Titan's global asymmetry in lake distribution and implications

Titan's global hydrocarbon cycle includes lakes and seas discovered and mapped by the Cassini Orbiter. Radar data show that poleward of about 60°N the lakes and seas are interpreted to be in a range of states, from filled to partially filled, to empty. In the south, observational radar coverage is significantly sparser, but the one pass obtained indicates a significant paucity in lakes relative to the north, which will be tested by further observations. Hemispheric differences where also apparent in telescopic observations. The asymmetry in lake distribution has been suggested to be due to a number of factors including 1) seasonal variations, 2) a topographic asymmetry, and 3) a seasonal asymmetry. With fewer empty lakes seen in the south and the basins' expected persistence for durations longer than the 29.7 year seasonal period , seasonal variations now seem an unlikely explanation for the asymmetry. A topographic, or subsurface lithologic asymmetry could be responsible for the precipitation, drainage, and infiltration rates, but large asymmetries have not been identified so far, using initial data of direct altimetry and SAR-Topo techniques from the Cassini Radar. Still, this remains a possibility. The observed asymmetry in lake distribution may be due to the asymmetry in Titan's seasons due the properties of Saturn's orbit. Saturn's obliquity is 26.7° (Titan's inclination and obliquity are 0.33° and 0.6°, respectively), its longitude of perihelion passage is Ls,p=279°.46 (near northern winter solstice) and orbital eccentricity is e=0.054. Hence southern summers are shorter and more intense than their northern counterparts and the asymmetry in insolation is about 12 percent (or twice that peak-to-peak). In this scenario, an enhancement in insolation driven evaporation, precipitation, or both would account for the unequal accumulation of hydrocarbon lakes. The candidate liquids, methane and ethane, have a volatility difference of a factor of nearly 104 at the relevant 92 K polar temperature, and a correspondingly shorter transport timescale. Methane, therefore, may be moved seasonally between the polar reservoirs while ethane's distribution would reflect a longer-term insolation history. However, depending on the rate with which the system may respond to changes, this asymmetry would disappear and reverse with Saturn's precession of perihelion passage, eccentricity variations, and position of spin axis. The period of Saturn's perihelion precession is roughly 60 kyr (currently -19.4889 arcsec/yr), and the other orbital parameter variations will further modulate the signal. If the volatile transport timescales are sufficiently fast and the lake distribution indeed records the forced seasonal asymmetry, then the lake regions on Titan, poleward of ~60° in both hemispheres, have a surface age younger than this orbital timescale. The upper limit on the age is very low, especially when considering the identification of several (~5) craters on the surface. These craters reside in the low latitudes and indicate a surface age of between 200 Myr and 1 Gyr.