Contrasting responses to orbital precession in Titan's and Earth's atmospheres

The atmospheres of Earth and of Saturn's moon Titan exhibit contrasting responses to orbital precession. Data collected by the Cassini Spacecraft indicate that lakes on Titan are primarily found in polar regions, preferentially in the north. It has been suggested that the hemispherical asymmetry in lake distribution is related to Saturns orbital precession, which changes the seasonal distribution of the solar radiation, but not the annual mean [Aharonson et al., 2009; Schneider et al., 2012]. In Titan's current climate, the longitude of perihelion is near the northern winter solstice, and the northern summer is longer and dimmer than the southern summer. Simulations with a Titan General Circulation Model (GCM) indicated that the longer northern summer leads to greater net precipitation in the annual mean and to the methane accumulation in the northern polar region [Schneider et al., 2012]. However, paleoclimate proxies on Earth indicate that annual-mean precipitation increases in the hemisphere with the brighter and shorter summer, and decreases in the other hemisphere. Thus, the annual-mean precipitation in Earth's and Titan's atmosphere exhibits contradictory responses to the planetary orbital precession.

Here we show that these different responses arise because of the different rotation rates of Titan and Earth. Changing of planetary rotation rates modifies the width of the Hadley cell, the meridional moisture transport, and the location of the precipitation, thus modifying the overall dynamical regime of the planetary atmosphere. This in turn drives the different responses to orbital precession on Earth and on Titan.