  Early Mars may have had a warmer and denser atmosphere allowing for the
presence of liquid water on the surface. However, climate model studies have
not been able to reproduce these conditions even with a CO2 atmosphere of
several bars. Recent 3D simulations of the early Mars climate show that mean
surface temperatures only slightly below 273K could be reached locally.
  We want to investigate the effect of increased partial pressures of N2 on
early Mars' surface temperature by including pressure broadening of absorption
lines and collision-induced N2-N2 absorption.
  A 1D radiative-convective cloud-free atmospheric model was used to calculate
temperature profiles and surface conditions. We performed a parameter study
varying the N2 partial pressures from 0 to 0.5bar at CO2 partial pressures
between 0.02bar and 3bar. These values are consistent with existing estimates
of the initial, pre-Noachian reservoir. Solar insolation was set to be
consistent with the late Noachian.
  Our 1D global mean simulations clearly show that enhanced N2 content in the
Martian atmosphere could have increased surface temperatures. An additional
greenhouse warming of up to 13K was found at a high N2 partial pressure of
0.5bar. Still, even at this N2 partial pressure, global mean surface
temperatures remained below 273K, i.e. the freezing point of water. However,
given the magnitude of the N2-induced surface warming and the results of recent
3D studies which show that local mean surface temperatures are not much lower
than 273K, our results imply that the presence of atmospheric N2 could have led
to almost continously habitable mean surface conditions in some regions. In
addition, atmospheric water column amounts increased by up to a factor of 6 in
response to the surface warming, indicating that precipitation might also
increase upon increasing N2 partial pressure.
