  Earth-like planets within the liquid water habitable zone of M type stars may
evolve into synchronous rotators. On these planets, the sub-stellar hemisphere
experiences perpetual daylight while the opposing anti-stellar hemisphere
experiences perpetual darkness. Because the night-side hemisphere has no direct
source of energy, the air over this side of the planet is prone to freeze out
and deposit on the surface, which could result in atmospheric collapse.
However, general circulation models (GCMs) have shown that atmospheric dynamics
can counteract this problem and provide sufficient energy transport to the
anti-stellar side.
  Here we use an idealized GCM to consider the impact of geothermal heating on
the habitability of synchronously rotating planets. Geothermal heating may be
expected due to tidal interactions with the host star, and the effects of
geothermal heating provide additional habitable surface area and may help to
induce melting of ice on the anti-stellar hemisphere. We also explore the
persistence of atmospheric asymmetries between the northern and southern
hemispheres, and we find that the direction of the meridional circulation (for
rapidly rotating planets) or the direction of zonal wind (for slowly rotating
planets) reverses on either side of the sub-stellar point. We show that the
zonal circulation approaches a theoretical state similar to a Walker
circulation only for slowly rotating planets, while rapidly rotating planets
show a zonal circulation with the opposite direction. We find that a
cross-polar circulation is present in all cases and provides an additional
mechanism of mass and energy transport from the sub-stellar to anti-stellar
point. Characterization of the atmospheres of synchronously rotating planets
should include consideration of hemispheric differences in meridional
circulation and examination of transport due to cross-polar flow.

Earth and Planetary Astrophysics;Atmospheric and Oceanic Physics