optimized
space missions to overcome these challenges are being studied. The LIFE
initiative focuses on the development of a space-based mid-infrared (MIR)
nulling interferometer probing the thermal emission of a large sample of
exoplanets.
  We derive first estimates for the signal-to-noise (S/N), spectral resolution
(R), and wavelength requirements for LIFE. Using an Earth-twin exoplanet as
reference case, we quantify how well planetary/atmospheric properties can be
constrained from MIR spectra of different quality.
  We simulate LIFE observations of an Earth-twin orbiting a G2V star at 10 pc
from the Sun with different levels of exozodiacal dust emissions. We combine a
cloud-free 1D radiative transfer model and the nested sampling algorithm to
retrieve planetary/atmospheric properties from input spectra of different
wavelength coverage, R, and S/N.
  We find that H2O, CO2, and O3 are detectable if S/N$\geq$10 (uncertainty
$\leq\pm1.0$ dex). We find upper limits for N2O (abundance $\leq10^{-3}$). CO,
N2, and O2 are unconstrained in all cases. The limit for a CH4 detection is R
$= 50$, S/N $=10$. We further correctly determine the exoplanet radius
(uncertainty $\leq\pm10\%$), surface temperature (uncertainty $\leq\pm20$K),
and surface pressure (uncertainty $\leq\pm0.5$ dex). With the current LIFE
design, the observation time required to reach the specified S/N amounts to
$\sim7$ weeks with 4x2m apertures.
  We conclude that a minimum wavelength coverage of $4-18.5\mu$m, a R of 50 and
an S/N of 10 is required. With the current assumptions, the atmospheric
characterization of several Earth-like exoplanets at a distance of 10 pc and
within a reasonable amount of observing time will require apertures $\geq2$
meters.
