Several clues indicate that Titan ’ s atmosphere has been depleted in methane during some period of its history , possibly as recently as 0.5-1 billion years ago .
It could also happen in the future .
Under these conditions , the atmosphere becomes only composed of nitrogen with a range of temperature and pressure allowing liquid or solid nitrogen to condense .
Here , we explore these exotic climates throughout Titan ’ s history with a 3D Global Climate Model ( GCM ) including the nitrogen cycle and the radiative effect of nitrogen clouds .
We show that for the last billion years , only small polar nitrogen lakes should have formed .
Yet , before 1 Ga , a significant part of the atmosphere could have condensed , forming deep nitrogen polar seas , which could have flowed and flooded the equatorial regions .
Alternatively , nitrogen could be frozen on the surface like on Triton , but this would require an initial surface albedo higher than 0.65 at 4 Ga .
Such a state could be stable even today if nitrogen ice albedo is higher than this value .
According to our model , nitrogen flows and rain may have been efficient to erode the surface .
Thus , we can speculate that a paleo-nitrogen cycle may explain the erosion and the age of Titan ’ s surface , and may have produced some of the present valley networks and shorelines .
Moreover , by diffusion of liquid nitrogen in the crust , a paleo-nitrogen cycle could be responsible of the flattening of the polar regions and be at the origin of the methane outgassing on Titan .