  Seasonal cycle of Martian relative humidity shows that, in the southern and
the northern RSL sites, humidity decreases when RSL increment while humidity is
high on the fading season. We suggest that the recurrence of RSL is caused by
the different angle of response between wet and dry grains. 2D DEM simulation
indicates that, when the ratio of a liquid bridge volume to a particle volume
is the order of 10^{-7}, the repose angle of Martian sand can increase to ~40
degrees due to cohesion, which is near the slope around RSL starting area.
Thus, if this amount of liquid bridge is generated by deliquescence or
capillary condensation of vapor and if the water evaporates following the
humidity and temperature cycles, Martian sand can induce avalanche on RSL
slope. When grains are wet again, sand and dust deposit on RSL, which may be
the mechanism of RSL disappearance and recharge of grains for the next season.
In our model, because liquid induces RSL indirectly (RSL itself is the granular
flow), the amount of liquid can be drastically reduced compared with the wet
mechanism. In this case, as well as temperature, the seasonal cycle of relative
humidity plays an important role in the recurrence of RSL.
